[PATCH 00/16] repack: incremental MIDX/bitmap-based repacking

[PATCH 00/16] repack: incremental MIDX/bitmap-based repacking

[Taylor Blau]

Note to the maintainer:

 * Now that tb/incremental-midx-part-3.2 is merged, this series is
   rebased on current 'master', which is 5361983c075 (The 22nd batch,
   2026-03-27) at the time of writing.

 * I suggest queueing as 'tb/incremental-midx-part-3.3'.

The below is a nearly identical cover letter to the one I sent for the
RFC-version[1] of this series. The only changes between the RFC version
and this one are:

 * A new commit to correctly handle certain cases when optimizing out
   MIDX writes.

 * Defined `strvec_init()` as a special case of `strvec_init_alloc()`
   based on feedback from Junio.

 * Minor ODB source-related fallout as a consequence of rebasing.

 * Add trace2 instrumentation to help follow what repack is doing in
   response to different packing configurations.

 * Fix various memory leaks.

The original cover letter is below:

------------------------------------------------------------------------

OVERVIEW
========

This series implements the third and final major component for an
incremental MIDX/bitmap-based repacking strategy. As a refresher, those
are:

 1. Refactoring code out of builtin/repack.c into individual compilation
    units outside of the builtin directory.

 2. Implementing MIDX layer compaction, i.e., the ability to combine a
    contiguous sequence of MIDX layers in an existing chain and replace
    them with a single layer representing the union of objects/packs
    among the compacted layers.

 3. (this series) An incremental repacking strategy that, unlike our
    current '--geometric' approach, does not rely on periodic
    all-into-one repacks.

BACKGROUND
==========

Today, a '--geometric' repack with '--write-midx' rewrites the entire
MIDX (and, when enabled, its reachability bitmap) on every invocation.
For most repositories this is acceptable. In large monorepos, these
costs can add up significantly, especially when those repositories are
repacked frequently.

The incremental MIDX support introduced in the earlier parts of this
effort allows us to append new layers to a MIDX chain without rewriting
anything. Combined with the support for MIDX compaction in 3.2, we now
have all of the building blocks needed to maintain an incrementally
growing and shrinking MIDX chain as part of the repack cycle.

STRATEGY
========

The incremental repacking strategy implemented in this series works
(roughly) as follows:

 1. Repack non-MIDX'd packs using an ordinary '--geometric' repack,
    optionally including packs from the tip MIDX layer if and only if it
    contains more than 'repack.midxNewLayerThreshold' number of packs.

 2. Prepare to write a new MIDX layer containing the freshly written
    pack(s) (and any surviving packs from the tip layer, if it was
    rewritten).

 3. Perform a compaction pass over adjacent MIDX layers to restore a
    geometric progression on object count among layers in the chain
    (determined by 'repack.midxSplitFactor').

 4. Write the new MIDX chain, link it into place, and remove redundant
    layers.

In effect, this approach encourages MIDX chains where:

 * older layers contain fewer, larger packs, and

 * newer layers contain many smaller packs.

As a result of the compaction pass, we prevent the chain itself from
ever growing too long.

This roughly matches the description of this algorithm I gave at Git
Merge last year, which is covered on slides 80-131 of this presentation:

    https://ttaylorr.com/presentations/git-merge-2025.pdf

THIS SERIES
===========

This series is organized roughly as follows:

 * The first three patches perform small-ish quality-of-life refactors
   within the MIDX machinery.

 * The next two patches introduce `--checksum-only` and `--base`
   options for `git multi-pack-index write` and `compact`, which are
   needed by the incremental repacking machinery to assemble MIDX
   chains without prematurely updating the chain file.

 * The next six patches prepare the repack infrastructure and pack
   geometry code for the new repacking strategy.

 * The final three patches introduce the new repacking machinery, expose
   it to users, and then extend it to work with non-geometric repacks in
   that order. The first two are the substantive patches of this series.

WHERE WE'RE AT
==============

This series delivers the final substantive component of this overall
effort to enable the new repacking strategy implemented here. There are
a couple of (comparatively much smaller) items that will be useful as
follow-on items, including:

 * "Reachability-infused" geometric repacking to emit small cruft packs
   to introduce other ways to update the set of cruft objects beyond a
   whole-repository traversal.

 * Richer bitmap configuration to determine which bitmap(s) to carry
   forward between adjacent MIDX layers when doing an incremental
   repack, to prevent an endless accumulation of reachability bitmaps.

Like I mentioned earlier, I think that both of those are significantly
smaller challenges than this and the previous series.

Thanks in advance for your review!

[1]: https://lore.kernel.org/git/cover.1771978829.git.me@ttaylorr.com/

Taylor Blau (16):
  midx-write: handle noop writes when converting incremental chains
  midx: use `string_list` for retained MIDX files
  strvec: introduce `strvec_init_alloc()`
  midx: use `strvec` for `keep_hashes`
  midx: introduce `--checksum-only` for incremental MIDX writes
  midx: support custom `--base` for incremental MIDX writes
  repack: track the ODB source via existing_packs
  midx: expose `midx_layer_contains_pack()`
  repack-midx: factor out `repack_prepare_midx_command()`
  repack-midx: extract `repack_fill_midx_stdin_packs()`
  repack-geometry: prepare for incremental MIDX repacking
  builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
  packfile: ensure `close_pack_revindex()` frees in-memory revindex
  repack: implement incremental MIDX repacking
  repack: introduce `--write-midx=incremental`
  repack: allow `--write-midx=incremental` without `--geometric`

 Documentation/config/repack.adoc        |  18 +
 Documentation/git-multi-pack-index.adoc |  19 +-
 Documentation/git-repack.adoc           |  44 +-
 builtin/multi-pack-index.c              |  48 +-
 builtin/repack.c                        |  90 ++-
 midx-write.c                            | 137 +++--
 midx.c                                  | 106 ++--
 midx.h                                  |  11 +-
 packfile.c                              |   2 +
 repack-geometry.c                       |  50 +-
 repack-midx.c                           | 717 +++++++++++++++++++++++-
 repack.c                                |  23 +-
 repack.h                                |  25 +-
 strvec.c                                |  15 +-
 strvec.h                                |   5 +
 t/meson.build                           |   1 +
 t/t5334-incremental-multi-pack-index.sh |  63 +++
 t/t5335-compact-multi-pack-index.sh     | 113 ++++
 t/t7705-repack-incremental-midx.sh      | 461 +++++++++++++++
 19 files changed, 1792 insertions(+), 156 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh


base-commit: 5361983c075154725be47b65cca9a2421789e410
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 01/16] midx-write: handle noop writes when converting incremental chains

[Taylor Blau]

When updating a MIDX, we optimize out writes that will result in an
identical MIDX as the one we already have on disk. See b3bab9d2729
(midx-write: extract function to test whether MIDX needs updating,
2025-12-10) for more details on exactly which writes are optimized out.

If `midx_needs_update()` can't rule out any of the obvious cases (e.g.,
the checksum is invalid, we're requesting a different version, or
performing compaction which always requires an update), then we compare
the packs we're writing to the packs we already know about. If there are
an equal number of packs being written as there are in any existing
MIDX layer(s), then we compare the packs by their name.

This comparison fails when we have an incremental MIDX chain with
at least two layers, since we do not recursively peel through earlier
layers, instead treating the `->pack_names` array of the tip MIDX layer
as containing all `m->num_packs + m->num_packs_in_base` packs.

Adjust this to instead look through the MIDX layers one by one when
comparing pack names. While we're at it, fix a typo above in the same
function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c                            | 18 ++++++++++--------
 t/t5334-incremental-multi-pack-index.sh | 16 ++++++++++++++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 0ff2e45aa7a..d0351bde493 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1152,7 +1152,7 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 
 	/*
 	 * Ensure that we have a valid checksum before consulting the
-	 * exisiting MIDX in order to determine if we can avoid an
+	 * existing MIDX in order to determine if we can avoid an
 	 * update.
 	 *
 	 * This is necessary because the given MIDX is loaded directly
@@ -1208,14 +1208,16 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 			BUG("same pack added twice?");
 	}
 
-	for (uint32_t i = 0; i < ctx->nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addstr(&buf, midx->pack_names[i]);
-		strbuf_strip_suffix(&buf, ".idx");
+	for (struct multi_pack_index *m = midx; m; m = m->base_midx) {
+		for (uint32_t i = 0; i < m->num_packs; i++) {
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, m->pack_names[i]);
+			strbuf_strip_suffix(&buf, ".idx");
 
-		if (!strset_contains(&packs, buf.buf))
-			goto out;
-		strset_remove(&packs, buf.buf);
+			if (!strset_contains(&packs, buf.buf))
+				goto out;
+			strset_remove(&packs, buf.buf);
+		}
 	}
 
 	needed = false;
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index 99c7d44d8e9..c9f5b4e87aa 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -132,4 +132,20 @@ test_expect_success 'relink existing MIDX layer' '
 
 '
 
+test_expect_success 'non-incremental write with existing incremental chain' '
+	git init non-incremental-write-with-existing &&
+	test_when_finished "rm -fr non-incremental-write-with-existing" &&
+
+	(
+		cd non-incremental-write-with-existing &&
+
+		git config set maintenance.auto false &&
+
+		write_midx_layer &&
+		write_midx_layer &&
+
+		git multi-pack-index write
+	)
+'
+
 test_done
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 02/16] midx: use `string_list` for retained MIDX files

[Taylor Blau]

Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
build a list of filenames to keep while pruning stale MIDX files. Today
they hand-roll an array instead of using a `string_list`, thus requiring
us to pass an additional length parameter, and makes lookups linear.

Replace the bare array with a `string_list` which can be passed around
as a single parameter. Though it improves lookup performance, the
difference is likely immeasurable given how small the keep_hashes array
typically is.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 53 +++++++++++++++++++++++------------------------------
 1 file changed, 23 insertions(+), 30 deletions(-)

diff --git a/midx.c b/midx.c
index 81d6ab11e6e..d322743f2db 100644
--- a/midx.c
+++ b/midx.c
@@ -758,8 +758,7 @@ int midx_checksum_valid(struct multi_pack_index *m)
 }
 
 struct clear_midx_data {
-	char **keep;
-	uint32_t keep_nr;
+	struct string_list keep;
 	const char *ext;
 };
 
@@ -767,15 +766,12 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 				const char *file_name, void *_data)
 {
 	struct clear_midx_data *data = _data;
-	uint32_t i;
 
 	if (!(starts_with(file_name, "multi-pack-index-") &&
 	      ends_with(file_name, data->ext)))
 		return;
-	for (i = 0; i < data->keep_nr; i++) {
-		if (!strcmp(data->keep[i], file_name))
-			return;
-	}
+	if (string_list_has_string(&data->keep, file_name))
+		return;
 	if (unlink(full_path))
 		die_errno(_("failed to remove %s"), full_path);
 }
@@ -783,48 +779,45 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash)
 {
-	struct clear_midx_data data;
-	memset(&data, 0, sizeof(struct clear_midx_data));
+	struct clear_midx_data data = {
+		.keep = STRING_LIST_INIT_DUP,
+		.ext = ext,
+	};
 
 	if (keep_hash) {
-		ALLOC_ARRAY(data.keep, 1);
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hash, ext);
 
-		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
-		data.keep_nr = 1;
+		string_list_insert(&data.keep, buf.buf);
+
+		strbuf_release(&buf);
 	}
-	data.ext = ext;
 
-	for_each_file_in_pack_dir(source->path,
-				  clear_midx_file_ext,
-				  &data);
+	for_each_file_in_pack_dir(source->path, clear_midx_file_ext, &data);
 
-	if (keep_hash)
-		free(data.keep[0]);
-	free(data.keep);
+	string_list_clear(&data.keep, 0);
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
 				      char **keep_hashes,
 				      uint32_t hashes_nr)
 {
-	struct clear_midx_data data;
+	struct clear_midx_data data = {
+		.keep = STRING_LIST_INIT_NODUP,
+		.ext = ext,
+	};
 	uint32_t i;
 
-	memset(&data, 0, sizeof(struct clear_midx_data));
-
-	ALLOC_ARRAY(data.keep, hashes_nr);
 	for (i = 0; i < hashes_nr; i++)
-		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
-				       ext);
-	data.keep_nr = hashes_nr;
-	data.ext = ext;
+		string_list_append(&data.keep,
+				   xstrfmt("multi-pack-index-%s.%s",
+					   keep_hashes[i], ext));
+	string_list_sort(&data.keep);
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
 				     clear_midx_file_ext, &data);
 
-	for (i = 0; i < hashes_nr; i++)
-		free(data.keep[i]);
-	free(data.keep);
+	string_list_clear(&data.keep, 0);
 }
 
 void clear_midx_file(struct repository *r)
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 03/16] strvec: introduce `strvec_init_alloc()`

[Taylor Blau]

When the caller knows upfront how many elements will be pushed onto a
`strvec`, it is useful to pre-allocate enough space in the array to fit
that many elements (and one additional slot to store NULL, indicating
the end of the list.)

Introduce `strvec_init_alloc()`, which allocates the backing array large
enough to hold `alloc` elements and the termination marker without
further reallocation. Reimplement `strvec_init()` as a special case of
`strvec_init_alloc()`, namely when `alloc` is zero.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 strvec.c | 15 +++++++++++++--
 strvec.h |  5 +++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/strvec.c b/strvec.c
index f8de79f5579..15371980b84 100644
--- a/strvec.c
+++ b/strvec.c
@@ -6,8 +6,19 @@ const char *empty_strvec[] = { NULL };
 
 void strvec_init(struct strvec *array)
 {
-	struct strvec blank = STRVEC_INIT;
-	memcpy(array, &blank, sizeof(*array));
+	strvec_init_alloc(array, 0);
+}
+
+void strvec_init_alloc(struct strvec *array, size_t alloc)
+{
+	if (!alloc) {
+		struct strvec blank = STRVEC_INIT;
+		memcpy(array, &blank, sizeof(*array));
+	} else {
+		CALLOC_ARRAY(array->v, st_add(alloc, 1));
+		array->nr = 0;
+		array->alloc = alloc + 1;
+	}
 }
 
 void strvec_push_nodup(struct strvec *array, char *value)
diff --git a/strvec.h b/strvec.h
index f74e061e141..34cb1f939f0 100644
--- a/strvec.h
+++ b/strvec.h
@@ -43,6 +43,11 @@ struct strvec {
  */
 void strvec_init(struct strvec *);
 
+/*
+ * Initializes an array large enough to store `alloc` elements.
+ */
+void strvec_init_alloc(struct strvec *, size_t alloc);
+
 /* Push a copy of a string onto the end of the array. */
 const char *strvec_push(struct strvec *, const char *);
 
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

The `keep_hashes` array in `write_midx_internal()` accumulates the
checksums of MIDX files that should be retained when pruning stale
entries from the MIDX chain. For similar reasons as in a previous
commit, rewrite this using a strvec, requiring us to pass one fewer
parameter.

Unlike the aforementioned previous commit, use a `strvec` instead of a
`string_list`, which provides a more ergonomic interface to adjust the
values at a particular index. The ordering is important here, as this
value is used to determine the contents of the resulting
`multi-pack-index-chain` file when writing with "--incremental".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 51 ++++++++++++++++++++++++++-------------------------
 midx.c       | 28 +++++++++++++++++-----------
 2 files changed, 43 insertions(+), 36 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index d0351bde493..9f7d2bbf4cb 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -29,8 +29,7 @@ extern void clear_midx_files_ext(struct odb_source *source, const char *ext,
 				 const char *keep_hash);
 extern void clear_incremental_midx_files_ext(struct odb_source *source,
 					     const char *ext,
-					     const char **keep_hashes,
-					     uint32_t hashes_nr);
+					     const struct strvec *keep_hashes);
 extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 				const char *idx_name);
 
@@ -1109,8 +1108,7 @@ static int link_midx_to_chain(struct multi_pack_index *m)
 }
 
 static void clear_midx_files(struct odb_source *source,
-			     const char **hashes, uint32_t hashes_nr,
-			     unsigned incremental)
+			     const struct strvec *hashes, unsigned incremental)
 {
 	/*
 	 * if incremental:
@@ -1124,13 +1122,15 @@ static void clear_midx_files(struct odb_source *source,
 	 */
 	struct strbuf buf = STRBUF_INIT;
 	const char *exts[] = { MIDX_EXT_BITMAP, MIDX_EXT_REV, MIDX_EXT_MIDX };
-	uint32_t i, j;
+	uint32_t i;
 
 	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		clear_incremental_midx_files_ext(source, exts[i],
-						 hashes, hashes_nr);
-		for (j = 0; j < hashes_nr; j++)
-			clear_midx_files_ext(source, exts[i], hashes[j]);
+		clear_incremental_midx_files_ext(source, exts[i], hashes);
+		if (hashes) {
+			for (size_t j = 0; j < hashes->nr; j++)
+				clear_midx_files_ext(source, exts[i],
+						     hashes->v[j]);
+		}
 	}
 
 	if (incremental)
@@ -1268,7 +1268,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = -1;
-	const char **keep_hashes = NULL;
+	struct strvec keep_hashes = STRVEC_INIT;
 	size_t keep_hashes_nr = 0;
 	struct chunkfile *cf;
 
@@ -1723,7 +1723,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	} else {
 		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
 	}
-	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
+	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
 
 	if (ctx.incremental) {
 		FILE *chainf = fdopen_lock_file(&lk, "w");
@@ -1760,39 +1760,45 @@ static int write_midx_internal(struct write_midx_opts *opts)
 			for (i = 0; i < num_layers_before_from; i++) {
 				uint32_t j = num_layers_before_from - i - 1;
 
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
+				keep_hashes.nr++;
 				m = m->base_midx;
 			}
 
-			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
+			keep_hashes.v[i] = xstrdup(hash_to_hex_algop(midx_hash,
 								   r->hash_algo));
+			keep_hashes.nr++;
 
 			i = 0;
 			for (m = ctx.m;
 			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
 			     m = m->base_midx) {
-				keep_hashes[keep_hashes_nr - i - 1] =
+				keep_hashes.v[keep_hashes_nr - i - 1] =
 					xstrdup(midx_get_checksum_hex(m));
+				keep_hashes.nr++;
 				i++;
 			}
 		} else {
-			keep_hashes[ctx.num_multi_pack_indexes_before] =
+			keep_hashes.v[ctx.num_multi_pack_indexes_before] =
 				xstrdup(hash_to_hex_algop(midx_hash,
 							  r->hash_algo));
+			keep_hashes.nr++;
 
 			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
 				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
 
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
+				keep_hashes.nr++;
 				m = m->base_midx;
 			}
 		}
 
 		for (uint32_t i = 0; i < keep_hashes_nr; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
+			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
 	} else {
-		keep_hashes[ctx.num_multi_pack_indexes_before] =
+		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
+		keep_hashes.nr++;
 	}
 
 	if (ctx.m || ctx.base_midx)
@@ -1801,8 +1807,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (commit_lock_file(&lk) < 0)
 		die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, keep_hashes, keep_hashes_nr,
-			 ctx.incremental);
+	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
 	result = 0;
 
 cleanup:
@@ -1818,11 +1823,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	free(ctx.entries);
 	free(ctx.pack_perm);
 	free(ctx.pack_order);
-	if (keep_hashes) {
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
-			free((char *)keep_hashes[i]);
-		free(keep_hashes);
-	}
+	strvec_clear(&keep_hashes);
 	strbuf_release(&midx_name);
 	close_midx(midx_to_free);
 
diff --git a/midx.c b/midx.c
index d322743f2db..e6b1fbe37d7 100644
--- a/midx.c
+++ b/midx.c
@@ -12,6 +12,7 @@
 #include "chunk-format.h"
 #include "pack-bitmap.h"
 #include "pack-revindex.h"
+#include "strvec.h"
 
 #define MIDX_PACK_ERROR ((void *)(intptr_t)-1)
 
@@ -19,8 +20,7 @@ int midx_checksum_valid(struct multi_pack_index *m);
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash);
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr);
+				      const struct strvec *keep_hashes);
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
@@ -799,20 +799,26 @@ void clear_midx_files_ext(struct odb_source *source, const char *ext,
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr)
+				      const struct strvec *keep_hashes)
 {
 	struct clear_midx_data data = {
-		.keep = STRING_LIST_INIT_NODUP,
+		.keep = STRING_LIST_INIT_DUP,
 		.ext = ext,
 	};
-	uint32_t i;
 
-	for (i = 0; i < hashes_nr; i++)
-		string_list_append(&data.keep,
-				   xstrfmt("multi-pack-index-%s.%s",
-					   keep_hashes[i], ext));
-	string_list_sort(&data.keep);
+	if (keep_hashes) {
+		struct strbuf buf = STRBUF_INIT;
+		for (size_t i = 0; i < keep_hashes->nr; i++) {
+			strbuf_reset(&buf);
+
+			strbuf_addf(&buf, "multi-pack-index-%s.%s",
+				    keep_hashes->v[i], ext);
+			string_list_append(&data.keep, buf.buf);
+		}
+
+		string_list_sort(&data.keep);
+		strbuf_release(&buf);
+	}
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
 				     clear_midx_file_ext, &data);
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 05/16] midx: introduce `--checksum-only` for incremental MIDX writes

[Taylor Blau]

When writing an incremental MIDX layer, the MIDX machinery writes the
new layer into the multi-pack-index.d directory and then updates the
multi-pack-index-chain file to include the freshly written layer.

Future callers however may not wish to immediately update the MIDX chain
itself, preferring instead to write out new layer(s) themselves before
atomically updating the chain. Concretely, the new incremental
MIDX-based repacking strategy will want to do exactly this (that is,
assemble the new MIDX chain itself before writing a new chain file and
atomically linking it into place).

Introduce a `--checksum-only` flag that:

 * writes the new MIDX layer into the multi-pack-index.d directory

 * prints its checksum

 * does not update the multi-pack-index-chain file.

The MIDX chain file (and thus, the lock protecting it) remain untouched,
allowing callers to assemble the chain themselves. This flag requires
`--incremental`, since the notion of a separate layer only makes sense
for incremental MIDXs.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc |  4 +--
 builtin/multi-pack-index.c              | 28 ++++++++++++++++--
 midx-write.c                            | 38 ++++++++++++++++---------
 midx.h                                  |  1 +
 t/t5334-incremental-multi-pack-index.sh | 17 +++++++++++
 t/t5335-compact-multi-pack-index.sh     | 36 +++++++++++++++++++++++
 6 files changed, 107 insertions(+), 17 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index 61256830141..657e0639f6a 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -11,9 +11,9 @@ SYNOPSIS
 [verse]
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
-		         [--refs-snapshot=<path>]
+		         [--refs-snapshot=<path>] [--[no-]checksum-only]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] <from> <to>
+		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 0f72d96c02d..17ab1525454 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,11 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>]")
+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] <from> <to>")
+	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -153,6 +153,9 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_BIT(0, "checksum-only", &opts.flags,
+			N_("write a MIDX layer without updating the MIDX chain"),
+			MIDX_WRITE_CHECKSUM_ONLY),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			 N_("write multi-pack index containing only given indexes")),
 		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
@@ -178,6 +181,15 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 	if (argc)
 		usage_with_options(builtin_multi_pack_index_write_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--checksum-only", "--incremental");
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -221,6 +233,9 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_BIT(0, "checksum-only", &opts.flags,
+			N_("write a MIDX layer without updating the MIDX chain"),
+			MIDX_WRITE_CHECKSUM_ONLY),
 		OPT_END(),
 	};
 
@@ -239,6 +254,15 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 	if (argc != 2)
 		usage_with_options(builtin_multi_pack_index_compact_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--checksum-only", "--incremental");
+		usage_with_options(builtin_multi_pack_index_compact_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(the_repository);
 
 	FREE_AND_NULL(options);
diff --git a/midx-write.c b/midx-write.c
index 9f7d2bbf4cb..2c6905173ba 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1600,11 +1600,14 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	}
 
 	if (ctx.incremental) {
-		struct strbuf lock_name = STRBUF_INIT;
+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
+			struct strbuf lock_name = STRBUF_INIT;
 
-		get_midx_chain_filename(opts->source, &lock_name);
-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
-		strbuf_release(&lock_name);
+			get_midx_chain_filename(opts->source, &lock_name);
+			hold_lock_file_for_update(&lk, lock_name.buf,
+						  LOCK_DIE_ON_ERROR);
+			strbuf_release(&lock_name);
+		}
 
 		incr = mks_tempfile_m(midx_name.buf, 0444);
 		if (!incr) {
@@ -1725,14 +1728,19 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	}
 	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
 
+	if (opts->flags & MIDX_WRITE_CHECKSUM_ONLY)
+		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));
+
 	if (ctx.incremental) {
-		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
 
-		if (!chainf) {
-			error_errno(_("unable to open multi-pack-index chain file"));
-			goto cleanup;
+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
+			FILE *chainf = fdopen_lock_file(&lk, "w");
+			if (!chainf) {
+				error_errno(_("unable to open multi-pack-index chain file"));
+				goto cleanup;
+			}
 		}
 
 		if (link_midx_to_chain(ctx.base_midx) < 0)
@@ -1793,8 +1801,10 @@ static int write_midx_internal(struct write_midx_opts *opts)
 			}
 		}
 
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY))
+			for (uint32_t i = 0; i < keep_hashes_nr; i++)
+				fprintf(get_lock_file_fp(&lk), "%s\n",
+					keep_hashes.v[i]);
 	} else {
 		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
@@ -1804,10 +1814,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.m || ctx.base_midx)
 		odb_close(ctx.repo->objects);
 
-	if (commit_lock_file(&lk) < 0)
-		die_errno(_("could not write multi-pack-index"));
+	if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
+		if (commit_lock_file(&lk) < 0)
+			die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+		clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+	}
 	result = 0;
 
 cleanup:
diff --git a/midx.h b/midx.h
index 08f3728e520..9f1acd7ace4 100644
--- a/midx.h
+++ b/midx.h
@@ -83,6 +83,7 @@ struct multi_pack_index {
 #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
 #define MIDX_WRITE_INCREMENTAL (1 << 5)
 #define MIDX_WRITE_COMPACT (1 << 6)
+#define MIDX_WRITE_CHECKSUM_ONLY (1 << 7)
 
 #define MIDX_EXT_REV "rev"
 #define MIDX_EXT_BITMAP "bitmap"
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index c9f5b4e87aa..ac249365f2b 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -96,6 +96,23 @@ test_expect_success 'show object from second pack' '
 	git cat-file -p 2.2
 '
 
+test_expect_success 'write MIDX layer with --checksum-only' '
+	test_commit checksum-only &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--checksum-only)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
+	test_must_fail git multi-pack-index write --bitmap --checksum-only 2>err &&
+	test_grep "cannot use --checksum-only without --incremental" err
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 40f3844282f..3ad910a9d1c 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -290,4 +290,40 @@ test_expect_success 'MIDX compaction with bitmaps (non-trivial)' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --checksum-only' '
+	git init midx-compact-with--checksum-only &&
+	(
+		cd midx-compact-with--checksum-only &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		layer="$(git multi-pack-index compact --incremental \
+			--checksum-only \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")")" &&
+
+		test_cmp "$midx_chain.bak" "$midx_chain" &&
+
+		# After writing the new layer, insert it into the chain
+		# manually. This is done in order to make $layer visible
+		# to the read-midx test helper below, and matches what
+		# the MIDX command would do without --checksum-only.
+		{
+			nth_line 1 "$midx_chain.bak" &&
+			echo $layer &&
+			nth_line 4 "$midx_chain.bak"
+		} >$midx_chain &&
+
+		test-tool read-midx $objdir $layer >midx.data &&
+		grep "^pack-B-.*\.idx" midx.data &&
+		grep "^pack-C-.*\.idx" midx.data
+
+	)
+'
+
 test_done
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 06/16] midx: support custom `--base` for incremental MIDX writes

[Taylor Blau]

Both `compact` and `write --incremental` fix the base of the resulting
MIDX layer: `compact` always places the compacted result on top of
"from's" immediate parent in the chain, and `write --incremental` always
appends a new layer to the existing tip. In both cases the base is not
configurable.

Future callers need additional flexibility. For instance, the incremental
MIDX-based repacking code may wish to write a layer based on some
intermediate ancestor rather than the current tip, or produce a root
layer when replacing the bottommost entries in the chain.

Introduce a new `--base` option for both subcommands to specify the
checksum of the MIDX layer to use as the base. The given checksum must
refer to a valid layer in the MIDX chain that is an ancestor of the
topmost layer being written or compacted.

The special value "none" is accepted to produce a root layer with no
parent. This will be needed when the incremental repacking machinery
determines that the bottommost layers of the chain should be replaced.

If no `--base` is given, behavior is unchanged: `compact` uses "from's"
immediate parent in the chain, and `write` appends to the existing tip.

For the `write` subcommand, `--base` requires `--checksum-only`. A plain
`write --incremental` appends a new layer to the live chain tip with no
mechanism to atomically replace it; overriding the base would produce a
layer that does not extend the tip, breaking chain invariants. With
`--checksum-only` the chain is left unmodified and the caller is
responsible for assembling a valid chain.

For `compact`, no such restriction applies. The compaction operation
atomically replaces the compacted range in the chain file, so writing
the result on top of any valid ancestor preserves chain invariants.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc | 17 +++++-
 builtin/multi-pack-index.c              | 24 ++++++--
 midx-write.c                            | 34 ++++++++++-
 midx.h                                  |  5 +-
 t/t5334-incremental-multi-pack-index.sh | 30 ++++++++++
 t/t5335-compact-multi-pack-index.sh     | 77 +++++++++++++++++++++++++
 6 files changed, 178 insertions(+), 9 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index 657e0639f6a..635105ad801 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -12,8 +12,10 @@ SYNOPSIS
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
 		         [--refs-snapshot=<path>] [--[no-]checksum-only]
+			 [--base=<checksum>]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
+		         [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]
+			 <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
@@ -83,6 +85,13 @@ marker).
 		and packs not present in an existing MIDX layer.
 		Migrates non-incremental MIDXs to incremental ones when
 		necessary.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base when writing a new incremental layer.
+		The special value `none` indicates that the new layer
+		should have no base (i.e., it becomes a root layer).
+		Requires `--checksum-only`.
 --
 
 compact::
@@ -97,6 +106,12 @@ compact::
 
 	--[no-]bitmap::
 		Control whether or not a multi-pack bitmap is written.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base for the compacted result, instead of using
+		the immediate parent of `<from>`. The special value
+		`none` indicates that the result should have no base.
 --
 
 verify::
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 17ab1525454..3a12baacb56 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,13 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]\n" \
+	   "  [--base=<checksum>]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
+	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]\n" \
+	   "  <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -63,6 +65,7 @@ static char const * const builtin_multi_pack_index_usage[] = {
 static struct opts_multi_pack_index {
 	char *object_dir;
 	const char *preferred_pack;
+	const char *incremental_base;
 	char *refs_snapshot;
 	unsigned long batch_size;
 	unsigned flags;
@@ -151,6 +154,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			   N_("pack for reuse when computing a multi-pack bitmap")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
 		OPT_BIT(0, "checksum-only", &opts.flags,
@@ -190,6 +195,13 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 				   options);
 	}
 
+	if (opts.incremental_base &&
+	    !(opts.flags & MIDX_WRITE_CHECKSUM_ONLY)) {
+		error(_("cannot use --base without --checksum-only"));
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -201,7 +213,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 
 		ret = write_midx_file_only(source, &packs,
 					   opts.preferred_pack,
-					   opts.refs_snapshot, opts.flags);
+					   opts.refs_snapshot,
+					   opts.incremental_base, opts.flags);
 
 		string_list_clear(&packs, 0);
 		free(opts.refs_snapshot);
@@ -229,6 +242,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 
 	struct option *options;
 	static struct option builtin_multi_pack_index_compact_options[] = {
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
@@ -290,7 +305,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			die(_("MIDX %s must be an ancestor of %s"), argv[0], argv[1]);
 	}
 
-	ret = write_midx_file_compact(source, from_midx, to_midx, opts.flags);
+	ret = write_midx_file_compact(source, from_midx, to_midx,
+				      opts.incremental_base, opts.flags);
 
 	return ret;
 }
diff --git a/midx-write.c b/midx-write.c
index 2c6905173ba..9298a5140fd 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1247,6 +1247,7 @@ struct write_midx_opts {
 
 	const char *preferred_pack_name;
 	const char *refs_snapshot;
+	const char *incremental_base;
 	unsigned flags;
 };
 
@@ -1329,11 +1330,32 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 	/*
 	 * If compacting MIDX layer(s) in the range [from, to], then the
-	 * compacted MIDX will share the same base MIDX as 'from'.
+	 * compacted MIDX will share the same base MIDX as 'from',
+	 * unless a custom --base is specified (see below).
 	 */
 	if (ctx.compact)
 		ctx.base_midx = ctx.compact_from->base_midx;
 
+	if (opts->incremental_base) {
+		if (!strcmp(opts->incremental_base, "none")) {
+			ctx.base_midx = NULL;
+		} else {
+			while (ctx.base_midx) {
+				const char *cmp = midx_get_checksum_hex(ctx.base_midx);
+				if (!strcmp(opts->incremental_base, cmp))
+					break;
+
+				ctx.base_midx = ctx.base_midx->base_midx;
+			}
+
+			if (!ctx.base_midx) {
+				error(_("could not find base MIDX '%s'"),
+				      opts->incremental_base);
+				goto cleanup;
+			}
+		}
+	}
+
 	ctx.nr = 0;
 	ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
 	ctx.info = NULL;
@@ -1846,7 +1868,8 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 int write_midx_file(struct odb_source *source,
 		    const char *preferred_pack_name,
-		    const char *refs_snapshot, unsigned flags)
+		    const char *refs_snapshot,
+		    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
@@ -1861,13 +1884,16 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags)
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.packs_to_include = packs_to_include,
 		.preferred_pack_name = preferred_pack_name,
 		.refs_snapshot = refs_snapshot,
+		.incremental_base = incremental_base,
 		.flags = flags,
 	};
 
@@ -1877,12 +1903,14 @@ int write_midx_file_only(struct odb_source *source,
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.compact_from = from,
 		.compact_to = to,
+		.incremental_base = incremental_base,
 		.flags = flags | MIDX_WRITE_COMPACT,
 	};
 
diff --git a/midx.h b/midx.h
index 9f1acd7ace4..e4a75ff2bef 100644
--- a/midx.h
+++ b/midx.h
@@ -132,10 +132,13 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags);
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags);
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct odb_source *source, unsigned flags);
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index ac249365f2b..7733cafb647 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -113,6 +113,36 @@ test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
 	test_grep "cannot use --checksum-only without --incremental" err
 '
 
+test_expect_success 'write MIDX layer with --base without --checksum-only' '
+	test_must_fail git multi-pack-index write --bitmap --incremental \
+		--base=none 2>err &&
+	test_grep "cannot use --base without --checksum-only" err
+'
+
+test_expect_success 'write MIDX layer with --base=none and --checksum-only' '
+	test_commit base-none &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--checksum-only --base=none)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write MIDX layer with --base=<hash> and --checksum-only' '
+	test_commit base-hash &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--checksum-only --base="$(nth_line 1 "$midx_chain")")" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 3ad910a9d1c..d8e1b03669a 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -304,6 +304,7 @@ test_expect_success 'MIDX compaction with --checksum-only' '
 
 		layer="$(git multi-pack-index compact --incremental \
 			--checksum-only \
+			--base="$(nth_line 1 "$midx_chain")" \
 			"$(nth_line 2 "$midx_chain")" \
 			"$(nth_line 3 "$midx_chain")")" &&
 
@@ -326,4 +327,80 @@ test_expect_success 'MIDX compaction with --checksum-only' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --base' '
+	git init midx-compact-with--base &&
+	(
+		cd midx-compact-with--base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 "$midx_chain" &&
+
+		cp "$midx_chain" "$midx_chain.bak" &&
+
+		git multi-pack-index compact --incremental \
+			--base="$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" \
+			"$(nth_line 4 "$midx_chain")" &&
+		test_line_count = 2 $midx_chain &&
+
+		nth_line 1 "$midx_chain.bak" >expect &&
+		nth_line 1 "$midx_chain" >actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with --base=none' '
+	git init midx-compact-base-none &&
+	(
+		cd midx-compact-base-none &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		# Compact the two bottommost layers (A and B) into a new
+		# root layer with no parent.
+		git multi-pack-index compact --incremental \
+			--base=none \
+			"$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 2 "$midx_chain")" &&
+
+		test_line_count = 3 $midx_chain &&
+
+		# The upper layers (C and D) should be preserved
+		# unchanged.
+		nth_line 3 "$midx_chain.bak" >expect &&
+		nth_line 4 "$midx_chain.bak" >>expect &&
+		nth_line 2 "$midx_chain" >actual &&
+		nth_line 3 "$midx_chain" >>actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with bogus --base checksum' '
+	git init midx-compact-bogus-base &&
+	(
+		cd midx-compact-bogus-base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C &&
+
+		test_must_fail git multi-pack-index compact --incremental \
+			--base=deadbeef \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" 2>err &&
+		test_grep "could not find base MIDX" err
+	)
+'
+
 test_done
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 07/16] repack: track the ODB source via existing_packs

[Taylor Blau]

Store the ODB source in the `existing_packs` struct and use that in
place of the raw `repo->objects->sources` access within `cmd_repack()`.

The source used is still assigned from the first source in the list, so
there are no functional changes in this commit. The changes instead
serve two purposes (one immediate, one not):

 - The incremental MIDX-based repacking machinery will need to know what
   source is being used to read the existing MIDX/chain (should one
   exist).

 - In the future, if "git repack" is taught how to operate on other
   object sources, this field will serve as the authoritative value for
   that source.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 5 ++---
 repack.c         | 2 ++
 repack.h         | 1 +
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index f6bb04bef72..44a95b56f23 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -402,7 +402,7 @@ int cmd_repack(int argc,
 		 * midx_has_unknown_packs() will make the decision for
 		 * us.
 		 */
-		if (!get_multi_pack_index(repo->objects->sources))
+		if (!get_multi_pack_index(existing.source))
 			midx_must_contain_cruft = 1;
 	}
 
@@ -549,8 +549,7 @@ int cmd_repack(int argc,
 		unsigned flags = 0;
 		if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL, 0))
 			flags |= MIDX_WRITE_INCREMENTAL;
-		write_midx_file(repo->objects->sources,
-				NULL, NULL, flags);
+		write_midx_file(existing.source, NULL, NULL, flags);
 	}
 
 cleanup:
diff --git a/repack.c b/repack.c
index 596841027af..2ee6b51420a 100644
--- a/repack.c
+++ b/repack.c
@@ -154,6 +154,8 @@ void existing_packs_collect(struct existing_packs *existing,
 			string_list_append(&existing->non_kept_packs, buf.buf);
 	}
 
+	existing->source = existing->repo->objects->sources;
+
 	string_list_sort(&existing->kept_packs);
 	string_list_sort(&existing->non_kept_packs);
 	string_list_sort(&existing->cruft_packs);
diff --git a/repack.h b/repack.h
index bc9f2e1a5de..c0e9f0ca647 100644
--- a/repack.h
+++ b/repack.h
@@ -56,6 +56,7 @@ struct packed_git;
 
 struct existing_packs {
 	struct repository *repo;
+	struct odb_source *source;
 	struct string_list kept_packs;
 	struct string_list non_kept_packs;
 	struct string_list cruft_packs;
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 08/16] midx: expose `midx_layer_contains_pack()`

[Taylor Blau]

Rename the function `midx_contains_pack_1()` to instead be called
`midx_layer_contains_pack()` and make it accessible. Unlike
`midx_contains_pack()` (which recurses through the entire chain), this
function checks only a single MIDX layer.

This will be used by a subsequent commit to determine whether a given
pack belongs to the tip MIDX layer specifically, rather than to any
layer in the chain.

No functional changes are present in this commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 6 +++---
 midx.h | 2 ++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index e6b1fbe37d7..7d23338aa3a 100644
--- a/midx.c
+++ b/midx.c
@@ -667,8 +667,8 @@ static int midx_pack_names_cmp(const void *a, const void *b, void *m_)
 		      m->pack_names[*(const size_t *)b]);
 }
 
-static int midx_contains_pack_1(struct multi_pack_index *m,
-				const char *idx_or_pack_name)
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name)
 {
 	uint32_t first = 0, last = m->num_packs;
 
@@ -709,7 +709,7 @@ static int midx_contains_pack_1(struct multi_pack_index *m,
 int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 {
 	for (; m; m = m->base_midx)
-		if (midx_contains_pack_1(m, idx_or_pack_name))
+		if (midx_layer_contains_pack(m, idx_or_pack_name))
 			return 1;
 	return 0;
 }
diff --git a/midx.h b/midx.h
index e4a75ff2bef..f211a38b9e7 100644
--- a/midx.h
+++ b/midx.h
@@ -119,6 +119,8 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 int fill_midx_entry(struct multi_pack_index *m, const struct object_id *oid, struct pack_entry *e);
 int midx_contains_pack(struct multi_pack_index *m,
 		       const char *idx_or_pack_name);
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name);
 int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id);
 int prepare_multi_pack_index_one(struct odb_source *source);
 
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 09/16] repack-midx: factor out `repack_prepare_midx_command()`

[Taylor Blau]

The `write_midx_included_packs()` function assembles and executes a
`git multi-pack-index write` command, constructing the argument list
inline.

Future commits will introduce additional callers that need to construct
similar `git multi-pack-index` commands (for both `write` and `compact`
subcommands), so extract the common portions of the command setup into a
reusable `repack_prepare_midx_command()` helper.

The extracted helper sets `git_cmd`, pushes the `multi-pack-index`
subcommand and verb, and handles `--progress`/`--no-progress` and
`--bitmap` flags. The remaining arguments that are specific to the
`write` subcommand (such as `--stdin-packs`) are left to the caller.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 0682b80c427..83151d4734a 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -275,6 +275,23 @@ static void remove_redundant_bitmaps(struct string_list *include,
 	strbuf_release(&path);
 }
 
+static void repack_prepare_midx_command(struct child_process *cmd,
+					struct repack_write_midx_opts *opts,
+					const char *verb)
+{
+	cmd->git_cmd = 1;
+
+	strvec_pushl(&cmd->args, "multi-pack-index", verb, NULL);
+
+	if (opts->show_progress)
+		strvec_push(&cmd->args, "--progress");
+	else
+		strvec_push(&cmd->args, "--no-progress");
+
+	if (opts->write_bitmaps)
+		strvec_push(&cmd->args, "--bitmap");
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
@@ -289,18 +306,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		goto done;
 
 	cmd.in = -1;
-	cmd.git_cmd = 1;
 
-	strvec_push(&cmd.args, "multi-pack-index");
-	strvec_pushl(&cmd.args, "write", "--stdin-packs", NULL);
-
-	if (opts->show_progress)
-		strvec_push(&cmd.args, "--progress");
-	else
-		strvec_push(&cmd.args, "--no-progress");
-
-	if (opts->write_bitmaps)
-		strvec_push(&cmd.args, "--bitmap");
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Taylor Blau]

The function `write_midx_included_packs()` manages the lifecycle of
writing packs to stdin when running `git multi-pack-index write` as a
child process.

Extract a standalone `repack_fill_midx_stdin_packs()` helper, which
handles `--stdin-packs` argument setup, starting the command, writing
pack names to its standard input, and finishing the command.

This simplifies `write_midx_included_packs()` and prepares for a
subsequent commit where the same helper is called with `cmd->out = -1`
to capture the MIDX's checksum from the command's standard output,
which is needed when writing MIDX layers with `--checksum-only`.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 83151d4734a..78f069c2151 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -292,23 +292,42 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 		strvec_push(&cmd->args, "--bitmap");
 }
 
+static int repack_fill_midx_stdin_packs(struct child_process *cmd,
+					struct string_list *include)
+{
+	struct string_list_item *item;
+	FILE *in;
+	int ret;
+
+	cmd->in = -1;
+
+	strvec_push(&cmd->args, "--stdin-packs");
+
+	ret = start_command(cmd);
+	if (ret)
+		return ret;
+
+	in = xfdopen(cmd->in, "w");
+	for_each_string_list_item(item, include)
+		fprintf(in, "%s\n", item->string);
+	fclose(in);
+
+	return finish_command(cmd);
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
 	struct string_list_item *item;
 	struct packed_git *preferred = pack_geometry_preferred_pack(opts->geometry);
-	FILE *in;
 	int ret = 0;
 
 	midx_included_packs(&include, opts);
 	if (!include.nr)
 		goto done;
 
-	cmd.in = -1;
-
 	repack_prepare_midx_command(&cmd, opts, "write");
-	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
@@ -350,16 +369,7 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = start_command(&cmd);
-	if (ret)
-		goto done;
-
-	in = xfdopen(cmd.in, "w");
-	for_each_string_list_item(item, &include)
-		fprintf(in, "%s\n", item->string);
-	fclose(in);
-
-	ret = finish_command(&cmd);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 11/16] repack-geometry: prepare for incremental MIDX repacking

[Taylor Blau]

Teach `pack_geometry_init()` to optionally restrict the set of
repacking candidates to only packs in the tip MIDX layer when a
`midx_layer_threshold` is configured. If the tip layer has fewer packs
than the threshold, those packs are excluded entirely; otherwise only
packs in that layer participate in the geometric repack.

Also track whether any tip-layer packs were included in the rollup
(`midx_tip_rewritten`), which a subsequent commit will use to decide
how to update the MIDX chain after repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-geometry.c | 37 +++++++++++++++++++++++++++++++++++++
 repack.h          |  4 ++++
 2 files changed, 41 insertions(+)

diff --git a/repack-geometry.c b/repack-geometry.c
index 7cebd0cb45f..d2065205f87 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -4,6 +4,7 @@
 #include "repack.h"
 #include "repository.h"
 #include "hex.h"
+#include "midx.h"
 #include "packfile.h"
 
 static uint32_t pack_geometry_weight(struct packed_git *p)
@@ -31,8 +32,30 @@ void pack_geometry_init(struct pack_geometry *geometry,
 {
 	struct packed_git *p;
 	struct strbuf buf = STRBUF_INIT;
+	struct multi_pack_index *m = get_multi_pack_index(existing->source);
 
 	repo_for_each_pack(existing->repo, p) {
+		if (geometry->midx_layer_threshold_set && m &&
+		    p->multi_pack_index) {
+			/*
+			 * When writing MIDX layers incrementally,
+			 * ignore packs unless they are in the most
+			 * recent MIDX layer *and* there are at least
+			 * 'midx_layer_threshold' packs in that layer.
+			 *
+			 * Otherwise 'p' is either in an older layer, or
+			 * the youngest layer does not have enough packs
+			 * to consider its packs as candidates for
+			 * repacking. In either of those cases we want
+			 * to ignore the pack.
+			 */
+			if (m->num_packs > geometry->midx_layer_threshold &&
+			    midx_layer_contains_pack(m, pack_basename(p)))
+				;
+			else
+				continue;
+		}
+
 		if (args->local && !p->pack_local)
 			/*
 			 * When asked to only repack local packfiles we skip
@@ -173,6 +196,20 @@ void pack_geometry_split(struct pack_geometry *geometry)
 	geometry->promisor_split = compute_pack_geometry_split(geometry->promisor_pack,
 							       geometry->promisor_pack_nr,
 							       geometry->split_factor);
+	for (uint32_t i = 0; i < geometry->split; i++) {
+		struct packed_git *p = geometry->pack[i];
+		/*
+		 * During incremental MIDX/bitmap repacking, any packs
+		 * included in the rollup are either (a) not MIDX'd, or
+		 * (b) contained in the tip layer iff it has more than
+		 * the threshold number of packs.
+		 *
+		 * In the latter case, we can safely conclude that the
+		 * tip of the MIDX chain will be rewritten.
+		 */
+		if (p->multi_pack_index)
+			geometry->midx_tip_rewritten = true;
+	}
 }
 
 struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry)
diff --git a/repack.h b/repack.h
index c0e9f0ca647..77d24ee45fb 100644
--- a/repack.h
+++ b/repack.h
@@ -108,6 +108,10 @@ struct pack_geometry {
 	uint32_t promisor_pack_nr, promisor_pack_alloc;
 	uint32_t promisor_split;
 
+	uint32_t midx_layer_threshold;
+	bool midx_layer_threshold_set;
+	bool midx_tip_rewritten;
+
 	int split_factor;
 };
 
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 12/16] builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

[Taylor Blau]

Change the --write-midx (-m) flag from an OPT_BOOL to an OPT_CALLBACK
that accepts an optional mode argument. Introduce an enum with
REPACK_WRITE_MIDX_NONE and REPACK_WRITE_MIDX_DEFAULT to distinguish
between the two states, and update all existing boolean checks
accordingly.

For now, passing no argument (or just `-m`) selects the default mode,
preserving existing behavior. A subsequent commit will add a new mode
for writing incremental MIDXs.

Extract repack_write_midx() as a dispatcher that selects the
appropriate MIDX-writing implementation based on the mode.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 50 ++++++++++++++++++++++++++++++++++++------------
 repack-midx.c    | 14 +++++++++++++-
 repack.h         |  8 +++++++-
 3 files changed, 58 insertions(+), 14 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 44a95b56f23..3a5042491d6 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -97,6 +97,24 @@ static int repack_config(const char *var, const char *value,
 	return git_default_config(var, value, ctx, cb);
 }
 
+static int option_parse_write_midx(const struct option *opt, const char *arg,
+				   int unset)
+{
+	enum repack_write_midx_mode *cfg = opt->value;
+
+	if (unset) {
+		*cfg = REPACK_WRITE_MIDX_NONE;
+		return 0;
+	}
+
+	if (!arg || !*arg)
+		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else
+		return error(_("unknown value for %s: %s"), opt->long_name, arg);
+
+	return 0;
+}
+
 int cmd_repack(int argc,
 	       const char **argv,
 	       const char *prefix,
@@ -119,7 +137,7 @@ int cmd_repack(int argc,
 	struct string_list keep_pack_list = STRING_LIST_INIT_NODUP;
 	struct pack_objects_args po_args = PACK_OBJECTS_ARGS_INIT;
 	struct pack_objects_args cruft_po_args = PACK_OBJECTS_ARGS_INIT;
-	int write_midx = 0;
+	enum repack_write_midx_mode write_midx = REPACK_WRITE_MIDX_NONE;
 	const char *cruft_expiration = NULL;
 	const char *expire_to = NULL;
 	const char *filter_to = NULL;
@@ -185,8 +203,14 @@ int cmd_repack(int argc,
 				N_("do not repack this pack")),
 		OPT_INTEGER('g', "geometric", &geometry.split_factor,
 			    N_("find a geometric progression with factor <N>")),
-		OPT_BOOL('m', "write-midx", &write_midx,
-			   N_("write a multi-pack index of the resulting packs")),
+		OPT_CALLBACK_F(0, "write-midx", &write_midx,
+			   N_("mode"),
+			   N_("write a multi-pack index of the resulting packs"),
+			   PARSE_OPT_OPTARG, option_parse_write_midx),
+		OPT_SET_INT_F('m', NULL, &write_midx,
+			   N_("write a multi-pack index of the resulting packs"),
+			   REPACK_WRITE_MIDX_DEFAULT,
+			   PARSE_OPT_HIDDEN),
 		OPT_STRING(0, "expire-to", &expire_to, N_("dir"),
 			   N_("pack prefix to store a pack containing pruned objects")),
 		OPT_STRING(0, "filter-to", &filter_to, N_("dir"),
@@ -221,14 +245,16 @@ int cmd_repack(int argc,
 		pack_everything |= ALL_INTO_ONE;
 
 	if (write_bitmaps < 0) {
-		if (!write_midx &&
+		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
 			write_bitmaps = 0;
 	}
 	if (po_args.pack_kept_objects < 0)
-		po_args.pack_kept_objects = write_bitmaps > 0 && !write_midx;
+		po_args.pack_kept_objects = write_bitmaps > 0 &&
+			write_midx == REPACK_WRITE_MIDX_NONE;
 
-	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
+	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) &&
+	    write_midx == REPACK_WRITE_MIDX_NONE)
 		die(_(incremental_bitmap_conflict_error));
 
 	if (write_bitmaps && po_args.local &&
@@ -244,7 +270,7 @@ int cmd_repack(int argc,
 		write_bitmaps = 0;
 	}
 
-	if (write_midx && write_bitmaps) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
 		strbuf_addf(&path, "%s/%s_XXXXXX",
@@ -297,7 +323,7 @@ int cmd_repack(int argc,
 	}
 	if (repo_has_promisor_remote(repo))
 		strvec_push(&cmd.args, "--exclude-promisor-objects");
-	if (!write_midx) {
+	if (write_midx == REPACK_WRITE_MIDX_NONE) {
 		if (write_bitmaps > 0)
 			strvec_push(&cmd.args, "--write-bitmap-index");
 		else if (write_bitmaps < 0)
@@ -504,7 +530,7 @@ int cmd_repack(int argc,
 	if (delete_redundant && pack_everything & ALL_INTO_ONE)
 		existing_packs_mark_for_deletion(&existing, &names);
 
-	if (write_midx) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE) {
 		struct repack_write_midx_opts opts = {
 			.existing = &existing,
 			.geometry = &geometry,
@@ -513,11 +539,11 @@ int cmd_repack(int argc,
 			.packdir = packdir,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
-			.midx_must_contain_cruft = midx_must_contain_cruft
+			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.mode = write_midx,
 		};
 
-		ret = write_midx_included_packs(&opts);
-
+		ret = repack_write_midx(&opts);
 		if (ret)
 			goto cleanup;
 	}
diff --git a/repack-midx.c b/repack-midx.c
index 78f069c2151..4a568a2a9b8 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -315,7 +315,7 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 	return finish_command(cmd);
 }
 
-int write_midx_included_packs(struct repack_write_midx_opts *opts)
+static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
@@ -378,3 +378,15 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 
 	return ret;
 }
+
+int repack_write_midx(struct repack_write_midx_opts *opts)
+{
+	switch (opts->mode) {
+	case REPACK_WRITE_MIDX_NONE:
+		BUG("write_midx mode is NONE?");
+	case REPACK_WRITE_MIDX_DEFAULT:
+		return write_midx_included_packs(opts);
+	default:
+		BUG("unhandled write_midx mode: %d", opts->mode);
+	}
+}
diff --git a/repack.h b/repack.h
index 77d24ee45fb..81907fcce7f 100644
--- a/repack.h
+++ b/repack.h
@@ -134,6 +134,11 @@ void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
 
+enum repack_write_midx_mode {
+	REPACK_WRITE_MIDX_NONE,
+	REPACK_WRITE_MIDX_DEFAULT,
+};
+
 struct repack_write_midx_opts {
 	struct existing_packs *existing;
 	struct pack_geometry *geometry;
@@ -143,10 +148,11 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	enum repack_write_midx_mode mode;
 };
 
 void midx_snapshot_refs(struct repository *repo, struct tempfile *f);
-int write_midx_included_packs(struct repack_write_midx_opts *opts);
+int repack_write_midx(struct repack_write_midx_opts *opts);
 
 int write_filtered_pack(const struct write_pack_opts *opts,
 			struct existing_packs *existing,
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 13/16] packfile: ensure `close_pack_revindex()` frees in-memory revindex

[Taylor Blau]

The following commit will introduce a case where we write a MIDX bitmap
over packs that do not themselves have on-disk *.rev files.

This case is supported within Git, and we will simply fall back to
generating the revindex in memory. But we don't ever release that
memory, causing a leak that is exposed by a test introduced in the
following commit.

(As far as I could find, we never free()'d memory allocated as a
byproduct of creating an in-memory revindex, likely because that code
predates the leak-checking niceties we have in the test suite now.)

Rectify this by calling `FREE_AND_NULL()` on the `p->revindex` field
when calling `close_pack_revindex()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 packfile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packfile.c b/packfile.c
index d4de9f3ffe8..97f7662e079 100644
--- a/packfile.c
+++ b/packfile.c
@@ -420,6 +420,8 @@ void close_pack_index(struct packed_git *p)
 
 static void close_pack_revindex(struct packed_git *p)
 {
+	FREE_AND_NULL(p->revindex);
+
 	if (!p->revindex_map)
 		return;
 
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

Implement the `write_midx_incremental()` function, which builds and
maintains an incremental MIDX chain as part of the geometric repacking
process.

Unlike the default mode which writes a single flat MIDX, the incremental
mode constructs a compaction plan that determines which MIDX layers to
write, compact, or copy, and then executes each step using `git
multi-pack-index` subcommands with the --checksum-only flag.

The repacking strategy works as follows:

 * Acquire the lock guarding the multi-pack-index-chain.

 * A new MIDX layer is always written containing the newly created
   pack(s). If the tip MIDX layer was rewritten during geometric
   repacking, any surviving packs from that layer are also included.

 * Starting from the new layer, adjacent MIDX layers are merged together
   as long as the accumulated object count exceeds half the object count
   of the next deeper layer (controlled by 'repack.midxSplitFactor').

 * Remaining layers in the chain are evaluated pairwise and either
   compacted or copied as-is, following the same merging condition.

 * Write the contents of the new multi-pack-index chain, atomically move
   it into place, and then release the lock.

 * Delete any now-unused MIDX layers.

After writing the new layer, the strategy is evaluated among the
existing MIDX layers in order from oldest to newest. Each step that
writes a new MIDX layer uses "--checksum-only" to avoid updating the
multi-pack-index-chain file. After all steps are complete, the new chain
file is written and then atomically moved into place.

At present, this functionality is exposed behind a new enum value,
`REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
subsequent commit will expose this mode via `git repack
--write-midx=incremental`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 578 +++++++++++++++++++++++++++++++++++++++++++++++++-
 repack.h      |   3 +
 2 files changed, 579 insertions(+), 2 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 4a568a2a9b8..a159b879775 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -2,12 +2,16 @@
 #include "repack.h"
 #include "hash.h"
 #include "hex.h"
+#include "lockfile.h"
+#include "midx.h"
 #include "odb.h"
 #include "oidset.h"
 #include "pack-bitmap.h"
+#include "path.h"
 #include "refs.h"
 #include "run-command.h"
 #include "tempfile.h"
+#include "trace2.h"
 
 struct midx_snapshot_ref_data {
 	struct repository *repo;
@@ -293,13 +297,16 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 }
 
 static int repack_fill_midx_stdin_packs(struct child_process *cmd,
-					struct string_list *include)
+					struct string_list *include,
+					struct string_list *out)
 {
 	struct string_list_item *item;
 	FILE *in;
 	int ret;
 
 	cmd->in = -1;
+	if (out)
+		cmd->out = -1;
 
 	strvec_push(&cmd->args, "--stdin-packs");
 
@@ -312,6 +319,17 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 		fprintf(in, "%s\n", item->string);
 	fclose(in);
 
+	if (out) {
+		struct strbuf buf = STRBUF_INIT;
+		FILE *outf = xfdopen(cmd->out, "r");
+
+		while (strbuf_getline(&buf, outf) != EOF)
+			string_list_append(out, buf.buf);
+		strbuf_release(&buf);
+
+		fclose(outf);
+	}
+
 	return finish_command(cmd);
 }
 
@@ -369,7 +387,7 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = repack_fill_midx_stdin_packs(&cmd, &include);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include, NULL);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
@@ -379,6 +397,560 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 	return ret;
 }
 
+struct midx_compaction_step {
+	union {
+		struct multi_pack_index *copy;
+		struct string_list write;
+		struct {
+			struct multi_pack_index *from;
+			struct multi_pack_index *to;
+		} compact;
+	} u;
+
+	uint32_t objects_nr;
+	char *csum;
+
+	enum {
+		MIDX_COMPACTION_STEP_UNKNOWN,
+		MIDX_COMPACTION_STEP_COPY,
+		MIDX_COMPACTION_STEP_WRITE,
+		MIDX_COMPACTION_STEP_COMPACT,
+	} type;
+};
+
+static const char *midx_compaction_step_base(const struct midx_compaction_step *step)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot use UNKNOWN step as a base");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_get_checksum_hex(step->u.copy);
+	case MIDX_COMPACTION_STEP_WRITE:
+		BUG("cannot use WRITE step as a base");
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_get_checksum_hex(step->u.compact.to);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static int midx_compaction_step_exec_copy(struct midx_compaction_step *step)
+{
+	step->csum = xstrdup(midx_get_checksum_hex(step->u.copy));
+	return 0;
+}
+
+static int midx_compaction_step_exec_write(struct midx_compaction_step *step,
+					   struct repack_write_midx_opts *opts,
+					   const char *base)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct string_list hash = STRING_LIST_INIT_DUP;
+	struct string_list_item *item;
+	const char *preferred_pack = NULL;
+	int ret = 0;
+
+	if (!step->u.write.nr) {
+		ret = error(_("no packs to write MIDX during compaction"));
+		goto out;
+	}
+
+	for_each_string_list_item(item, &step->u.write) {
+		if (item->util)
+			preferred_pack = item->string;
+	}
+
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only", NULL);
+	strvec_pushf(&cmd.args, "--base=%s", base ? base : "none");
+
+	if (preferred_pack) {
+		struct strbuf buf = STRBUF_INIT;
+
+		strbuf_addstr(&buf, preferred_pack);
+		strbuf_strip_suffix(&buf, ".idx");
+		strbuf_addstr(&buf, ".pack");
+
+		strvec_pushf(&cmd.args, "--preferred-pack=%s", buf.buf);
+
+		strbuf_release(&buf);
+	}
+
+	ret = repack_fill_midx_stdin_packs(&cmd, &step->u.write, &hash);
+	if (hash.nr != 1) {
+		ret = error(_("expected exactly one line during MIDX write, "
+			      "got: %"PRIuMAX),
+			    (uintmax_t)hash.nr);
+		goto out;
+	}
+
+	step->csum = xstrdup(hash.items[0].string);
+
+out:
+	string_list_clear(&hash, 0);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec_compact(struct midx_compaction_step *step,
+					     struct repack_write_midx_opts *opts)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf buf = STRBUF_INIT;
+	FILE *out = NULL;
+	int ret;
+
+	repack_prepare_midx_command(&cmd, opts, "compact");
+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only",
+		     midx_get_checksum_hex(step->u.compact.from),
+		     midx_get_checksum_hex(step->u.compact.to), NULL);
+
+	cmd.out = -1;
+
+	ret = start_command(&cmd);
+	if (ret)
+		goto out;
+
+	out = xfdopen(cmd.out, "r");
+	while (strbuf_getline_lf(&buf, out) != EOF) {
+		if (step->csum) {
+			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
+			goto out;
+		}
+		step->csum = strbuf_detach(&buf, NULL);
+	}
+
+	ret = finish_command(&cmd);
+
+out:
+	if (out)
+		fclose(out);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec(struct midx_compaction_step *step,
+				     struct repack_write_midx_opts *opts,
+				     const char *base)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot execute UNKNOWN midx compaction step");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_compaction_step_exec_copy(step);
+	case MIDX_COMPACTION_STEP_WRITE:
+		return midx_compaction_step_exec_write(step, opts, base);
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_compaction_step_exec_compact(step, opts);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static void midx_compaction_step_release(struct midx_compaction_step *step)
+{
+	if (step->type == MIDX_COMPACTION_STEP_WRITE)
+		string_list_clear(&step->u.write, 0);
+	free(step->csum);
+}
+
+static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
+					    struct midx_compaction_step **steps_p,
+					    size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step step = { 0 };
+	struct strbuf buf = STRBUF_INIT;
+	size_t steps_nr = 0, steps_alloc = 0;
+	uint32_t i;
+	int ret = 0;
+
+	trace2_region_enter("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	for (i = 0; m && i < m->num_packs + m->num_packs_in_base; i++) {
+		if (prepare_midx_pack(m, i)) {
+			ret = error(_("could not load pack %"PRIu32" from MIDX"),
+				    i);
+			goto out;
+		}
+	}
+
+	trace2_region_enter("repack", "steps:write", opts->existing->repo);
+
+	/*
+	 * The first MIDX in the resulting chain is always going to be
+	 * new.
+	 *
+	 * At a minimum, it will include all of the newly written packs.
+	 * If there is an existing MIDX whose tip layer contains packs
+	 * that were repacked, it will also include any of its packs
+	 * which were *not* rolled up as part of the geometric repack
+	 * (if any), and the previous tip will be replaced.
+	 *
+	 * It may grow to include the packs from zero or more MIDXs from
+	 * the old chain, beginning either at the old tip (if the MIDX
+	 * was *not* rewritten) or the old tip's base MIDX layer
+	 * (otherwise).
+	 */
+	step.type = MIDX_COMPACTION_STEP_WRITE;
+	string_list_init_dup(&step.u.write);
+
+	for (i = 0; i < opts->names->nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "pack-%s.idx", opts->names->items[i].string);
+		string_list_append(&step.u.write, buf.buf);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:fresh",
+				   step.u.write.items[step.u.write.nr - 1].string);
+	}
+	for (i = 0; i < opts->geometry->split; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:fresh:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * Now handle any existing packs which were *not* rewritten.
+	 *
+	 * The list of packs in opts->geometry only contains MIDX'd
+	 * packs from the newest layer when that layer has more than
+	 * 'repack.midxNewLayerThreshold' number of packs.
+	 *
+	 * If the MIDX tip was rewritten (that is, one or more of those
+	 * packs appear below the split line), then add all packs above
+	 * the split line to the new layer, as the old one is no longer
+	 * usable.
+	 *
+	 * If the MIDX tip was not rewritten (that is, all MIDX'd packs
+	 * from the youngest layer appear below the split line, or were
+	 * not included in the geometric repack at all because there
+	 * were too few of them), ignore them since we'll retain the
+	 * existing layer as-is.
+	 */
+	for (i = opts->geometry->split; i < opts->geometry->pack_nr; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		struct string_list_item *item;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, pack_basename(p));
+		strbuf_strip_suffix(&buf, ".pack");
+		strbuf_addstr(&buf, ".idx");
+
+		if (p->multi_pack_index &&
+		    !opts->geometry->midx_tip_rewritten) {
+			trace2_data_string("repack", opts->existing->repo,
+					   "exclude:unmodified", buf.buf);
+			continue;
+		}
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified", buf.buf);
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified:midx",
+				   p->multi_pack_index ? "true" : "false");
+
+		item = string_list_append(&step.u.write, buf.buf);
+		if (p->multi_pack_index || i == opts->geometry->pack_nr - 1)
+			item->util = (void *)1; /* mark as preferred */
+
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:unmodified:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * If the MIDX tip was rewritten, then we no longer consider it
+	 * a candidate for compaction, since it will not exist in the
+	 * MIDX chain being built.
+	 */
+	if (opts->geometry->midx_tip_rewritten)
+		m = m->base_midx;
+
+	trace2_data_string("repack", opts->existing->repo, "midx:rewrote-tip",
+			   opts->geometry->midx_tip_rewritten ? "true" : "false");
+
+	trace2_region_enter("repack", "compact", opts->existing->repo);
+
+	/*
+	 * Compact additional MIDX layers into this proposed one until
+	 * the merging condition is violated.
+	 */
+	while (m) {
+		uint32_t preferred_pack_idx;
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "candidate", midx_get_checksum_hex(m));
+
+		if (step.objects_nr < m->num_objects / opts->midx_split_factor) {
+			/*
+			 * Stop compacting MIDX layer as soon as the
+			 * merged size is less than half the size of the
+			 * next layer in the chain.
+			 */
+			trace2_data_string("repack", opts->existing->repo,
+					   "compact", "violated");
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "objects_nr",
+					   (uintmax_t)step.objects_nr);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "next_objects_nr",
+					   (uintmax_t)m->num_objects);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "split_factor",
+					   (uintmax_t)opts->midx_split_factor);
+
+			break;
+		}
+
+		if (midx_preferred_pack(m, &preferred_pack_idx) < 0) {
+			ret = error(_("could not find preferred pack for MIDX "
+				      "%s"), midx_get_checksum_hex(m));
+			goto out;
+		}
+
+		for (i = 0; i < m->num_packs; i++) {
+			struct string_list_item *item;
+			uint32_t pack_int_id = i + m->num_packs_in_base;
+			struct packed_git *p = nth_midxed_pack(m, pack_int_id);
+
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, pack_basename(p));
+			strbuf_strip_suffix(&buf, ".pack");
+			strbuf_addstr(&buf, ".idx");
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "midx:pack", buf.buf);
+
+			item = string_list_append(&step.u.write, buf.buf);
+			if (pack_int_id == preferred_pack_idx)
+				item->util = (void *)1; /* mark as preferred */
+		}
+
+		if (unsigned_add_overflows(step.objects_nr, m->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+		step.objects_nr += m->num_objects;
+
+		m = m->base_midx;
+	}
+
+	if (step.u.write.nr > 0) {
+		/*
+		 * As long as there is at least one new pack to write
+		 * (and thus the MIDX is non-empty), add it to the plan.
+		 */
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+		steps[steps_nr++] = step;
+	}
+
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:objects_nr", (uintmax_t)step.objects_nr);
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:packs_nr", (uintmax_t)step.u.write.nr);
+
+	trace2_region_leave("repack", "compact", opts->existing->repo);
+	trace2_region_leave("repack", "steps:write", opts->existing->repo);
+
+	trace2_region_enter("repack", "steps:rest", opts->existing->repo);
+
+	/*
+	 * Then start over, repeat, and either compact or keep as-is
+	 * each MIDX layer until we have exhausted the chain.
+	 *
+	 * Finally, evaluate the remainder of the chain (if any) and
+	 * either compact a sequence of adjacent layers, or keep
+	 * individual layers as-is according to the same merging
+	 * condition as above.
+	 */
+	while (m) {
+		struct multi_pack_index *next = m;
+
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+
+		memset(&step, 0, sizeof(step));
+		step.type = MIDX_COMPACTION_STEP_UNKNOWN;
+
+		trace2_region_enter("repack", "step", opts->existing->repo);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "from", midx_get_checksum_hex(m));
+
+		while (next) {
+			uint32_t proposed_objects_nr;
+			if (unsigned_add_overflows(step.objects_nr, next->num_objects)) {
+				ret = error(_("too many objects in MIDX compaction step"));
+				trace2_region_leave("repack", "step", opts->existing->repo);
+				goto out;
+			}
+
+			proposed_objects_nr = step.objects_nr + next->num_objects;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "proposed",
+					   midx_get_checksum_hex(next));
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "proposed:objects_nr",
+					   (uintmax_t)next->num_objects);
+
+			if (!next->base_midx) {
+				/*
+				 * If we are at the end of the MIDX
+				 * chain, there is nothing to compact,
+				 * so mark it and stop.
+				 */
+				step.objects_nr = proposed_objects_nr;
+				break;
+			}
+
+			if (proposed_objects_nr < next->base_midx->num_objects / opts->midx_split_factor) {
+				/*
+				 * If there is a MIDX following this
+				 * one, but our accumulated size is less
+				 * than half of its size, compacting
+				 * them would violate the merging
+				 * condition, so stop here.
+				 */
+
+				trace2_data_string("repack", opts->existing->repo,
+						   "compact:violated:at",
+						   midx_get_checksum_hex(next->base_midx));
+				trace2_data_intmax("repack", opts->existing->repo,
+						   "compact:violated:at:objects_nr",
+						   (uintmax_t)next->base_midx->num_objects);
+				break;
+			}
+
+			/*
+			 * Otherwise, it is OK to compact the next layer
+			 * into this one. Do so, and then continue
+			 * through the remainder of the chain.
+			 */
+			step.objects_nr = proposed_objects_nr;
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "step:objects_nr",
+					   (uintmax_t)step.objects_nr);
+			next = next->base_midx;
+		}
+
+		if (m == next) {
+			step.type = MIDX_COMPACTION_STEP_COPY;
+			step.u.copy = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "copy");
+		} else {
+			step.type = MIDX_COMPACTION_STEP_COMPACT;
+			step.u.compact.from = next;
+			step.u.compact.to = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "to", midx_get_checksum_hex(m));
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "compact");
+		}
+
+		m = next->base_midx;
+		steps[steps_nr++] = step;
+		trace2_region_leave("repack", "step", opts->existing->repo);
+	}
+
+	trace2_region_leave("repack", "steps:rest", opts->existing->repo);
+
+out:
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+
+	strbuf_release(&buf);
+
+	trace2_region_leave("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	return ret;
+}
+
+static int write_midx_incremental(struct repack_write_midx_opts *opts)
+{
+	struct midx_compaction_step *steps = NULL;
+	struct strbuf lock_name = STRBUF_INIT;
+	struct lock_file lf;
+	size_t steps_nr = 0;
+	size_t i;
+	int ret = 0;
+
+	get_midx_chain_filename(opts->existing->source, &lock_name);
+	if (safe_create_leading_directories(opts->existing->repo,
+					    lock_name.buf))
+		die_errno(_("unable to create leading directories of %s"),
+			  lock_name.buf);
+	hold_lock_file_for_update(&lf, lock_name.buf, LOCK_DIE_ON_ERROR);
+
+	if (!fdopen_lock_file(&lf, "w")) {
+		ret = error_errno(_("unable to open multi-pack-index chain file"));
+		goto done;
+	}
+
+	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+		ret = error(_("unable to generate compaction plan"));
+		goto done;
+	}
+
+	for (i = 0; i < steps_nr; i++) {
+		struct midx_compaction_step *step = &steps[i];
+		char *base = NULL;
+
+		if (i + 1 < steps_nr)
+			base = xstrdup(midx_compaction_step_base(&steps[i + 1]));
+
+		if (midx_compaction_step_exec(step, opts, base) < 0) {
+			ret = error(_("unable to execute compaction step %"PRIuMAX),
+				    (uintmax_t)i);
+			free(base);
+			goto done;
+		}
+
+		free(base);
+	}
+
+	i = steps_nr;
+	while (i--) {
+		struct midx_compaction_step *step = &steps[i];
+		if (!step->csum)
+			BUG("missing result for compaction step %"PRIuMAX,
+			    (uintmax_t)i);
+		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+	}
+
+	commit_lock_file(&lf);
+
+done:
+	strbuf_release(&lock_name);
+	for (i = 0; i < steps_nr; i++)
+		midx_compaction_step_release(&steps[i]);
+	free(steps);
+	return ret;
+}
+
 int repack_write_midx(struct repack_write_midx_opts *opts)
 {
 	switch (opts->mode) {
@@ -386,6 +958,8 @@ int repack_write_midx(struct repack_write_midx_opts *opts)
 		BUG("write_midx mode is NONE?");
 	case REPACK_WRITE_MIDX_DEFAULT:
 		return write_midx_included_packs(opts);
+	case REPACK_WRITE_MIDX_INCREMENTAL:
+		return write_midx_incremental(opts);
 	default:
 		BUG("unhandled write_midx mode: %d", opts->mode);
 	}
diff --git a/repack.h b/repack.h
index 81907fcce7f..831ccfb1c6c 100644
--- a/repack.h
+++ b/repack.h
@@ -137,6 +137,7 @@ struct tempfile;
 enum repack_write_midx_mode {
 	REPACK_WRITE_MIDX_NONE,
 	REPACK_WRITE_MIDX_DEFAULT,
+	REPACK_WRITE_MIDX_INCREMENTAL,
 };
 
 struct repack_write_midx_opts {
@@ -148,6 +149,8 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 	enum repack_write_midx_mode mode;
 };
 
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 15/16] repack: introduce `--write-midx=incremental`

[Taylor Blau]

Expose the incremental MIDX repacking mode (implemented in an earlier
commit) via a new --write-midx=incremental option for `git repack`.

Add "incremental" as a recognized argument to the --write-midx
OPT_CALLBACK, mapping it to REPACK_WRITE_MIDX_INCREMENTAL. When this
mode is active and --geometric is in use, set the midx_layer_threshold
on the pack geometry so that only packs in sufficiently large tip layers
are considered for repacking.

Two new configuration options control the compaction behavior:

 - repack.midxSplitFactor (default: 2): the factor used in the
   geometric merging condition for MIDX layers.

 - repack.midxNewLayerThreshold (default: 8): the minimum number of
   packs in the tip MIDX layer before its packs are considered as
   candidates for geometric repacking.

Add tests exercising the new mode across a variety of scenarios
including basic geometric violations, multi-round chain integrity,
branching and merging histories, cross-layer object uniqueness, and
threshold-based compaction.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/config/repack.adoc   |  18 ++
 Documentation/git-repack.adoc      |  39 ++-
 builtin/repack.c                   |  38 ++-
 midx.c                             |  31 ++
 midx.h                             |   3 +
 repack-geometry.c                  |  13 +-
 repack-midx.c                      |   5 +
 repack.c                           |  21 +-
 repack.h                           |   9 +-
 t/meson.build                      |   1 +
 t/t7705-repack-incremental-midx.sh | 436 +++++++++++++++++++++++++++++
 11 files changed, 593 insertions(+), 21 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh

diff --git a/Documentation/config/repack.adoc b/Documentation/config/repack.adoc
index e9e78dcb198..054de9f8795 100644
--- a/Documentation/config/repack.adoc
+++ b/Documentation/config/repack.adoc
@@ -46,3 +46,21 @@ repack.midxMustContainCruft::
 	`--write-midx`. When false, cruft packs are only included in the MIDX
 	when necessary (e.g., because they might be required to form a
 	reachability closure with MIDX bitmaps). Defaults to true.
+
+repack.midxSplitFactor::
+	The factor used in the geometric merging condition when
+	compacting incremental MIDX layers during `git repack` when
+	invoked with the `--write-midx=incremental` option.
++
+Adjacent layers are merged when the accumulated object count of the
+newer layer exceeds `1/<N>` of the object count of the next deeper
+layer. Defaults to 2.
+
+repack.midxNewLayerThreshold::
+	The minimum number of packs in the tip MIDX layer before those
+	packs are considered as candidates for geometric repacking
+	during `git repack --write-midx=incremental`.
++
+When the tip layer has fewer packs than this threshold, those packs are
+excluded from the geometric repack entirely, and are thus left
+unmodified. Defaults to 8.
diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 673ce910837..27a99cc46f4 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
-	[--write-midx] [--name-hash-version=<n>] [--path-walk]
+	[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]
 
 DESCRIPTION
 -----------
@@ -250,9 +250,42 @@ pack as the preferred pack for object selection by the MIDX (see
 linkgit:git-multi-pack-index[1]).
 
 -m::
---write-midx::
+--write-midx[=<mode>]::
 	Write a multi-pack index (see linkgit:git-multi-pack-index[1])
-	containing the non-redundant packs.
+	containing the non-redundant packs. The following modes are
+	available:
++
+--
+	`default`;;
+		Write a single MIDX covering all packs. This is the
+		default when `--write-midx` is given without an
+		explicit mode.
+
+	`incremental`;;
+		Write an incremental MIDX chain instead of a single
+		flat MIDX. This mode requires `--geometric`.
++
+The incremental mode maintains a chain of MIDX layers that is compacted
+over time using a geometric merging strategy. Each repack creates a new
+tip layer containing the newly written pack(s). Adjacent layers are then
+merged whenever the newer layer's object count exceeds
+`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
+that do not meet this condition are retained as-is.
++
+The result is that newer (tip) layers tend to contain many small packs
+with relatively few objects, while older (deeper) layers contain fewer,
+larger packs covering more objects. Because compaction is driven by the
+tip of the chain, newer layers are also rewritten more frequently than
+older ones, which are only touched when enough objects have accumulated
+to justify merging into them. This keeps the total number of layers
+logarithmic relative to the total number of objects.
++
+Only packs in the tip MIDX layer are considered as candidates for the
+geometric repack; packs in deeper layers are left untouched. If the tip
+layer contains fewer packs than `repack.midxNewLayerThreshold`, those
+packs are excluded from the geometry entirely, and a new layer is
+created for any new pack(s) without disturbing the existing chain.
+--
 
 --name-hash-version=<n>::
 	Provide this argument to the underlying `git pack-objects` process.
diff --git a/builtin/repack.c b/builtin/repack.c
index 3a5042491d6..9e070f35868 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -33,7 +33,7 @@ static int midx_must_contain_cruft = 1;
 static const char *const git_repack_usage[] = {
 	N_("git repack [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]\n"
 	   "[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]\n"
-	   "[--write-midx] [--name-hash-version=<n>] [--path-walk]"),
+	   "[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]"),
 	NULL
 };
 
@@ -42,9 +42,14 @@ static const char incremental_bitmap_conflict_error[] = N_(
 "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
 );
 
+#define DEFAULT_MIDX_SPLIT_FACTOR 2
+#define DEFAULT_MIDX_NEW_LAYER_THRESHOLD 8
+
 struct repack_config_ctx {
 	struct pack_objects_args *po_args;
 	struct pack_objects_args *cruft_po_args;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 };
 
 static int repack_config(const char *var, const char *value,
@@ -94,6 +99,16 @@ static int repack_config(const char *var, const char *value,
 		midx_must_contain_cruft = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "repack.midxsplitfactor")) {
+		repack_ctx->midx_split_factor = git_config_int(var, value,
+							       ctx->kvi);
+		return 0;
+	}
+	if (!strcmp(var, "repack.midxnewlayerthreshold")) {
+		repack_ctx->midx_new_layer_threshold = git_config_int(var, value,
+								      ctx->kvi);
+		return 0;
+	}
 	return git_default_config(var, value, ctx, cb);
 }
 
@@ -109,6 +124,8 @@ static int option_parse_write_midx(const struct option *opt, const char *arg,
 
 	if (!arg || !*arg)
 		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else if (!strcmp(arg, "incremental"))
+		*cfg = REPACK_WRITE_MIDX_INCREMENTAL;
 	else
 		return error(_("unknown value for %s: %s"), opt->long_name, arg);
 
@@ -223,6 +240,8 @@ int cmd_repack(int argc,
 	memset(&config_ctx, 0, sizeof(config_ctx));
 	config_ctx.po_args = &po_args;
 	config_ctx.cruft_po_args = &cruft_po_args;
+	config_ctx.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR;
+	config_ctx.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD;
 
 	repo_config(repo, repack_config, &config_ctx);
 
@@ -244,6 +263,9 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
+	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
+		die(_("--write-midx=incremental requires --geometric"));
+
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
@@ -293,6 +315,10 @@ int cmd_repack(int argc,
 	if (geometry.split_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
+		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL) {
+			geometry.midx_layer_threshold = config_ctx.midx_new_layer_threshold;
+			geometry.midx_layer_threshold_set = true;
+		}
 		pack_geometry_init(&geometry, &existing, &po_args);
 		pack_geometry_split(&geometry);
 	}
@@ -540,6 +566,8 @@ int cmd_repack(int argc,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
 			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.midx_split_factor = config_ctx.midx_split_factor,
+			.midx_new_layer_threshold = config_ctx.midx_new_layer_threshold,
 			.mode = write_midx,
 		};
 
@@ -552,11 +580,15 @@ int cmd_repack(int argc,
 
 	if (delete_redundant) {
 		int opts = 0;
-		existing_packs_remove_redundant(&existing, packdir);
+		bool wrote_incremental_midx = write_midx == REPACK_WRITE_MIDX_INCREMENTAL;
+
+		existing_packs_remove_redundant(&existing, packdir,
+						wrote_incremental_midx);
 
 		if (geometry.split_factor)
 			pack_geometry_remove_redundant(&geometry, &names,
-						       &existing, packdir);
+						       &existing, packdir,
+						       wrote_incremental_midx);
 		if (show_progress)
 			opts |= PRUNE_PACKED_VERBOSE;
 		prune_packed_objects(opts);
diff --git a/midx.c b/midx.c
index 7d23338aa3a..fd9e72a3941 100644
--- a/midx.c
+++ b/midx.c
@@ -852,6 +852,37 @@ void clear_midx_file(struct repository *r)
 	strbuf_release(&midx);
 }
 
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes)
+{
+	struct strbuf chain = STRBUF_INIT;
+
+	get_midx_chain_filename(r->objects->sources, &chain);
+
+	if (r->objects) {
+		struct odb_source *source = r->objects->sources;
+		for (source = r->objects->sources; source; source = source->next) {
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			if (files->packed->midx)
+				close_midx(files->packed->midx);
+			files->packed->midx = NULL;
+		}
+	}
+
+	if (!keep_hashes && remove_path(chain.buf))
+		die(_("failed to clear multi-pack-index chain at %s"),
+		    chain.buf);
+
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_BITMAP,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_REV,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_MIDX,
+					 keep_hashes);
+
+	strbuf_release(&chain);
+}
+
 static int verify_midx_error;
 
 __attribute__((format (printf, 1, 2)))
diff --git a/midx.h b/midx.h
index f211a38b9e7..b45da0a3144 100644
--- a/midx.h
+++ b/midx.h
@@ -9,6 +9,7 @@ struct repository;
 struct bitmapped_pack;
 struct git_hash_algo;
 struct odb_source;
+struct strvec;
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION_V1 1
@@ -143,6 +144,8 @@ int write_midx_file_compact(struct odb_source *source,
 			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes);
 int verify_midx_file(struct odb_source *source, unsigned flags);
 int expire_midx_packs(struct odb_source *source, unsigned flags);
 int midx_repack(struct odb_source *source, size_t batch_size, unsigned flags);
diff --git a/repack-geometry.c b/repack-geometry.c
index d2065205f87..5b554da89f1 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -251,7 +251,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 				   uint32_t pack_nr,
 				   struct string_list *names,
 				   struct existing_packs *existing,
-				   const char *packdir)
+				   const char *packdir,
+				   bool wrote_incremental_midx)
 {
 	const struct git_hash_algo *algop = existing->repo->hash_algo;
 	struct strbuf buf = STRBUF_INIT;
@@ -271,7 +272,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 		    (string_list_has_string(&existing->kept_packs, buf.buf)))
 			continue;
 
-		repack_remove_redundant_pack(existing->repo, packdir, buf.buf);
+		repack_remove_redundant_pack(existing->repo, packdir, buf.buf,
+					     wrote_incremental_midx);
 	}
 
 	strbuf_release(&buf);
@@ -280,12 +282,13 @@ static void remove_redundant_packs(struct packed_git **pack,
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir)
+				    const char *packdir,
+				    bool wrote_incremental_midx)
 {
 	remove_redundant_packs(geometry->pack, geometry->split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 	remove_redundant_packs(geometry->promisor_pack, geometry->promisor_split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 }
 
 void pack_geometry_release(struct pack_geometry *geometry)
diff --git a/repack-midx.c b/repack-midx.c
index a159b879775..5bf5df03d5d 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -894,6 +894,7 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 	struct midx_compaction_step *steps = NULL;
 	struct strbuf lock_name = STRBUF_INIT;
 	struct lock_file lf;
+	struct strvec keep_hashes = STRVEC_INIT;
 	size_t steps_nr = 0;
 	size_t i;
 	int ret = 0;
@@ -939,11 +940,15 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 			BUG("missing result for compaction step %"PRIuMAX,
 			    (uintmax_t)i);
 		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+		strvec_push(&keep_hashes, step->csum);
 	}
 
 	commit_lock_file(&lf);
 
+	clear_incremental_midx_files(opts->existing->repo, &keep_hashes);
+
 done:
+	strvec_clear(&keep_hashes);
 	strbuf_release(&lock_name);
 	for (i = 0; i < steps_nr; i++)
 		midx_compaction_step_release(&steps[i]);
diff --git a/repack.c b/repack.c
index 2ee6b51420a..be2cc0e9d44 100644
--- a/repack.c
+++ b/repack.c
@@ -55,14 +55,18 @@ void pack_objects_args_release(struct pack_objects_args *args)
 }
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name)
+				  const char *base_name,
+				  bool wrote_incremental_midx)
 {
 	struct strbuf buf = STRBUF_INIT;
 	struct odb_source *source = repo->objects->sources;
 	struct multi_pack_index *m = get_multi_pack_index(source);
 	strbuf_addf(&buf, "%s.pack", base_name);
-	if (m && source->local && midx_contains_pack(m, buf.buf))
+	if (m && source->local && midx_contains_pack(m, buf.buf)) {
 		clear_midx_file(repo);
+		if (!wrote_incremental_midx)
+			clear_incremental_midx_files(repo, NULL);
+	}
 	strbuf_insertf(&buf, 0, "%s/", dir_name);
 	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
@@ -252,23 +256,26 @@ void existing_packs_mark_for_deletion(struct existing_packs *existing,
 
 static void remove_redundant_packs_1(struct repository *repo,
 				     struct string_list *packs,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	struct string_list_item *item;
 	for_each_string_list_item(item, packs) {
 		if (!existing_pack_is_marked_for_deletion(item))
 			continue;
-		repack_remove_redundant_pack(repo, packdir, item->string);
+		repack_remove_redundant_pack(repo, packdir, item->string,
+					     wrote_incremental_midx);
 	}
 }
 
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	remove_redundant_packs_1(existing->repo, &existing->non_kept_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 	remove_redundant_packs_1(existing->repo, &existing->cruft_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 }
 
 void existing_packs_release(struct existing_packs *existing)
diff --git a/repack.h b/repack.h
index 831ccfb1c6c..d2876f569a6 100644
--- a/repack.h
+++ b/repack.h
@@ -34,7 +34,8 @@ void prepare_pack_objects(struct child_process *cmd,
 void pack_objects_args_release(struct pack_objects_args *args);
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name);
+				  const char *base_name,
+				  bool wrote_incremental_midx);
 
 struct write_pack_opts {
 	struct pack_objects_args *po_args;
@@ -84,7 +85,8 @@ void existing_packs_retain_cruft(struct existing_packs *existing,
 void existing_packs_mark_for_deletion(struct existing_packs *existing,
 				      struct string_list *names);
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir);
+				     const char *packdir,
+				     bool wrote_incremental_midx);
 void existing_packs_release(struct existing_packs *existing);
 
 struct generated_pack;
@@ -129,7 +131,8 @@ struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry);
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir);
+				    const char *packdir,
+				    bool wrote_incremental_midx);
 void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
diff --git a/t/meson.build b/t/meson.build
index 919a0ae4cbb..3fc38d123b1 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -950,6 +950,7 @@ integration_tests = [
   't7702-repack-cyclic-alternate.sh',
   't7703-repack-geometric.sh',
   't7704-repack-cruft.sh',
+  't7705-repack-incremental-midx.sh',
   't7800-difftool.sh',
   't7810-grep.sh',
   't7811-grep-open.sh',
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
new file mode 100755
index 00000000000..f81c2c67060
--- /dev/null
+++ b/t/t7705-repack-incremental-midx.sh
@@ -0,0 +1,436 @@
+#!/bin/sh
+
+test_description='git repack --write-midx=incremental'
+
+. ./test-lib.sh
+
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
+
+objdir=.git/objects
+packdir=$objdir/pack
+midxdir=$packdir/multi-pack-index.d
+midx_chain=$midxdir/multi-pack-index-chain
+
+# incrementally_repack N
+#
+# Make "N" new commits, each stored in their own pack, and then repacked
+# with the --write-midx=incremental strategy.
+incrementally_repack () {
+	for i in $(test_seq 1 "$1")
+	do
+		test_commit "$i" &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify || return 1
+	done
+}
+
+# Create packs with geometrically increasing sizes so that they
+# satisfy the geometric progression and survive a --geometric=2
+# repack without being rolled up. Creates 3 packs containing 1,
+# 2, and 6 commits (3, 6, and 18 objects) respectively.
+create_geometric_packs () {
+	test_commit "small" &&
+	git repack -d &&
+
+	test_commit_bulk --message="medium" 2 &&
+	test_commit_bulk --message="large" 6 &&
+
+	git repack --geometric=2 -d --write-midx=incremental \
+		--write-bitmap-index
+}
+
+# create_layer <test_commit_bulk args>
+#
+# Creates a new MIDX layer with the contents of "test_commit_bulk $@".
+create_layer () {
+	test_commit_bulk "$@" &&
+
+	git multi-pack-index write --incremental --bitmap
+}
+
+# create_layers
+#
+# Reads lines of "<message> <nr>" from stdin and creates a new MIDX
+# layer for each line. See create_layer above for more.
+create_layers () {
+	while read msg nr
+	do
+		create_layer --message="$msg" "$nr" || return 1
+	done
+}
+
+test_expect_success '--write-midx=incremental requires --geometric' '
+	test_must_fail git repack --write-midx=incremental 2>err &&
+
+	test_grep -- "--write-midx=incremental requires --geometric" err
+'
+
+test_expect_success 'below layer threshold, tip packs excluded' '
+	git init below-layer-threshold-tip-packs-excluded &&
+	(
+		cd below-layer-threshold-tip-packs-excluded &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 4 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create 3 packs forming a geometric progression by
+		# object count such that they are unmodified by the
+		# initial repack. The MIDX chain thusly contains a
+		# single layer with three packs.
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a new commit. Since the layer threshold is
+		# unmet, a new MIDX layer is added on top of the
+		# existing one.
+		test_commit extra &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+		test_line_count = 1 packs.new &&
+
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs repacked' '
+	git init above-layer-threshold-tip-packs-repacked &&
+	(
+		cd above-layer-threshold-tip-packs-repacked &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Same setup, but with the layer threshold set to 2.
+		# Since the tip MIDX layer meets that threshold, its
+		# packs are considered repack candidates.
+		create_geometric_packs &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Perturb the existing progression such that it is
+		# rolled up into a single new pack, invalidating the
+		# existing MIDX layer and replacing it with a new one.
+		test_commit extra &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip layer preserved' '
+	git init above-layer-threshold-tip-layer-preserved &&
+	(
+		cd above-layer-threshold-tip-layer-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		test_commit_bulk --message="medium" 2 &&
+		test_commit_bulk --message="large" 6 &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 1 "$midx_chain" &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Create objects to form a pack satisfying the geometric
+		# progression (thus preserving the tip layer), but not
+		# so large that it meets the layer merging condition.
+		test_commit_bulk --message="small" 1 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		test_line_count = 1 packs.new &&
+		test_line_count = 3 packs.after &&
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs preserved' '
+	git init above-layer-threshold-tip-packs-preserved &&
+	(
+		cd above-layer-threshold-tip-packs-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Same setup as above, but this time the new objects do
+		# not satisfy the new layer merging condition, resulting
+		# in a new tip layer.
+		test_commit_bulk --message="huge" 18 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+		test_line_count = 1 packs.new &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'new tip absorbs multiple layers' '
+	git init new-tip-absorbs-multiple-layers &&
+	(
+		cd new-tip-absorbs-multiple-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a 4-layer chain where each layer is too small to
+		# absorb the one below it. The sizes must satisfy L(n) <
+		# L(n-1)/2 for each adjacent pair:
+		#
+		#   L0 (oldest): 75 obj (25 commits)
+		#   L1:          21 obj  (7 commits, 21 < 75/2)
+		#   L2:           9 obj  (3 commits,  9 < 21/2)
+		#   L3 (tip):     3 obj  (1 commit,   3 <  9/2)
+		create_layers <<-\EOF &&
+		L0 25
+		L1 7
+		L2 3
+		L3 1
+		EOF
+
+		test_line_count = 4 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Now add a new commit. The merging condition is
+		# satisfied between L3-L1, but violated at L0, which is
+		# too large relative to the accumulated size.
+		#
+		# As a result, the chain shrinks from 4 to 2 layers.
+		test_commit new &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'compaction of older layers' '
+	git init compaction-of-older-layers &&
+	(
+		cd compaction-of-older-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a chain with two small layers at the bottom
+		# and a larger barrier layer on top, producing a
+		# chain that violates the compaction invariant, since
+		# the two small layers would normally have been merged.
+		create_layers <<-\EOF &&
+		one 2
+		two 4
+		barrier 54
+		EOF
+
+		cp $midx_chain $midx_chain.before &&
+
+		# Running an incremental repack compacts the two
+		# small layers at the bottom of the chain as a
+		# separate step in the compaction plan.
+		test_commit another &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'geometric rollup with surviving tip packs' '
+	git init geometric-rollup-with-surviving-tip-packs &&
+	(
+		cd geometric-rollup-with-surviving-tip-packs &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create a pack large enough to anchor the geometric
+		# progression when small packs are added alongside it.
+		create_layer --message="big" 5 &&
+
+		test_line_count = 1 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a small number of objects such that the
+		# progression is unbothered. Note that the existing pack
+		# is considered a repack candidate as the new layer
+		# threshold is set to 1.
+		test_commit small-1 &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		cp $midx_chain $midx_chain.before
+	)
+'
+
+test_expect_success 'kept packs are excluded from repack' '
+	git init kept-packs-excluded-from-repack &&
+	(
+		cd kept-packs-excluded-from-repack &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create two equal-sized packs, marking one as kept.
+		for i in A B
+		do
+			test_commit "$i" && git repack -d || return 1
+		done &&
+
+		keep=$(ls $packdir/pack-*.idx | head -n 1) &&
+		touch "${keep%.idx}.keep" &&
+
+		# The kept pack is excluded as a repacking candidate
+		# entirely, so no rollup occurs as there is only one
+		# non-kept pack. A new MIDX layer is written containing
+		# that pack.
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count = 1 actual.packs &&
+		test_grep ! "$keep" actual.packs &&
+
+		git multi-pack-index verify &&
+
+		# All objects (from both kept and non-kept packs)
+		# must still be accessible.
+		git fsck
+	)
+'
+
+test_expect_success 'incremental MIDX with --max-pack-size' '
+	git init incremental-midx-with--max-pack-size &&
+	(
+		cd incremental-midx-with--max-pack-size &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		# Now add enough data that a small --max-pack-size will
+		# cause pack-objects to split its output. Create objects
+		# large enough to fill multiple packs.
+		test-tool genrandom foo 1M >big1 &&
+		test-tool genrandom bar 1M >big2 &&
+		git add big1 big2 &&
+		test_tick &&
+		git commit -a -m "big blobs" &&
+		git repack -d &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index --max-pack-size=1M &&
+
+		test_line_count = 1 "$midx_chain" &&
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count -gt 1 actual.packs &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'noop repack preserves valid MIDX chain' '
+	git init noop-repack-preserves-valid-midx-chain &&
+	(
+		cd noop-repack-preserves-valid-midx-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		git multi-pack-index verify &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Running again with no new objects should not break
+		# the MIDX chain. It produces "Nothing new to pack."
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_cmp $midx_chain.before $midx_chain &&
+
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
+test_expect_success 'repack -ad removes stale incremental chain' '
+	git init repack--ad-removes-stale-incremental-chain &&
+	(
+		cd repack--ad-removes-stale-incremental-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layers <<-\EOF &&
+		one 1
+		two 1
+		EOF
+
+		test_path_is_file $midx_chain &&
+		test_line_count = 2 $midx_chain &&
+
+		git repack -ad &&
+
+		test_path_is_missing $packdir/multi-pack-index &&
+		test_dir_is_empty $midxdir
+	)
+'
+
+test_done
-- 
2.53.0.729.g817728289e1.dirty

[PATCH 16/16] repack: allow `--write-midx=incremental` without `--geometric`

[Taylor Blau]

Previously, `--write-midx=incremental` required `--geometric` and would
die() without it. Relax this restriction so that incremental MIDX
repacking can be used independently.

Without `--geometric`, the behavior is append-only: a single new MIDX
layer is created containing whatever packs were written by the repack
and appended to the existing chain (or a new chain is started). Existing
layers are preserved as-is with no compaction or merging.

Implement this via a new repack_make_midx_append_plan() that builds a
plan consisting of a WRITE step for the freshly written packs followed
by COPY steps for every existing MIDX layer. The existing compaction
plan (repack_make_midx_compaction_plan) is used only when `--geometric`
is active.

Update the documentation to describe the behavior with and without
`--geometric`, and replace the test that enforced the old restriction
with one exercising append-only incremental MIDX repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-repack.adoc      | 19 +++++----
 builtin/repack.c                   |  3 --
 repack-midx.c                      | 64 ++++++++++++++++++++++++++++--
 t/t7705-repack-incremental-midx.sh | 35 +++++++++++++---
 4 files changed, 103 insertions(+), 18 deletions(-)

diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 27a99cc46f4..72c42015e23 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -263,14 +263,19 @@ linkgit:git-multi-pack-index[1]).
 
 	`incremental`;;
 		Write an incremental MIDX chain instead of a single
-		flat MIDX. This mode requires `--geometric`.
+		flat MIDX.
 +
-The incremental mode maintains a chain of MIDX layers that is compacted
-over time using a geometric merging strategy. Each repack creates a new
-tip layer containing the newly written pack(s). Adjacent layers are then
-merged whenever the newer layer's object count exceeds
-`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
-that do not meet this condition are retained as-is.
+Without `--geometric`, a new MIDX layer is appended to the existing
+chain (or a new chain is started) containing whatever packs were written
+by the repack. Existing layers are preserved as-is.
++
+When combined with `--geometric`, the incremental mode maintains a chain
+of MIDX layers that is compacted over time using a geometric merging
+strategy. Each repack creates a new tip layer containing the newly
+written pack(s). Adjacent layers are then merged whenever the newer
+layer's object count exceeds `1/repack.midxSplitFactor` of the next
+deeper layer's count. Layers that do not meet this condition are
+retained as-is.
 +
 The result is that newer (tip) layers tend to contain many small packs
 with relatively few objects, while older (deeper) layers contain fewer,
diff --git a/builtin/repack.c b/builtin/repack.c
index 9e070f35868..8e187322fe4 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -263,9 +263,6 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
-	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
-		die(_("--write-midx=incremental requires --geometric"));
-
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
diff --git a/repack-midx.c b/repack-midx.c
index 5bf5df03d5d..055aa4c1356 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -555,6 +555,60 @@ static void midx_compaction_step_release(struct midx_compaction_step *step)
 	free(step->csum);
 }
 
+/*
+ * Build an append-only MIDX plan: a single WRITE step for the freshly
+ * written packs, plus COPY steps for every existing layer.  No
+ * compaction or merging is performed.
+ */
+static void repack_make_midx_append_plan(struct repack_write_midx_opts *opts,
+					 struct midx_compaction_step **steps_p,
+					 size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step *step;
+	size_t steps_nr = 0, steps_alloc = 0;
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	if (opts->names->nr) {
+		struct strbuf buf = STRBUF_INIT;
+		uint32_t i;
+
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_WRITE;
+		string_list_init_dup(&step->u.write);
+
+		for (i = 0; i < opts->names->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "pack-%s.idx",
+				    opts->names->items[i].string);
+			string_list_append(&step->u.write, buf.buf);
+		}
+
+		strbuf_release(&buf);
+	}
+
+	for (; m; m = m->base_midx) {
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_COPY;
+		step->u.copy = m;
+		step->objects_nr = m->num_objects;
+	}
+
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+}
+
 static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
 					    struct midx_compaction_step **steps_p,
 					    size_t *steps_nr_p)
@@ -911,9 +965,13 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 		goto done;
 	}
 
-	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
-		ret = error(_("unable to generate compaction plan"));
-		goto done;
+	if (opts->geometry->split_factor) {
+		if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+			ret = error(_("unable to generate compaction plan"));
+			goto done;
+		}
+	} else {
+		repack_make_midx_append_plan(opts, &steps, &steps_nr);
 	}
 
 	for (i = 0; i < steps_nr; i++) {
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
index f81c2c67060..562554e69b4 100755
--- a/t/t7705-repack-incremental-midx.sh
+++ b/t/t7705-repack-incremental-midx.sh
@@ -63,10 +63,36 @@ create_layers () {
 	done
 }
 
-test_expect_success '--write-midx=incremental requires --geometric' '
-	test_must_fail git repack --write-midx=incremental 2>err &&
+test_expect_success '--write-midx=incremental without --geometric' '
+	git init incremental-without-geometric &&
+	(
+		cd incremental-without-geometric &&
 
-	test_grep -- "--write-midx=incremental requires --geometric" err
+		git config maintenance.auto false &&
+
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# A second repack appends a new layer without
+		# disturbing the existing one.
+		test_commit third &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 2 $midx_chain &&
+		head -n 1 $midx_chain.before >expect &&
+		head -n 1 $midx_chain >actual &&
+		test_cmp expect actual &&
+
+		git fsck
+	)
 '
 
 test_expect_success 'below layer threshold, tip packs excluded' '
@@ -334,8 +360,7 @@ test_expect_success 'kept packs are excluded from repack' '
 		# entirely, so no rollup occurs as there is only one
 		# non-kept pack. A new MIDX layer is written containing
 		# that pack.
-		git repack --geometric=2 -d --write-midx=incremental \
-			--write-bitmap-index &&
+		git repack --geometric=2 -d --write-midx=incremental &&
 
 		test-tool read-midx $objdir >actual &&
 		grep "^pack-.*\.idx$" actual >actual.packs &&
-- 
2.53.0.729.g817728289e1.dirty

Re: [PATCH 01/16] midx-write: handle noop writes when converting incremental chains

[Jeff King]

On Sun, Mar 29, 2026 at 05:40:53PM -0400, Taylor Blau wrote:

> This comparison fails when we have an incremental MIDX chain with
> at least two layers, since we do not recursively peel through earlier
> layers, instead treating the `->pack_names` array of the tip MIDX layer
> as containing all `m->num_packs + m->num_packs_in_base` packs.
> 
> Adjust this to instead look through the MIDX layers one by one when
> comparing pack names. While we're at it, fix a typo above in the same
> function.

Makes sense.

> @@ -1208,14 +1208,16 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
>  			BUG("same pack added twice?");
>  	}
>  
> -	for (uint32_t i = 0; i < ctx->nr; i++) {
> -		strbuf_reset(&buf);
> -		strbuf_addstr(&buf, midx->pack_names[i]);
> -		strbuf_strip_suffix(&buf, ".idx");
> +	for (struct multi_pack_index *m = midx; m; m = m->base_midx) {
> +		for (uint32_t i = 0; i < m->num_packs; i++) {
> +			strbuf_reset(&buf);
> +			strbuf_addstr(&buf, m->pack_names[i]);
> +			strbuf_strip_suffix(&buf, ".idx");
>  
> -		if (!strset_contains(&packs, buf.buf))
> -			goto out;
> -		strset_remove(&packs, buf.buf);
> +			if (!strset_contains(&packs, buf.buf))
> +				goto out;
> +			strset_remove(&packs, buf.buf);
> +		}

I wondered here if we would also need to adjust the check of the count,
but it is already OK. Just above the context is:

          if (ctx->nr != midx->num_packs + midx->num_packs_in_base)
                  goto out;

which handles the layered midx. Since we were counting up to ctx->nr
before, which includes packs in base midx layers, I guess these old
lines:

> -	for (uint32_t i = 0; i < ctx->nr; i++) {
> [...]
> -		strbuf_addstr(&buf, midx->pack_names[i]);

was probably reading garbage off the end of the array.

Now we iterate through each layer individually. We assume that the total
count matches ctx->nr, otherwise the midx struct is totally corrupt. We
could count them and double-check, I guess, but I don't think there's
much point (it would be a BUG() if they were mismatched).

So looks good to me.

-Peff

Re: [PATCH 02/16] midx: use `string_list` for retained MIDX files

[Jeff King]

On Sun, Mar 29, 2026 at 05:40:55PM -0400, Taylor Blau wrote:

> @@ -767,15 +766,12 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
>  				const char *file_name, void *_data)
>  {
>  	struct clear_midx_data *data = _data;
> -	uint32_t i;
>  
>  	if (!(starts_with(file_name, "multi-pack-index-") &&
>  	      ends_with(file_name, data->ext)))
>  		return;
> -	for (i = 0; i < data->keep_nr; i++) {
> -		if (!strcmp(data->keep[i], file_name))
> -			return;
> -	}
> +	if (string_list_has_string(&data->keep, file_name))
> +		return;

We are using has_string() here, so &data->keep will need to be sorted
(just keeping it in mind as we read further).

>  	if (keep_hash) {
> -		ALLOC_ARRAY(data.keep, 1);
> +		struct strbuf buf = STRBUF_INIT;
> +		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hash, ext);
>  
> -		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
> -		data.keep_nr = 1;
> +		string_list_insert(&data.keep, buf.buf);

OK, we use insert() here. But there is no difference between insert and
append, since the list only has one element. It is sorted either way. ;)

And we do not have to worry about being accidentally-quadratic because
there is no loop.

> -	ALLOC_ARRAY(data.keep, hashes_nr);
>  	for (i = 0; i < hashes_nr; i++)
> -		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
> -				       ext);
> -	data.keep_nr = hashes_nr;
> -	data.ext = ext;
> +		string_list_append(&data.keep,
> +				   xstrfmt("multi-pack-index-%s.%s",
> +					   keep_hashes[i], ext));
> +	string_list_sort(&data.keep);

OK, and here we really do have a list, but we do append+sort to avoid
being quadratic. Good.

Since AFAICT we only care about set inclusion, I probably would have
used a strset instead, but for the numbers we are talking about it is
unlikely to matter much either way.

So looking good.

-Peff

Re: [PATCH 03/16] strvec: introduce `strvec_init_alloc()`

[Jeff King]

On Sun, Mar 29, 2026 at 05:40:58PM -0400, Taylor Blau wrote:

> When the caller knows upfront how many elements will be pushed onto a
> `strvec`, it is useful to pre-allocate enough space in the array to fit
> that many elements (and one additional slot to store NULL, indicating
> the end of the list.)

The main reason I didn't add an "alloc" parameter to the initializer
originally is that if you know how many elements you are going to push,
you probably don't need a strvec in the first place (you can just make
an array). But let's wait and see how it gets used in a future patch.

> +void strvec_init_alloc(struct strvec *array, size_t alloc)
> +{
> +	if (!alloc) {
> +		struct strvec blank = STRVEC_INIT;
> +		memcpy(array, &blank, sizeof(*array));
> +	} else {
> +		CALLOC_ARRAY(array->v, st_add(alloc, 1));
> +		array->nr = 0;
> +		array->alloc = alloc + 1;
> +	}
>  }

We use the blank+memcpy approach so that we don't duplicate knowledge of
which elements are in the struct (and which might need to be
initialized). But now there's an implicit zero-ing happening when alloc
is non-zero. Can we write this as:

  struct strvec blank = STRVEC_INIT;
  memcpy(array, &blank, sizeof(*array));
  if (alloc) {
	CALLOC_ARRAY(array->v, st_add(alloc, 1));
	array->alloc = alloc + 1;
  }

The worst case is a useless write of a few struct elements, but I
suspect that an optimizing compiler can even get rid of those.

I doubt we are going to add new elements to a strvec, so this is
arguably just pedantry. It just feels like it's easy enough to do now
while we're thinking about it.

  As an aside, it is sad that we have to CALLOC() ourselves here, as
  there is no reusable "grow" operation. Probably not worth refactoring
  it out, though, though.

-Peff

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:00PM -0400, Taylor Blau wrote:

> -	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
> +	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
>  
>  	if (ctx.incremental) {
>  		FILE *chainf = fdopen_lock_file(&lk, "w");
> @@ -1760,39 +1760,45 @@ static int write_midx_internal(struct write_midx_opts *opts)
>  			for (i = 0; i < num_layers_before_from; i++) {
>  				uint32_t j = num_layers_before_from - i - 1;
>  
> -				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
> +				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
> +				keep_hashes.nr++;

Gross, we are just manipulating the innards of the strvec ourselves?

Is it really worth doing this (and adding init_alloc()) versus just:

  strvec_init(&keep_hashes);
  for (...)
	strvec_push(midx_get_checksum_hex(m));

? That's amortized linear-time, and it's not like the number of midx
layers is going to be large anyway.

>  				m = m->base_midx;
>  			}
>  
> -			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
> +			keep_hashes.v[i] = xstrdup(hash_to_hex_algop(midx_hash,
>  								   r->hash_algo));
> +			keep_hashes.nr++;

OK, this could be a push, too.

>  
>  			i = 0;
>  			for (m = ctx.m;
>  			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
>  			     m = m->base_midx) {
> -				keep_hashes[keep_hashes_nr - i - 1] =
> +				keep_hashes.v[keep_hashes_nr - i - 1] =
>  					xstrdup(midx_get_checksum_hex(m));
> +				keep_hashes.nr++;

But what is this? We're filling in from the back side of the array? I
mean...yeah, that's something that strvec_push() can't do. But I can't
help but feel like it might be simpler and more obvious to adjust the
iteration to build the array in order.

I dunno. Maybe that is hard to do. But if so, I question whether moving
to a strvec is worth it here, since we are not treating it as an opaque
type anymore. And it is not buying us much to use it (we get to pass one
parameter versus two, though that is easily solved with a struct, and we
get to use _clear() instead of our own free loop).

>  void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
> -				      char **keep_hashes,
> -				      uint32_t hashes_nr)
> +				      const struct strvec *keep_hashes)
>  {
>  	struct clear_midx_data data = {
> -		.keep = STRING_LIST_INIT_NODUP,
> +		.keep = STRING_LIST_INIT_DUP,
>  		.ext = ext,
>  	};
> -	uint32_t i;
>  
> -	for (i = 0; i < hashes_nr; i++)
> -		string_list_append(&data.keep,
> -				   xstrfmt("multi-pack-index-%s.%s",
> -					   keep_hashes[i], ext));
> -	string_list_sort(&data.keep);
> +	if (keep_hashes) {
> +		struct strbuf buf = STRBUF_INIT;
> +		for (size_t i = 0; i < keep_hashes->nr; i++) {
> +			strbuf_reset(&buf);
> +
> +			strbuf_addf(&buf, "multi-pack-index-%s.%s",
> +				    keep_hashes->v[i], ext);
> +			string_list_append(&data.keep, buf.buf);
> +		}
> +
> +		string_list_sort(&data.keep);
> +		strbuf_release(&buf);
> +	}

This hunk was unexpected. We move from xstrfmt() to a strbuf, but does
that have anything to do with the rest of the patch?

Also, I don't think using a strbuf really buys us anything. We are
reusing the strbuf for each format operation, but then we copy into the
string list anyway. So there is one allocation per loop iteration either
way.

Also also, the original was leaking the strings, right? The string_list
was initialized as NODUP, but we assigned allocate xstrfmt() results to
it. But because of the nodup, string_list_clear() won't free them.
It should have been:

  .keep = STRING_LIST_INIT_DUP,
  [...]
  string_list_append_nodup(&data.keep, xstfmrt(...));
  [...]
  string_list_clear(&data.keep);

in patch 2.

-Peff

Re: [PATCH 05/16] midx: introduce `--checksum-only` for incremental MIDX writes

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:03PM -0400, Taylor Blau wrote:

> When writing an incremental MIDX layer, the MIDX machinery writes the
> new layer into the multi-pack-index.d directory and then updates the
> multi-pack-index-chain file to include the freshly written layer.
> 
> Future callers however may not wish to immediately update the MIDX chain
> itself, preferring instead to write out new layer(s) themselves before
> atomically updating the chain. Concretely, the new incremental
> MIDX-based repacking strategy will want to do exactly this (that is,
> assemble the new MIDX chain itself before writing a new chain file and
> atomically linking it into place).
> 
> Introduce a `--checksum-only` flag that:
> 
>  * writes the new MIDX layer into the multi-pack-index.d directory
> 
>  * prints its checksum
> 
>  * does not update the multi-pack-index-chain file.

Your goal makes sense, but that is not at all what I would have expected
a "--checksum-only" option to do. From the name I expected it to just
compute or verify checksums and not write anything to disk. I dunno.
Maybe it should be "--no-write-chain-file" or something?

This is perhaps on the border of bike-shedding, and maybe the fact that
it is an option of "multi-pack-index write" would be enough to clarify.

> @@ -221,6 +233,9 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
>  			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
>  		OPT_BIT(0, "incremental", &opts.flags,
>  			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
> +		OPT_BIT(0, "checksum-only", &opts.flags,
> +			N_("write a MIDX layer without updating the MIDX chain"),
> +			MIDX_WRITE_CHECKSUM_ONLY),
>  		OPT_END(),
>  	};
>  
> @@ -239,6 +254,15 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
>  	if (argc != 2)
>  		usage_with_options(builtin_multi_pack_index_compact_usage,
>  				   options);
> +
> +	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
> +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
> +		error(_("cannot use %s without %s"),
> +		      "--checksum-only", "--incremental");
> +		usage_with_options(builtin_multi_pack_index_compact_usage,
> +				   options);
> +	}
> +

OK. It's sad that we have to duplicate these bits between the write and
compact operations, but I'm not sure how much sense it would make to
factor out common options (both how much work it is, and how much
duplication we could actually drop in practice).

>  	source = handle_object_dir_option(the_repository);
>  
>  	FREE_AND_NULL(options);
> diff --git a/midx-write.c b/midx-write.c
> index 9f7d2bbf4cb..2c6905173ba 100644
> --- a/midx-write.c
> +++ b/midx-write.c
> @@ -1600,11 +1600,14 @@ static int write_midx_internal(struct write_midx_opts *opts)
>  	}
>  
>  	if (ctx.incremental) {
> -		struct strbuf lock_name = STRBUF_INIT;
> +		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
> +			struct strbuf lock_name = STRBUF_INIT;
>  
> -		get_midx_chain_filename(opts->source, &lock_name);
> -		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
> -		strbuf_release(&lock_name);
> +			get_midx_chain_filename(opts->source, &lock_name);
> +			hold_lock_file_for_update(&lk, lock_name.buf,
> +						  LOCK_DIE_ON_ERROR);
> +			strbuf_release(&lock_name);
> +		}

OK, we skip the chain-file write here (or least the start of it).

> @@ -1725,14 +1728,19 @@ static int write_midx_internal(struct write_midx_opts *opts)
>  	}
>  	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
>  
> +	if (opts->flags & MIDX_WRITE_CHECKSUM_ONLY)
> +		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));

And then output the hash here instead.

Just thinking on the name I suggested, it implies the action from the
first hunk above, but not this one (whereas your name sort of does the
opposite). I don't think there is any reason to have _two_ options,
since you'd always want to do them together (or not at all). I still
like my suggestion better. ;)


>  	if (ctx.incremental) {
> -		FILE *chainf = fdopen_lock_file(&lk, "w");
>  		struct strbuf final_midx_name = STRBUF_INIT;
>  		struct multi_pack_index *m = ctx.base_midx;
>  
> -		if (!chainf) {
> -			error_errno(_("unable to open multi-pack-index chain file"));
> -			goto cleanup;
> +		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
> +			FILE *chainf = fdopen_lock_file(&lk, "w");
> +			if (!chainf) {
> +				error_errno(_("unable to open multi-pack-index chain file"));
> +				goto cleanup;
> +			}
>  		}

OK, and this is the actual write which is skipped. I guess this could
also be conditional on is_lock_file_locked(&lk), assuming we initialize
it with LOCK_INIT. In some ways that seems easier to analyze to me,
since you don't have to realize that "lk" is only active when
CHECKSUM_ONLY is not set. But I don't think there's another reason we'd
choose not to take a lock file, so I think it may be six of one and half
a dozen of the other.

> @@ -1793,8 +1801,10 @@ static int write_midx_internal(struct write_midx_opts *opts)
>  			}
>  		}
>  
> -		for (uint32_t i = 0; i < keep_hashes_nr; i++)
> -			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
> +		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY))
> +			for (uint32_t i = 0; i < keep_hashes_nr; i++)
> +				fprintf(get_lock_file_fp(&lk), "%s\n",
> +					keep_hashes.v[i]);
>  	} else {
>  		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
>  			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
> @@ -1804,10 +1814,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
>  	if (ctx.m || ctx.base_midx)
>  		odb_close(ctx.repo->objects);
>  
> -	if (commit_lock_file(&lk) < 0)
> -		die_errno(_("could not write multi-pack-index"));
> +	if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
> +		if (commit_lock_file(&lk) < 0)
> +			die_errno(_("could not write multi-pack-index"));
>  
> -	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
> +		clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
> +	}
>  	result = 0;

Ditto on these conditionals.

I'm out of time to keep reviewing, so I'll stop here for now. I'll try
to continue with the rest late tonight or tomorrow.

-Peff

Re: [PATCH 01/16] midx-write: handle noop writes when converting incremental chains

[Taylor Blau]

On Mon, Mar 30, 2026 at 06:33:46PM -0400, Jeff King wrote:
> Now we iterate through each layer individually. We assume that the total
> count matches ctx->nr, otherwise the midx struct is totally corrupt. We
> could count them and double-check, I guess, but I don't think there's
> much point (it would be a BUG() if they were mismatched).

Yeah, I agree. The omitted context:

        if (ctx->nr != midx->num_packs + midx->num_packs_in_base)
                goto out;

tells us that if we get to this spot, we expect our 'ctx->nr' to be equal
to 'midx->num_packs + midx->num_packs_in_base'.

As soon as we find a non-already-MIDX'd pack, we 'goto out', so I don't
think it makes much sense to build up a count and check it at the end
even after we know we can't optimize out the write.

We could check that the ->num_packs of each of the tip's base MIDXs
match ->num_packs_in_base, but the MIDX internals handle that for us by
how we construct that field, since num_packs_in_base is *not* read from
the on-disk MIDX.

Thanks,
Taylor

Re: [PATCH 02/16] midx: use `string_list` for retained MIDX files

[Taylor Blau]

On Mon, Mar 30, 2026 at 06:38:49PM -0400, Jeff King wrote:
> Since AFAICT we only care about set inclusion, I probably would have
> used a strset instead, but for the numbers we are talking about it is
> unlikely to matter much either way.

Yeah, a strset makes more sense here. I adjusted it locally to use that
over a string_list. Thanks!

Thanks,
Taylor

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

On Mon, Mar 30, 2026 at 07:01:30PM -0400, Jeff King wrote:
> On Sun, Mar 29, 2026 at 05:41:00PM -0400, Taylor Blau wrote:
>
> > -	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
> > +	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
> >
> >  	if (ctx.incremental) {
> >  		FILE *chainf = fdopen_lock_file(&lk, "w");
> > @@ -1760,39 +1760,45 @@ static int write_midx_internal(struct write_midx_opts *opts)
> >  			for (i = 0; i < num_layers_before_from; i++) {
> >  				uint32_t j = num_layers_before_from - i - 1;
> >
> > -				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
> > +				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
> > +				keep_hashes.nr++;
>
> Gross, we are just manipulating the innards of the strvec ourselves?
>
> Is it really worth doing this (and adding init_alloc()) versus just:
>
>   strvec_init(&keep_hashes);
>   for (...)
> 	strvec_push(midx_get_checksum_hex(m));
>
> ? That's amortized linear-time, and it's not like the number of midx
> layers is going to be large anyway.

Yeah, this is all pretty disgusting. It's in service of what you noted
below where we need to fill in the array out-of-order, but that's really
bending strvec around MIDX-specific awkwardness, which I dislike.

> I dunno. Maybe that is hard to do. But if so, I question whether moving
> to a strvec is worth it here, since we are not treating it as an opaque
> type anymore. And it is not buying us much to use it (we get to pass one
> parameter versus two, though that is easily solved with a struct, and we
> get to use _clear() instead of our own free loop).

I think it is worth it to move to a strvec here to avoid having to
manage our own memory and pass the length around separately, but I
dislike the way that I did it in this patch.

I tried adjusting this patch to juggle the MIDX layers in a way that
would allow us to just push to the strvec. It ended up being less
awkward/difficult than I thought, so I think we should go with that and
drop the previous patch.

> >  void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
> > -				      char **keep_hashes,
> > -				      uint32_t hashes_nr)
> > +				      const struct strvec *keep_hashes)
> >  {
> >  	struct clear_midx_data data = {
> > -		.keep = STRING_LIST_INIT_NODUP,
> > +		.keep = STRING_LIST_INIT_DUP,
> >  		.ext = ext,
> >  	};
> > -	uint32_t i;
> >
> > -	for (i = 0; i < hashes_nr; i++)
> > -		string_list_append(&data.keep,
> > -				   xstrfmt("multi-pack-index-%s.%s",
> > -					   keep_hashes[i], ext));
> > -	string_list_sort(&data.keep);
> > +	if (keep_hashes) {
> > +		struct strbuf buf = STRBUF_INIT;
> > +		for (size_t i = 0; i < keep_hashes->nr; i++) {
> > +			strbuf_reset(&buf);
> > +
> > +			strbuf_addf(&buf, "multi-pack-index-%s.%s",
> > +				    keep_hashes->v[i], ext);
> > +			string_list_append(&data.keep, buf.buf);
> > +		}
> > +
> > +		string_list_sort(&data.keep);
> > +		strbuf_release(&buf);
> > +	}
>
> This hunk was unexpected. We move from xstrfmt() to a strbuf, but does
> that have anything to do with the rest of the patch?

Yeah, this looks like it was left over from when I was writing this
patch and trying out different approaches, and I didn't notice that I
left it in when sending this patch out.

> Also also, the original was leaking the strings, right? The string_list
> was initialized as NODUP, but we assigned allocate xstrfmt() results to
> it. But because of the nodup, string_list_clear() won't free them.
> It should have been:
>
>   .keep = STRING_LIST_INIT_DUP,
>   [...]
>   string_list_append_nodup(&data.keep, xstfmrt(...));
>   [...]
>   string_list_clear(&data.keep);
>
> in patch 2.

Good catch, that's right, but partially obviated by the fact that we're moving to
strset here.

Thanks,
Taylor

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

On Tue, Mar 31, 2026 at 06:26:04PM -0400, Taylor Blau wrote:
> > Also also, the original was leaking the strings, right? The string_list
> > was initialized as NODUP, but we assigned allocate xstrfmt() results to
> > it. But because of the nodup, string_list_clear() won't free them.
> > It should have been:
> >
> >   .keep = STRING_LIST_INIT_DUP,
> >   [...]
> >   string_list_append_nodup(&data.keep, xstfmrt(...));
> >   [...]
> >   string_list_clear(&data.keep);
> >
> > in patch 2.
>
> Good catch, that's right, but partially obviated by the fact that we're moving to
> strset here.

...actually, I think this is a little more subtle than I was hoping for.

If we move to strset, we obviously still need to ensure that
constructing .keep doesn't leak strings. The strset API has a
strdup_strings option (via the strmap underneath), but does not
explicitly free keys on clear.

As far as I can tell, the strset API is not designed to take ownership
of the strings themselves, so to avoid leaks we would have to do
something like:

--- 8< ---
diff --git a/midx.c b/midx.c
index 4976ed4fac3..1c17a8657ff 100644
--- a/midx.c
+++ b/midx.c
@@ -802,12 +802,14 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
 				      char **keep_hashes,
 				      uint32_t hashes_nr)
 {
-	struct clear_midx_data data = {
-		.keep = STRSET_INIT,
-		.ext = ext,
-	};
+	struct clear_midx_data data = { .ext = ext };
+	struct hashmap_iter iter;
+	struct strmap_entry *e;
+
 	uint32_t i;

+	strset_init_with_options(&data.keep, NULL, 0);
+
 	for (i = 0; i < hashes_nr; i++)
 		strset_add(&data.keep, xstrfmt("multi-pack-index-%s.%s",
 					       keep_hashes[i], ext));
@@ -815,6 +817,9 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
 				     clear_midx_file_ext, &data);

+	hashmap_for_each_entry((struct hashmap*)&data.keep.map, &iter, e, ent) {
+		free((void *)e->key);
+	}
 	strset_clear(&data.keep);
 }
--- >8 ---

which is... pretty gross. I think we may be better off sticking with a
string_list for 'keep' here internally.

Thanks,
Taylor

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Jeff King]

On Tue, Mar 31, 2026 at 06:50:49PM -0400, Taylor Blau wrote:

> If we move to strset, we obviously still need to ensure that
> constructing .keep doesn't leak strings. The strset API has a
> strdup_strings option (via the strmap underneath), but does not
> explicitly free keys on clear.
> 
> As far as I can tell, the strset API is not designed to take ownership
> of the strings themselves, so to avoid leaks we would have to do
> something like:

It's based on strmap, which handles ownership. If you use STRMAP_INIT or
strmap_init(), then strdup_strings is set to 1.

Or do you mean that you can't transfer ownership into the strset from an
existing string? I think that is true, and if you are generating the
strings with xstrfmt() then yeah, you have two copies in the loop and
have to free one.

That is probably actually fine for the sizes we are talking about, but
if you want to care you can either do the reusable-strbuf thing, or
we can add a strset_add_nodup().

> @@ -815,6 +817,9 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
>  	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
>  				     clear_midx_file_ext, &data);
> 
> +	hashmap_for_each_entry((struct hashmap*)&data.keep.map, &iter, e, ent) {
> +		free((void *)e->key);
> +	}
>  	strset_clear(&data.keep);
>  }
> --- >8 ---
> 
> which is... pretty gross. I think we may be better off sticking with a
> string_list for 'keep' here internally.

Yeah, this is horrible, and this kind of "we own it, but only kind of
and strdup_strings is not set" game has been the cause of many bugs on
the string-list side.

Having a "nodup" variant is less awful, but still error prone and leads
to complications. For example, if strset_add_nodup() sees a duplicate,
what does it do with the input?

So I would probably just either do:

  for (...) {
	item = xstrfmt(...);
	strset_add(&set, item);
	free(item);
  }

or if you care about the extra allocation, then:

  struct strbuf item = STRBUF_INIT;
  for (...) {
	strbuf_reset(&item);
	strbuf_addf(&item, ...);
	strset_add(&set, item.buf);
  }
  strbuf_release(&item);

-Peff

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

On Tue, Mar 31, 2026 at 07:17:13PM -0400, Jeff King wrote:
> > @@ -815,6 +817,9 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
> >  	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
> >  				     clear_midx_file_ext, &data);
> >
> > +	hashmap_for_each_entry((struct hashmap*)&data.keep.map, &iter, e, ent) {
> > +		free((void *)e->key);
> > +	}
> >  	strset_clear(&data.keep);
> >  }
> > --- >8 ---
> >
> > which is... pretty gross. I think we may be better off sticking with a
> > string_list for 'keep' here internally.
>
> Yeah, this is horrible, and this kind of "we own it, but only kind of
> and strdup_strings is not set" game has been the cause of many bugs on
> the string-list side.

Indeed ;-).

> Having a "nodup" variant is less awful, but still error prone and leads
> to complications. For example, if strset_add_nodup() sees a duplicate,
> what does it do with the input?
>
> So I would probably just either do:
>
>   for (...) {
> 	item = xstrfmt(...);
> 	strset_add(&set, item);
> 	free(item);
>   }
>
> or if you care about the extra allocation, then:
>
>   struct strbuf item = STRBUF_INIT;
>   for (...) {
> 	strbuf_reset(&item);
> 	strbuf_addf(&item, ...);
> 	strset_add(&set, item.buf);
>   }
>   strbuf_release(&item);

I think the latter of these two makes sense to me.

I'm not necessarily of the opinion that the strset API should *never*
have DUP/NODUP semantics like the string_list API does. But I do think
that we should punt on taking something like this up as long as
possible.

Thanks,
Taylor

Re: [PATCH 04/16] midx: use `strvec` for `keep_hashes`

[Jeff King]

On Wed, Apr 01, 2026 at 11:41:29AM -0400, Taylor Blau wrote:

> I think the latter of these two makes sense to me.
> 
> I'm not necessarily of the opinion that the strset API should *never*
> have DUP/NODUP semantics like the string_list API does. But I do think
> that we should punt on taking something like this up as long as
> possible.

Yup, I completely agree.

-Peff

Re: [PATCH 05/16] midx: introduce `--checksum-only` for incremental MIDX writes

[Taylor Blau]

On Mon, Mar 30, 2026 at 07:15:47PM -0400, Jeff King wrote:
> Your goal makes sense, but that is not at all what I would have expected
> a "--checksum-only" option to do. From the name I expected it to just
> compute or verify checksums and not write anything to disk. I dunno.
> Maybe it should be "--no-write-chain-file" or something?
>
> This is perhaps on the border of bike-shedding, and maybe the fact that
> it is an option of "multi-pack-index write" would be enough to clarify.

Heh :-).

I think that it definitely qualifies as bike-shedding, but I'm glad you
brought it up. I hadn't considered the name being potentially confusing,
but now that you mention it I can't un-see it.

Calling this something like '--no-write-chain-file' feels like a good
choice to me, and doesn't paint us into a corner if we want to later add
an option that does something like you describe above.

(As an aside, we already have 'git multi-pack-index verify', which is
probably the closest thing to what you're describing, so perhaps the
concern is lessened by that fact. But even if we already have that
functionality laying around, I think that "--no-write-chain-file" is a
more descriptive name for the command-line flag than "--checksum-only".)

> > @@ -239,6 +254,15 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
> >  	if (argc != 2)
> >  		usage_with_options(builtin_multi_pack_index_compact_usage,
> >  				   options);
> > +
> > +	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
> > +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
> > +		error(_("cannot use %s without %s"),
> > +		      "--checksum-only", "--incremental");
> > +		usage_with_options(builtin_multi_pack_index_compact_usage,
> > +				   options);
> > +	}
> > +
>
> OK. It's sad that we have to duplicate these bits between the write and
> compact operations, but I'm not sure how much sense it would make to
> factor out common options (both how much work it is, and how much
> duplication we could actually drop in practice).

I agree. I looked at unifying these a little further, but decided
against it as it ended up making things more complex than it was worth
IMHO.

> >  	if (ctx.incremental) {
> > -		FILE *chainf = fdopen_lock_file(&lk, "w");
> >  		struct strbuf final_midx_name = STRBUF_INIT;
> >  		struct multi_pack_index *m = ctx.base_midx;
> >
> > -		if (!chainf) {
> > -			error_errno(_("unable to open multi-pack-index chain file"));
> > -			goto cleanup;
> > +		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
> > +			FILE *chainf = fdopen_lock_file(&lk, "w");
> > +			if (!chainf) {
> > +				error_errno(_("unable to open multi-pack-index chain file"));
> > +				goto cleanup;
> > +			}
> >  		}
>
> OK, and this is the actual write which is skipped. I guess this could
> also be conditional on is_lock_file_locked(&lk), assuming we initialize
> it with LOCK_INIT. In some ways that seems easier to analyze to me,
> since you don't have to realize that "lk" is only active when
> CHECKSUM_ONLY is not set. But I don't think there's another reason we'd
> choose not to take a lock file, so I think it may be six of one and half
> a dozen of the other.

Yeah, I think this is more or less equivalent. In some ways I like the
explicitness of the conditional as it is currently written, but I think
your suggestion reads clearer.

It ends up producing nice code like:

    if (is_lock_file_locked(&lk)) {
        FILE *chainf = fdopen_lock_file(&lk, "w");
        /* ... */
    }

which is pleasing to me in the sense that it reads like "if the lockfile
is held, then do something with it".

I think there is a small risk of coupling the state of the lockfile with
the MIDX_NO_WRITE_CHAIN bit in the flags, but it's easily mitigated by
an ASSERT(opts->flags & MIDX_NO_WRITE_CHAIN) in the else branch of the
conditional above.

There are a few such conditionals, and repeatedly checking that the flag
bits are set is wasteful, so I check it only once in the big "if
(ctx->incremental)" block, and then once again in the top-level
write_midx_internal() function before we do the final commit and
cleanup.

Thanks,
Taylor

Re: [PATCH 06/16] midx: support custom `--base` for incremental MIDX writes

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:06PM -0400, Taylor Blau wrote:

> Introduce a new `--base` option for both subcommands to specify the
> checksum of the MIDX layer to use as the base. The given checksum must
> refer to a valid layer in the MIDX chain that is an ancestor of the
> topmost layer being written or compacted.

OK, that makes sense.

> The special value "none" is accepted to produce a root layer with no
> parent. This will be needed when the incremental repacking machinery
> determines that the bottommost layers of the chain should be replaced.

My gut feeling on reading is that a chain based on "none" is really just
a non-incremental midx, so we could just write that. But:

  1. I don't recall off-hand how expensive it is to convert a
     non-incremental to an incremental one. I.e., can we just rename the
     file and write out a chain link and it becomes incremental?

  2. Even if it is cheap to convert, I can believe it makes geometric
     repacking easier to think about if we just always think about it as
     an incremental midx.

And perhaps (3), I am just wrong that they are the same thing. ;) I
haven't thought it through all that carefully.

> If no `--base` is given, behavior is unchanged: `compact` uses "from's"
> immediate parent in the chain, and `write` appends to the existing tip.
> 
> For the `write` subcommand, `--base` requires `--checksum-only`. A plain
> `write --incremental` appends a new layer to the live chain tip with no
> mechanism to atomically replace it; overriding the base would produce a
> layer that does not extend the tip, breaking chain invariants. With
> `--checksum-only` the chain is left unmodified and the caller is
> responsible for assembling a valid chain.

OK, makes sense. If we are using --base we are doing something clever
and weird, and the chain tip should be left untouched. I guess "midx
write" would never delete an incremental (it is always just adding one
more to the chain), so we don't have to worry about that. Presumably
we'll eventually write out the new chain tip from the caller, which will
then have to remember to delete the now-orphaned chain elements. But
that's kind of the point of the new geometric approach, so it seems like
things are falling into place.

> For `compact`, no such restriction applies. The compaction operation
> atomically replaces the compacted range in the chain file, so writing
> the result on top of any valid ancestor preserves chain invariants.

OK, so there's an implicit base..tip compaction here (as opposed to
without --base, an implicit root..trip compaction?). Makes sense.

> @@ -83,6 +85,13 @@ marker).
>  		and packs not present in an existing MIDX layer.
>  		Migrates non-incremental MIDXs to incremental ones when
>  		necessary.
> +
> +	--base=<checksum>::
> +		Specify the checksum of an existing MIDX layer to use
> +		as the base when writing a new incremental layer.
> +		The special value `none` indicates that the new layer
> +		should have no base (i.e., it becomes a root layer).
> +		Requires `--checksum-only`.

Not really a critique of this patch, but I'd guess that a lot of these
options are getting esoteric enough that it is hard to understand 

> [...]

The patch looked as advertised, and I didn't find anything in it to
comment on.

-Peff

Re: [PATCH 07/16] repack: track the ODB source via existing_packs

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:08PM -0400, Taylor Blau wrote:

> Store the ODB source in the `existing_packs` struct and use that in
> place of the raw `repo->objects->sources` access within `cmd_repack()`.
> 
> The source used is still assigned from the first source in the list, so
> there are no functional changes in this commit. The changes instead
> serve two purposes (one immediate, one not):
> 
>  - The incremental MIDX-based repacking machinery will need to know what
>    source is being used to read the existing MIDX/chain (should one
>    exist).
> 
>  - In the future, if "git repack" is taught how to operate on other
>    object sources, this field will serve as the authoritative value for
>    that source.

Makes sense. I wonder if we still need the repo field here:

> diff --git a/repack.h b/repack.h
> index bc9f2e1a5de..c0e9f0ca647 100644
> --- a/repack.h
> +++ b/repack.h
> @@ -56,6 +56,7 @@ struct packed_git;
>  
>  struct existing_packs {
>  	struct repository *repo;
> +	struct odb_source *source;
>  	struct string_list kept_packs;
>  	struct string_list non_kept_packs;
>  	struct string_list cruft_packs;

since we are going to mostly be operating on one odb source. But I think
it gets weird asking about things like "which hash algorithm to use",
because then you end up back at source->odb->repo. Plus there are some
parts that seem to look at the whole repo.

So I think it gets complicated and is well off the path to our goal, so
not worth thinking further about for now.

-Peff

Re: [PATCH 08/16] midx: expose `midx_layer_contains_pack()`

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:11PM -0400, Taylor Blau wrote:

> Rename the function `midx_contains_pack_1()` to instead be called
> `midx_layer_contains_pack()` and make it accessible. Unlike
> `midx_contains_pack()` (which recurses through the entire chain), this
> function checks only a single MIDX layer.
> 
> This will be used by a subsequent commit to determine whether a given
> pack belongs to the tip MIDX layer specifically, rather than to any
> layer in the chain.

Yup, that is a much better name.

-Peff

Re: [PATCH 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:16PM -0400, Taylor Blau wrote:

> This simplifies `write_midx_included_packs()` and prepares for a
> subsequent commit where the same helper is called with `cmd->out = -1`
> to capture the MIDX's checksum from the command's standard output,
> which is needed when writing MIDX layers with `--checksum-only`.

Hmm, that will involve us feeding both "in" and "out", so we'll have to
consider deadlock. We're not there yet, though... :)

-Peff

Re: [PATCH 11/16] repack-geometry: prepare for incremental MIDX repacking

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:18PM -0400, Taylor Blau wrote:

> Teach `pack_geometry_init()` to optionally restrict the set of
> repacking candidates to only packs in the tip MIDX layer when a
> `midx_layer_threshold` is configured. If the tip layer has fewer packs
> than the threshold, those packs are excluded entirely; otherwise only
> packs in that layer participate in the geometric repack.

My first question of course is "so how do we set that threshold?". But
it looks like that is not done yet in this commit, so this is mostly
dead code preparing for later (and thus no tests). OK.

-Peff

Re: [PATCH 12/16] builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:21PM -0400, Taylor Blau wrote:

> diff --git a/repack.h b/repack.h
> index 77d24ee45fb..81907fcce7f 100644
> --- a/repack.h
> +++ b/repack.h
> @@ -134,6 +134,11 @@ void pack_geometry_release(struct pack_geometry *geometry);
>  
>  struct tempfile;
>  
> +enum repack_write_midx_mode {
> +	REPACK_WRITE_MIDX_NONE,
> +	REPACK_WRITE_MIDX_DEFAULT,
> +};

Since the name of the variable did not change, the compiler would not
catch any old cases that did:

  if (write_midx)

for us. Which made me wonder if we should set _NONE to 0 explicitly here
so that continues to work, in case we missed a spot.

But after manually checking each spot that uses the enum (and there are
not that many), they have all been faithfully converted. So it's
probably not worth trying to preserve that compatibility (and anyway,
once we introduce a third mode, it's not clear that such a site would be
correct anymore anyway).

-Peff

Re: [PATCH 13/16] packfile: ensure `close_pack_revindex()` frees in-memory revindex

[Jeff King]

On Sun, Mar 29, 2026 at 05:41:23PM -0400, Taylor Blau wrote:

> The following commit will introduce a case where we write a MIDX bitmap
> over packs that do not themselves have on-disk *.rev files.
> 
> This case is supported within Git, and we will simply fall back to
> generating the revindex in memory. But we don't ever release that
> memory, causing a leak that is exposed by a test introduced in the
> following commit.
> 
> (As far as I could find, we never free()'d memory allocated as a
> byproduct of creating an in-memory revindex, likely because that code
> predates the leak-checking niceties we have in the test suite now.)

Hmmm, I'm surprised we don't trigger this leak during the course of
odb_free(). But I guess we do that fairly rarely. It mostly happens via
repo_clear(), and we never do that for the "main" repo (just for
submodules). And it's unlikely that we'd have loaded a revindex for the
submodule for most operations (mostly we need them for generating packs
or when using bitmaps).

So I can't see an obvious way that this would be triggered in practice
with the current code. But I agree it is a leak waiting to happen, as
nobody ever clears p->revindex.

> diff --git a/packfile.c b/packfile.c
> index d4de9f3ffe8..97f7662e079 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -420,6 +420,8 @@ void close_pack_index(struct packed_git *p)
>  
>  static void close_pack_revindex(struct packed_git *p)
>  {
> +	FREE_AND_NULL(p->revindex);
> +
>  	if (!p->revindex_map)
>  		return;

The fact that this is called "close" made me wonder if some code might
want to just relinquish access to on-disk resources, and then pay the
cost when we have to unexpectedly re-compute the in-memory revindex.

But this function and its siblings are really about resetting the whole
state of the packed_git struct. So the behavior you're proposing matches
that.

-Peff

Re: [PATCH 06/16] midx: support custom `--base` for incremental MIDX writes

[Taylor Blau]

On Tue, Apr 07, 2026 at 01:57:53AM -0400, Jeff King wrote:
> On Sun, Mar 29, 2026 at 05:41:06PM -0400, Taylor Blau wrote:
>
> > Introduce a new `--base` option for both subcommands to specify the
> > checksum of the MIDX layer to use as the base. The given checksum must
> > refer to a valid layer in the MIDX chain that is an ancestor of the
> > topmost layer being written or compacted.
>
> OK, that makes sense.
>
> > The special value "none" is accepted to produce a root layer with no
> > parent. This will be needed when the incremental repacking machinery
> > determines that the bottommost layers of the chain should be replaced.
>
> My gut feeling on reading is that a chain based on "none" is really just
> a non-incremental midx, so we could just write that. But:
>
>   1. I don't recall off-hand how expensive it is to convert a
>      non-incremental to an incremental one. I.e., can we just rename the
>      file and write out a chain link and it becomes incremental?
>
>   2. Even if it is cheap to convert, I can believe it makes geometric
>      repacking easier to think about if we just always think about it as
>      an incremental midx.
>
> And perhaps (3), I am just wrong that they are the same thing. ;) I
> haven't thought it through all that carefully.

In this case we are truly writing an incremental MIDX layer without any
parent. So it's logically similar from a non-incremental MIDX in the
sense that there is only a single layer, but it is still an incremental
MIDX nonetheless.

We have to write these as incremental layers, since we can't replace the
main $GIT_DIR/objects/pack/multi-pack-index, so that takes (1) off of
the table.

> > If no `--base` is given, behavior is unchanged: `compact` uses "from's"
> > immediate parent in the chain, and `write` appends to the existing tip.
> >
> > For the `write` subcommand, `--base` requires `--checksum-only`. A plain
> > `write --incremental` appends a new layer to the live chain tip with no
> > mechanism to atomically replace it; overriding the base would produce a
> > layer that does not extend the tip, breaking chain invariants. With
> > `--checksum-only` the chain is left unmodified and the caller is
> > responsible for assembling a valid chain.
>
> OK, makes sense. If we are using --base we are doing something clever
> and weird, and the chain tip should be left untouched. I guess "midx
> write" would never delete an incremental (it is always just adding one
> more to the chain), so we don't have to worry about that. Presumably
> we'll eventually write out the new chain tip from the caller, which will
> then have to remember to delete the now-orphaned chain elements. But
> that's kind of the point of the new geometric approach, so it seems like
> things are falling into place.

Yup, that is driven at the 'repack' layer and will appear later on in
this series.

Thanks,
Taylor

Re: [PATCH 07/16] repack: track the ODB source via existing_packs

[Taylor Blau]

On Tue, Apr 07, 2026 at 02:04:39AM -0400, Jeff King wrote:
> Makes sense. I wonder if we still need the repo field here:
>
> > diff --git a/repack.h b/repack.h
> > index bc9f2e1a5de..c0e9f0ca647 100644
> > --- a/repack.h
> > +++ b/repack.h
> > @@ -56,6 +56,7 @@ struct packed_git;
> >
> >  struct existing_packs {
> >  	struct repository *repo;
> > +	struct odb_source *source;
> >  	struct string_list kept_packs;
> >  	struct string_list non_kept_packs;
> >  	struct string_list cruft_packs;
>
> since we are going to mostly be operating on one odb source. But I think
> it gets weird asking about things like "which hash algorithm to use",
> because then you end up back at source->odb->repo. Plus there are some
> parts that seem to look at the whole repo.

I think that doing so is possible, though it's not totally
straightforward. There are a handful of places where you really truly
do want to work with the repo (e.g., when calling trace2_region_enter(),
which requires a repository pointer as its final argument).

I do think that we are probably over-relying on the repository pointer
in the existing_packs struct after this patch, especially with the ODB
source refactorings that have been going on.

> So I think it gets complicated and is well off the path to our goal, so
> not worth thinking further about for now.

I agree.

Thanks,
Taylor

Re: [PATCH 00/16] repack: incremental MIDX/bitmap-based repacking

[Taylor Blau]

On Sun, Mar 29, 2026 at 05:40:50PM -0400, Taylor Blau wrote:
> OVERVIEW
> ========
>
> This series implements the third and final major component for an
> incremental MIDX/bitmap-based repacking strategy. As a refresher, those
> are:

Thanks for the review on the first round. Review seems to have
stabilized (I somehow missed the handful of messages that Peff sent on
the 7th, but just read through them and only came up with minimal
changes as a result).

For now, I'll hold off on sending a v2 until we are on the other side of
2.54 next week. In the meantime, here is a quick summary of the main
changes that that version will contain:

 - Use a strset instead of a string_list for keeping track of the MIDX
   layers to retain when calling `either clear_midx_files_ext()` or
   `clear_incremental_midx_files_ext()`.

 - A new patch to rewrite the logic for determining which MIDX layers
   comprise the new chain via keep_hashes to build the array in order.
   The subsequent patch converts that into a strvec, which no longer
   requires direct manipulation.

 - The new "--checksum-only" option has been renamed to
   "--no-write-chain-file", and various small implementation tweaks
   (e.g., relying on `is_lock_file_locked()` to determine whether we
   should update the chain file as opposed to reading the flags).

For the curious, the v2 that I'll send is currently in my fork
(available from git@github.com:ttaylorr/git.git) in a branch called
'tb/incremental-midx-part-3.3'. A range-diff of what I will likely send
is below:

--- 8< ---
 1:  0c7a68ca5a7 =  1:  2db38d41b96 midx-write: handle noop writes when converting incremental chains
 2:  1391552dfc6 !  2:  99e2c8f0cd6 midx: use `string_list` for retained MIDX files
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>

      ## Commit message ##
    -    midx: use `string_list` for retained MIDX files
    +    midx: use `strset` for retained MIDX files

         Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
         build a list of filenames to keep while pruning stale MIDX files. Today
    -    they hand-roll an array instead of using a `string_list`, thus requiring
    -    us to pass an additional length parameter, and makes lookups linear.
    +    they hand-roll an array instead of using a `strset`, thus requiring us
    +    to pass an additional length parameter, and makes lookups linear.

    -    Replace the bare array with a `string_list` which can be passed around
    -    as a single parameter. Though it improves lookup performance, the
    -    difference is likely immeasurable given how small the keep_hashes array
    -    typically is.
    +    Replace the bare array with a `strset` which can be passed around as a
    +    single parameter. Though it improves lookup performance, the difference
    +    is likely immeasurable given how small the keep_hashes array typically
    +    is.

         Signed-off-by: Taylor Blau <me@ttaylorr.com>

    @@ midx.c: int midx_checksum_valid(struct multi_pack_index *m)
      struct clear_midx_data {
     -	char **keep;
     -	uint32_t keep_nr;
    -+	struct string_list keep;
    ++	struct strset keep;
      	const char *ext;
      };

    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -		if (!strcmp(data->keep[i], file_name))
     -			return;
     -	}
    -+	if (string_list_has_string(&data->keep, file_name))
    ++	if (strset_contains(&data->keep, file_name))
     +		return;
      	if (unlink(full_path))
      		die_errno(_("failed to remove %s"), full_path);
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	struct clear_midx_data data;
     -	memset(&data, 0, sizeof(struct clear_midx_data));
     +	struct clear_midx_data data = {
    -+		.keep = STRING_LIST_INIT_DUP,
    ++		.keep = STRSET_INIT,
     +		.ext = ext,
     +	};

    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_

     -		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
     -		data.keep_nr = 1;
    -+		string_list_insert(&data.keep, buf.buf);
    ++		strset_add(&data.keep, buf.buf);
     +
     +		strbuf_release(&buf);
      	}
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	if (keep_hash)
     -		free(data.keep[0]);
     -	free(data.keep);
    -+	string_list_clear(&data.keep, 0);
    ++	strset_clear(&data.keep);
      }

      void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
      {
     -	struct clear_midx_data data;
     +	struct clear_midx_data data = {
    -+		.keep = STRING_LIST_INIT_NODUP,
    ++		.keep = STRSET_INIT,
     +		.ext = ext,
     +	};
    ++	struct strbuf buf = STRBUF_INIT;
      	uint32_t i;

     -	memset(&data, 0, sizeof(struct clear_midx_data));
    --
    ++	for (i = 0; i < hashes_nr; i++) {
    ++		strbuf_reset(&buf);
    ++		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
    ++			    ext);
    +
     -	ALLOC_ARRAY(data.keep, hashes_nr);
    - 	for (i = 0; i < hashes_nr; i++)
    +-	for (i = 0; i < hashes_nr; i++)
     -		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
     -				       ext);
     -	data.keep_nr = hashes_nr;
     -	data.ext = ext;
    -+		string_list_append(&data.keep,
    -+				   xstrfmt("multi-pack-index-%s.%s",
    -+					   keep_hashes[i], ext));
    -+	string_list_sort(&data.keep);
    ++		strset_add(&data.keep, buf.buf);
    ++	}

      	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
      				     clear_midx_file_ext, &data);
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	for (i = 0; i < hashes_nr; i++)
     -		free(data.keep[i]);
     -	free(data.keep);
    -+	string_list_clear(&data.keep, 0);
    ++	strbuf_release(&buf);
    ++	strset_clear(&data.keep);
      }

      void clear_midx_file(struct repository *r)
 3:  aea6aad6953 <  -:  ----------- strvec: introduce `strvec_init_alloc()`
 -:  ----------- >  3:  04f1c23884c midx: build `keep_hashes` array in order
 4:  5fc72d5049a !  4:  a71c8024c1f midx: use `strvec` for `keep_hashes`
    @@ Commit message
         value is used to determine the contents of the resulting
         `multi-pack-index-chain` file when writing with "--incremental".

    +    Since the previous commit already builds the array in forward order, the
    +    conversion is straightforward: replace indexed assignments with
    +    `strvec_push()`, drop the pre-counting and `CALLOC_ARRAY()`, and
    +    simplify cleanup via `strvec_clear()`.
    +
         Signed-off-by: Taylor Blau <me@ttaylorr.com>

      ## midx-write.c ##
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	int dropped_packs = 0;
      	int result = -1;
     -	const char **keep_hashes = NULL;
    +-	size_t keep_hashes_nr = 0;
     +	struct strvec keep_hashes = STRVEC_INIT;
    - 	size_t keep_hashes_nr = 0;
      	struct chunkfile *cf;

    + 	trace2_region_enter("midx", "write_midx_internal", r);
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 	} else {
    - 		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
    - 	}
    + 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
    + 		die(_("too many multi-pack-indexes"));
    +
    +-	if (ctx.compact) {
    +-		struct multi_pack_index *m;
    +-
    +-		/*
    +-		 * Keep all MIDX layers excluding those in the range [from, to].
    +-		 */
    +-		for (m = ctx.base_midx; m; m = m->base_midx)
    +-			keep_hashes_nr++;
    +-		for (m = ctx.m;
    +-		     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
    +-		     m = m->base_midx)
    +-			keep_hashes_nr++;
    +-
    +-		keep_hashes_nr++; /* include the compacted layer */
    +-	} else {
    +-		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
    +-	}
     -	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
    -+	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
    -
    +-
      	if (ctx.incremental) {
      		FILE *chainf = fdopen_lock_file(&lk, "w");
    -@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 			for (i = 0; i < num_layers_before_from; i++) {
    - 				uint32_t j = num_layers_before_from - i - 1;
    + 		struct strbuf final_midx_name = STRBUF_INIT;
    + 		struct multi_pack_index *m = ctx.base_midx;
    + 		struct multi_pack_index **layers = NULL;
    + 		size_t layers_nr = 0, layers_alloc = 0;
    +-		size_t j = 0;

    --				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				m = m->base_midx;
    + 		if (!chainf) {
    + 			error_errno(_("unable to open multi-pack-index chain file"));
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 				layers[layers_nr++] = mp;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));

    --			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
    -+			keep_hashes.v[i] = xstrdup(hash_to_hex_algop(midx_hash,
    - 								   r->hash_algo));
    -+			keep_hashes.nr++;
    +-			keep_hashes[j++] =
    +-				xstrdup(hash_to_hex_algop(midx_hash,
    +-							  r->hash_algo));
    ++			strvec_push(&keep_hashes,
    ++				    hash_to_hex_algop(midx_hash,
    ++						     r->hash_algo));

    - 			i = 0;
    - 			for (m = ctx.m;
    - 			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
    - 			     m = m->base_midx) {
    --				keep_hashes[keep_hashes_nr - i - 1] =
    -+				keep_hashes.v[keep_hashes_nr - i - 1] =
    - 					xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				i++;
    + 			for (mp = ctx.m;
    + 			     mp && midx_hashcmp(mp, ctx.compact_to,
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 				layers[layers_nr++] = mp;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));
      		} else {
    --			keep_hashes[ctx.num_multi_pack_indexes_before] =
    -+			keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 				xstrdup(hash_to_hex_algop(midx_hash,
    - 							  r->hash_algo));
    -+			keep_hashes.nr++;
    -
    - 			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
    - 				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
    -
    --				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				m = m->base_midx;
    + 			for (; m; m = m->base_midx) {
    + 				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
    + 				layers[layers_nr++] = m;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));
    +
    +-			keep_hashes[j++] =
    +-				xstrdup(hash_to_hex_algop(midx_hash,
    +-							  r->hash_algo));
    ++			strvec_push(&keep_hashes,
    ++				    hash_to_hex_algop(midx_hash,
    ++						     r->hash_algo));
      		}

    - 		for (uint32_t i = 0; i < keep_hashes_nr; i++)
    +-		ASSERT(j == keep_hashes_nr);
    +-
    + 		free(layers);
    +
    +-		for (uint32_t i = 0; i < j; i++)
     -			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
    ++		for (size_t i = 0; i < keep_hashes.nr; i++)
     +			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
      	} else {
     -		keep_hashes[ctx.num_multi_pack_indexes_before] =
    -+		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    -+		keep_hashes.nr++;
    +-			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    ++		strvec_push(&keep_hashes,
    ++			    hash_to_hex_algop(midx_hash, r->hash_algo));
      	}

      	if (ctx.m || ctx.base_midx)
    @@ midx.c: void clear_midx_files_ext(struct odb_source *source, const char *ext,
     +				      const struct strvec *keep_hashes)
      {
      	struct clear_midx_data data = {
    --		.keep = STRING_LIST_INIT_NODUP,
    -+		.keep = STRING_LIST_INIT_DUP,
    + 		.keep = STRSET_INIT,
      		.ext = ext,
      	};
    + 	struct strbuf buf = STRBUF_INIT;
     -	uint32_t i;

    --	for (i = 0; i < hashes_nr; i++)
    --		string_list_append(&data.keep,
    --				   xstrfmt("multi-pack-index-%s.%s",
    --					   keep_hashes[i], ext));
    --	string_list_sort(&data.keep);
    +-	for (i = 0; i < hashes_nr; i++) {
    +-		strbuf_reset(&buf);
    +-		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
    +-			    ext);
     +	if (keep_hashes) {
    -+		struct strbuf buf = STRBUF_INIT;
     +		for (size_t i = 0; i < keep_hashes->nr; i++) {
     +			strbuf_reset(&buf);
    -+
     +			strbuf_addf(&buf, "multi-pack-index-%s.%s",
     +				    keep_hashes->v[i], ext);
    -+			string_list_append(&data.keep, buf.buf);
    +
    +-		strset_add(&data.keep, buf.buf);
    ++			strset_add(&data.keep, buf.buf);
     +		}
    -+
    -+		string_list_sort(&data.keep);
    -+		strbuf_release(&buf);
    -+	}
    + 	}

      	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
    - 				     clear_midx_file_ext, &data);
 5:  41cb5471bb9 !  5:  51ffdb1382d midx: introduce `--checksum-only` for incremental MIDX writes
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>

      ## Commit message ##
    -    midx: introduce `--checksum-only` for incremental MIDX writes
    +    midx: introduce `--no-write-chain-file` for incremental MIDX writes

         When writing an incremental MIDX layer, the MIDX machinery writes the
         new layer into the multi-pack-index.d directory and then updates the
    @@ Commit message
         assemble the new MIDX chain itself before writing a new chain file and
         atomically linking it into place).

    -    Introduce a `--checksum-only` flag that:
    +    Introduce a `--no-write-chain-file` flag that:

          * writes the new MIDX layer into the multi-pack-index.d directory

    @@ Documentation/git-multi-pack-index.adoc: SYNOPSIS
      'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
      		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
     -		         [--refs-snapshot=<path>]
    -+		         [--refs-snapshot=<path>] [--[no-]checksum-only]
    ++		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
      'git multi-pack-index' [<options>] compact [--[no-]incremental]
     -		         [--[no-]bitmap] <from> <to>
    -+		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
    ++		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
      'git multi-pack-index' [<options>] verify
      'git multi-pack-index' [<options>] expire
      'git multi-pack-index' [<options>] repack [--batch-size=<size>]
    +@@ Documentation/git-multi-pack-index.adoc: marker).
    + 		and packs not present in an existing MIDX layer.
    + 		Migrates non-incremental MIDXs to incremental ones when
    + 		necessary.
    ++
    ++	--[no-]write-chain-file::
    ++		When used with `--incremental`, write a new MIDX layer
    ++		but do not update the multi-pack-index-chain file.
    ++		The checksum of the new layer is printed to standard
    ++		output, allowing the caller to assemble and write the
    ++		chain itself. Requires `--incremental`.
    + --
    +
    + compact::
    +@@ Documentation/git-multi-pack-index.adoc: compact::
    +
    + 	--[no-]bitmap::
    + 		Control whether or not a multi-pack bitmap is written.
    ++
    ++	--[no-]write-chain-file::
    ++		When used with `--incremental`, write a new compacted
    ++		MIDX layer but do not update the multi-pack-index-chain
    ++		file. The checksum of the new layer is printed to
    ++		standard output. Requires `--incremental`.
    + --
    +
    + verify::

      ## builtin/multi-pack-index.c ##
     @@
    @@ builtin/multi-pack-index.c
      	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
      	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
     -	   "  [--refs-snapshot=<path>]")
    -+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
    ++	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")

      #define BUILTIN_MIDX_COMPACT_USAGE \
      	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
     -	   "  [--[no-]bitmap] <from> <to>")
    -+	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
    ++	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")

      #define BUILTIN_MIDX_VERIFY_USAGE \
      	N_("git multi-pack-index [<options>] verify")
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
      			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    -+		OPT_BIT(0, "checksum-only", &opts.flags,
    -+			N_("write a MIDX layer without updating the MIDX chain"),
    -+			MIDX_WRITE_CHECKSUM_ONLY),
    ++		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
    ++			N_("write the multi-pack-index chain file"),
    ++			MIDX_WRITE_NO_CHAIN),
      		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
      			 N_("write multi-pack index containing only given indexes")),
      		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
      		usage_with_options(builtin_multi_pack_index_write_usage,
      				   options);
     +
    -+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
    ++	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
     +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
     +		error(_("cannot use %s without %s"),
    -+		      "--checksum-only", "--incremental");
    ++		      "--no-write-chain-file", "--incremental");
     +		usage_with_options(builtin_multi_pack_index_write_usage,
     +				   options);
     +	}
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    -+		OPT_BIT(0, "checksum-only", &opts.flags,
    -+			N_("write a MIDX layer without updating the MIDX chain"),
    -+			MIDX_WRITE_CHECKSUM_ONLY),
    ++		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
    ++			N_("write the multi-pack-index chain file"),
    ++			MIDX_WRITE_NO_CHAIN),
      		OPT_END(),
      	};

    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      		usage_with_options(builtin_multi_pack_index_compact_usage,
      				   options);
     +
    -+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
    ++	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
     +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
     +		error(_("cannot use %s without %s"),
    -+		      "--checksum-only", "--incremental");
    ++		      "--no-write-chain-file", "--incremental");
     +		usage_with_options(builtin_multi_pack_index_compact_usage,
     +				   options);
     +	}
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      	FREE_AND_NULL(options);

      ## midx-write.c ##
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 	unsigned char midx_hash[GIT_MAX_RAWSZ];
    + 	uint32_t start_pack;
    + 	struct hashfile *f = NULL;
    +-	struct lock_file lk;
    ++	struct lock_file lk = LOCK_INIT;
    + 	struct tempfile *incr;
    + 	struct write_midx_context ctx = {
    + 		.preferred_pack_idx = NO_PREFERRED_PACK,
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	}

      	if (ctx.incremental) {
     -		struct strbuf lock_name = STRBUF_INIT;
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++		if (!(opts->flags & MIDX_WRITE_NO_CHAIN)) {
     +			struct strbuf lock_name = STRBUF_INIT;

     -		get_midx_chain_filename(opts->source, &lock_name);
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      		incr = mks_tempfile_m(midx_name.buf, 0444);
      		if (!incr) {
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 	}
    - 	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
    + 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
    + 		die(_("too many multi-pack-indexes"));

    -+	if (opts->flags & MIDX_WRITE_CHECKSUM_ONLY)
    ++	if (!is_lock_file_locked(&lk))
     +		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));
    ++	else if (opts->flags & MIDX_WRITE_NO_CHAIN)
    ++		BUG("lockfile held with MIDX_WRITE_NO_CHAIN set?");
     +
      	if (ctx.incremental) {
     -		FILE *chainf = fdopen_lock_file(&lk, "w");
      		struct strbuf final_midx_name = STRBUF_INIT;
      		struct multi_pack_index *m = ctx.base_midx;
    + 		struct multi_pack_index **layers = NULL;
    + 		size_t layers_nr = 0, layers_alloc = 0;

     -		if (!chainf) {
     -			error_errno(_("unable to open multi-pack-index chain file"));
     -			goto cleanup;
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++		if (is_lock_file_locked(&lk)){
     +			FILE *chainf = fdopen_lock_file(&lk, "w");
     +			if (!chainf) {
     +				error_errno(_("unable to open multi-pack-index chain file"));
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)

      		if (link_midx_to_chain(ctx.base_midx) < 0)
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 			}
    - 		}

    --		for (uint32_t i = 0; i < keep_hashes_nr; i++)
    + 		free(layers);
    +
    +-		for (size_t i = 0; i < keep_hashes.nr; i++)
     -			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY))
    -+			for (uint32_t i = 0; i < keep_hashes_nr; i++)
    ++		if (is_lock_file_locked(&lk))
    ++			for (size_t i = 0; i < keep_hashes.nr; i++)
     +				fprintf(get_lock_file_fp(&lk), "%s\n",
     +					keep_hashes.v[i]);
      	} else {
    - 		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    + 		strvec_push(&keep_hashes,
    + 			    hash_to_hex_algop(midx_hash, r->hash_algo));
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	if (ctx.m || ctx.base_midx)
      		odb_close(ctx.repo->objects);

     -	if (commit_lock_file(&lk) < 0)
     -		die_errno(_("could not write multi-pack-index"));
    -+	if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++	if (is_lock_file_locked(&lk)) {
     +		if (commit_lock_file(&lk) < 0)
     +			die_errno(_("could not write multi-pack-index"));

    @@ midx.h: struct multi_pack_index {
      #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
      #define MIDX_WRITE_INCREMENTAL (1 << 5)
      #define MIDX_WRITE_COMPACT (1 << 6)
    -+#define MIDX_WRITE_CHECKSUM_ONLY (1 << 7)
    ++#define MIDX_WRITE_NO_CHAIN (1 << 7)

      #define MIDX_EXT_REV "rev"
      #define MIDX_EXT_BITMAP "bitmap"
    @@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'show object from s
      	git cat-file -p 2.2
      '

    -+test_expect_success 'write MIDX layer with --checksum-only' '
    -+	test_commit checksum-only &&
    ++test_expect_success 'write MIDX layer with --no-write-chain-file' '
    ++	test_commit no-write-chain-file &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only)" &&
    ++		--no-write-chain-file)" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
     +'
     +
    -+test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
    -+	test_must_fail git multi-pack-index write --bitmap --checksum-only 2>err &&
    -+	test_grep "cannot use --checksum-only without --incremental" err
    ++test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file' '
    ++	test_must_fail git multi-pack-index write --bitmap --no-write-chain-file 2>err &&
    ++	test_grep "cannot use --no-write-chain-file without --incremental" err
     +'
     +
      for reuse in false single multi
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
      	)
      '

    -+test_expect_success 'MIDX compaction with --checksum-only' '
    -+	git init midx-compact-with--checksum-only &&
    ++test_expect_success 'MIDX compaction with --no-write-chain-file' '
    ++	git init midx-compact-with--no-write-chain-file &&
     +	(
    -+		cd midx-compact-with--checksum-only &&
    ++		cd midx-compact-with--no-write-chain-file &&
     +
     +		git config maintenance.auto false &&
     +
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
     +		cp "$midx_chain" "$midx_chain".bak &&
     +
     +		layer="$(git multi-pack-index compact --incremental \
    -+			--checksum-only \
    ++			--no-write-chain-file \
     +			"$(nth_line 2 "$midx_chain")" \
     +			"$(nth_line 3 "$midx_chain")")" &&
     +
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
     +		# After writing the new layer, insert it into the chain
     +		# manually. This is done in order to make $layer visible
     +		# to the read-midx test helper below, and matches what
    -+		# the MIDX command would do without --checksum-only.
    ++		# the MIDX command would do without --no-write-chain-file.
     +		{
     +			nth_line 1 "$midx_chain.bak" &&
     +			echo $layer &&
 6:  6b42d284b79 !  6:  48893baa339 midx: support custom `--base` for incremental MIDX writes
    @@ Commit message
         If no `--base` is given, behavior is unchanged: `compact` uses "from's"
         immediate parent in the chain, and `write` appends to the existing tip.

    -    For the `write` subcommand, `--base` requires `--checksum-only`. A plain
    +    For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
         `write --incremental` appends a new layer to the live chain tip with no
         mechanism to atomically replace it; overriding the base would produce a
         layer that does not extend the tip, breaking chain invariants. With
    -    `--checksum-only` the chain is left unmodified and the caller is
    +    `--no-write-chain-file` the chain is left unmodified and the caller is
         responsible for assembling a valid chain.

         For `compact`, no such restriction applies. The compaction operation
    @@ Documentation/git-multi-pack-index.adoc
     @@ Documentation/git-multi-pack-index.adoc: SYNOPSIS
      'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
      		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
    - 		         [--refs-snapshot=<path>] [--[no-]checksum-only]
    + 		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
     +			 [--base=<checksum>]
      'git multi-pack-index' [<options>] compact [--[no-]incremental]
    --		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
    -+		         [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]
    +-		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
    ++		         [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]
     +			 <from> <to>
      'git multi-pack-index' [<options>] verify
      'git multi-pack-index' [<options>] expire
      'git multi-pack-index' [<options>] repack [--batch-size=<size>]
     @@ Documentation/git-multi-pack-index.adoc: marker).
    - 		and packs not present in an existing MIDX layer.
    - 		Migrates non-incremental MIDXs to incremental ones when
    - 		necessary.
    + 		The checksum of the new layer is printed to standard
    + 		output, allowing the caller to assemble and write the
    + 		chain itself. Requires `--incremental`.
     +
     +	--base=<checksum>::
     +		Specify the checksum of an existing MIDX layer to use
     +		as the base when writing a new incremental layer.
     +		The special value `none` indicates that the new layer
     +		should have no base (i.e., it becomes a root layer).
    -+		Requires `--checksum-only`.
    ++		Requires `--no-write-chain-file`.
      --

      compact::
     @@ Documentation/git-multi-pack-index.adoc: compact::
    -
    - 	--[no-]bitmap::
    - 		Control whether or not a multi-pack bitmap is written.
    + 		MIDX layer but do not update the multi-pack-index-chain
    + 		file. The checksum of the new layer is printed to
    + 		standard output. Requires `--incremental`.
     +
     +	--base=<checksum>::
     +		Specify the checksum of an existing MIDX layer to use
    @@ builtin/multi-pack-index.c
      #define BUILTIN_MIDX_WRITE_USAGE \
      	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
      	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
    --	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
    -+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]\n" \
    +-	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
    ++	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]\n" \
     +	   "  [--base=<checksum>]")

      #define BUILTIN_MIDX_COMPACT_USAGE \
      	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
    --	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
    -+	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]\n" \
    +-	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
    ++	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]\n" \
     +	   "  <from> <to>")

      #define BUILTIN_MIDX_VERIFY_USAGE \
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
     +			   N_("base MIDX for incremental writes")),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    - 		OPT_BIT(0, "checksum-only", &opts.flags,
    + 		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
     @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, const char **argv,
      				   options);
      	}

     +	if (opts.incremental_base &&
    -+	    !(opts.flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    -+		error(_("cannot use --base without --checksum-only"));
    ++	    !(opts.flags & MIDX_WRITE_NO_CHAIN)) {
    ++		error(_("cannot use --base without --no-write-chain-file"));
     +		usage_with_options(builtin_multi_pack_index_write_usage,
     +				   options);
     +	}
    @@ midx.h: int write_midx_file(struct odb_source *source,
      int verify_midx_file(struct odb_source *source, unsigned flags);

      ## t/t5334-incremental-multi-pack-index.sh ##
    -@@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
    - 	test_grep "cannot use --checksum-only without --incremental" err
    +@@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file
    + 	test_grep "cannot use --no-write-chain-file without --incremental" err
      '

    -+test_expect_success 'write MIDX layer with --base without --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base without --no-write-chain-file' '
     +	test_must_fail git multi-pack-index write --bitmap --incremental \
     +		--base=none 2>err &&
    -+	test_grep "cannot use --base without --checksum-only" err
    ++	test_grep "cannot use --base without --no-write-chain-file" err
     +'
     +
    -+test_expect_success 'write MIDX layer with --base=none and --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base=none and --no-write-chain-file' '
     +	test_commit base-none &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only --base=none)" &&
    ++		--no-write-chain-file --base=none)" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
     +'
     +
    -+test_expect_success 'write MIDX layer with --base=<hash> and --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base=<hash> and --no-write-chain-file' '
     +	test_commit base-hash &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only --base="$(nth_line 1 "$midx_chain")")" &&
    ++		--no-write-chain-file --base="$(nth_line 1 "$midx_chain")")" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
    @@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremen
      	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '

      ## t/t5335-compact-multi-pack-index.sh ##
    -@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --checksum-only' '
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '

      		layer="$(git multi-pack-index compact --incremental \
    - 			--checksum-only \
    + 			--no-write-chain-file \
     +			--base="$(nth_line 1 "$midx_chain")" \
      			"$(nth_line 2 "$midx_chain")" \
      			"$(nth_line 3 "$midx_chain")")" &&

    -@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --checksum-only' '
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '
      	)
      '

 7:  2d377f53407 =  7:  b4bfaa699ba repack: track the ODB source via existing_packs
 8:  6af23849f1d =  8:  901f1401bb1 midx: expose `midx_layer_contains_pack()`
 9:  5c9fea0fc78 =  9:  dced4f38fda repack-midx: factor out `repack_prepare_midx_command()`
10:  cd5eba0f665 = 10:  42a70e4edbe repack-midx: extract `repack_fill_midx_stdin_packs()`
11:  d2c667e8413 = 11:  6eb1277edca repack-geometry: prepare for incremental MIDX repacking
12:  56dc6d55974 = 12:  a1272a25fca builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
13:  e304e316ac4 = 13:  f0c3152c085 packfile: ensure `close_pack_revindex()` frees in-memory revindex
14:  8d5aa3d6fa7 ! 14:  961413b89f3 repack: implement incremental MIDX repacking
    @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opt
     +	}
     +
     +	repack_prepare_midx_command(&cmd, opts, "write");
    -+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only", NULL);
    ++	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);
     +	strvec_pushf(&cmd.args, "--base=%s", base ? base : "none");
     +
     +	if (preferred_pack) {
    @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opt
     +	int ret;
     +
     +	repack_prepare_midx_command(&cmd, opts, "compact");
    -+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only",
    ++	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file",
     +		     midx_get_checksum_hex(step->u.compact.from),
     +		     midx_get_checksum_hex(step->u.compact.to), NULL);
     +
15:  e8e22d7958d = 15:  eade33bf8cb repack: introduce `--write-midx=incremental`
16:  79d5a12a390 = 16:  e674ba7741b repack: allow `--write-midx=incremental` without `--geometric`
--- >8 ---


Thanks,
Taylor

Re: [PATCH 11/16] repack-geometry: prepare for incremental MIDX repacking

[Elijah Newren]

Hi,

On Sun, Mar 29, 2026 at 2:41 PM Taylor Blau <me@ttaylorr.com> wrote:
>
> ...If the tip layer has fewer packs
> than the threshold, those packs are excluded entirely; otherwise only
> packs in that layer participate in the geometric repack.

"fewer packs than the threshold"; I'll come back to this below...

>         repo_for_each_pack(existing->repo, p) {
> +               if (geometry->midx_layer_threshold_set && m &&
> +                   p->multi_pack_index) {
> +                       /*
> +                        * When writing MIDX layers incrementally,
> +                        * ignore packs unless they are in the most
> +                        * recent MIDX layer *and* there are at least
> +                        * 'midx_layer_threshold' packs in that layer.
> +                        *
> +                        * Otherwise 'p' is either in an older layer, or
> +                        * the youngest layer does not have enough packs
> +                        * to consider its packs as candidates for
> +                        * repacking. In either of those cases we want
> +                        * to ignore the pack.
> +                        */
> +                       if (m->num_packs > geometry->midx_layer_threshold &&
> +                           midx_layer_contains_pack(m, pack_basename(p)))
> +                               ;
> +                       else
> +                               continue;

Should the commit message use "no more packs than" rather than "fewer
packs than"?  Alternatively, should this be >=  rather than >?  Or am
I just mis-reading?

Also, the empty-if looks a bit weird; would it make sense to invert
the condition to get rid of the empty-if?  In other words, something
like:

    if (m->num_packs <= geometry->midx_layer_threshold ||
       !midx_layer_contains_pack(m, pack_basename(p)))
            continue;

> @@ -173,6 +196,20 @@ void pack_geometry_split(struct pack_geometry *geometry)
>         geometry->promisor_split = compute_pack_geometry_split(geometry->promisor_pack,
>                                                                geometry->promisor_pack_nr,
>                                                                geometry->split_factor);
> +       for (uint32_t i = 0; i < geometry->split; i++) {
> +               struct packed_git *p = geometry->pack[i];
> +               /*
> +                * During incremental MIDX/bitmap repacking, any packs
> +                * included in the rollup are either (a) not MIDX'd, or
> +                * (b) contained in the tip layer iff it has more than
> +                * the threshold number of packs.

This wording looks consistent with the previous check, but not the
commit message.  If the commit message is deemed correct, then you'd
probably need to update not only the above comparison but this comment
as well.

Re: [PATCH 14/16] repack: implement incremental MIDX repacking

[Elijah Newren]

On Sun, Mar 29, 2026 at 2:41 PM Taylor Blau <me@ttaylorr.com> wrote:
>
> At present, this functionality is exposed behind a new enum value,
> `REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
> subsequent commit will expose this mode via `git repack
> --write-midx=incremental`.

Noted, although...

> +       /*
> +        * Compact additional MIDX layers into this proposed one until
> +        * the merging condition is violated.
> +        */
> +       while (m) {
> +               uint32_t preferred_pack_idx;
> +
> +               trace2_data_string("repack", opts->existing->repo,
> +                                  "candidate", midx_get_checksum_hex(m));
> +
> +               if (step.objects_nr < m->num_objects / opts->midx_split_factor) {

Here the new value is used...

> +                       if (proposed_objects_nr < next->base_midx->num_objects / opts->midx_split_factor) {

...and here...

>  struct repack_write_midx_opts {
> @@ -148,6 +149,8 @@ struct repack_write_midx_opts {
>         int show_progress;
>         int write_bitmaps;
>         int midx_must_contain_cruft;
> +       int midx_split_factor;
> +       int midx_new_layer_threshold;
>         enum repack_write_midx_mode mode;
>  };

...and here it is defined.  There is also another new value defined
here (midx_new_layer_threshold) that isn't used anywhere in this patch
yet, but I'll focus on midx_split_factor.

Although, midx_split_factor is defined here, it isn't explicitly
initialized anywhere.  As far as I can tell, it's only initialized
through a designated initializer in builtin/repack.c with
midx_split_factor not appearing as one of the designated fields and
thus getting a value of 0.  That would result in a divide by
zero...although, as you noted in the commit message, this code isn't
yet active.

Still, would it make sense to initialize opts.midx_split_factor in
builtin/repack.c to something non-zero as part of this patch?

Re: [PATCH 15/16] repack: introduce `--write-midx=incremental`

[Elijah Newren]

On Sun, Mar 29, 2026 at 2:41 PM Taylor Blau <me@ttaylorr.com> wrote:
>
> +       if (!strcmp(var, "repack.midxsplitfactor")) {
> +               repack_ctx->midx_split_factor = git_config_int(var, value,
> +                                                              ctx->kvi);
> +               return 0;
> +       }
> +       if (!strcmp(var, "repack.midxnewlayerthreshold")) {
> +               repack_ctx->midx_new_layer_threshold = git_config_int(var, value,
> +                                                                     ctx->kvi);
> +               return 0;
> +       }

Should there be any sanity checking on appropriate values beyond
is-an-integer, such as requiring midx_split_factor > 1 ?

Re: [PATCH 11/16] repack-geometry: prepare for incremental MIDX repacking

[Taylor Blau]

On Thu, Apr 16, 2026 at 03:51:32PM -0700, Elijah Newren wrote:
> Hi,
>
> On Sun, Mar 29, 2026 at 2:41 PM Taylor Blau <me@ttaylorr.com> wrote:
> >
> > ...If the tip layer has fewer packs
> > than the threshold, those packs are excluded entirely; otherwise only
> > packs in that layer participate in the geometric repack.
>
> "fewer packs than the threshold"; I'll come back to this below...
>
> >         repo_for_each_pack(existing->repo, p) {
> > +               if (geometry->midx_layer_threshold_set && m &&
> > +                   p->multi_pack_index) {
> > +                       /*
> > +                        * When writing MIDX layers incrementally,
> > +                        * ignore packs unless they are in the most
> > +                        * recent MIDX layer *and* there are at least
> > +                        * 'midx_layer_threshold' packs in that layer.
> > +                        *
> > +                        * Otherwise 'p' is either in an older layer, or
> > +                        * the youngest layer does not have enough packs
> > +                        * to consider its packs as candidates for
> > +                        * repacking. In either of those cases we want
> > +                        * to ignore the pack.
> > +                        */
> > +                       if (m->num_packs > geometry->midx_layer_threshold &&
> > +                           midx_layer_contains_pack(m, pack_basename(p)))
> > +                               ;
> > +                       else
> > +                               continue;
>
> Should the commit message use "no more packs than" rather than "fewer
> packs than"?  Alternatively, should this be >=  rather than >?  Or am
> I just mis-reading?

I think the goal was to have "fewer packs than" so the commit message is
right here and the implementation is wrong.

> Also, the empty-if looks a bit weird; would it make sense to invert
> the condition to get rid of the empty-if?  In other words, something
> like:
>
>     if (m->num_packs <= geometry->midx_layer_threshold ||
>        !midx_layer_contains_pack(m, pack_basename(p)))
>             continue;

I sometimes will use the empty-if style when the conditional is easier
to explain in the positive than the negative, but reading it out like
this I think that the conditional as written here is still clear. Thanks
for the suggestion!

With the above "no more than" vs "fewer than", I changed this as
follows:

--- 8< ---
diff --git a/repack-geometry.c b/repack-geometry.c
index 5b554da89f1..0469dc6496a 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -49,10 +49,8 @@ void pack_geometry_init(struct pack_geometry *geometry,
 			 * repacking. In either of those cases we want
 			 * to ignore the pack.
 			 */
-			if (m->num_packs > geometry->midx_layer_threshold &&
-			    midx_layer_contains_pack(m, pack_basename(p)))
-				;
-			else
+			if (m->num_packs < geometry->midx_layer_threshold ||
+			    !midx_layer_contains_pack(m, pack_basename(p)))
 				continue;
 		}
--- >8 ---

> > @@ -173,6 +196,20 @@ void pack_geometry_split(struct pack_geometry *geometry)
> >         geometry->promisor_split = compute_pack_geometry_split(geometry->promisor_pack,
> >                                                                geometry->promisor_pack_nr,
> >                                                                geometry->split_factor);
> > +       for (uint32_t i = 0; i < geometry->split; i++) {
> > +               struct packed_git *p = geometry->pack[i];
> > +               /*
> > +                * During incremental MIDX/bitmap repacking, any packs
> > +                * included in the rollup are either (a) not MIDX'd, or
> > +                * (b) contained in the tip layer iff it has more than
> > +                * the threshold number of packs.
>
> This wording looks consistent with the previous check, but not the
> commit message.  If the commit message is deemed correct, then you'd
> probably need to update not only the above comparison but this comment
> as well.

Great catch, I would have definitely missed this if you hadn't pointed
it out.

Thanks,
Taylor

Re: [PATCH 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

On Thu, Apr 16, 2026 at 03:53:10PM -0700, Elijah Newren wrote:
> Although, midx_split_factor is defined here, it isn't explicitly
> initialized anywhere.  As far as I can tell, it's only initialized
> through a designated initializer in builtin/repack.c with
> midx_split_factor not appearing as one of the designated fields and
> thus getting a value of 0.  That would result in a divide by
> zero...although, as you noted in the commit message, this code isn't
> yet active.
>
> Still, would it make sense to initialize opts.midx_split_factor in
> builtin/repack.c to something non-zero as part of this patch?

Good point. I had avoided initializing it because this code is all dead
as of this point in the series, and I didn't want to touch the repack
builtin until we actually wire it all up.

But I think the point you raise here is very fair, so I introduced the
constants in this patch and used them in the initializer, then adjust
them in the subsequent patch to use the given values instead of the
defaults.

Thanks,
Taylor

Re: [PATCH 15/16] repack: introduce `--write-midx=incremental`

[Taylor Blau]

On Thu, Apr 16, 2026 at 03:53:28PM -0700, Elijah Newren wrote:
> On Sun, Mar 29, 2026 at 2:41 PM Taylor Blau <me@ttaylorr.com> wrote:
> >
> > +       if (!strcmp(var, "repack.midxsplitfactor")) {
> > +               repack_ctx->midx_split_factor = git_config_int(var, value,
> > +                                                              ctx->kvi);
> > +               return 0;
> > +       }
> > +       if (!strcmp(var, "repack.midxnewlayerthreshold")) {
> > +               repack_ctx->midx_new_layer_threshold = git_config_int(var, value,
> > +                                                                     ctx->kvi);
> > +               return 0;
> > +       }
>
> Should there be any sanity checking on appropriate values beyond
> is-an-integer, such as requiring midx_split_factor > 1 ?

Very reasonable, I added some basic sanity checking for both.

Thanks,
Taylor

[PATCH v2 00/16] repack: incremental MIDX/bitmap-based repacking

[Taylor Blau]

[Note to the maintainer, this is rebased onto current 'master', which is
94f057755b7 (Git 2.54, 2026-04-19) at the time of writing.]

This a modest-sized reroll of my series to implement the last remaining
component of the incremental MIDX/bitmap-based repacking strategy that I
have been working on.

The main changes since last time are described in [1], which I sent to
the list last week just before the 2.54 release was tagged. For
convenience, a copy of the main changes are below:

 - Use a strset instead of a string_list for keeping track of the MIDX
   layers to retain when calling `either clear_midx_files_ext()` or
   `clear_incremental_midx_files_ext()`.

 - A new patch to rewrite the logic for determining which MIDX layers
   comprise the new chain via keep_hashes to build the array in order.
   The subsequent patch converts that into a strvec, which no longer
   requires direct manipulation.

 - The new "--checksum-only" option has been renamed to
   "--no-write-chain-file", and various small implementation tweaks
   (e.g., relying on `is_lock_file_locked()` to determine whether we
   should update the chain file as opposed to reading the flags).

As usual, a range-diff is included below as well for convenience. Thanks
in advance for reviewing!

[1]: https://lore.kernel.org/git/ad7B7cxKP7tNCXUf@nand.local

Taylor Blau (16):
  midx-write: handle noop writes when converting incremental chains
  midx: use `strset` for retained MIDX files
  midx: build `keep_hashes` array in order
  midx: use `strvec` for `keep_hashes`
  midx: introduce `--no-write-chain-file` for incremental MIDX writes
  midx: support custom `--base` for incremental MIDX writes
  repack: track the ODB source via existing_packs
  midx: expose `midx_layer_contains_pack()`
  repack-midx: factor out `repack_prepare_midx_command()`
  repack-midx: extract `repack_fill_midx_stdin_packs()`
  repack-geometry: prepare for incremental MIDX repacking
  builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
  packfile: ensure `close_pack_revindex()` frees in-memory revindex
  repack: implement incremental MIDX repacking
  repack: introduce `--write-midx=incremental`
  repack: allow `--write-midx=incremental` without `--geometric`

 Documentation/config/repack.adoc        |  18 +
 Documentation/git-multi-pack-index.adoc |  32 +-
 Documentation/git-repack.adoc           |  44 +-
 builtin/multi-pack-index.c              |  48 +-
 builtin/repack.c                        | 102 +++-
 midx-write.c                            | 206 ++++---
 midx.c                                  | 104 ++--
 midx.h                                  |  11 +-
 packfile.c                              |   2 +
 repack-geometry.c                       |  48 +-
 repack-midx.c                           | 717 +++++++++++++++++++++++-
 repack.c                                |  58 +-
 repack.h                                |  26 +-
 t/meson.build                           |   1 +
 t/t5334-incremental-multi-pack-index.sh |  63 +++
 t/t5335-compact-multi-pack-index.sh     | 113 ++++
 t/t7705-repack-incremental-midx.sh      | 525 +++++++++++++++++
 17 files changed, 1918 insertions(+), 200 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh

Range-diff against v1:
 1:  0c7a68ca5a7 =  1:  d6c27317c25 midx-write: handle noop writes when converting incremental chains
 2:  1391552dfc6 !  2:  629c8d23116 midx: use `string_list` for retained MIDX files
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>
     
      ## Commit message ##
    -    midx: use `string_list` for retained MIDX files
    +    midx: use `strset` for retained MIDX files
     
         Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
         build a list of filenames to keep while pruning stale MIDX files. Today
    -    they hand-roll an array instead of using a `string_list`, thus requiring
    -    us to pass an additional length parameter, and makes lookups linear.
    +    they hand-roll an array instead of using a `strset`, thus requiring us
    +    to pass an additional length parameter, and makes lookups linear.
     
    -    Replace the bare array with a `string_list` which can be passed around
    -    as a single parameter. Though it improves lookup performance, the
    -    difference is likely immeasurable given how small the keep_hashes array
    -    typically is.
    +    Replace the bare array with a `strset` which can be passed around as a
    +    single parameter. Though it improves lookup performance, the difference
    +    is likely immeasurable given how small the keep_hashes array typically
    +    is.
     
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
    @@ midx.c: int midx_checksum_valid(struct multi_pack_index *m)
      struct clear_midx_data {
     -	char **keep;
     -	uint32_t keep_nr;
    -+	struct string_list keep;
    ++	struct strset keep;
      	const char *ext;
      };
      
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -		if (!strcmp(data->keep[i], file_name))
     -			return;
     -	}
    -+	if (string_list_has_string(&data->keep, file_name))
    ++	if (strset_contains(&data->keep, file_name))
     +		return;
      	if (unlink(full_path))
      		die_errno(_("failed to remove %s"), full_path);
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	struct clear_midx_data data;
     -	memset(&data, 0, sizeof(struct clear_midx_data));
     +	struct clear_midx_data data = {
    -+		.keep = STRING_LIST_INIT_DUP,
    ++		.keep = STRSET_INIT,
     +		.ext = ext,
     +	};
      
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
      
     -		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
     -		data.keep_nr = 1;
    -+		string_list_insert(&data.keep, buf.buf);
    ++		strset_add(&data.keep, buf.buf);
     +
     +		strbuf_release(&buf);
      	}
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	if (keep_hash)
     -		free(data.keep[0]);
     -	free(data.keep);
    -+	string_list_clear(&data.keep, 0);
    ++	strset_clear(&data.keep);
      }
      
      void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
      {
     -	struct clear_midx_data data;
     +	struct clear_midx_data data = {
    -+		.keep = STRING_LIST_INIT_NODUP,
    ++		.keep = STRSET_INIT,
     +		.ext = ext,
     +	};
    ++	struct strbuf buf = STRBUF_INIT;
      	uint32_t i;
      
     -	memset(&data, 0, sizeof(struct clear_midx_data));
    --
    ++	for (i = 0; i < hashes_nr; i++) {
    ++		strbuf_reset(&buf);
    ++		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
    ++			    ext);
    + 
     -	ALLOC_ARRAY(data.keep, hashes_nr);
    - 	for (i = 0; i < hashes_nr; i++)
    +-	for (i = 0; i < hashes_nr; i++)
     -		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
     -				       ext);
     -	data.keep_nr = hashes_nr;
     -	data.ext = ext;
    -+		string_list_append(&data.keep,
    -+				   xstrfmt("multi-pack-index-%s.%s",
    -+					   keep_hashes[i], ext));
    -+	string_list_sort(&data.keep);
    ++		strset_add(&data.keep, buf.buf);
    ++	}
      
      	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
      				     clear_midx_file_ext, &data);
    @@ midx.c: static void clear_midx_file_ext(const char *full_path, size_t full_path_
     -	for (i = 0; i < hashes_nr; i++)
     -		free(data.keep[i]);
     -	free(data.keep);
    -+	string_list_clear(&data.keep, 0);
    ++	strbuf_release(&buf);
    ++	strset_clear(&data.keep);
      }
      
      void clear_midx_file(struct repository *r)
 3:  aea6aad6953 !  3:  e303bf6a4ac strvec: introduce `strvec_init_alloc()`
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>
     
      ## Commit message ##
    -    strvec: introduce `strvec_init_alloc()`
    +    midx: build `keep_hashes` array in order
     
    -    When the caller knows upfront how many elements will be pushed onto a
    -    `strvec`, it is useful to pre-allocate enough space in the array to fit
    -    that many elements (and one additional slot to store NULL, indicating
    -    the end of the list.)
    +    Instead of filling the keep_hashes array using reverse indexing (e.g.,
    +    `keep_hashes[count - i - 1]`) while traversing linked lists forward,
    +    collect linked list nodes into a temporary `layers` array and then
    +    iterate it backwards to fill `keep_hashes` sequentially.
     
    -    Introduce `strvec_init_alloc()`, which allocates the backing array large
    -    enough to hold `alloc` elements and the termination marker without
    -    further reallocation. Reimplement `strvec_init()` as a special case of
    -    `strvec_init_alloc()`, namely when `alloc` is zero.
    +    This makes the filling logic easier to follow, since each segment of the
    +    array is filled with a simple forward-marching index. Moreover, this
    +    change prepares us for a subsequent commit that will switch to using a
    +    `strvec`.
     
    -    Helped-by: Junio C Hamano <gitster@pobox.com>
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
    - ## strvec.c ##
    -@@ strvec.c: const char *empty_strvec[] = { NULL };
    + ## midx-write.c ##
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 		FILE *chainf = fdopen_lock_file(&lk, "w");
    + 		struct strbuf final_midx_name = STRBUF_INIT;
    + 		struct multi_pack_index *m = ctx.base_midx;
    ++		struct multi_pack_index **layers = NULL;
    ++		size_t layers_nr = 0, layers_alloc = 0;
    ++		size_t j = 0;
      
    - void strvec_init(struct strvec *array)
    - {
    --	struct strvec blank = STRVEC_INIT;
    --	memcpy(array, &blank, sizeof(*array));
    -+	strvec_init_alloc(array, 0);
    -+}
    -+
    -+void strvec_init_alloc(struct strvec *array, size_t alloc)
    -+{
    -+	if (!alloc) {
    -+		struct strvec blank = STRVEC_INIT;
    -+		memcpy(array, &blank, sizeof(*array));
    -+	} else {
    -+		CALLOC_ARRAY(array->v, st_add(alloc, 1));
    -+		array->nr = 0;
    -+		array->alloc = alloc + 1;
    -+	}
    - }
    + 		if (!chainf) {
    + 			error_errno(_("unable to open multi-pack-index chain file"));
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 		strbuf_release(&final_midx_name);
    + 
    + 		if (ctx.compact) {
    +-			struct multi_pack_index *m;
    +-			uint32_t num_layers_before_from = 0;
    +-			uint32_t i;
    ++			struct multi_pack_index *mp;
    + 
    +-			for (m = ctx.base_midx; m; m = m->base_midx)
    +-				num_layers_before_from++;
    +-
    +-			m = ctx.base_midx;
    +-			for (i = 0; i < num_layers_before_from; i++) {
    +-				uint32_t j = num_layers_before_from - i - 1;
    +-
    +-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    +-				m = m->base_midx;
    ++			for (mp = ctx.base_midx; mp; mp = mp->base_midx) {
    ++				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
    ++				layers[layers_nr++] = mp;
    + 			}
    ++			while (layers_nr)
    ++				keep_hashes[j++] =
    ++					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
      
    - void strvec_push_nodup(struct strvec *array, char *value)
    -
    - ## strvec.h ##
    -@@ strvec.h: struct strvec {
    -  */
    - void strvec_init(struct strvec *);
    +-			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
    +-								   r->hash_algo));
    ++			keep_hashes[j++] =
    ++				xstrdup(hash_to_hex_algop(midx_hash,
    ++							  r->hash_algo));
      
    -+/*
    -+ * Initializes an array large enough to store `alloc` elements.
    -+ */
    -+void strvec_init_alloc(struct strvec *, size_t alloc);
    +-			i = 0;
    +-			for (m = ctx.m;
    +-			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
    +-			     m = m->base_midx) {
    +-				keep_hashes[keep_hashes_nr - i - 1] =
    +-					xstrdup(midx_get_checksum_hex(m));
    +-				i++;
    ++			for (mp = ctx.m;
    ++			     mp && midx_hashcmp(mp, ctx.compact_to,
    ++						r->hash_algo);
    ++			     mp = mp->base_midx) {
    ++				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
    ++				layers[layers_nr++] = mp;
    + 			}
    ++			while (layers_nr)
    ++				keep_hashes[j++] =
    ++					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    + 		} else {
    +-			keep_hashes[ctx.num_multi_pack_indexes_before] =
    ++			for (; m; m = m->base_midx) {
    ++				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
    ++				layers[layers_nr++] = m;
    ++			}
    ++			while (layers_nr)
    ++				keep_hashes[j++] =
    ++					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
     +
    - /* Push a copy of a string onto the end of the array. */
    - const char *strvec_push(struct strvec *, const char *);
    ++			keep_hashes[j++] =
    + 				xstrdup(hash_to_hex_algop(midx_hash,
    + 							  r->hash_algo));
    +-
    +-			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
    +-				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
    +-
    +-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    +-				m = m->base_midx;
    +-			}
    + 		}
      
    +-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
    ++		ASSERT(j == keep_hashes_nr);
    ++
    ++		free(layers);
    ++
    ++		for (uint32_t i = 0; i < j; i++)
    + 			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
    + 	} else {
    + 		keep_hashes[ctx.num_multi_pack_indexes_before] =
 4:  5fc72d5049a !  4:  42d76c70060 midx: use `strvec` for `keep_hashes`
    @@ Commit message
         value is used to determine the contents of the resulting
         `multi-pack-index-chain` file when writing with "--incremental".
     
    +    Since the previous commit already builds the array in forward order, the
    +    conversion is straightforward: replace indexed assignments with
    +    `strvec_push()`, drop the pre-counting and `CALLOC_ARRAY()`, and
    +    simplify cleanup via `strvec_clear()`.
    +
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
      ## midx-write.c ##
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	int dropped_packs = 0;
      	int result = -1;
     -	const char **keep_hashes = NULL;
    +-	size_t keep_hashes_nr = 0;
     +	struct strvec keep_hashes = STRVEC_INIT;
    - 	size_t keep_hashes_nr = 0;
      	struct chunkfile *cf;
      
    + 	trace2_region_enter("midx", "write_midx_internal", r);
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 	} else {
    - 		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
    - 	}
    + 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
    + 		die(_("too many multi-pack-indexes"));
    + 
    +-	if (ctx.compact) {
    +-		struct multi_pack_index *m;
    +-
    +-		/*
    +-		 * Keep all MIDX layers excluding those in the range [from, to].
    +-		 */
    +-		for (m = ctx.base_midx; m; m = m->base_midx)
    +-			keep_hashes_nr++;
    +-		for (m = ctx.m;
    +-		     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
    +-		     m = m->base_midx)
    +-			keep_hashes_nr++;
    +-
    +-		keep_hashes_nr++; /* include the compacted layer */
    +-	} else {
    +-		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
    +-	}
     -	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
    -+	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
    - 
    +-
      	if (ctx.incremental) {
      		FILE *chainf = fdopen_lock_file(&lk, "w");
    -@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 			for (i = 0; i < num_layers_before_from; i++) {
    - 				uint32_t j = num_layers_before_from - i - 1;
    + 		struct strbuf final_midx_name = STRBUF_INIT;
    + 		struct multi_pack_index *m = ctx.base_midx;
    + 		struct multi_pack_index **layers = NULL;
    + 		size_t layers_nr = 0, layers_alloc = 0;
    +-		size_t j = 0;
      
    --				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				m = m->base_midx;
    + 		if (!chainf) {
    + 			error_errno(_("unable to open multi-pack-index chain file"));
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 				layers[layers_nr++] = mp;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));
      
    --			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
    -+			keep_hashes.v[i] = xstrdup(hash_to_hex_algop(midx_hash,
    - 								   r->hash_algo));
    -+			keep_hashes.nr++;
    +-			keep_hashes[j++] =
    +-				xstrdup(hash_to_hex_algop(midx_hash,
    +-							  r->hash_algo));
    ++			strvec_push(&keep_hashes,
    ++				    hash_to_hex_algop(midx_hash,
    ++						     r->hash_algo));
      
    - 			i = 0;
    - 			for (m = ctx.m;
    - 			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
    - 			     m = m->base_midx) {
    --				keep_hashes[keep_hashes_nr - i - 1] =
    -+				keep_hashes.v[keep_hashes_nr - i - 1] =
    - 					xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				i++;
    + 			for (mp = ctx.m;
    + 			     mp && midx_hashcmp(mp, ctx.compact_to,
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 				layers[layers_nr++] = mp;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));
      		} else {
    --			keep_hashes[ctx.num_multi_pack_indexes_before] =
    -+			keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 				xstrdup(hash_to_hex_algop(midx_hash,
    - 							  r->hash_algo));
    -+			keep_hashes.nr++;
    - 
    - 			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
    - 				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
    - 
    --				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.v[j] = xstrdup(midx_get_checksum_hex(m));
    -+				keep_hashes.nr++;
    - 				m = m->base_midx;
    + 			for (; m; m = m->base_midx) {
    + 				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
    + 				layers[layers_nr++] = m;
      			}
    + 			while (layers_nr)
    +-				keep_hashes[j++] =
    +-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
    ++				strvec_push(&keep_hashes,
    ++					    midx_get_checksum_hex(layers[--layers_nr]));
    + 
    +-			keep_hashes[j++] =
    +-				xstrdup(hash_to_hex_algop(midx_hash,
    +-							  r->hash_algo));
    ++			strvec_push(&keep_hashes,
    ++				    hash_to_hex_algop(midx_hash,
    ++						     r->hash_algo));
      		}
      
    - 		for (uint32_t i = 0; i < keep_hashes_nr; i++)
    +-		ASSERT(j == keep_hashes_nr);
    +-
    + 		free(layers);
    + 
    +-		for (uint32_t i = 0; i < j; i++)
     -			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
    ++		for (size_t i = 0; i < keep_hashes.nr; i++)
     +			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
      	} else {
     -		keep_hashes[ctx.num_multi_pack_indexes_before] =
    -+		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    -+		keep_hashes.nr++;
    +-			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    ++		strvec_push(&keep_hashes,
    ++			    hash_to_hex_algop(midx_hash, r->hash_algo));
      	}
      
      	if (ctx.m || ctx.base_midx)
    @@ midx.c: void clear_midx_files_ext(struct odb_source *source, const char *ext,
     +				      const struct strvec *keep_hashes)
      {
      	struct clear_midx_data data = {
    --		.keep = STRING_LIST_INIT_NODUP,
    -+		.keep = STRING_LIST_INIT_DUP,
    + 		.keep = STRSET_INIT,
      		.ext = ext,
      	};
    + 	struct strbuf buf = STRBUF_INIT;
     -	uint32_t i;
      
    --	for (i = 0; i < hashes_nr; i++)
    --		string_list_append(&data.keep,
    --				   xstrfmt("multi-pack-index-%s.%s",
    --					   keep_hashes[i], ext));
    --	string_list_sort(&data.keep);
    +-	for (i = 0; i < hashes_nr; i++) {
    +-		strbuf_reset(&buf);
    +-		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
    +-			    ext);
     +	if (keep_hashes) {
    -+		struct strbuf buf = STRBUF_INIT;
     +		for (size_t i = 0; i < keep_hashes->nr; i++) {
     +			strbuf_reset(&buf);
    -+
     +			strbuf_addf(&buf, "multi-pack-index-%s.%s",
     +				    keep_hashes->v[i], ext);
    -+			string_list_append(&data.keep, buf.buf);
    + 
    +-		strset_add(&data.keep, buf.buf);
    ++			strset_add(&data.keep, buf.buf);
     +		}
    -+
    -+		string_list_sort(&data.keep);
    -+		strbuf_release(&buf);
    -+	}
    + 	}
      
      	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
    - 				     clear_midx_file_ext, &data);
 5:  41cb5471bb9 !  5:  2c80aa34fac midx: introduce `--checksum-only` for incremental MIDX writes
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>
     
      ## Commit message ##
    -    midx: introduce `--checksum-only` for incremental MIDX writes
    +    midx: introduce `--no-write-chain-file` for incremental MIDX writes
     
         When writing an incremental MIDX layer, the MIDX machinery writes the
         new layer into the multi-pack-index.d directory and then updates the
    @@ Commit message
         assemble the new MIDX chain itself before writing a new chain file and
         atomically linking it into place).
     
    -    Introduce a `--checksum-only` flag that:
    +    Introduce a `--no-write-chain-file` flag that:
     
          * writes the new MIDX layer into the multi-pack-index.d directory
     
    @@ Documentation/git-multi-pack-index.adoc: SYNOPSIS
      'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
      		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
     -		         [--refs-snapshot=<path>]
    -+		         [--refs-snapshot=<path>] [--[no-]checksum-only]
    ++		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
      'git multi-pack-index' [<options>] compact [--[no-]incremental]
     -		         [--[no-]bitmap] <from> <to>
    -+		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
    ++		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
      'git multi-pack-index' [<options>] verify
      'git multi-pack-index' [<options>] expire
      'git multi-pack-index' [<options>] repack [--batch-size=<size>]
    +@@ Documentation/git-multi-pack-index.adoc: marker).
    + 		and packs not present in an existing MIDX layer.
    + 		Migrates non-incremental MIDXs to incremental ones when
    + 		necessary.
    ++
    ++	--[no-]write-chain-file::
    ++		When used with `--incremental`, write a new MIDX layer
    ++		but do not update the multi-pack-index-chain file.
    ++		The checksum of the new layer is printed to standard
    ++		output, allowing the caller to assemble and write the
    ++		chain itself. Requires `--incremental`.
    + --
    + 
    + compact::
    +@@ Documentation/git-multi-pack-index.adoc: compact::
    + 
    + 	--[no-]bitmap::
    + 		Control whether or not a multi-pack bitmap is written.
    ++
    ++	--[no-]write-chain-file::
    ++		When used with `--incremental`, write a new compacted
    ++		MIDX layer but do not update the multi-pack-index-chain
    ++		file. The checksum of the new layer is printed to
    ++		standard output. Requires `--incremental`.
    + --
    + +
    + Note that the compact command requires writing a version-2 midx that
     
      ## builtin/multi-pack-index.c ##
     @@
    @@ builtin/multi-pack-index.c
      	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
      	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
     -	   "  [--refs-snapshot=<path>]")
    -+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
    ++	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
      
      #define BUILTIN_MIDX_COMPACT_USAGE \
      	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
     -	   "  [--[no-]bitmap] <from> <to>")
    -+	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
    ++	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
      
      #define BUILTIN_MIDX_VERIFY_USAGE \
      	N_("git multi-pack-index [<options>] verify")
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
      			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    -+		OPT_BIT(0, "checksum-only", &opts.flags,
    -+			N_("write a MIDX layer without updating the MIDX chain"),
    -+			MIDX_WRITE_CHECKSUM_ONLY),
    ++		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
    ++			N_("write the multi-pack-index chain file"),
    ++			MIDX_WRITE_NO_CHAIN),
      		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
      			 N_("write multi-pack index containing only given indexes")),
      		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
      		usage_with_options(builtin_multi_pack_index_write_usage,
      				   options);
     +
    -+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
    ++	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
     +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
     +		error(_("cannot use %s without %s"),
    -+		      "--checksum-only", "--incremental");
    ++		      "--no-write-chain-file", "--incremental");
     +		usage_with_options(builtin_multi_pack_index_write_usage,
     +				   options);
     +	}
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    -+		OPT_BIT(0, "checksum-only", &opts.flags,
    -+			N_("write a MIDX layer without updating the MIDX chain"),
    -+			MIDX_WRITE_CHECKSUM_ONLY),
    ++		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
    ++			N_("write the multi-pack-index chain file"),
    ++			MIDX_WRITE_NO_CHAIN),
      		OPT_END(),
      	};
      
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      		usage_with_options(builtin_multi_pack_index_compact_usage,
      				   options);
     +
    -+	if (opts.flags & MIDX_WRITE_CHECKSUM_ONLY &&
    ++	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
     +	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
     +		error(_("cannot use %s without %s"),
    -+		      "--checksum-only", "--incremental");
    ++		      "--no-write-chain-file", "--incremental");
     +		usage_with_options(builtin_multi_pack_index_compact_usage,
     +				   options);
     +	}
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, co
      	FREE_AND_NULL(options);
     
      ## midx-write.c ##
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 	unsigned char midx_hash[GIT_MAX_RAWSZ];
    + 	uint32_t start_pack;
    + 	struct hashfile *f = NULL;
    +-	struct lock_file lk;
    ++	struct lock_file lk = LOCK_INIT;
    + 	struct tempfile *incr;
    + 	struct write_midx_context ctx = {
    + 		.preferred_pack_idx = NO_PREFERRED_PACK,
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	}
      
      	if (ctx.incremental) {
     -		struct strbuf lock_name = STRBUF_INIT;
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++		if (!(opts->flags & MIDX_WRITE_NO_CHAIN)) {
     +			struct strbuf lock_name = STRBUF_INIT;
      
     -		get_midx_chain_filename(opts->source, &lock_name);
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      		incr = mks_tempfile_m(midx_name.buf, 0444);
      		if (!incr) {
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 	}
    - 	strvec_init_alloc(&keep_hashes, keep_hashes_nr);
    + 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
    + 		die(_("too many multi-pack-indexes"));
      
    -+	if (opts->flags & MIDX_WRITE_CHECKSUM_ONLY)
    ++	if (!is_lock_file_locked(&lk))
     +		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));
    ++	else if (opts->flags & MIDX_WRITE_NO_CHAIN)
    ++		BUG("lockfile held with MIDX_WRITE_NO_CHAIN set?");
     +
      	if (ctx.incremental) {
     -		FILE *chainf = fdopen_lock_file(&lk, "w");
      		struct strbuf final_midx_name = STRBUF_INIT;
      		struct multi_pack_index *m = ctx.base_midx;
    + 		struct multi_pack_index **layers = NULL;
    + 		size_t layers_nr = 0, layers_alloc = 0;
      
     -		if (!chainf) {
     -			error_errno(_("unable to open multi-pack-index chain file"));
     -			goto cleanup;
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++		if (is_lock_file_locked(&lk)){
     +			FILE *chainf = fdopen_lock_file(&lk, "w");
     +			if (!chainf) {
     +				error_errno(_("unable to open multi-pack-index chain file"));
    @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      
      		if (link_midx_to_chain(ctx.base_midx) < 0)
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    - 			}
    - 		}
      
    --		for (uint32_t i = 0; i < keep_hashes_nr; i++)
    + 		free(layers);
    + 
    +-		for (size_t i = 0; i < keep_hashes.nr; i++)
     -			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
    -+		if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY))
    -+			for (uint32_t i = 0; i < keep_hashes_nr; i++)
    ++		if (is_lock_file_locked(&lk))
    ++			for (size_t i = 0; i < keep_hashes.nr; i++)
     +				fprintf(get_lock_file_fp(&lk), "%s\n",
     +					keep_hashes.v[i]);
      	} else {
    - 		keep_hashes.v[ctx.num_multi_pack_indexes_before] =
    - 			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
    + 		strvec_push(&keep_hashes,
    + 			    hash_to_hex_algop(midx_hash, r->hash_algo));
     @@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
      	if (ctx.m || ctx.base_midx)
      		odb_close(ctx.repo->objects);
      
     -	if (commit_lock_file(&lk) < 0)
     -		die_errno(_("could not write multi-pack-index"));
    -+	if (!(opts->flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    ++	if (is_lock_file_locked(&lk)) {
     +		if (commit_lock_file(&lk) < 0)
     +			die_errno(_("could not write multi-pack-index"));
      
    @@ midx.h: struct multi_pack_index {
      #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
      #define MIDX_WRITE_INCREMENTAL (1 << 5)
      #define MIDX_WRITE_COMPACT (1 << 6)
    -+#define MIDX_WRITE_CHECKSUM_ONLY (1 << 7)
    ++#define MIDX_WRITE_NO_CHAIN (1 << 7)
      
      #define MIDX_EXT_REV "rev"
      #define MIDX_EXT_BITMAP "bitmap"
    @@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'show object from s
      	git cat-file -p 2.2
      '
      
    -+test_expect_success 'write MIDX layer with --checksum-only' '
    -+	test_commit checksum-only &&
    ++test_expect_success 'write MIDX layer with --no-write-chain-file' '
    ++	test_commit no-write-chain-file &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only)" &&
    ++		--no-write-chain-file)" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
     +'
     +
    -+test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
    -+	test_must_fail git multi-pack-index write --bitmap --checksum-only 2>err &&
    -+	test_grep "cannot use --checksum-only without --incremental" err
    ++test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file' '
    ++	test_must_fail git multi-pack-index write --bitmap --no-write-chain-file 2>err &&
    ++	test_grep "cannot use --no-write-chain-file without --incremental" err
     +'
     +
      for reuse in false single multi
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
      	)
      '
      
    -+test_expect_success 'MIDX compaction with --checksum-only' '
    -+	git init midx-compact-with--checksum-only &&
    ++test_expect_success 'MIDX compaction with --no-write-chain-file' '
    ++	git init midx-compact-with--no-write-chain-file &&
     +	(
    -+		cd midx-compact-with--checksum-only &&
    ++		cd midx-compact-with--no-write-chain-file &&
     +
     +		git config maintenance.auto false &&
     +
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
     +		cp "$midx_chain" "$midx_chain".bak &&
     +
     +		layer="$(git multi-pack-index compact --incremental \
    -+			--checksum-only \
    ++			--no-write-chain-file \
     +			"$(nth_line 2 "$midx_chain")" \
     +			"$(nth_line 3 "$midx_chain")")" &&
     +
    @@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with b
     +		# After writing the new layer, insert it into the chain
     +		# manually. This is done in order to make $layer visible
     +		# to the read-midx test helper below, and matches what
    -+		# the MIDX command would do without --checksum-only.
    ++		# the MIDX command would do without --no-write-chain-file.
     +		{
     +			nth_line 1 "$midx_chain.bak" &&
     +			echo $layer &&
 6:  6b42d284b79 !  6:  2a05f4b86f3 midx: support custom `--base` for incremental MIDX writes
    @@ Commit message
         If no `--base` is given, behavior is unchanged: `compact` uses "from's"
         immediate parent in the chain, and `write` appends to the existing tip.
     
    -    For the `write` subcommand, `--base` requires `--checksum-only`. A plain
    +    For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
         `write --incremental` appends a new layer to the live chain tip with no
         mechanism to atomically replace it; overriding the base would produce a
         layer that does not extend the tip, breaking chain invariants. With
    -    `--checksum-only` the chain is left unmodified and the caller is
    +    `--no-write-chain-file` the chain is left unmodified and the caller is
         responsible for assembling a valid chain.
     
         For `compact`, no such restriction applies. The compaction operation
    @@ Documentation/git-multi-pack-index.adoc
     @@ Documentation/git-multi-pack-index.adoc: SYNOPSIS
      'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
      		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
    - 		         [--refs-snapshot=<path>] [--[no-]checksum-only]
    + 		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
     +			 [--base=<checksum>]
      'git multi-pack-index' [<options>] compact [--[no-]incremental]
    --		         [--[no-]bitmap] [--[no-]checksum-only] <from> <to>
    -+		         [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]
    +-		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
    ++		         [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]
     +			 <from> <to>
      'git multi-pack-index' [<options>] verify
      'git multi-pack-index' [<options>] expire
      'git multi-pack-index' [<options>] repack [--batch-size=<size>]
     @@ Documentation/git-multi-pack-index.adoc: marker).
    - 		and packs not present in an existing MIDX layer.
    - 		Migrates non-incremental MIDXs to incremental ones when
    - 		necessary.
    + 		The checksum of the new layer is printed to standard
    + 		output, allowing the caller to assemble and write the
    + 		chain itself. Requires `--incremental`.
     +
     +	--base=<checksum>::
     +		Specify the checksum of an existing MIDX layer to use
     +		as the base when writing a new incremental layer.
     +		The special value `none` indicates that the new layer
     +		should have no base (i.e., it becomes a root layer).
    -+		Requires `--checksum-only`.
    ++		Requires `--no-write-chain-file`.
      --
      
      compact::
     @@ Documentation/git-multi-pack-index.adoc: compact::
    - 
    - 	--[no-]bitmap::
    - 		Control whether or not a multi-pack bitmap is written.
    + 		MIDX layer but do not update the multi-pack-index-chain
    + 		file. The checksum of the new layer is printed to
    + 		standard output. Requires `--incremental`.
     +
     +	--base=<checksum>::
     +		Specify the checksum of an existing MIDX layer to use
    @@ Documentation/git-multi-pack-index.adoc: compact::
     +		the immediate parent of `<from>`. The special value
     +		`none` indicates that the result should have no base.
      --
    - 
    - verify::
    + +
    + Note that the compact command requires writing a version-2 midx that
     
      ## builtin/multi-pack-index.c ##
     @@
      #define BUILTIN_MIDX_WRITE_USAGE \
      	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
      	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
    --	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]")
    -+	   "  [--refs-snapshot=<path>] [--[no-]checksum-only]\n" \
    +-	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
    ++	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]\n" \
     +	   "  [--base=<checksum>]")
      
      #define BUILTIN_MIDX_COMPACT_USAGE \
      	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
    --	   "  [--[no-]bitmap] [--[no-]checksum-only] <from> <to>")
    -+	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]checksum-only]\n" \
    +-	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
    ++	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]\n" \
     +	   "  <from> <to>")
      
      #define BUILTIN_MIDX_VERIFY_USAGE \
    @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, cons
     +			   N_("base MIDX for incremental writes")),
      		OPT_BIT(0, "incremental", &opts.flags,
      			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    - 		OPT_BIT(0, "checksum-only", &opts.flags,
    + 		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
     @@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, const char **argv,
      				   options);
      	}
      
     +	if (opts.incremental_base &&
    -+	    !(opts.flags & MIDX_WRITE_CHECKSUM_ONLY)) {
    -+		error(_("cannot use --base without --checksum-only"));
    ++	    !(opts.flags & MIDX_WRITE_NO_CHAIN)) {
    ++		error(_("cannot use --base without --no-write-chain-file"));
     +		usage_with_options(builtin_multi_pack_index_write_usage,
     +				   options);
     +	}
    @@ midx.h: int write_midx_file(struct odb_source *source,
      int verify_midx_file(struct odb_source *source, unsigned flags);
     
      ## t/t5334-incremental-multi-pack-index.sh ##
    -@@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremental MIDX layer with --checksum-only' '
    - 	test_grep "cannot use --checksum-only without --incremental" err
    +@@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file
    + 	test_grep "cannot use --no-write-chain-file without --incremental" err
      '
      
    -+test_expect_success 'write MIDX layer with --base without --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base without --no-write-chain-file' '
     +	test_must_fail git multi-pack-index write --bitmap --incremental \
     +		--base=none 2>err &&
    -+	test_grep "cannot use --base without --checksum-only" err
    ++	test_grep "cannot use --base without --no-write-chain-file" err
     +'
     +
    -+test_expect_success 'write MIDX layer with --base=none and --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base=none and --no-write-chain-file' '
     +	test_commit base-none &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only --base=none)" &&
    ++		--no-write-chain-file --base=none)" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
     +'
     +
    -+test_expect_success 'write MIDX layer with --base=<hash> and --checksum-only' '
    ++test_expect_success 'write MIDX layer with --base=<hash> and --no-write-chain-file' '
     +	test_commit base-hash &&
     +	git repack -d &&
     +
     +	cp "$midx_chain" "$midx_chain.bak" &&
     +	layer="$(git multi-pack-index write --bitmap --incremental \
    -+		--checksum-only --base="$(nth_line 1 "$midx_chain")")" &&
    ++		--no-write-chain-file --base="$(nth_line 1 "$midx_chain")")" &&
     +
     +	test_cmp "$midx_chain.bak" "$midx_chain" &&
     +	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
    @@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremen
      	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
     
      ## t/t5335-compact-multi-pack-index.sh ##
    -@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --checksum-only' '
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '
      
      		layer="$(git multi-pack-index compact --incremental \
    - 			--checksum-only \
    + 			--no-write-chain-file \
     +			--base="$(nth_line 1 "$midx_chain")" \
      			"$(nth_line 2 "$midx_chain")" \
      			"$(nth_line 3 "$midx_chain")")" &&
      
    -@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --checksum-only' '
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '
      	)
      '
      
 7:  2d377f53407 =  7:  92aba3d366f repack: track the ODB source via existing_packs
 8:  6af23849f1d =  8:  d3ac65c1f11 midx: expose `midx_layer_contains_pack()`
 9:  5c9fea0fc78 =  9:  1bd2f194c6f repack-midx: factor out `repack_prepare_midx_command()`
10:  cd5eba0f665 = 10:  44f522ea04d repack-midx: extract `repack_fill_midx_stdin_packs()`
11:  d2c667e8413 ! 11:  f5642a46bbd repack-geometry: prepare for incremental MIDX repacking
    @@ repack-geometry.c: void pack_geometry_init(struct pack_geometry *geometry,
     +			 * repacking. In either of those cases we want
     +			 * to ignore the pack.
     +			 */
    -+			if (m->num_packs > geometry->midx_layer_threshold &&
    -+			    midx_layer_contains_pack(m, pack_basename(p)))
    -+				;
    -+			else
    ++			if (m->num_packs < geometry->midx_layer_threshold ||
    ++			    !midx_layer_contains_pack(m, pack_basename(p)))
     +				continue;
     +		}
     +
    @@ repack-geometry.c: void pack_geometry_split(struct pack_geometry *geometry)
     +		/*
     +		 * During incremental MIDX/bitmap repacking, any packs
     +		 * included in the rollup are either (a) not MIDX'd, or
    -+		 * (b) contained in the tip layer iff it has more than
    ++		 * (b) contained in the tip layer iff it has at least
     +		 * the threshold number of packs.
     +		 *
     +		 * In the latter case, we can safely conclude that the
12:  56dc6d55974 = 12:  9fdcb253a96 builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
13:  e304e316ac4 = 13:  1e1b957bf12 packfile: ensure `close_pack_revindex()` frees in-memory revindex
14:  8d5aa3d6fa7 ! 14:  93e152fb6aa repack: implement incremental MIDX repacking
    @@ Commit message
     
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
    + ## builtin/repack.c ##
    +@@ builtin/repack.c: static const char incremental_bitmap_conflict_error[] = N_(
    + "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
    + );
    + 
    ++#define DEFAULT_MIDX_SPLIT_FACTOR 2
    ++#define DEFAULT_MIDX_NEW_LAYER_THRESHOLD 8
    ++
    + struct repack_config_ctx {
    + 	struct pack_objects_args *po_args;
    + 	struct pack_objects_args *cruft_po_args;
    +@@ builtin/repack.c: int cmd_repack(int argc,
    + 			.show_progress = show_progress,
    + 			.write_bitmaps = write_bitmaps > 0,
    + 			.midx_must_contain_cruft = midx_must_contain_cruft,
    ++			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
    ++			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
    + 			.mode = write_midx,
    + 		};
    + 
    +
      ## repack-midx.c ##
     @@
      #include "repack.h"
    @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opt
     +	}
     +
     +	repack_prepare_midx_command(&cmd, opts, "write");
    -+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only", NULL);
    ++	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);
     +	strvec_pushf(&cmd.args, "--base=%s", base ? base : "none");
     +
     +	if (preferred_pack) {
    @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opt
     +	int ret;
     +
     +	repack_prepare_midx_command(&cmd, opts, "compact");
    -+	strvec_pushl(&cmd.args, "--incremental", "--checksum-only",
    ++	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file",
     +		     midx_get_checksum_hex(step->u.compact.from),
     +		     midx_get_checksum_hex(step->u.compact.to), NULL);
     +
15:  e8e22d7958d ! 15:  6119f15d3e8 repack: introduce `--write-midx=incremental`
    @@ Documentation/config/repack.adoc: repack.midxMustContainCruft::
     ++
     +Adjacent layers are merged when the accumulated object count of the
     +newer layer exceeds `1/<N>` of the object count of the next deeper
    -+layer. Defaults to 2.
    ++layer. Must be at least 2. Defaults to 2.
     +
     +repack.midxNewLayerThreshold::
     +	The minimum number of packs in the tip MIDX layer before those
    @@ Documentation/config/repack.adoc: repack.midxMustContainCruft::
     ++
     +When the tip layer has fewer packs than this threshold, those packs are
     +excluded from the geometric repack entirely, and are thus left
    -+unmodified. Defaults to 8.
    ++unmodified. Must be at least 1. Defaults to 8.
     
      ## Documentation/git-repack.adoc ##
     @@ Documentation/git-repack.adoc: SYNOPSIS
    @@ builtin/repack.c: static int midx_must_contain_cruft = 1;
      };
      
     @@ builtin/repack.c: static const char incremental_bitmap_conflict_error[] = N_(
    - "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
    - );
    - 
    -+#define DEFAULT_MIDX_SPLIT_FACTOR 2
    -+#define DEFAULT_MIDX_NEW_LAYER_THRESHOLD 8
    -+
      struct repack_config_ctx {
      	struct pack_objects_args *po_args;
      	struct pack_objects_args *cruft_po_args;
    @@ builtin/repack.c: int cmd_repack(int argc,
      	if (write_bitmaps < 0) {
      		if (write_midx == REPACK_WRITE_MIDX_NONE &&
      		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
    +@@ builtin/repack.c: int cmd_repack(int argc,
    + 		write_bitmaps = 0;
    + 	}
    + 
    ++	if (config_ctx.midx_split_factor < 2)
    ++		die(_("invalid value for %s: %d"), "--midx-split-factor",
    ++		    config_ctx.midx_split_factor);
    ++	if (config_ctx.midx_new_layer_threshold < 1)
    ++		die(_("invalid value for %s: %d"), "--midx-new-layer-threshold",
    ++		    config_ctx.midx_new_layer_threshold);
    ++
    + 	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
    + 		struct strbuf path = STRBUF_INIT;
    + 
     @@ builtin/repack.c: int cmd_repack(int argc,
      	if (geometry.split_factor) {
      		if (pack_everything)
    @@ builtin/repack.c: int cmd_repack(int argc,
      		pack_geometry_init(&geometry, &existing, &po_args);
      		pack_geometry_split(&geometry);
      	}
    +@@ builtin/repack.c: int cmd_repack(int argc,
    + 				       packtmp);
    + 	/* End of pack replacement. */
    + 
    +-	if (delete_redundant && pack_everything & ALL_INTO_ONE)
    ++	if (delete_redundant && pack_everything & ALL_INTO_ONE) {
    ++		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL)
    ++			existing_packs_retain_midx_packs(&existing);
    + 		existing_packs_mark_for_deletion(&existing, &names);
    ++	}
    + 
    + 	if (write_midx != REPACK_WRITE_MIDX_NONE) {
    + 		struct repack_write_midx_opts opts = {
     @@ builtin/repack.c: int cmd_repack(int argc,
      			.show_progress = show_progress,
      			.write_bitmaps = write_bitmaps > 0,
      			.midx_must_contain_cruft = midx_must_contain_cruft,
    +-			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
    +-			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
     +			.midx_split_factor = config_ctx.midx_split_factor,
     +			.midx_new_layer_threshold = config_ctx.midx_new_layer_threshold,
      			.mode = write_midx,
    @@ repack.c: void pack_objects_args_release(struct pack_objects_args *args)
      	unlink_pack_path(buf.buf, 1);
      	strbuf_release(&buf);
     @@ repack.c: void existing_packs_mark_for_deletion(struct existing_packs *existing,
    + 					   &existing->cruft_packs);
    + }
      
    ++/*
    ++ * Mark every pack that is referenced by the existing MIDX chain as
    ++ * retained, so that a subsequent call to
    ++ * existing_packs_mark_for_deletion() will not mark them for deletion.
    ++ *
    ++ * This is used when writing an incremental MIDX layer on top of an
    ++ * existing chain: retained layers continue to reference the same
    ++ * packs on disk, so those packs must not be unlinked even if the
    ++ * freshly-written pack supersedes them.
    ++ */
    ++void existing_packs_retain_midx_packs(struct existing_packs *existing)
    ++{
    ++	struct string_list_item *item;
    ++	struct strbuf buf = STRBUF_INIT;
    ++
    ++	for_each_string_list_item(item, &existing->midx_packs) {
    ++		struct string_list_item *found;
    ++
    ++		strbuf_reset(&buf);
    ++		strbuf_addstr(&buf, item->string);
    ++		strbuf_strip_suffix(&buf, ".pack");
    ++		strbuf_strip_suffix(&buf, ".idx");
    ++
    ++		found = string_list_lookup(&existing->non_kept_packs, buf.buf);
    ++		if (found)
    ++			existing_packs_mark_retained(found);
    ++
    ++		found = string_list_lookup(&existing->cruft_packs, buf.buf);
    ++		if (found)
    ++			existing_packs_mark_retained(found);
    ++	}
    ++
    ++	strbuf_release(&buf);
    ++}
    ++
      static void remove_redundant_packs_1(struct repository *repo,
      				     struct string_list *packs,
     -				     const char *packdir)
    @@ repack.h: void prepare_pack_objects(struct child_process *cmd,
      struct write_pack_opts {
      	struct pack_objects_args *po_args;
     @@ repack.h: void existing_packs_retain_cruft(struct existing_packs *existing,
    + 				 struct packed_git *cruft);
      void existing_packs_mark_for_deletion(struct existing_packs *existing,
      				      struct string_list *names);
    ++void existing_packs_retain_midx_packs(struct existing_packs *existing);
      void existing_packs_remove_redundant(struct existing_packs *existing,
     -				     const char *packdir);
     +				     const char *packdir,
    @@ t/t7705-repack-incremental-midx.sh (new)
     +	)
     +'
     +
    ++test_expect_success 'repack -ad --write-midx=incremental is safe' '
    ++	git init ad-incremental-midx &&
    ++	(
    ++		cd ad-incremental-midx &&
    ++
    ++		git config maintenance.auto false &&
    ++
    ++		# Build a MIDX chain with multiple layers referencing
    ++		# distinct packs.
    ++		test_commit first &&
    ++		git repack -d &&
    ++
    ++		test_commit second &&
    ++		git repack -d --write-midx=incremental &&
    ++
    ++		git multi-pack-index verify &&
    ++		test_line_count = 1 $midx_chain &&
    ++
    ++		# Now do a full -ad repack. The new pack contains all
    ++		# objects, but any retained MIDX layers still reference
    ++		# the now-deleted packs.
    ++		test_commit third &&
    ++		git repack -ad --write-midx=incremental &&
    ++
    ++		git multi-pack-index verify &&
    ++		git fsck &&
    ++		git rev-list --all --objects >/dev/null
    ++	)
    ++'
    ++
    ++test_expect_success 'repack rejects invalid midxSplitFactor' '
    ++	test_when_finished "rm -fr bad-split-factor" &&
    ++	git init bad-split-factor &&
    ++	(
    ++		cd bad-split-factor &&
    ++		test_commit base &&
    ++
    ++		for v in 0 1 -1
    ++		do
    ++			test_must_fail git -c repack.midxSplitFactor=$v \
    ++				repack -d --geometric=2 --write-midx=incremental 2>err &&
    ++			test_grep "invalid value for --midx-split-factor" err ||
    ++			return 1
    ++		done
    ++	)
    ++'
    ++
    ++test_expect_success 'repack rejects invalid midxNewLayerThreshold' '
    ++	test_when_finished "rm -fr bad-layer-threshold" &&
    ++	git init bad-layer-threshold &&
    ++	(
    ++		cd bad-layer-threshold &&
    ++		test_commit base &&
    ++
    ++		for v in 0 -1
    ++		do
    ++			test_must_fail git -c repack.midxNewLayerThreshold=$v \
    ++				repack -d --geometric=2 --write-midx=incremental 2>err &&
    ++			test_grep "invalid value for --midx-new-layer-threshold" err ||
    ++			return 1
    ++		done
    ++	)
    ++'
    ++
     +test_done
16:  79d5a12a390 = 16:  d9acef1334a repack: allow `--write-midx=incremental` without `--geometric`

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 01/16] midx-write: handle noop writes when converting incremental chains

[Taylor Blau]

When updating a MIDX, we optimize out writes that will result in an
identical MIDX as the one we already have on disk. See b3bab9d2729
(midx-write: extract function to test whether MIDX needs updating,
2025-12-10) for more details on exactly which writes are optimized out.

If `midx_needs_update()` can't rule out any of the obvious cases (e.g.,
the checksum is invalid, we're requesting a different version, or
performing compaction which always requires an update), then we compare
the packs we're writing to the packs we already know about. If there are
an equal number of packs being written as there are in any existing
MIDX layer(s), then we compare the packs by their name.

This comparison fails when we have an incremental MIDX chain with
at least two layers, since we do not recursively peel through earlier
layers, instead treating the `->pack_names` array of the tip MIDX layer
as containing all `m->num_packs + m->num_packs_in_base` packs.

Adjust this to instead look through the MIDX layers one by one when
comparing pack names. While we're at it, fix a typo above in the same
function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c                            | 18 ++++++++++--------
 t/t5334-incremental-multi-pack-index.sh | 16 ++++++++++++++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index a25cab75aba..9328f65a201 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1152,7 +1152,7 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 
 	/*
 	 * Ensure that we have a valid checksum before consulting the
-	 * exisiting MIDX in order to determine if we can avoid an
+	 * existing MIDX in order to determine if we can avoid an
 	 * update.
 	 *
 	 * This is necessary because the given MIDX is loaded directly
@@ -1208,14 +1208,16 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 			BUG("same pack added twice?");
 	}
 
-	for (uint32_t i = 0; i < ctx->nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addstr(&buf, midx->pack_names[i]);
-		strbuf_strip_suffix(&buf, ".idx");
+	for (struct multi_pack_index *m = midx; m; m = m->base_midx) {
+		for (uint32_t i = 0; i < m->num_packs; i++) {
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, m->pack_names[i]);
+			strbuf_strip_suffix(&buf, ".idx");
 
-		if (!strset_contains(&packs, buf.buf))
-			goto out;
-		strset_remove(&packs, buf.buf);
+			if (!strset_contains(&packs, buf.buf))
+				goto out;
+			strset_remove(&packs, buf.buf);
+		}
 	}
 
 	needed = false;
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index 99c7d44d8e9..c9f5b4e87aa 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -132,4 +132,20 @@ test_expect_success 'relink existing MIDX layer' '
 
 '
 
+test_expect_success 'non-incremental write with existing incremental chain' '
+	git init non-incremental-write-with-existing &&
+	test_when_finished "rm -fr non-incremental-write-with-existing" &&
+
+	(
+		cd non-incremental-write-with-existing &&
+
+		git config set maintenance.auto false &&
+
+		write_midx_layer &&
+		write_midx_layer &&
+
+		git multi-pack-index write
+	)
+'
+
 test_done
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 02/16] midx: use `strset` for retained MIDX files

[Taylor Blau]

Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
build a list of filenames to keep while pruning stale MIDX files. Today
they hand-roll an array instead of using a `strset`, thus requiring us
to pass an additional length parameter, and makes lookups linear.

Replace the bare array with a `strset` which can be passed around as a
single parameter. Though it improves lookup performance, the difference
is likely immeasurable given how small the keep_hashes array typically
is.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 57 +++++++++++++++++++++++++++------------------------------
 1 file changed, 27 insertions(+), 30 deletions(-)

diff --git a/midx.c b/midx.c
index 81d6ab11e6e..f75e3c9fa6d 100644
--- a/midx.c
+++ b/midx.c
@@ -758,8 +758,7 @@ int midx_checksum_valid(struct multi_pack_index *m)
 }
 
 struct clear_midx_data {
-	char **keep;
-	uint32_t keep_nr;
+	struct strset keep;
 	const char *ext;
 };
 
@@ -767,15 +766,12 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 				const char *file_name, void *_data)
 {
 	struct clear_midx_data *data = _data;
-	uint32_t i;
 
 	if (!(starts_with(file_name, "multi-pack-index-") &&
 	      ends_with(file_name, data->ext)))
 		return;
-	for (i = 0; i < data->keep_nr; i++) {
-		if (!strcmp(data->keep[i], file_name))
-			return;
-	}
+	if (strset_contains(&data->keep, file_name))
+		return;
 	if (unlink(full_path))
 		die_errno(_("failed to remove %s"), full_path);
 }
@@ -783,48 +779,49 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash)
 {
-	struct clear_midx_data data;
-	memset(&data, 0, sizeof(struct clear_midx_data));
+	struct clear_midx_data data = {
+		.keep = STRSET_INIT,
+		.ext = ext,
+	};
 
 	if (keep_hash) {
-		ALLOC_ARRAY(data.keep, 1);
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hash, ext);
 
-		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
-		data.keep_nr = 1;
+		strset_add(&data.keep, buf.buf);
+
+		strbuf_release(&buf);
 	}
-	data.ext = ext;
 
-	for_each_file_in_pack_dir(source->path,
-				  clear_midx_file_ext,
-				  &data);
+	for_each_file_in_pack_dir(source->path, clear_midx_file_ext, &data);
 
-	if (keep_hash)
-		free(data.keep[0]);
-	free(data.keep);
+	strset_clear(&data.keep);
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
 				      char **keep_hashes,
 				      uint32_t hashes_nr)
 {
-	struct clear_midx_data data;
+	struct clear_midx_data data = {
+		.keep = STRSET_INIT,
+		.ext = ext,
+	};
+	struct strbuf buf = STRBUF_INIT;
 	uint32_t i;
 
-	memset(&data, 0, sizeof(struct clear_midx_data));
+	for (i = 0; i < hashes_nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
+			    ext);
 
-	ALLOC_ARRAY(data.keep, hashes_nr);
-	for (i = 0; i < hashes_nr; i++)
-		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
-				       ext);
-	data.keep_nr = hashes_nr;
-	data.ext = ext;
+		strset_add(&data.keep, buf.buf);
+	}
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
 				     clear_midx_file_ext, &data);
 
-	for (i = 0; i < hashes_nr; i++)
-		free(data.keep[i]);
-	free(data.keep);
+	strbuf_release(&buf);
+	strset_clear(&data.keep);
 }
 
 void clear_midx_file(struct repository *r)
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 03/16] midx: build `keep_hashes` array in order

[Taylor Blau]

Instead of filling the keep_hashes array using reverse indexing (e.g.,
`keep_hashes[count - i - 1]`) while traversing linked lists forward,
collect linked list nodes into a temporary `layers` array and then
iterate it backwards to fill `keep_hashes` sequentially.

This makes the filling logic easier to follow, since each segment of the
array is filled with a simple forward-marching index. Moreover, this
change prepares us for a subsequent commit that will switch to using a
`strvec`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 66 ++++++++++++++++++++++++++++------------------------
 1 file changed, 36 insertions(+), 30 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 9328f65a201..55c778a97cb 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1731,6 +1731,9 @@ static int write_midx_internal(struct write_midx_opts *opts)
 		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
+		struct multi_pack_index **layers = NULL;
+		size_t layers_nr = 0, layers_alloc = 0;
+		size_t j = 0;
 
 		if (!chainf) {
 			error_errno(_("unable to open multi-pack-index chain file"));
@@ -1751,46 +1754,49 @@ static int write_midx_internal(struct write_midx_opts *opts)
 		strbuf_release(&final_midx_name);
 
 		if (ctx.compact) {
-			struct multi_pack_index *m;
-			uint32_t num_layers_before_from = 0;
-			uint32_t i;
+			struct multi_pack_index *mp;
 
-			for (m = ctx.base_midx; m; m = m->base_midx)
-				num_layers_before_from++;
-
-			m = ctx.base_midx;
-			for (i = 0; i < num_layers_before_from; i++) {
-				uint32_t j = num_layers_before_from - i - 1;
-
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
-				m = m->base_midx;
+			for (mp = ctx.base_midx; mp; mp = mp->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = mp;
 			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
-								   r->hash_algo));
+			keep_hashes[j++] =
+				xstrdup(hash_to_hex_algop(midx_hash,
+							  r->hash_algo));
 
-			i = 0;
-			for (m = ctx.m;
-			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
-			     m = m->base_midx) {
-				keep_hashes[keep_hashes_nr - i - 1] =
-					xstrdup(midx_get_checksum_hex(m));
-				i++;
+			for (mp = ctx.m;
+			     mp && midx_hashcmp(mp, ctx.compact_to,
+						r->hash_algo);
+			     mp = mp->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = mp;
 			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
 		} else {
-			keep_hashes[ctx.num_multi_pack_indexes_before] =
+			for (; m; m = m->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = m;
+			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+
+			keep_hashes[j++] =
 				xstrdup(hash_to_hex_algop(midx_hash,
 							  r->hash_algo));
-
-			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
-				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
-
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
-				m = m->base_midx;
-			}
 		}
 
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
+		ASSERT(j == keep_hashes_nr);
+
+		free(layers);
+
+		for (uint32_t i = 0; i < j; i++)
 			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
 	} else {
 		keep_hashes[ctx.num_multi_pack_indexes_before] =
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

The `keep_hashes` array in `write_midx_internal()` accumulates the
checksums of MIDX files that should be retained when pruning stale
entries from the MIDX chain. For similar reasons as in a previous
commit, rewrite this using a strvec, requiring us to pass one fewer
parameter.

Unlike the aforementioned previous commit, use a `strvec` instead of a
`string_list`, which provides a more ergonomic interface to adjust the
values at a particular index. The ordering is important here, as this
value is used to determine the contents of the resulting
`multi-pack-index-chain` file when writing with "--incremental".

Since the previous commit already builds the array in forward order, the
conversion is straightforward: replace indexed assignments with
`strvec_push()`, drop the pre-counting and `CALLOC_ARRAY()`, and
simplify cleanup via `strvec_clear()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 84 ++++++++++++++++++----------------------------------
 midx.c       | 20 ++++++-------
 2 files changed, 38 insertions(+), 66 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 55c778a97cb..5d9409a9741 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -29,8 +29,7 @@ extern void clear_midx_files_ext(struct odb_source *source, const char *ext,
 				 const char *keep_hash);
 extern void clear_incremental_midx_files_ext(struct odb_source *source,
 					     const char *ext,
-					     const char **keep_hashes,
-					     uint32_t hashes_nr);
+					     const struct strvec *keep_hashes);
 extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 				const char *idx_name);
 
@@ -1109,8 +1108,7 @@ static int link_midx_to_chain(struct multi_pack_index *m)
 }
 
 static void clear_midx_files(struct odb_source *source,
-			     const char **hashes, uint32_t hashes_nr,
-			     unsigned incremental)
+			     const struct strvec *hashes, unsigned incremental)
 {
 	/*
 	 * if incremental:
@@ -1124,13 +1122,15 @@ static void clear_midx_files(struct odb_source *source,
 	 */
 	struct strbuf buf = STRBUF_INIT;
 	const char *exts[] = { MIDX_EXT_BITMAP, MIDX_EXT_REV, MIDX_EXT_MIDX };
-	uint32_t i, j;
+	uint32_t i;
 
 	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		clear_incremental_midx_files_ext(source, exts[i],
-						 hashes, hashes_nr);
-		for (j = 0; j < hashes_nr; j++)
-			clear_midx_files_ext(source, exts[i], hashes[j]);
+		clear_incremental_midx_files_ext(source, exts[i], hashes);
+		if (hashes) {
+			for (size_t j = 0; j < hashes->nr; j++)
+				clear_midx_files_ext(source, exts[i],
+						     hashes->v[j]);
+		}
 	}
 
 	if (incremental)
@@ -1267,8 +1267,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = -1;
-	const char **keep_hashes = NULL;
-	size_t keep_hashes_nr = 0;
+	struct strvec keep_hashes = STRVEC_INIT;
 	struct chunkfile *cf;
 
 	trace2_region_enter("midx", "write_midx_internal", r);
@@ -1708,32 +1707,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
 		die(_("too many multi-pack-indexes"));
 
-	if (ctx.compact) {
-		struct multi_pack_index *m;
-
-		/*
-		 * Keep all MIDX layers excluding those in the range [from, to].
-		 */
-		for (m = ctx.base_midx; m; m = m->base_midx)
-			keep_hashes_nr++;
-		for (m = ctx.m;
-		     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
-		     m = m->base_midx)
-			keep_hashes_nr++;
-
-		keep_hashes_nr++; /* include the compacted layer */
-	} else {
-		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
-	}
-	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
-
 	if (ctx.incremental) {
 		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
 		struct multi_pack_index **layers = NULL;
 		size_t layers_nr = 0, layers_alloc = 0;
-		size_t j = 0;
 
 		if (!chainf) {
 			error_errno(_("unable to open multi-pack-index chain file"));
@@ -1761,12 +1740,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 				layers[layers_nr++] = mp;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[j++] =
-				xstrdup(hash_to_hex_algop(midx_hash,
-							  r->hash_algo));
+			strvec_push(&keep_hashes,
+				    hash_to_hex_algop(midx_hash,
+						     r->hash_algo));
 
 			for (mp = ctx.m;
 			     mp && midx_hashcmp(mp, ctx.compact_to,
@@ -1776,31 +1755,29 @@ static int write_midx_internal(struct write_midx_opts *opts)
 				layers[layers_nr++] = mp;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 		} else {
 			for (; m; m = m->base_midx) {
 				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
 				layers[layers_nr++] = m;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[j++] =
-				xstrdup(hash_to_hex_algop(midx_hash,
-							  r->hash_algo));
+			strvec_push(&keep_hashes,
+				    hash_to_hex_algop(midx_hash,
+						     r->hash_algo));
 		}
 
-		ASSERT(j == keep_hashes_nr);
-
 		free(layers);
 
-		for (uint32_t i = 0; i < j; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
+		for (size_t i = 0; i < keep_hashes.nr; i++)
+			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
 	} else {
-		keep_hashes[ctx.num_multi_pack_indexes_before] =
-			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
+		strvec_push(&keep_hashes,
+			    hash_to_hex_algop(midx_hash, r->hash_algo));
 	}
 
 	if (ctx.m || ctx.base_midx)
@@ -1809,8 +1786,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (commit_lock_file(&lk) < 0)
 		die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, keep_hashes, keep_hashes_nr,
-			 ctx.incremental);
+	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
 	result = 0;
 
 cleanup:
@@ -1826,11 +1802,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	free(ctx.entries);
 	free(ctx.pack_perm);
 	free(ctx.pack_order);
-	if (keep_hashes) {
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
-			free((char *)keep_hashes[i]);
-		free(keep_hashes);
-	}
+	strvec_clear(&keep_hashes);
 	strbuf_release(&midx_name);
 	close_midx(midx_to_free);
 
diff --git a/midx.c b/midx.c
index f75e3c9fa6d..bcb8c999015 100644
--- a/midx.c
+++ b/midx.c
@@ -12,6 +12,7 @@
 #include "chunk-format.h"
 #include "pack-bitmap.h"
 #include "pack-revindex.h"
+#include "strvec.h"
 
 #define MIDX_PACK_ERROR ((void *)(intptr_t)-1)
 
@@ -19,8 +20,7 @@ int midx_checksum_valid(struct multi_pack_index *m);
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash);
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr);
+				      const struct strvec *keep_hashes);
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
@@ -799,22 +799,22 @@ void clear_midx_files_ext(struct odb_source *source, const char *ext,
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr)
+				      const struct strvec *keep_hashes)
 {
 	struct clear_midx_data data = {
 		.keep = STRSET_INIT,
 		.ext = ext,
 	};
 	struct strbuf buf = STRBUF_INIT;
-	uint32_t i;
 
-	for (i = 0; i < hashes_nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
-			    ext);
+	if (keep_hashes) {
+		for (size_t i = 0; i < keep_hashes->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "multi-pack-index-%s.%s",
+				    keep_hashes->v[i], ext);
 
-		strset_add(&data.keep, buf.buf);
+			strset_add(&data.keep, buf.buf);
+		}
 	}
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 05/16] midx: introduce `--no-write-chain-file` for incremental MIDX writes

[Taylor Blau]

When writing an incremental MIDX layer, the MIDX machinery writes the
new layer into the multi-pack-index.d directory and then updates the
multi-pack-index-chain file to include the freshly written layer.

Future callers however may not wish to immediately update the MIDX chain
itself, preferring instead to write out new layer(s) themselves before
atomically updating the chain. Concretely, the new incremental
MIDX-based repacking strategy will want to do exactly this (that is,
assemble the new MIDX chain itself before writing a new chain file and
atomically linking it into place).

Introduce a `--no-write-chain-file` flag that:

 * writes the new MIDX layer into the multi-pack-index.d directory

 * prints its checksum

 * does not update the multi-pack-index-chain file.

The MIDX chain file (and thus, the lock protecting it) remain untouched,
allowing callers to assemble the chain themselves. This flag requires
`--incremental`, since the notion of a separate layer only makes sense
for incremental MIDXs.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc | 17 ++++++++--
 builtin/multi-pack-index.c              | 28 +++++++++++++++--
 midx-write.c                            | 42 ++++++++++++++++---------
 midx.h                                  |  1 +
 t/t5334-incremental-multi-pack-index.sh | 17 ++++++++++
 t/t5335-compact-multi-pack-index.sh     | 36 +++++++++++++++++++++
 6 files changed, 123 insertions(+), 18 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index 3a5aa227784..c26196815e2 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -11,9 +11,9 @@ SYNOPSIS
 [verse]
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
-		         [--refs-snapshot=<path>]
+		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] <from> <to>
+		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
@@ -83,6 +83,13 @@ marker).
 		and packs not present in an existing MIDX layer.
 		Migrates non-incremental MIDXs to incremental ones when
 		necessary.
+
+	--[no-]write-chain-file::
+		When used with `--incremental`, write a new MIDX layer
+		but do not update the multi-pack-index-chain file.
+		The checksum of the new layer is printed to standard
+		output, allowing the caller to assemble and write the
+		chain itself. Requires `--incremental`.
 --
 
 compact::
@@ -97,6 +104,12 @@ compact::
 
 	--[no-]bitmap::
 		Control whether or not a multi-pack bitmap is written.
+
+	--[no-]write-chain-file::
+		When used with `--incremental`, write a new compacted
+		MIDX layer but do not update the multi-pack-index-chain
+		file. The checksum of the new layer is printed to
+		standard output. Requires `--incremental`.
 --
 +
 Note that the compact command requires writing a version-2 midx that
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 0f72d96c02d..f861b4b8394 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,11 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>]")
+	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] <from> <to>")
+	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -153,6 +153,9 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
+			N_("write the multi-pack-index chain file"),
+			MIDX_WRITE_NO_CHAIN),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			 N_("write multi-pack index containing only given indexes")),
 		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
@@ -178,6 +181,15 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 	if (argc)
 		usage_with_options(builtin_multi_pack_index_write_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--no-write-chain-file", "--incremental");
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -221,6 +233,9 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
+			N_("write the multi-pack-index chain file"),
+			MIDX_WRITE_NO_CHAIN),
 		OPT_END(),
 	};
 
@@ -239,6 +254,15 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 	if (argc != 2)
 		usage_with_options(builtin_multi_pack_index_compact_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--no-write-chain-file", "--incremental");
+		usage_with_options(builtin_multi_pack_index_compact_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(the_repository);
 
 	FREE_AND_NULL(options);
diff --git a/midx-write.c b/midx-write.c
index 5d9409a9741..38c898e5ff5 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1257,7 +1257,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	unsigned char midx_hash[GIT_MAX_RAWSZ];
 	uint32_t start_pack;
 	struct hashfile *f = NULL;
-	struct lock_file lk;
+	struct lock_file lk = LOCK_INIT;
 	struct tempfile *incr;
 	struct write_midx_context ctx = {
 		.preferred_pack_idx = NO_PREFERRED_PACK,
@@ -1601,11 +1601,14 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	}
 
 	if (ctx.incremental) {
-		struct strbuf lock_name = STRBUF_INIT;
+		if (!(opts->flags & MIDX_WRITE_NO_CHAIN)) {
+			struct strbuf lock_name = STRBUF_INIT;
 
-		get_midx_chain_filename(opts->source, &lock_name);
-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
-		strbuf_release(&lock_name);
+			get_midx_chain_filename(opts->source, &lock_name);
+			hold_lock_file_for_update(&lk, lock_name.buf,
+						  LOCK_DIE_ON_ERROR);
+			strbuf_release(&lock_name);
+		}
 
 		incr = mks_tempfile_m(midx_name.buf, 0444);
 		if (!incr) {
@@ -1707,16 +1710,23 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
 		die(_("too many multi-pack-indexes"));
 
+	if (!is_lock_file_locked(&lk))
+		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));
+	else if (opts->flags & MIDX_WRITE_NO_CHAIN)
+		BUG("lockfile held with MIDX_WRITE_NO_CHAIN set?");
+
 	if (ctx.incremental) {
-		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
 		struct multi_pack_index **layers = NULL;
 		size_t layers_nr = 0, layers_alloc = 0;
 
-		if (!chainf) {
-			error_errno(_("unable to open multi-pack-index chain file"));
-			goto cleanup;
+		if (is_lock_file_locked(&lk)){
+			FILE *chainf = fdopen_lock_file(&lk, "w");
+			if (!chainf) {
+				error_errno(_("unable to open multi-pack-index chain file"));
+				goto cleanup;
+			}
 		}
 
 		if (link_midx_to_chain(ctx.base_midx) < 0)
@@ -1773,8 +1783,10 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 		free(layers);
 
-		for (size_t i = 0; i < keep_hashes.nr; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
+		if (is_lock_file_locked(&lk))
+			for (size_t i = 0; i < keep_hashes.nr; i++)
+				fprintf(get_lock_file_fp(&lk), "%s\n",
+					keep_hashes.v[i]);
 	} else {
 		strvec_push(&keep_hashes,
 			    hash_to_hex_algop(midx_hash, r->hash_algo));
@@ -1783,10 +1795,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.m || ctx.base_midx)
 		odb_close(ctx.repo->objects);
 
-	if (commit_lock_file(&lk) < 0)
-		die_errno(_("could not write multi-pack-index"));
+	if (is_lock_file_locked(&lk)) {
+		if (commit_lock_file(&lk) < 0)
+			die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+		clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+	}
 	result = 0;
 
 cleanup:
diff --git a/midx.h b/midx.h
index 08f3728e520..5b193882dcf 100644
--- a/midx.h
+++ b/midx.h
@@ -83,6 +83,7 @@ struct multi_pack_index {
 #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
 #define MIDX_WRITE_INCREMENTAL (1 << 5)
 #define MIDX_WRITE_COMPACT (1 << 6)
+#define MIDX_WRITE_NO_CHAIN (1 << 7)
 
 #define MIDX_EXT_REV "rev"
 #define MIDX_EXT_BITMAP "bitmap"
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index c9f5b4e87aa..66d6894761b 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -96,6 +96,23 @@ test_expect_success 'show object from second pack' '
 	git cat-file -p 2.2
 '
 
+test_expect_success 'write MIDX layer with --no-write-chain-file' '
+	test_commit no-write-chain-file &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file' '
+	test_must_fail git multi-pack-index write --bitmap --no-write-chain-file 2>err &&
+	test_grep "cannot use --no-write-chain-file without --incremental" err
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 40f3844282f..1a65d48b62b 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -290,4 +290,40 @@ test_expect_success 'MIDX compaction with bitmaps (non-trivial)' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --no-write-chain-file' '
+	git init midx-compact-with--no-write-chain-file &&
+	(
+		cd midx-compact-with--no-write-chain-file &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		layer="$(git multi-pack-index compact --incremental \
+			--no-write-chain-file \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")")" &&
+
+		test_cmp "$midx_chain.bak" "$midx_chain" &&
+
+		# After writing the new layer, insert it into the chain
+		# manually. This is done in order to make $layer visible
+		# to the read-midx test helper below, and matches what
+		# the MIDX command would do without --no-write-chain-file.
+		{
+			nth_line 1 "$midx_chain.bak" &&
+			echo $layer &&
+			nth_line 4 "$midx_chain.bak"
+		} >$midx_chain &&
+
+		test-tool read-midx $objdir $layer >midx.data &&
+		grep "^pack-B-.*\.idx" midx.data &&
+		grep "^pack-C-.*\.idx" midx.data
+
+	)
+'
+
 test_done
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 06/16] midx: support custom `--base` for incremental MIDX writes

[Taylor Blau]

Both `compact` and `write --incremental` fix the base of the resulting
MIDX layer: `compact` always places the compacted result on top of
"from's" immediate parent in the chain, and `write --incremental` always
appends a new layer to the existing tip. In both cases the base is not
configurable.

Future callers need additional flexibility. For instance, the incremental
MIDX-based repacking code may wish to write a layer based on some
intermediate ancestor rather than the current tip, or produce a root
layer when replacing the bottommost entries in the chain.

Introduce a new `--base` option for both subcommands to specify the
checksum of the MIDX layer to use as the base. The given checksum must
refer to a valid layer in the MIDX chain that is an ancestor of the
topmost layer being written or compacted.

The special value "none" is accepted to produce a root layer with no
parent. This will be needed when the incremental repacking machinery
determines that the bottommost layers of the chain should be replaced.

If no `--base` is given, behavior is unchanged: `compact` uses "from's"
immediate parent in the chain, and `write` appends to the existing tip.

For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
`write --incremental` appends a new layer to the live chain tip with no
mechanism to atomically replace it; overriding the base would produce a
layer that does not extend the tip, breaking chain invariants. With
`--no-write-chain-file` the chain is left unmodified and the caller is
responsible for assembling a valid chain.

For `compact`, no such restriction applies. The compaction operation
atomically replaces the compacted range in the chain file, so writing
the result on top of any valid ancestor preserves chain invariants.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc | 17 +++++-
 builtin/multi-pack-index.c              | 24 ++++++--
 midx-write.c                            | 34 ++++++++++-
 midx.h                                  |  5 +-
 t/t5334-incremental-multi-pack-index.sh | 30 ++++++++++
 t/t5335-compact-multi-pack-index.sh     | 77 +++++++++++++++++++++++++
 6 files changed, 178 insertions(+), 9 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index c26196815e2..c6d23aeeb9a 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -12,8 +12,10 @@ SYNOPSIS
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
 		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
+			 [--base=<checksum>]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
+		         [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]
+			 <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
@@ -90,6 +92,13 @@ marker).
 		The checksum of the new layer is printed to standard
 		output, allowing the caller to assemble and write the
 		chain itself. Requires `--incremental`.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base when writing a new incremental layer.
+		The special value `none` indicates that the new layer
+		should have no base (i.e., it becomes a root layer).
+		Requires `--no-write-chain-file`.
 --
 
 compact::
@@ -110,6 +119,12 @@ compact::
 		MIDX layer but do not update the multi-pack-index-chain
 		file. The checksum of the new layer is printed to
 		standard output. Requires `--incremental`.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base for the compacted result, instead of using
+		the immediate parent of `<from>`. The special value
+		`none` indicates that the result should have no base.
 --
 +
 Note that the compact command requires writing a version-2 midx that
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index f861b4b8394..00ffb36394d 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,13 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
+	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]\n" \
+	   "  [--base=<checksum>]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
+	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]\n" \
+	   "  <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -63,6 +65,7 @@ static char const * const builtin_multi_pack_index_usage[] = {
 static struct opts_multi_pack_index {
 	char *object_dir;
 	const char *preferred_pack;
+	const char *incremental_base;
 	char *refs_snapshot;
 	unsigned long batch_size;
 	unsigned flags;
@@ -151,6 +154,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			   N_("pack for reuse when computing a multi-pack bitmap")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
 		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
@@ -190,6 +195,13 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 				   options);
 	}
 
+	if (opts.incremental_base &&
+	    !(opts.flags & MIDX_WRITE_NO_CHAIN)) {
+		error(_("cannot use --base without --no-write-chain-file"));
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -201,7 +213,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 
 		ret = write_midx_file_only(source, &packs,
 					   opts.preferred_pack,
-					   opts.refs_snapshot, opts.flags);
+					   opts.refs_snapshot,
+					   opts.incremental_base, opts.flags);
 
 		string_list_clear(&packs, 0);
 		free(opts.refs_snapshot);
@@ -229,6 +242,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 
 	struct option *options;
 	static struct option builtin_multi_pack_index_compact_options[] = {
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
@@ -290,7 +305,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			die(_("MIDX %s must be an ancestor of %s"), argv[0], argv[1]);
 	}
 
-	ret = write_midx_file_compact(source, from_midx, to_midx, opts.flags);
+	ret = write_midx_file_compact(source, from_midx, to_midx,
+				      opts.incremental_base, opts.flags);
 
 	return ret;
 }
diff --git a/midx-write.c b/midx-write.c
index 38c898e5ff5..561e9eedc0e 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1247,6 +1247,7 @@ struct write_midx_opts {
 
 	const char *preferred_pack_name;
 	const char *refs_snapshot;
+	const char *incremental_base;
 	unsigned flags;
 };
 
@@ -1330,11 +1331,32 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 	/*
 	 * If compacting MIDX layer(s) in the range [from, to], then the
-	 * compacted MIDX will share the same base MIDX as 'from'.
+	 * compacted MIDX will share the same base MIDX as 'from',
+	 * unless a custom --base is specified (see below).
 	 */
 	if (ctx.compact)
 		ctx.base_midx = ctx.compact_from->base_midx;
 
+	if (opts->incremental_base) {
+		if (!strcmp(opts->incremental_base, "none")) {
+			ctx.base_midx = NULL;
+		} else {
+			while (ctx.base_midx) {
+				const char *cmp = midx_get_checksum_hex(ctx.base_midx);
+				if (!strcmp(opts->incremental_base, cmp))
+					break;
+
+				ctx.base_midx = ctx.base_midx->base_midx;
+			}
+
+			if (!ctx.base_midx) {
+				error(_("could not find base MIDX '%s'"),
+				      opts->incremental_base);
+				goto cleanup;
+			}
+		}
+	}
+
 	ctx.nr = 0;
 	ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
 	ctx.info = NULL;
@@ -1827,7 +1849,8 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 int write_midx_file(struct odb_source *source,
 		    const char *preferred_pack_name,
-		    const char *refs_snapshot, unsigned flags)
+		    const char *refs_snapshot,
+		    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
@@ -1842,13 +1865,16 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags)
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.packs_to_include = packs_to_include,
 		.preferred_pack_name = preferred_pack_name,
 		.refs_snapshot = refs_snapshot,
+		.incremental_base = incremental_base,
 		.flags = flags,
 	};
 
@@ -1858,12 +1884,14 @@ int write_midx_file_only(struct odb_source *source,
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.compact_from = from,
 		.compact_to = to,
+		.incremental_base = incremental_base,
 		.flags = flags | MIDX_WRITE_COMPACT,
 	};
 
diff --git a/midx.h b/midx.h
index 5b193882dcf..77dd66de02b 100644
--- a/midx.h
+++ b/midx.h
@@ -132,10 +132,13 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags);
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags);
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct odb_source *source, unsigned flags);
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index 66d6894761b..68a103d13d2 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -113,6 +113,36 @@ test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file
 	test_grep "cannot use --no-write-chain-file without --incremental" err
 '
 
+test_expect_success 'write MIDX layer with --base without --no-write-chain-file' '
+	test_must_fail git multi-pack-index write --bitmap --incremental \
+		--base=none 2>err &&
+	test_grep "cannot use --base without --no-write-chain-file" err
+'
+
+test_expect_success 'write MIDX layer with --base=none and --no-write-chain-file' '
+	test_commit base-none &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file --base=none)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write MIDX layer with --base=<hash> and --no-write-chain-file' '
+	test_commit base-hash &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file --base="$(nth_line 1 "$midx_chain")")" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 1a65d48b62b..ec1dafe89fc 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -304,6 +304,7 @@ test_expect_success 'MIDX compaction with --no-write-chain-file' '
 
 		layer="$(git multi-pack-index compact --incremental \
 			--no-write-chain-file \
+			--base="$(nth_line 1 "$midx_chain")" \
 			"$(nth_line 2 "$midx_chain")" \
 			"$(nth_line 3 "$midx_chain")")" &&
 
@@ -326,4 +327,80 @@ test_expect_success 'MIDX compaction with --no-write-chain-file' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --base' '
+	git init midx-compact-with--base &&
+	(
+		cd midx-compact-with--base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 "$midx_chain" &&
+
+		cp "$midx_chain" "$midx_chain.bak" &&
+
+		git multi-pack-index compact --incremental \
+			--base="$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" \
+			"$(nth_line 4 "$midx_chain")" &&
+		test_line_count = 2 $midx_chain &&
+
+		nth_line 1 "$midx_chain.bak" >expect &&
+		nth_line 1 "$midx_chain" >actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with --base=none' '
+	git init midx-compact-base-none &&
+	(
+		cd midx-compact-base-none &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		# Compact the two bottommost layers (A and B) into a new
+		# root layer with no parent.
+		git multi-pack-index compact --incremental \
+			--base=none \
+			"$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 2 "$midx_chain")" &&
+
+		test_line_count = 3 $midx_chain &&
+
+		# The upper layers (C and D) should be preserved
+		# unchanged.
+		nth_line 3 "$midx_chain.bak" >expect &&
+		nth_line 4 "$midx_chain.bak" >>expect &&
+		nth_line 2 "$midx_chain" >actual &&
+		nth_line 3 "$midx_chain" >>actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with bogus --base checksum' '
+	git init midx-compact-bogus-base &&
+	(
+		cd midx-compact-bogus-base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C &&
+
+		test_must_fail git multi-pack-index compact --incremental \
+			--base=deadbeef \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" 2>err &&
+		test_grep "could not find base MIDX" err
+	)
+'
+
 test_done
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 07/16] repack: track the ODB source via existing_packs

[Taylor Blau]

Store the ODB source in the `existing_packs` struct and use that in
place of the raw `repo->objects->sources` access within `cmd_repack()`.

The source used is still assigned from the first source in the list, so
there are no functional changes in this commit. The changes instead
serve two purposes (one immediate, one not):

 - The incremental MIDX-based repacking machinery will need to know what
   source is being used to read the existing MIDX/chain (should one
   exist).

 - In the future, if "git repack" is taught how to operate on other
   object sources, this field will serve as the authoritative value for
   that source.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 5 ++---
 repack.c         | 2 ++
 repack.h         | 1 +
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 4c5a82c2c8d..24be147d39a 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -417,7 +417,7 @@ int cmd_repack(int argc,
 		 * midx_has_unknown_packs() will make the decision for
 		 * us.
 		 */
-		if (!get_multi_pack_index(repo->objects->sources))
+		if (!get_multi_pack_index(existing.source))
 			midx_must_contain_cruft = 1;
 	}
 
@@ -564,8 +564,7 @@ int cmd_repack(int argc,
 		unsigned flags = 0;
 		if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL, 0))
 			flags |= MIDX_WRITE_INCREMENTAL;
-		write_midx_file(repo->objects->sources,
-				NULL, NULL, flags);
+		write_midx_file(existing.source, NULL, NULL, flags);
 	}
 
 cleanup:
diff --git a/repack.c b/repack.c
index 596841027af..2ee6b51420a 100644
--- a/repack.c
+++ b/repack.c
@@ -154,6 +154,8 @@ void existing_packs_collect(struct existing_packs *existing,
 			string_list_append(&existing->non_kept_packs, buf.buf);
 	}
 
+	existing->source = existing->repo->objects->sources;
+
 	string_list_sort(&existing->kept_packs);
 	string_list_sort(&existing->non_kept_packs);
 	string_list_sort(&existing->cruft_packs);
diff --git a/repack.h b/repack.h
index bc9f2e1a5de..c0e9f0ca647 100644
--- a/repack.h
+++ b/repack.h
@@ -56,6 +56,7 @@ struct packed_git;
 
 struct existing_packs {
 	struct repository *repo;
+	struct odb_source *source;
 	struct string_list kept_packs;
 	struct string_list non_kept_packs;
 	struct string_list cruft_packs;
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 08/16] midx: expose `midx_layer_contains_pack()`

[Taylor Blau]

Rename the function `midx_contains_pack_1()` to instead be called
`midx_layer_contains_pack()` and make it accessible. Unlike
`midx_contains_pack()` (which recurses through the entire chain), this
function checks only a single MIDX layer.

This will be used by a subsequent commit to determine whether a given
pack belongs to the tip MIDX layer specifically, rather than to any
layer in the chain.

No functional changes are present in this commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 6 +++---
 midx.h | 2 ++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index bcb8c999015..dc86c8e7fee 100644
--- a/midx.c
+++ b/midx.c
@@ -667,8 +667,8 @@ static int midx_pack_names_cmp(const void *a, const void *b, void *m_)
 		      m->pack_names[*(const size_t *)b]);
 }
 
-static int midx_contains_pack_1(struct multi_pack_index *m,
-				const char *idx_or_pack_name)
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name)
 {
 	uint32_t first = 0, last = m->num_packs;
 
@@ -709,7 +709,7 @@ static int midx_contains_pack_1(struct multi_pack_index *m,
 int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 {
 	for (; m; m = m->base_midx)
-		if (midx_contains_pack_1(m, idx_or_pack_name))
+		if (midx_layer_contains_pack(m, idx_or_pack_name))
 			return 1;
 	return 0;
 }
diff --git a/midx.h b/midx.h
index 77dd66de02b..3ee12dd08ec 100644
--- a/midx.h
+++ b/midx.h
@@ -119,6 +119,8 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 int fill_midx_entry(struct multi_pack_index *m, const struct object_id *oid, struct pack_entry *e);
 int midx_contains_pack(struct multi_pack_index *m,
 		       const char *idx_or_pack_name);
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name);
 int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id);
 int prepare_multi_pack_index_one(struct odb_source *source);
 
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 09/16] repack-midx: factor out `repack_prepare_midx_command()`

[Taylor Blau]

The `write_midx_included_packs()` function assembles and executes a
`git multi-pack-index write` command, constructing the argument list
inline.

Future commits will introduce additional callers that need to construct
similar `git multi-pack-index` commands (for both `write` and `compact`
subcommands), so extract the common portions of the command setup into a
reusable `repack_prepare_midx_command()` helper.

The extracted helper sets `git_cmd`, pushes the `multi-pack-index`
subcommand and verb, and handles `--progress`/`--no-progress` and
`--bitmap` flags. The remaining arguments that are specific to the
`write` subcommand (such as `--stdin-packs`) are left to the caller.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 0682b80c427..83151d4734a 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -275,6 +275,23 @@ static void remove_redundant_bitmaps(struct string_list *include,
 	strbuf_release(&path);
 }
 
+static void repack_prepare_midx_command(struct child_process *cmd,
+					struct repack_write_midx_opts *opts,
+					const char *verb)
+{
+	cmd->git_cmd = 1;
+
+	strvec_pushl(&cmd->args, "multi-pack-index", verb, NULL);
+
+	if (opts->show_progress)
+		strvec_push(&cmd->args, "--progress");
+	else
+		strvec_push(&cmd->args, "--no-progress");
+
+	if (opts->write_bitmaps)
+		strvec_push(&cmd->args, "--bitmap");
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
@@ -289,18 +306,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		goto done;
 
 	cmd.in = -1;
-	cmd.git_cmd = 1;
 
-	strvec_push(&cmd.args, "multi-pack-index");
-	strvec_pushl(&cmd.args, "write", "--stdin-packs", NULL);
-
-	if (opts->show_progress)
-		strvec_push(&cmd.args, "--progress");
-	else
-		strvec_push(&cmd.args, "--no-progress");
-
-	if (opts->write_bitmaps)
-		strvec_push(&cmd.args, "--bitmap");
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Taylor Blau]

The function `write_midx_included_packs()` manages the lifecycle of
writing packs to stdin when running `git multi-pack-index write` as a
child process.

Extract a standalone `repack_fill_midx_stdin_packs()` helper, which
handles `--stdin-packs` argument setup, starting the command, writing
pack names to its standard input, and finishing the command.

This simplifies `write_midx_included_packs()` and prepares for a
subsequent commit where the same helper is called with `cmd->out = -1`
to capture the MIDX's checksum from the command's standard output,
which is needed when writing MIDX layers with `--checksum-only`.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 83151d4734a..78f069c2151 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -292,23 +292,42 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 		strvec_push(&cmd->args, "--bitmap");
 }
 
+static int repack_fill_midx_stdin_packs(struct child_process *cmd,
+					struct string_list *include)
+{
+	struct string_list_item *item;
+	FILE *in;
+	int ret;
+
+	cmd->in = -1;
+
+	strvec_push(&cmd->args, "--stdin-packs");
+
+	ret = start_command(cmd);
+	if (ret)
+		return ret;
+
+	in = xfdopen(cmd->in, "w");
+	for_each_string_list_item(item, include)
+		fprintf(in, "%s\n", item->string);
+	fclose(in);
+
+	return finish_command(cmd);
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
 	struct string_list_item *item;
 	struct packed_git *preferred = pack_geometry_preferred_pack(opts->geometry);
-	FILE *in;
 	int ret = 0;
 
 	midx_included_packs(&include, opts);
 	if (!include.nr)
 		goto done;
 
-	cmd.in = -1;
-
 	repack_prepare_midx_command(&cmd, opts, "write");
-	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
@@ -350,16 +369,7 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = start_command(&cmd);
-	if (ret)
-		goto done;
-
-	in = xfdopen(cmd.in, "w");
-	for_each_string_list_item(item, &include)
-		fprintf(in, "%s\n", item->string);
-	fclose(in);
-
-	ret = finish_command(&cmd);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 11/16] repack-geometry: prepare for incremental MIDX repacking

[Taylor Blau]

Teach `pack_geometry_init()` to optionally restrict the set of
repacking candidates to only packs in the tip MIDX layer when a
`midx_layer_threshold` is configured. If the tip layer has fewer packs
than the threshold, those packs are excluded entirely; otherwise only
packs in that layer participate in the geometric repack.

Also track whether any tip-layer packs were included in the rollup
(`midx_tip_rewritten`), which a subsequent commit will use to decide
how to update the MIDX chain after repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-geometry.c | 35 +++++++++++++++++++++++++++++++++++
 repack.h          |  4 ++++
 2 files changed, 39 insertions(+)

diff --git a/repack-geometry.c b/repack-geometry.c
index 7cebd0cb45f..2408b8a3cc2 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -4,6 +4,7 @@
 #include "repack.h"
 #include "repository.h"
 #include "hex.h"
+#include "midx.h"
 #include "packfile.h"
 
 static uint32_t pack_geometry_weight(struct packed_git *p)
@@ -31,8 +32,28 @@ void pack_geometry_init(struct pack_geometry *geometry,
 {
 	struct packed_git *p;
 	struct strbuf buf = STRBUF_INIT;
+	struct multi_pack_index *m = get_multi_pack_index(existing->source);
 
 	repo_for_each_pack(existing->repo, p) {
+		if (geometry->midx_layer_threshold_set && m &&
+		    p->multi_pack_index) {
+			/*
+			 * When writing MIDX layers incrementally,
+			 * ignore packs unless they are in the most
+			 * recent MIDX layer *and* there are at least
+			 * 'midx_layer_threshold' packs in that layer.
+			 *
+			 * Otherwise 'p' is either in an older layer, or
+			 * the youngest layer does not have enough packs
+			 * to consider its packs as candidates for
+			 * repacking. In either of those cases we want
+			 * to ignore the pack.
+			 */
+			if (m->num_packs < geometry->midx_layer_threshold ||
+			    !midx_layer_contains_pack(m, pack_basename(p)))
+				continue;
+		}
+
 		if (args->local && !p->pack_local)
 			/*
 			 * When asked to only repack local packfiles we skip
@@ -173,6 +194,20 @@ void pack_geometry_split(struct pack_geometry *geometry)
 	geometry->promisor_split = compute_pack_geometry_split(geometry->promisor_pack,
 							       geometry->promisor_pack_nr,
 							       geometry->split_factor);
+	for (uint32_t i = 0; i < geometry->split; i++) {
+		struct packed_git *p = geometry->pack[i];
+		/*
+		 * During incremental MIDX/bitmap repacking, any packs
+		 * included in the rollup are either (a) not MIDX'd, or
+		 * (b) contained in the tip layer iff it has at least
+		 * the threshold number of packs.
+		 *
+		 * In the latter case, we can safely conclude that the
+		 * tip of the MIDX chain will be rewritten.
+		 */
+		if (p->multi_pack_index)
+			geometry->midx_tip_rewritten = true;
+	}
 }
 
 struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry)
diff --git a/repack.h b/repack.h
index c0e9f0ca647..77d24ee45fb 100644
--- a/repack.h
+++ b/repack.h
@@ -108,6 +108,10 @@ struct pack_geometry {
 	uint32_t promisor_pack_nr, promisor_pack_alloc;
 	uint32_t promisor_split;
 
+	uint32_t midx_layer_threshold;
+	bool midx_layer_threshold_set;
+	bool midx_tip_rewritten;
+
 	int split_factor;
 };
 
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 12/16] builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

[Taylor Blau]

Change the --write-midx (-m) flag from an OPT_BOOL to an OPT_CALLBACK
that accepts an optional mode argument. Introduce an enum with
REPACK_WRITE_MIDX_NONE and REPACK_WRITE_MIDX_DEFAULT to distinguish
between the two states, and update all existing boolean checks
accordingly.

For now, passing no argument (or just `-m`) selects the default mode,
preserving existing behavior. A subsequent commit will add a new mode
for writing incremental MIDXs.

Extract repack_write_midx() as a dispatcher that selects the
appropriate MIDX-writing implementation based on the mode.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 50 ++++++++++++++++++++++++++++++++++++------------
 repack-midx.c    | 14 +++++++++++++-
 repack.h         |  8 +++++++-
 3 files changed, 58 insertions(+), 14 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 24be147d39a..5d366340c34 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -97,6 +97,24 @@ static int repack_config(const char *var, const char *value,
 	return git_default_config(var, value, ctx, cb);
 }
 
+static int option_parse_write_midx(const struct option *opt, const char *arg,
+				   int unset)
+{
+	enum repack_write_midx_mode *cfg = opt->value;
+
+	if (unset) {
+		*cfg = REPACK_WRITE_MIDX_NONE;
+		return 0;
+	}
+
+	if (!arg || !*arg)
+		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else
+		return error(_("unknown value for %s: %s"), opt->long_name, arg);
+
+	return 0;
+}
+
 int cmd_repack(int argc,
 	       const char **argv,
 	       const char *prefix,
@@ -119,7 +137,7 @@ int cmd_repack(int argc,
 	struct string_list keep_pack_list = STRING_LIST_INIT_NODUP;
 	struct pack_objects_args po_args = PACK_OBJECTS_ARGS_INIT;
 	struct pack_objects_args cruft_po_args = PACK_OBJECTS_ARGS_INIT;
-	int write_midx = 0;
+	enum repack_write_midx_mode write_midx = REPACK_WRITE_MIDX_NONE;
 	const char *cruft_expiration = NULL;
 	const char *expire_to = NULL;
 	const char *filter_to = NULL;
@@ -185,8 +203,14 @@ int cmd_repack(int argc,
 				N_("do not repack this pack")),
 		OPT_INTEGER('g', "geometric", &geometry.split_factor,
 			    N_("find a geometric progression with factor <N>")),
-		OPT_BOOL('m', "write-midx", &write_midx,
-			   N_("write a multi-pack index of the resulting packs")),
+		OPT_CALLBACK_F(0, "write-midx", &write_midx,
+			   N_("mode"),
+			   N_("write a multi-pack index of the resulting packs"),
+			   PARSE_OPT_OPTARG, option_parse_write_midx),
+		OPT_SET_INT_F('m', NULL, &write_midx,
+			   N_("write a multi-pack index of the resulting packs"),
+			   REPACK_WRITE_MIDX_DEFAULT,
+			   PARSE_OPT_HIDDEN),
 		OPT_STRING(0, "expire-to", &expire_to, N_("dir"),
 			   N_("pack prefix to store a pack containing pruned objects")),
 		OPT_STRING(0, "filter-to", &filter_to, N_("dir"),
@@ -221,14 +245,16 @@ int cmd_repack(int argc,
 		pack_everything |= ALL_INTO_ONE;
 
 	if (write_bitmaps < 0) {
-		if (!write_midx &&
+		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
 			write_bitmaps = 0;
 	}
 	if (po_args.pack_kept_objects < 0)
-		po_args.pack_kept_objects = write_bitmaps > 0 && !write_midx;
+		po_args.pack_kept_objects = write_bitmaps > 0 &&
+			write_midx == REPACK_WRITE_MIDX_NONE;
 
-	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
+	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) &&
+	    write_midx == REPACK_WRITE_MIDX_NONE)
 		die(_(incremental_bitmap_conflict_error));
 
 	if (write_bitmaps && po_args.local &&
@@ -244,7 +270,7 @@ int cmd_repack(int argc,
 		write_bitmaps = 0;
 	}
 
-	if (write_midx && write_bitmaps) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
 		strbuf_addf(&path, "%s/%s_XXXXXX",
@@ -297,7 +323,7 @@ int cmd_repack(int argc,
 	}
 	if (repo_has_promisor_remote(repo))
 		strvec_push(&cmd.args, "--exclude-promisor-objects");
-	if (!write_midx) {
+	if (write_midx == REPACK_WRITE_MIDX_NONE) {
 		if (write_bitmaps > 0)
 			strvec_push(&cmd.args, "--write-bitmap-index");
 		else if (write_bitmaps < 0)
@@ -519,7 +545,7 @@ int cmd_repack(int argc,
 	if (delete_redundant && pack_everything & ALL_INTO_ONE)
 		existing_packs_mark_for_deletion(&existing, &names);
 
-	if (write_midx) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE) {
 		struct repack_write_midx_opts opts = {
 			.existing = &existing,
 			.geometry = &geometry,
@@ -528,11 +554,11 @@ int cmd_repack(int argc,
 			.packdir = packdir,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
-			.midx_must_contain_cruft = midx_must_contain_cruft
+			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.mode = write_midx,
 		};
 
-		ret = write_midx_included_packs(&opts);
-
+		ret = repack_write_midx(&opts);
 		if (ret)
 			goto cleanup;
 	}
diff --git a/repack-midx.c b/repack-midx.c
index 78f069c2151..4a568a2a9b8 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -315,7 +315,7 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 	return finish_command(cmd);
 }
 
-int write_midx_included_packs(struct repack_write_midx_opts *opts)
+static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
@@ -378,3 +378,15 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 
 	return ret;
 }
+
+int repack_write_midx(struct repack_write_midx_opts *opts)
+{
+	switch (opts->mode) {
+	case REPACK_WRITE_MIDX_NONE:
+		BUG("write_midx mode is NONE?");
+	case REPACK_WRITE_MIDX_DEFAULT:
+		return write_midx_included_packs(opts);
+	default:
+		BUG("unhandled write_midx mode: %d", opts->mode);
+	}
+}
diff --git a/repack.h b/repack.h
index 77d24ee45fb..81907fcce7f 100644
--- a/repack.h
+++ b/repack.h
@@ -134,6 +134,11 @@ void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
 
+enum repack_write_midx_mode {
+	REPACK_WRITE_MIDX_NONE,
+	REPACK_WRITE_MIDX_DEFAULT,
+};
+
 struct repack_write_midx_opts {
 	struct existing_packs *existing;
 	struct pack_geometry *geometry;
@@ -143,10 +148,11 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	enum repack_write_midx_mode mode;
 };
 
 void midx_snapshot_refs(struct repository *repo, struct tempfile *f);
-int write_midx_included_packs(struct repack_write_midx_opts *opts);
+int repack_write_midx(struct repack_write_midx_opts *opts);
 
 int write_filtered_pack(const struct write_pack_opts *opts,
 			struct existing_packs *existing,
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 13/16] packfile: ensure `close_pack_revindex()` frees in-memory revindex

[Taylor Blau]

The following commit will introduce a case where we write a MIDX bitmap
over packs that do not themselves have on-disk *.rev files.

This case is supported within Git, and we will simply fall back to
generating the revindex in memory. But we don't ever release that
memory, causing a leak that is exposed by a test introduced in the
following commit.

(As far as I could find, we never free()'d memory allocated as a
byproduct of creating an in-memory revindex, likely because that code
predates the leak-checking niceties we have in the test suite now.)

Rectify this by calling `FREE_AND_NULL()` on the `p->revindex` field
when calling `close_pack_revindex()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 packfile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packfile.c b/packfile.c
index b012d648ada..a1e88fdb223 100644
--- a/packfile.c
+++ b/packfile.c
@@ -420,6 +420,8 @@ void close_pack_index(struct packed_git *p)
 
 static void close_pack_revindex(struct packed_git *p)
 {
+	FREE_AND_NULL(p->revindex);
+
 	if (!p->revindex_map)
 		return;
 
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

Implement the `write_midx_incremental()` function, which builds and
maintains an incremental MIDX chain as part of the geometric repacking
process.

Unlike the default mode which writes a single flat MIDX, the incremental
mode constructs a compaction plan that determines which MIDX layers to
write, compact, or copy, and then executes each step using `git
multi-pack-index` subcommands with the --checksum-only flag.

The repacking strategy works as follows:

 * Acquire the lock guarding the multi-pack-index-chain.

 * A new MIDX layer is always written containing the newly created
   pack(s). If the tip MIDX layer was rewritten during geometric
   repacking, any surviving packs from that layer are also included.

 * Starting from the new layer, adjacent MIDX layers are merged together
   as long as the accumulated object count exceeds half the object count
   of the next deeper layer (controlled by 'repack.midxSplitFactor').

 * Remaining layers in the chain are evaluated pairwise and either
   compacted or copied as-is, following the same merging condition.

 * Write the contents of the new multi-pack-index chain, atomically move
   it into place, and then release the lock.

 * Delete any now-unused MIDX layers.

After writing the new layer, the strategy is evaluated among the
existing MIDX layers in order from oldest to newest. Each step that
writes a new MIDX layer uses "--checksum-only" to avoid updating the
multi-pack-index-chain file. After all steps are complete, the new chain
file is written and then atomically moved into place.

At present, this functionality is exposed behind a new enum value,
`REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
subsequent commit will expose this mode via `git repack
--write-midx=incremental`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c |   5 +
 repack-midx.c    | 578 ++++++++++++++++++++++++++++++++++++++++++++++-
 repack.h         |   3 +
 3 files changed, 584 insertions(+), 2 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 5d366340c34..75c57736780 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -42,6 +42,9 @@ static const char incremental_bitmap_conflict_error[] = N_(
 "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
 );
 
+#define DEFAULT_MIDX_SPLIT_FACTOR 2
+#define DEFAULT_MIDX_NEW_LAYER_THRESHOLD 8
+
 struct repack_config_ctx {
 	struct pack_objects_args *po_args;
 	struct pack_objects_args *cruft_po_args;
@@ -555,6 +558,8 @@ int cmd_repack(int argc,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
 			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
+			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
 			.mode = write_midx,
 		};
 
diff --git a/repack-midx.c b/repack-midx.c
index 4a568a2a9b8..8f3720772b8 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -2,12 +2,16 @@
 #include "repack.h"
 #include "hash.h"
 #include "hex.h"
+#include "lockfile.h"
+#include "midx.h"
 #include "odb.h"
 #include "oidset.h"
 #include "pack-bitmap.h"
+#include "path.h"
 #include "refs.h"
 #include "run-command.h"
 #include "tempfile.h"
+#include "trace2.h"
 
 struct midx_snapshot_ref_data {
 	struct repository *repo;
@@ -293,13 +297,16 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 }
 
 static int repack_fill_midx_stdin_packs(struct child_process *cmd,
-					struct string_list *include)
+					struct string_list *include,
+					struct string_list *out)
 {
 	struct string_list_item *item;
 	FILE *in;
 	int ret;
 
 	cmd->in = -1;
+	if (out)
+		cmd->out = -1;
 
 	strvec_push(&cmd->args, "--stdin-packs");
 
@@ -312,6 +319,17 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 		fprintf(in, "%s\n", item->string);
 	fclose(in);
 
+	if (out) {
+		struct strbuf buf = STRBUF_INIT;
+		FILE *outf = xfdopen(cmd->out, "r");
+
+		while (strbuf_getline(&buf, outf) != EOF)
+			string_list_append(out, buf.buf);
+		strbuf_release(&buf);
+
+		fclose(outf);
+	}
+
 	return finish_command(cmd);
 }
 
@@ -369,7 +387,7 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = repack_fill_midx_stdin_packs(&cmd, &include);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include, NULL);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
@@ -379,6 +397,560 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 	return ret;
 }
 
+struct midx_compaction_step {
+	union {
+		struct multi_pack_index *copy;
+		struct string_list write;
+		struct {
+			struct multi_pack_index *from;
+			struct multi_pack_index *to;
+		} compact;
+	} u;
+
+	uint32_t objects_nr;
+	char *csum;
+
+	enum {
+		MIDX_COMPACTION_STEP_UNKNOWN,
+		MIDX_COMPACTION_STEP_COPY,
+		MIDX_COMPACTION_STEP_WRITE,
+		MIDX_COMPACTION_STEP_COMPACT,
+	} type;
+};
+
+static const char *midx_compaction_step_base(const struct midx_compaction_step *step)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot use UNKNOWN step as a base");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_get_checksum_hex(step->u.copy);
+	case MIDX_COMPACTION_STEP_WRITE:
+		BUG("cannot use WRITE step as a base");
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_get_checksum_hex(step->u.compact.to);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static int midx_compaction_step_exec_copy(struct midx_compaction_step *step)
+{
+	step->csum = xstrdup(midx_get_checksum_hex(step->u.copy));
+	return 0;
+}
+
+static int midx_compaction_step_exec_write(struct midx_compaction_step *step,
+					   struct repack_write_midx_opts *opts,
+					   const char *base)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct string_list hash = STRING_LIST_INIT_DUP;
+	struct string_list_item *item;
+	const char *preferred_pack = NULL;
+	int ret = 0;
+
+	if (!step->u.write.nr) {
+		ret = error(_("no packs to write MIDX during compaction"));
+		goto out;
+	}
+
+	for_each_string_list_item(item, &step->u.write) {
+		if (item->util)
+			preferred_pack = item->string;
+	}
+
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);
+	strvec_pushf(&cmd.args, "--base=%s", base ? base : "none");
+
+	if (preferred_pack) {
+		struct strbuf buf = STRBUF_INIT;
+
+		strbuf_addstr(&buf, preferred_pack);
+		strbuf_strip_suffix(&buf, ".idx");
+		strbuf_addstr(&buf, ".pack");
+
+		strvec_pushf(&cmd.args, "--preferred-pack=%s", buf.buf);
+
+		strbuf_release(&buf);
+	}
+
+	ret = repack_fill_midx_stdin_packs(&cmd, &step->u.write, &hash);
+	if (hash.nr != 1) {
+		ret = error(_("expected exactly one line during MIDX write, "
+			      "got: %"PRIuMAX),
+			    (uintmax_t)hash.nr);
+		goto out;
+	}
+
+	step->csum = xstrdup(hash.items[0].string);
+
+out:
+	string_list_clear(&hash, 0);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec_compact(struct midx_compaction_step *step,
+					     struct repack_write_midx_opts *opts)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf buf = STRBUF_INIT;
+	FILE *out = NULL;
+	int ret;
+
+	repack_prepare_midx_command(&cmd, opts, "compact");
+	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file",
+		     midx_get_checksum_hex(step->u.compact.from),
+		     midx_get_checksum_hex(step->u.compact.to), NULL);
+
+	cmd.out = -1;
+
+	ret = start_command(&cmd);
+	if (ret)
+		goto out;
+
+	out = xfdopen(cmd.out, "r");
+	while (strbuf_getline_lf(&buf, out) != EOF) {
+		if (step->csum) {
+			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
+			goto out;
+		}
+		step->csum = strbuf_detach(&buf, NULL);
+	}
+
+	ret = finish_command(&cmd);
+
+out:
+	if (out)
+		fclose(out);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec(struct midx_compaction_step *step,
+				     struct repack_write_midx_opts *opts,
+				     const char *base)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot execute UNKNOWN midx compaction step");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_compaction_step_exec_copy(step);
+	case MIDX_COMPACTION_STEP_WRITE:
+		return midx_compaction_step_exec_write(step, opts, base);
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_compaction_step_exec_compact(step, opts);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static void midx_compaction_step_release(struct midx_compaction_step *step)
+{
+	if (step->type == MIDX_COMPACTION_STEP_WRITE)
+		string_list_clear(&step->u.write, 0);
+	free(step->csum);
+}
+
+static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
+					    struct midx_compaction_step **steps_p,
+					    size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step step = { 0 };
+	struct strbuf buf = STRBUF_INIT;
+	size_t steps_nr = 0, steps_alloc = 0;
+	uint32_t i;
+	int ret = 0;
+
+	trace2_region_enter("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	for (i = 0; m && i < m->num_packs + m->num_packs_in_base; i++) {
+		if (prepare_midx_pack(m, i)) {
+			ret = error(_("could not load pack %"PRIu32" from MIDX"),
+				    i);
+			goto out;
+		}
+	}
+
+	trace2_region_enter("repack", "steps:write", opts->existing->repo);
+
+	/*
+	 * The first MIDX in the resulting chain is always going to be
+	 * new.
+	 *
+	 * At a minimum, it will include all of the newly written packs.
+	 * If there is an existing MIDX whose tip layer contains packs
+	 * that were repacked, it will also include any of its packs
+	 * which were *not* rolled up as part of the geometric repack
+	 * (if any), and the previous tip will be replaced.
+	 *
+	 * It may grow to include the packs from zero or more MIDXs from
+	 * the old chain, beginning either at the old tip (if the MIDX
+	 * was *not* rewritten) or the old tip's base MIDX layer
+	 * (otherwise).
+	 */
+	step.type = MIDX_COMPACTION_STEP_WRITE;
+	string_list_init_dup(&step.u.write);
+
+	for (i = 0; i < opts->names->nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "pack-%s.idx", opts->names->items[i].string);
+		string_list_append(&step.u.write, buf.buf);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:fresh",
+				   step.u.write.items[step.u.write.nr - 1].string);
+	}
+	for (i = 0; i < opts->geometry->split; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:fresh:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * Now handle any existing packs which were *not* rewritten.
+	 *
+	 * The list of packs in opts->geometry only contains MIDX'd
+	 * packs from the newest layer when that layer has more than
+	 * 'repack.midxNewLayerThreshold' number of packs.
+	 *
+	 * If the MIDX tip was rewritten (that is, one or more of those
+	 * packs appear below the split line), then add all packs above
+	 * the split line to the new layer, as the old one is no longer
+	 * usable.
+	 *
+	 * If the MIDX tip was not rewritten (that is, all MIDX'd packs
+	 * from the youngest layer appear below the split line, or were
+	 * not included in the geometric repack at all because there
+	 * were too few of them), ignore them since we'll retain the
+	 * existing layer as-is.
+	 */
+	for (i = opts->geometry->split; i < opts->geometry->pack_nr; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		struct string_list_item *item;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, pack_basename(p));
+		strbuf_strip_suffix(&buf, ".pack");
+		strbuf_addstr(&buf, ".idx");
+
+		if (p->multi_pack_index &&
+		    !opts->geometry->midx_tip_rewritten) {
+			trace2_data_string("repack", opts->existing->repo,
+					   "exclude:unmodified", buf.buf);
+			continue;
+		}
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified", buf.buf);
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified:midx",
+				   p->multi_pack_index ? "true" : "false");
+
+		item = string_list_append(&step.u.write, buf.buf);
+		if (p->multi_pack_index || i == opts->geometry->pack_nr - 1)
+			item->util = (void *)1; /* mark as preferred */
+
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:unmodified:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * If the MIDX tip was rewritten, then we no longer consider it
+	 * a candidate for compaction, since it will not exist in the
+	 * MIDX chain being built.
+	 */
+	if (opts->geometry->midx_tip_rewritten)
+		m = m->base_midx;
+
+	trace2_data_string("repack", opts->existing->repo, "midx:rewrote-tip",
+			   opts->geometry->midx_tip_rewritten ? "true" : "false");
+
+	trace2_region_enter("repack", "compact", opts->existing->repo);
+
+	/*
+	 * Compact additional MIDX layers into this proposed one until
+	 * the merging condition is violated.
+	 */
+	while (m) {
+		uint32_t preferred_pack_idx;
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "candidate", midx_get_checksum_hex(m));
+
+		if (step.objects_nr < m->num_objects / opts->midx_split_factor) {
+			/*
+			 * Stop compacting MIDX layer as soon as the
+			 * merged size is less than half the size of the
+			 * next layer in the chain.
+			 */
+			trace2_data_string("repack", opts->existing->repo,
+					   "compact", "violated");
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "objects_nr",
+					   (uintmax_t)step.objects_nr);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "next_objects_nr",
+					   (uintmax_t)m->num_objects);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "split_factor",
+					   (uintmax_t)opts->midx_split_factor);
+
+			break;
+		}
+
+		if (midx_preferred_pack(m, &preferred_pack_idx) < 0) {
+			ret = error(_("could not find preferred pack for MIDX "
+				      "%s"), midx_get_checksum_hex(m));
+			goto out;
+		}
+
+		for (i = 0; i < m->num_packs; i++) {
+			struct string_list_item *item;
+			uint32_t pack_int_id = i + m->num_packs_in_base;
+			struct packed_git *p = nth_midxed_pack(m, pack_int_id);
+
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, pack_basename(p));
+			strbuf_strip_suffix(&buf, ".pack");
+			strbuf_addstr(&buf, ".idx");
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "midx:pack", buf.buf);
+
+			item = string_list_append(&step.u.write, buf.buf);
+			if (pack_int_id == preferred_pack_idx)
+				item->util = (void *)1; /* mark as preferred */
+		}
+
+		if (unsigned_add_overflows(step.objects_nr, m->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+		step.objects_nr += m->num_objects;
+
+		m = m->base_midx;
+	}
+
+	if (step.u.write.nr > 0) {
+		/*
+		 * As long as there is at least one new pack to write
+		 * (and thus the MIDX is non-empty), add it to the plan.
+		 */
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+		steps[steps_nr++] = step;
+	}
+
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:objects_nr", (uintmax_t)step.objects_nr);
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:packs_nr", (uintmax_t)step.u.write.nr);
+
+	trace2_region_leave("repack", "compact", opts->existing->repo);
+	trace2_region_leave("repack", "steps:write", opts->existing->repo);
+
+	trace2_region_enter("repack", "steps:rest", opts->existing->repo);
+
+	/*
+	 * Then start over, repeat, and either compact or keep as-is
+	 * each MIDX layer until we have exhausted the chain.
+	 *
+	 * Finally, evaluate the remainder of the chain (if any) and
+	 * either compact a sequence of adjacent layers, or keep
+	 * individual layers as-is according to the same merging
+	 * condition as above.
+	 */
+	while (m) {
+		struct multi_pack_index *next = m;
+
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+
+		memset(&step, 0, sizeof(step));
+		step.type = MIDX_COMPACTION_STEP_UNKNOWN;
+
+		trace2_region_enter("repack", "step", opts->existing->repo);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "from", midx_get_checksum_hex(m));
+
+		while (next) {
+			uint32_t proposed_objects_nr;
+			if (unsigned_add_overflows(step.objects_nr, next->num_objects)) {
+				ret = error(_("too many objects in MIDX compaction step"));
+				trace2_region_leave("repack", "step", opts->existing->repo);
+				goto out;
+			}
+
+			proposed_objects_nr = step.objects_nr + next->num_objects;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "proposed",
+					   midx_get_checksum_hex(next));
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "proposed:objects_nr",
+					   (uintmax_t)next->num_objects);
+
+			if (!next->base_midx) {
+				/*
+				 * If we are at the end of the MIDX
+				 * chain, there is nothing to compact,
+				 * so mark it and stop.
+				 */
+				step.objects_nr = proposed_objects_nr;
+				break;
+			}
+
+			if (proposed_objects_nr < next->base_midx->num_objects / opts->midx_split_factor) {
+				/*
+				 * If there is a MIDX following this
+				 * one, but our accumulated size is less
+				 * than half of its size, compacting
+				 * them would violate the merging
+				 * condition, so stop here.
+				 */
+
+				trace2_data_string("repack", opts->existing->repo,
+						   "compact:violated:at",
+						   midx_get_checksum_hex(next->base_midx));
+				trace2_data_intmax("repack", opts->existing->repo,
+						   "compact:violated:at:objects_nr",
+						   (uintmax_t)next->base_midx->num_objects);
+				break;
+			}
+
+			/*
+			 * Otherwise, it is OK to compact the next layer
+			 * into this one. Do so, and then continue
+			 * through the remainder of the chain.
+			 */
+			step.objects_nr = proposed_objects_nr;
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "step:objects_nr",
+					   (uintmax_t)step.objects_nr);
+			next = next->base_midx;
+		}
+
+		if (m == next) {
+			step.type = MIDX_COMPACTION_STEP_COPY;
+			step.u.copy = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "copy");
+		} else {
+			step.type = MIDX_COMPACTION_STEP_COMPACT;
+			step.u.compact.from = next;
+			step.u.compact.to = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "to", midx_get_checksum_hex(m));
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "compact");
+		}
+
+		m = next->base_midx;
+		steps[steps_nr++] = step;
+		trace2_region_leave("repack", "step", opts->existing->repo);
+	}
+
+	trace2_region_leave("repack", "steps:rest", opts->existing->repo);
+
+out:
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+
+	strbuf_release(&buf);
+
+	trace2_region_leave("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	return ret;
+}
+
+static int write_midx_incremental(struct repack_write_midx_opts *opts)
+{
+	struct midx_compaction_step *steps = NULL;
+	struct strbuf lock_name = STRBUF_INIT;
+	struct lock_file lf;
+	size_t steps_nr = 0;
+	size_t i;
+	int ret = 0;
+
+	get_midx_chain_filename(opts->existing->source, &lock_name);
+	if (safe_create_leading_directories(opts->existing->repo,
+					    lock_name.buf))
+		die_errno(_("unable to create leading directories of %s"),
+			  lock_name.buf);
+	hold_lock_file_for_update(&lf, lock_name.buf, LOCK_DIE_ON_ERROR);
+
+	if (!fdopen_lock_file(&lf, "w")) {
+		ret = error_errno(_("unable to open multi-pack-index chain file"));
+		goto done;
+	}
+
+	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+		ret = error(_("unable to generate compaction plan"));
+		goto done;
+	}
+
+	for (i = 0; i < steps_nr; i++) {
+		struct midx_compaction_step *step = &steps[i];
+		char *base = NULL;
+
+		if (i + 1 < steps_nr)
+			base = xstrdup(midx_compaction_step_base(&steps[i + 1]));
+
+		if (midx_compaction_step_exec(step, opts, base) < 0) {
+			ret = error(_("unable to execute compaction step %"PRIuMAX),
+				    (uintmax_t)i);
+			free(base);
+			goto done;
+		}
+
+		free(base);
+	}
+
+	i = steps_nr;
+	while (i--) {
+		struct midx_compaction_step *step = &steps[i];
+		if (!step->csum)
+			BUG("missing result for compaction step %"PRIuMAX,
+			    (uintmax_t)i);
+		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+	}
+
+	commit_lock_file(&lf);
+
+done:
+	strbuf_release(&lock_name);
+	for (i = 0; i < steps_nr; i++)
+		midx_compaction_step_release(&steps[i]);
+	free(steps);
+	return ret;
+}
+
 int repack_write_midx(struct repack_write_midx_opts *opts)
 {
 	switch (opts->mode) {
@@ -386,6 +958,8 @@ int repack_write_midx(struct repack_write_midx_opts *opts)
 		BUG("write_midx mode is NONE?");
 	case REPACK_WRITE_MIDX_DEFAULT:
 		return write_midx_included_packs(opts);
+	case REPACK_WRITE_MIDX_INCREMENTAL:
+		return write_midx_incremental(opts);
 	default:
 		BUG("unhandled write_midx mode: %d", opts->mode);
 	}
diff --git a/repack.h b/repack.h
index 81907fcce7f..831ccfb1c6c 100644
--- a/repack.h
+++ b/repack.h
@@ -137,6 +137,7 @@ struct tempfile;
 enum repack_write_midx_mode {
 	REPACK_WRITE_MIDX_NONE,
 	REPACK_WRITE_MIDX_DEFAULT,
+	REPACK_WRITE_MIDX_INCREMENTAL,
 };
 
 struct repack_write_midx_opts {
@@ -148,6 +149,8 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 	enum repack_write_midx_mode mode;
 };
 
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 15/16] repack: introduce `--write-midx=incremental`

[Taylor Blau]

Expose the incremental MIDX repacking mode (implemented in an earlier
commit) via a new --write-midx=incremental option for `git repack`.

Add "incremental" as a recognized argument to the --write-midx
OPT_CALLBACK, mapping it to REPACK_WRITE_MIDX_INCREMENTAL. When this
mode is active and --geometric is in use, set the midx_layer_threshold
on the pack geometry so that only packs in sufficiently large tip layers
are considered for repacking.

Two new configuration options control the compaction behavior:

 - repack.midxSplitFactor (default: 2): the factor used in the
   geometric merging condition for MIDX layers.

 - repack.midxNewLayerThreshold (default: 8): the minimum number of
   packs in the tip MIDX layer before its packs are considered as
   candidates for geometric repacking.

Add tests exercising the new mode across a variety of scenarios
including basic geometric violations, multi-round chain integrity,
branching and merging histories, cross-layer object uniqueness, and
threshold-based compaction.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/config/repack.adoc   |  18 ++
 Documentation/git-repack.adoc      |  39 ++-
 builtin/repack.c                   |  49 ++-
 midx.c                             |  31 ++
 midx.h                             |   3 +
 repack-geometry.c                  |  13 +-
 repack-midx.c                      |   5 +
 repack.c                           |  56 +++-
 repack.h                           |  10 +-
 t/meson.build                      |   1 +
 t/t7705-repack-incremental-midx.sh | 500 +++++++++++++++++++++++++++++
 11 files changed, 701 insertions(+), 24 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh

diff --git a/Documentation/config/repack.adoc b/Documentation/config/repack.adoc
index e9e78dcb198..4c22a499f62 100644
--- a/Documentation/config/repack.adoc
+++ b/Documentation/config/repack.adoc
@@ -46,3 +46,21 @@ repack.midxMustContainCruft::
 	`--write-midx`. When false, cruft packs are only included in the MIDX
 	when necessary (e.g., because they might be required to form a
 	reachability closure with MIDX bitmaps). Defaults to true.
+
+repack.midxSplitFactor::
+	The factor used in the geometric merging condition when
+	compacting incremental MIDX layers during `git repack` when
+	invoked with the `--write-midx=incremental` option.
++
+Adjacent layers are merged when the accumulated object count of the
+newer layer exceeds `1/<N>` of the object count of the next deeper
+layer. Must be at least 2. Defaults to 2.
+
+repack.midxNewLayerThreshold::
+	The minimum number of packs in the tip MIDX layer before those
+	packs are considered as candidates for geometric repacking
+	during `git repack --write-midx=incremental`.
++
+When the tip layer has fewer packs than this threshold, those packs are
+excluded from the geometric repack entirely, and are thus left
+unmodified. Must be at least 1. Defaults to 8.
diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 673ce910837..27a99cc46f4 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
-	[--write-midx] [--name-hash-version=<n>] [--path-walk]
+	[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]
 
 DESCRIPTION
 -----------
@@ -250,9 +250,42 @@ pack as the preferred pack for object selection by the MIDX (see
 linkgit:git-multi-pack-index[1]).
 
 -m::
---write-midx::
+--write-midx[=<mode>]::
 	Write a multi-pack index (see linkgit:git-multi-pack-index[1])
-	containing the non-redundant packs.
+	containing the non-redundant packs. The following modes are
+	available:
++
+--
+	`default`;;
+		Write a single MIDX covering all packs. This is the
+		default when `--write-midx` is given without an
+		explicit mode.
+
+	`incremental`;;
+		Write an incremental MIDX chain instead of a single
+		flat MIDX. This mode requires `--geometric`.
++
+The incremental mode maintains a chain of MIDX layers that is compacted
+over time using a geometric merging strategy. Each repack creates a new
+tip layer containing the newly written pack(s). Adjacent layers are then
+merged whenever the newer layer's object count exceeds
+`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
+that do not meet this condition are retained as-is.
++
+The result is that newer (tip) layers tend to contain many small packs
+with relatively few objects, while older (deeper) layers contain fewer,
+larger packs covering more objects. Because compaction is driven by the
+tip of the chain, newer layers are also rewritten more frequently than
+older ones, which are only touched when enough objects have accumulated
+to justify merging into them. This keeps the total number of layers
+logarithmic relative to the total number of objects.
++
+Only packs in the tip MIDX layer are considered as candidates for the
+geometric repack; packs in deeper layers are left untouched. If the tip
+layer contains fewer packs than `repack.midxNewLayerThreshold`, those
+packs are excluded from the geometry entirely, and a new layer is
+created for any new pack(s) without disturbing the existing chain.
+--
 
 --name-hash-version=<n>::
 	Provide this argument to the underlying `git pack-objects` process.
diff --git a/builtin/repack.c b/builtin/repack.c
index 75c57736780..5ffa18e085e 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -33,7 +33,7 @@ static int midx_must_contain_cruft = 1;
 static const char *const git_repack_usage[] = {
 	N_("git repack [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]\n"
 	   "[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]\n"
-	   "[--write-midx] [--name-hash-version=<n>] [--path-walk]"),
+	   "[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]"),
 	NULL
 };
 
@@ -48,6 +48,8 @@ static const char incremental_bitmap_conflict_error[] = N_(
 struct repack_config_ctx {
 	struct pack_objects_args *po_args;
 	struct pack_objects_args *cruft_po_args;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 };
 
 static int repack_config(const char *var, const char *value,
@@ -97,6 +99,16 @@ static int repack_config(const char *var, const char *value,
 		midx_must_contain_cruft = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "repack.midxsplitfactor")) {
+		repack_ctx->midx_split_factor = git_config_int(var, value,
+							       ctx->kvi);
+		return 0;
+	}
+	if (!strcmp(var, "repack.midxnewlayerthreshold")) {
+		repack_ctx->midx_new_layer_threshold = git_config_int(var, value,
+								      ctx->kvi);
+		return 0;
+	}
 	return git_default_config(var, value, ctx, cb);
 }
 
@@ -112,6 +124,8 @@ static int option_parse_write_midx(const struct option *opt, const char *arg,
 
 	if (!arg || !*arg)
 		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else if (!strcmp(arg, "incremental"))
+		*cfg = REPACK_WRITE_MIDX_INCREMENTAL;
 	else
 		return error(_("unknown value for %s: %s"), opt->long_name, arg);
 
@@ -226,6 +240,8 @@ int cmd_repack(int argc,
 	memset(&config_ctx, 0, sizeof(config_ctx));
 	config_ctx.po_args = &po_args;
 	config_ctx.cruft_po_args = &cruft_po_args;
+	config_ctx.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR;
+	config_ctx.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD;
 
 	repo_config(repo, repack_config, &config_ctx);
 
@@ -247,6 +263,9 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
+	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
+		die(_("--write-midx=incremental requires --geometric"));
+
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
@@ -273,6 +292,13 @@ int cmd_repack(int argc,
 		write_bitmaps = 0;
 	}
 
+	if (config_ctx.midx_split_factor < 2)
+		die(_("invalid value for %s: %d"), "--midx-split-factor",
+		    config_ctx.midx_split_factor);
+	if (config_ctx.midx_new_layer_threshold < 1)
+		die(_("invalid value for %s: %d"), "--midx-new-layer-threshold",
+		    config_ctx.midx_new_layer_threshold);
+
 	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
@@ -296,6 +322,10 @@ int cmd_repack(int argc,
 	if (geometry.split_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
+		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL) {
+			geometry.midx_layer_threshold = config_ctx.midx_new_layer_threshold;
+			geometry.midx_layer_threshold_set = true;
+		}
 		pack_geometry_init(&geometry, &existing, &po_args);
 		pack_geometry_split(&geometry);
 	}
@@ -545,8 +575,11 @@ int cmd_repack(int argc,
 				       packtmp);
 	/* End of pack replacement. */
 
-	if (delete_redundant && pack_everything & ALL_INTO_ONE)
+	if (delete_redundant && pack_everything & ALL_INTO_ONE) {
+		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL)
+			existing_packs_retain_midx_packs(&existing);
 		existing_packs_mark_for_deletion(&existing, &names);
+	}
 
 	if (write_midx != REPACK_WRITE_MIDX_NONE) {
 		struct repack_write_midx_opts opts = {
@@ -558,8 +591,8 @@ int cmd_repack(int argc,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
 			.midx_must_contain_cruft = midx_must_contain_cruft,
-			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
-			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
+			.midx_split_factor = config_ctx.midx_split_factor,
+			.midx_new_layer_threshold = config_ctx.midx_new_layer_threshold,
 			.mode = write_midx,
 		};
 
@@ -572,11 +605,15 @@ int cmd_repack(int argc,
 
 	if (delete_redundant) {
 		int opts = 0;
-		existing_packs_remove_redundant(&existing, packdir);
+		bool wrote_incremental_midx = write_midx == REPACK_WRITE_MIDX_INCREMENTAL;
+
+		existing_packs_remove_redundant(&existing, packdir,
+						wrote_incremental_midx);
 
 		if (geometry.split_factor)
 			pack_geometry_remove_redundant(&geometry, &names,
-						       &existing, packdir);
+						       &existing, packdir,
+						       wrote_incremental_midx);
 		if (show_progress)
 			opts |= PRUNE_PACKED_VERBOSE;
 		prune_packed_objects(opts);
diff --git a/midx.c b/midx.c
index dc86c8e7fee..cd31fa20788 100644
--- a/midx.c
+++ b/midx.c
@@ -850,6 +850,37 @@ void clear_midx_file(struct repository *r)
 	strbuf_release(&midx);
 }
 
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes)
+{
+	struct strbuf chain = STRBUF_INIT;
+
+	get_midx_chain_filename(r->objects->sources, &chain);
+
+	if (r->objects) {
+		struct odb_source *source = r->objects->sources;
+		for (source = r->objects->sources; source; source = source->next) {
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			if (files->packed->midx)
+				close_midx(files->packed->midx);
+			files->packed->midx = NULL;
+		}
+	}
+
+	if (!keep_hashes && remove_path(chain.buf))
+		die(_("failed to clear multi-pack-index chain at %s"),
+		    chain.buf);
+
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_BITMAP,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_REV,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_MIDX,
+					 keep_hashes);
+
+	strbuf_release(&chain);
+}
+
 static int verify_midx_error;
 
 __attribute__((format (printf, 1, 2)))
diff --git a/midx.h b/midx.h
index 3ee12dd08ec..63853a03a47 100644
--- a/midx.h
+++ b/midx.h
@@ -9,6 +9,7 @@ struct repository;
 struct bitmapped_pack;
 struct git_hash_algo;
 struct odb_source;
+struct strvec;
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION_V1 1
@@ -143,6 +144,8 @@ int write_midx_file_compact(struct odb_source *source,
 			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes);
 int verify_midx_file(struct odb_source *source, unsigned flags);
 int expire_midx_packs(struct odb_source *source, unsigned flags);
 int midx_repack(struct odb_source *source, size_t batch_size, unsigned flags);
diff --git a/repack-geometry.c b/repack-geometry.c
index 2408b8a3cc2..2064683dcfe 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -249,7 +249,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 				   uint32_t pack_nr,
 				   struct string_list *names,
 				   struct existing_packs *existing,
-				   const char *packdir)
+				   const char *packdir,
+				   bool wrote_incremental_midx)
 {
 	const struct git_hash_algo *algop = existing->repo->hash_algo;
 	struct strbuf buf = STRBUF_INIT;
@@ -269,7 +270,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 		    (string_list_has_string(&existing->kept_packs, buf.buf)))
 			continue;
 
-		repack_remove_redundant_pack(existing->repo, packdir, buf.buf);
+		repack_remove_redundant_pack(existing->repo, packdir, buf.buf,
+					     wrote_incremental_midx);
 	}
 
 	strbuf_release(&buf);
@@ -278,12 +280,13 @@ static void remove_redundant_packs(struct packed_git **pack,
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir)
+				    const char *packdir,
+				    bool wrote_incremental_midx)
 {
 	remove_redundant_packs(geometry->pack, geometry->split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 	remove_redundant_packs(geometry->promisor_pack, geometry->promisor_split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 }
 
 void pack_geometry_release(struct pack_geometry *geometry)
diff --git a/repack-midx.c b/repack-midx.c
index 8f3720772b8..6a0358096ec 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -894,6 +894,7 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 	struct midx_compaction_step *steps = NULL;
 	struct strbuf lock_name = STRBUF_INIT;
 	struct lock_file lf;
+	struct strvec keep_hashes = STRVEC_INIT;
 	size_t steps_nr = 0;
 	size_t i;
 	int ret = 0;
@@ -939,11 +940,15 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 			BUG("missing result for compaction step %"PRIuMAX,
 			    (uintmax_t)i);
 		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+		strvec_push(&keep_hashes, step->csum);
 	}
 
 	commit_lock_file(&lf);
 
+	clear_incremental_midx_files(opts->existing->repo, &keep_hashes);
+
 done:
+	strvec_clear(&keep_hashes);
 	strbuf_release(&lock_name);
 	for (i = 0; i < steps_nr; i++)
 		midx_compaction_step_release(&steps[i]);
diff --git a/repack.c b/repack.c
index 2ee6b51420a..571dabb665e 100644
--- a/repack.c
+++ b/repack.c
@@ -55,14 +55,18 @@ void pack_objects_args_release(struct pack_objects_args *args)
 }
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name)
+				  const char *base_name,
+				  bool wrote_incremental_midx)
 {
 	struct strbuf buf = STRBUF_INIT;
 	struct odb_source *source = repo->objects->sources;
 	struct multi_pack_index *m = get_multi_pack_index(source);
 	strbuf_addf(&buf, "%s.pack", base_name);
-	if (m && source->local && midx_contains_pack(m, buf.buf))
+	if (m && source->local && midx_contains_pack(m, buf.buf)) {
 		clear_midx_file(repo);
+		if (!wrote_incremental_midx)
+			clear_incremental_midx_files(repo, NULL);
+	}
 	strbuf_insertf(&buf, 0, "%s/", dir_name);
 	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
@@ -250,25 +254,63 @@ void existing_packs_mark_for_deletion(struct existing_packs *existing,
 					   &existing->cruft_packs);
 }
 
+/*
+ * Mark every pack that is referenced by the existing MIDX chain as
+ * retained, so that a subsequent call to
+ * existing_packs_mark_for_deletion() will not mark them for deletion.
+ *
+ * This is used when writing an incremental MIDX layer on top of an
+ * existing chain: retained layers continue to reference the same
+ * packs on disk, so those packs must not be unlinked even if the
+ * freshly-written pack supersedes them.
+ */
+void existing_packs_retain_midx_packs(struct existing_packs *existing)
+{
+	struct string_list_item *item;
+	struct strbuf buf = STRBUF_INIT;
+
+	for_each_string_list_item(item, &existing->midx_packs) {
+		struct string_list_item *found;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, item->string);
+		strbuf_strip_suffix(&buf, ".pack");
+		strbuf_strip_suffix(&buf, ".idx");
+
+		found = string_list_lookup(&existing->non_kept_packs, buf.buf);
+		if (found)
+			existing_packs_mark_retained(found);
+
+		found = string_list_lookup(&existing->cruft_packs, buf.buf);
+		if (found)
+			existing_packs_mark_retained(found);
+	}
+
+	strbuf_release(&buf);
+}
+
 static void remove_redundant_packs_1(struct repository *repo,
 				     struct string_list *packs,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	struct string_list_item *item;
 	for_each_string_list_item(item, packs) {
 		if (!existing_pack_is_marked_for_deletion(item))
 			continue;
-		repack_remove_redundant_pack(repo, packdir, item->string);
+		repack_remove_redundant_pack(repo, packdir, item->string,
+					     wrote_incremental_midx);
 	}
 }
 
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	remove_redundant_packs_1(existing->repo, &existing->non_kept_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 	remove_redundant_packs_1(existing->repo, &existing->cruft_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 }
 
 void existing_packs_release(struct existing_packs *existing)
diff --git a/repack.h b/repack.h
index 831ccfb1c6c..f9fbc895f02 100644
--- a/repack.h
+++ b/repack.h
@@ -34,7 +34,8 @@ void prepare_pack_objects(struct child_process *cmd,
 void pack_objects_args_release(struct pack_objects_args *args);
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name);
+				  const char *base_name,
+				  bool wrote_incremental_midx);
 
 struct write_pack_opts {
 	struct pack_objects_args *po_args;
@@ -83,8 +84,10 @@ void existing_packs_retain_cruft(struct existing_packs *existing,
 				 struct packed_git *cruft);
 void existing_packs_mark_for_deletion(struct existing_packs *existing,
 				      struct string_list *names);
+void existing_packs_retain_midx_packs(struct existing_packs *existing);
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir);
+				     const char *packdir,
+				     bool wrote_incremental_midx);
 void existing_packs_release(struct existing_packs *existing);
 
 struct generated_pack;
@@ -129,7 +132,8 @@ struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry);
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir);
+				    const char *packdir,
+				    bool wrote_incremental_midx);
 void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
diff --git a/t/meson.build b/t/meson.build
index 7528e5cda5f..25f0d823d8e 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -951,6 +951,7 @@ integration_tests = [
   't7702-repack-cyclic-alternate.sh',
   't7703-repack-geometric.sh',
   't7704-repack-cruft.sh',
+  't7705-repack-incremental-midx.sh',
   't7800-difftool.sh',
   't7810-grep.sh',
   't7811-grep-open.sh',
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
new file mode 100755
index 00000000000..af1a1483b8c
--- /dev/null
+++ b/t/t7705-repack-incremental-midx.sh
@@ -0,0 +1,500 @@
+#!/bin/sh
+
+test_description='git repack --write-midx=incremental'
+
+. ./test-lib.sh
+
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
+
+objdir=.git/objects
+packdir=$objdir/pack
+midxdir=$packdir/multi-pack-index.d
+midx_chain=$midxdir/multi-pack-index-chain
+
+# incrementally_repack N
+#
+# Make "N" new commits, each stored in their own pack, and then repacked
+# with the --write-midx=incremental strategy.
+incrementally_repack () {
+	for i in $(test_seq 1 "$1")
+	do
+		test_commit "$i" &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify || return 1
+	done
+}
+
+# Create packs with geometrically increasing sizes so that they
+# satisfy the geometric progression and survive a --geometric=2
+# repack without being rolled up. Creates 3 packs containing 1,
+# 2, and 6 commits (3, 6, and 18 objects) respectively.
+create_geometric_packs () {
+	test_commit "small" &&
+	git repack -d &&
+
+	test_commit_bulk --message="medium" 2 &&
+	test_commit_bulk --message="large" 6 &&
+
+	git repack --geometric=2 -d --write-midx=incremental \
+		--write-bitmap-index
+}
+
+# create_layer <test_commit_bulk args>
+#
+# Creates a new MIDX layer with the contents of "test_commit_bulk $@".
+create_layer () {
+	test_commit_bulk "$@" &&
+
+	git multi-pack-index write --incremental --bitmap
+}
+
+# create_layers
+#
+# Reads lines of "<message> <nr>" from stdin and creates a new MIDX
+# layer for each line. See create_layer above for more.
+create_layers () {
+	while read msg nr
+	do
+		create_layer --message="$msg" "$nr" || return 1
+	done
+}
+
+test_expect_success '--write-midx=incremental requires --geometric' '
+	test_must_fail git repack --write-midx=incremental 2>err &&
+
+	test_grep -- "--write-midx=incremental requires --geometric" err
+'
+
+test_expect_success 'below layer threshold, tip packs excluded' '
+	git init below-layer-threshold-tip-packs-excluded &&
+	(
+		cd below-layer-threshold-tip-packs-excluded &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 4 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create 3 packs forming a geometric progression by
+		# object count such that they are unmodified by the
+		# initial repack. The MIDX chain thusly contains a
+		# single layer with three packs.
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a new commit. Since the layer threshold is
+		# unmet, a new MIDX layer is added on top of the
+		# existing one.
+		test_commit extra &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+		test_line_count = 1 packs.new &&
+
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs repacked' '
+	git init above-layer-threshold-tip-packs-repacked &&
+	(
+		cd above-layer-threshold-tip-packs-repacked &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Same setup, but with the layer threshold set to 2.
+		# Since the tip MIDX layer meets that threshold, its
+		# packs are considered repack candidates.
+		create_geometric_packs &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Perturb the existing progression such that it is
+		# rolled up into a single new pack, invalidating the
+		# existing MIDX layer and replacing it with a new one.
+		test_commit extra &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip layer preserved' '
+	git init above-layer-threshold-tip-layer-preserved &&
+	(
+		cd above-layer-threshold-tip-layer-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		test_commit_bulk --message="medium" 2 &&
+		test_commit_bulk --message="large" 6 &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 1 "$midx_chain" &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Create objects to form a pack satisfying the geometric
+		# progression (thus preserving the tip layer), but not
+		# so large that it meets the layer merging condition.
+		test_commit_bulk --message="small" 1 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		test_line_count = 1 packs.new &&
+		test_line_count = 3 packs.after &&
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs preserved' '
+	git init above-layer-threshold-tip-packs-preserved &&
+	(
+		cd above-layer-threshold-tip-packs-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Same setup as above, but this time the new objects do
+		# not satisfy the new layer merging condition, resulting
+		# in a new tip layer.
+		test_commit_bulk --message="huge" 18 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+		test_line_count = 1 packs.new &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'new tip absorbs multiple layers' '
+	git init new-tip-absorbs-multiple-layers &&
+	(
+		cd new-tip-absorbs-multiple-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a 4-layer chain where each layer is too small to
+		# absorb the one below it. The sizes must satisfy L(n) <
+		# L(n-1)/2 for each adjacent pair:
+		#
+		#   L0 (oldest): 75 obj (25 commits)
+		#   L1:          21 obj  (7 commits, 21 < 75/2)
+		#   L2:           9 obj  (3 commits,  9 < 21/2)
+		#   L3 (tip):     3 obj  (1 commit,   3 <  9/2)
+		create_layers <<-\EOF &&
+		L0 25
+		L1 7
+		L2 3
+		L3 1
+		EOF
+
+		test_line_count = 4 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Now add a new commit. The merging condition is
+		# satisfied between L3-L1, but violated at L0, which is
+		# too large relative to the accumulated size.
+		#
+		# As a result, the chain shrinks from 4 to 2 layers.
+		test_commit new &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'compaction of older layers' '
+	git init compaction-of-older-layers &&
+	(
+		cd compaction-of-older-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a chain with two small layers at the bottom
+		# and a larger barrier layer on top, producing a
+		# chain that violates the compaction invariant, since
+		# the two small layers would normally have been merged.
+		create_layers <<-\EOF &&
+		one 2
+		two 4
+		barrier 54
+		EOF
+
+		cp $midx_chain $midx_chain.before &&
+
+		# Running an incremental repack compacts the two
+		# small layers at the bottom of the chain as a
+		# separate step in the compaction plan.
+		test_commit another &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'geometric rollup with surviving tip packs' '
+	git init geometric-rollup-with-surviving-tip-packs &&
+	(
+		cd geometric-rollup-with-surviving-tip-packs &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create a pack large enough to anchor the geometric
+		# progression when small packs are added alongside it.
+		create_layer --message="big" 5 &&
+
+		test_line_count = 1 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a small number of objects such that the
+		# progression is unbothered. Note that the existing pack
+		# is considered a repack candidate as the new layer
+		# threshold is set to 1.
+		test_commit small-1 &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		cp $midx_chain $midx_chain.before
+	)
+'
+
+test_expect_success 'kept packs are excluded from repack' '
+	git init kept-packs-excluded-from-repack &&
+	(
+		cd kept-packs-excluded-from-repack &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create two equal-sized packs, marking one as kept.
+		for i in A B
+		do
+			test_commit "$i" && git repack -d || return 1
+		done &&
+
+		keep=$(ls $packdir/pack-*.idx | head -n 1) &&
+		touch "${keep%.idx}.keep" &&
+
+		# The kept pack is excluded as a repacking candidate
+		# entirely, so no rollup occurs as there is only one
+		# non-kept pack. A new MIDX layer is written containing
+		# that pack.
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count = 1 actual.packs &&
+		test_grep ! "$keep" actual.packs &&
+
+		git multi-pack-index verify &&
+
+		# All objects (from both kept and non-kept packs)
+		# must still be accessible.
+		git fsck
+	)
+'
+
+test_expect_success 'incremental MIDX with --max-pack-size' '
+	git init incremental-midx-with--max-pack-size &&
+	(
+		cd incremental-midx-with--max-pack-size &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		# Now add enough data that a small --max-pack-size will
+		# cause pack-objects to split its output. Create objects
+		# large enough to fill multiple packs.
+		test-tool genrandom foo 1M >big1 &&
+		test-tool genrandom bar 1M >big2 &&
+		git add big1 big2 &&
+		test_tick &&
+		git commit -a -m "big blobs" &&
+		git repack -d &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index --max-pack-size=1M &&
+
+		test_line_count = 1 "$midx_chain" &&
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count -gt 1 actual.packs &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'noop repack preserves valid MIDX chain' '
+	git init noop-repack-preserves-valid-midx-chain &&
+	(
+		cd noop-repack-preserves-valid-midx-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		git multi-pack-index verify &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Running again with no new objects should not break
+		# the MIDX chain. It produces "Nothing new to pack."
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_cmp $midx_chain.before $midx_chain &&
+
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
+test_expect_success 'repack -ad removes stale incremental chain' '
+	git init repack--ad-removes-stale-incremental-chain &&
+	(
+		cd repack--ad-removes-stale-incremental-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layers <<-\EOF &&
+		one 1
+		two 1
+		EOF
+
+		test_path_is_file $midx_chain &&
+		test_line_count = 2 $midx_chain &&
+
+		git repack -ad &&
+
+		test_path_is_missing $packdir/multi-pack-index &&
+		test_dir_is_empty $midxdir
+	)
+'
+
+test_expect_success 'repack -ad --write-midx=incremental is safe' '
+	git init ad-incremental-midx &&
+	(
+		cd ad-incremental-midx &&
+
+		git config maintenance.auto false &&
+
+		# Build a MIDX chain with multiple layers referencing
+		# distinct packs.
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack -d --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+
+		# Now do a full -ad repack. The new pack contains all
+		# objects, but any retained MIDX layers still reference
+		# the now-deleted packs.
+		test_commit third &&
+		git repack -ad --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		git fsck &&
+		git rev-list --all --objects >/dev/null
+	)
+'
+
+test_expect_success 'repack rejects invalid midxSplitFactor' '
+	test_when_finished "rm -fr bad-split-factor" &&
+	git init bad-split-factor &&
+	(
+		cd bad-split-factor &&
+		test_commit base &&
+
+		for v in 0 1 -1
+		do
+			test_must_fail git -c repack.midxSplitFactor=$v \
+				repack -d --geometric=2 --write-midx=incremental 2>err &&
+			test_grep "invalid value for --midx-split-factor" err ||
+			return 1
+		done
+	)
+'
+
+test_expect_success 'repack rejects invalid midxNewLayerThreshold' '
+	test_when_finished "rm -fr bad-layer-threshold" &&
+	git init bad-layer-threshold &&
+	(
+		cd bad-layer-threshold &&
+		test_commit base &&
+
+		for v in 0 -1
+		do
+			test_must_fail git -c repack.midxNewLayerThreshold=$v \
+				repack -d --geometric=2 --write-midx=incremental 2>err &&
+			test_grep "invalid value for --midx-new-layer-threshold" err ||
+			return 1
+		done
+	)
+'
+
+test_done
-- 
2.54.0.9.gb905fd5d0ae

[PATCH v2 16/16] repack: allow `--write-midx=incremental` without `--geometric`

[Taylor Blau]

Previously, `--write-midx=incremental` required `--geometric` and would
die() without it. Relax this restriction so that incremental MIDX
repacking can be used independently.

Without `--geometric`, the behavior is append-only: a single new MIDX
layer is created containing whatever packs were written by the repack
and appended to the existing chain (or a new chain is started). Existing
layers are preserved as-is with no compaction or merging.

Implement this via a new repack_make_midx_append_plan() that builds a
plan consisting of a WRITE step for the freshly written packs followed
by COPY steps for every existing MIDX layer. The existing compaction
plan (repack_make_midx_compaction_plan) is used only when `--geometric`
is active.

Update the documentation to describe the behavior with and without
`--geometric`, and replace the test that enforced the old restriction
with one exercising append-only incremental MIDX repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-repack.adoc      | 19 +++++----
 builtin/repack.c                   |  3 --
 repack-midx.c                      | 64 ++++++++++++++++++++++++++++--
 t/t7705-repack-incremental-midx.sh | 35 +++++++++++++---
 4 files changed, 103 insertions(+), 18 deletions(-)

diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 27a99cc46f4..72c42015e23 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -263,14 +263,19 @@ linkgit:git-multi-pack-index[1]).
 
 	`incremental`;;
 		Write an incremental MIDX chain instead of a single
-		flat MIDX. This mode requires `--geometric`.
+		flat MIDX.
 +
-The incremental mode maintains a chain of MIDX layers that is compacted
-over time using a geometric merging strategy. Each repack creates a new
-tip layer containing the newly written pack(s). Adjacent layers are then
-merged whenever the newer layer's object count exceeds
-`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
-that do not meet this condition are retained as-is.
+Without `--geometric`, a new MIDX layer is appended to the existing
+chain (or a new chain is started) containing whatever packs were written
+by the repack. Existing layers are preserved as-is.
++
+When combined with `--geometric`, the incremental mode maintains a chain
+of MIDX layers that is compacted over time using a geometric merging
+strategy. Each repack creates a new tip layer containing the newly
+written pack(s). Adjacent layers are then merged whenever the newer
+layer's object count exceeds `1/repack.midxSplitFactor` of the next
+deeper layer's count. Layers that do not meet this condition are
+retained as-is.
 +
 The result is that newer (tip) layers tend to contain many small packs
 with relatively few objects, while older (deeper) layers contain fewer,
diff --git a/builtin/repack.c b/builtin/repack.c
index 5ffa18e085e..1524a9c13ad 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -263,9 +263,6 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
-	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
-		die(_("--write-midx=incremental requires --geometric"));
-
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
diff --git a/repack-midx.c b/repack-midx.c
index 6a0358096ec..b1c429b8cee 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -555,6 +555,60 @@ static void midx_compaction_step_release(struct midx_compaction_step *step)
 	free(step->csum);
 }
 
+/*
+ * Build an append-only MIDX plan: a single WRITE step for the freshly
+ * written packs, plus COPY steps for every existing layer.  No
+ * compaction or merging is performed.
+ */
+static void repack_make_midx_append_plan(struct repack_write_midx_opts *opts,
+					 struct midx_compaction_step **steps_p,
+					 size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step *step;
+	size_t steps_nr = 0, steps_alloc = 0;
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	if (opts->names->nr) {
+		struct strbuf buf = STRBUF_INIT;
+		uint32_t i;
+
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_WRITE;
+		string_list_init_dup(&step->u.write);
+
+		for (i = 0; i < opts->names->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "pack-%s.idx",
+				    opts->names->items[i].string);
+			string_list_append(&step->u.write, buf.buf);
+		}
+
+		strbuf_release(&buf);
+	}
+
+	for (; m; m = m->base_midx) {
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_COPY;
+		step->u.copy = m;
+		step->objects_nr = m->num_objects;
+	}
+
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+}
+
 static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
 					    struct midx_compaction_step **steps_p,
 					    size_t *steps_nr_p)
@@ -911,9 +965,13 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 		goto done;
 	}
 
-	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
-		ret = error(_("unable to generate compaction plan"));
-		goto done;
+	if (opts->geometry->split_factor) {
+		if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+			ret = error(_("unable to generate compaction plan"));
+			goto done;
+		}
+	} else {
+		repack_make_midx_append_plan(opts, &steps, &steps_nr);
 	}
 
 	for (i = 0; i < steps_nr; i++) {
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
index af1a1483b8c..25a8c40e8ee 100755
--- a/t/t7705-repack-incremental-midx.sh
+++ b/t/t7705-repack-incremental-midx.sh
@@ -63,10 +63,36 @@ create_layers () {
 	done
 }
 
-test_expect_success '--write-midx=incremental requires --geometric' '
-	test_must_fail git repack --write-midx=incremental 2>err &&
+test_expect_success '--write-midx=incremental without --geometric' '
+	git init incremental-without-geometric &&
+	(
+		cd incremental-without-geometric &&
 
-	test_grep -- "--write-midx=incremental requires --geometric" err
+		git config maintenance.auto false &&
+
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# A second repack appends a new layer without
+		# disturbing the existing one.
+		test_commit third &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 2 $midx_chain &&
+		head -n 1 $midx_chain.before >expect &&
+		head -n 1 $midx_chain >actual &&
+		test_cmp expect actual &&
+
+		git fsck
+	)
 '
 
 test_expect_success 'below layer threshold, tip packs excluded' '
@@ -334,8 +360,7 @@ test_expect_success 'kept packs are excluded from repack' '
 		# entirely, so no rollup occurs as there is only one
 		# non-kept pack. A new MIDX layer is written containing
 		# that pack.
-		git repack --geometric=2 -d --write-midx=incremental \
-			--write-bitmap-index &&
+		git repack --geometric=2 -d --write-midx=incremental &&
 
 		test-tool read-midx $objdir >actual &&
 		grep "^pack-.*\.idx$" actual >actual.packs &&
-- 
2.54.0.9.gb905fd5d0ae

Re: [PATCH v2 15/16] repack: introduce `--write-midx=incremental`

[Taylor Blau]

On Tue, Apr 21, 2026 at 04:37:57PM -0400, Taylor Blau wrote:
> ---
>  Documentation/config/repack.adoc   |  18 ++
>  Documentation/git-repack.adoc      |  39 ++-
>  builtin/repack.c                   |  49 ++-
>  midx.c                             |  31 ++
>  midx.h                             |   3 +
>  repack-geometry.c                  |  13 +-
>  repack-midx.c                      |   5 +
>  repack.c                           |  56 +++-
>  repack.h                           |  10 +-
>  t/meson.build                      |   1 +
>  t/t7705-repack-incremental-midx.sh | 500 +++++++++++++++++++++++++++++
>  11 files changed, 701 insertions(+), 24 deletions(-)
>  create mode 100755 t/t7705-repack-incremental-midx.sh

I squashed a new test into this commit, but meant to squash it into the
latter. That test depends on functionality that is not yet introduced
until the subsequent commit, hence t7705 fails on this commit.

Since the maintainer is offline currently, I'll avoid re-submitting for
now since this won't get queued anyways. Once there is some review, I'll
re-submit the series with the test squashed into the right commit,
alongside any changes from that review.

Sorry about that.

Thanks,
Taylor

Re: [PATCH v2 00/16] repack: incremental MIDX/bitmap-based repacking

[Elijah Newren]

On Tue, Apr 21, 2026 at 1:37 PM Taylor Blau <me@ttaylorr.com> wrote:
>
> [Note to the maintainer, this is rebased onto current 'master', which is
> 94f057755b7 (Git 2.54, 2026-04-19) at the time of writing.]
>
> This a modest-sized reroll of my series to implement the last remaining
> component of the incremental MIDX/bitmap-based repacking strategy that I
> have been working on.
>
> The main changes since last time are described in [1], which I sent to
> the list last week just before the 2.54 release was tagged. For
> convenience, a copy of the main changes are below:
>
>  - Use a strset instead of a string_list for keeping track of the MIDX
>    layers to retain when calling `either clear_midx_files_ext()` or
>    `clear_incremental_midx_files_ext()`.
>
>  - A new patch to rewrite the logic for determining which MIDX layers
>    comprise the new chain via keep_hashes to build the array in order.
>    The subsequent patch converts that into a strvec, which no longer
>    requires direct manipulation.
>
>  - The new "--checksum-only" option has been renamed to
>    "--no-write-chain-file", and various small implementation tweaks
>    (e.g., relying on `is_lock_file_locked()` to determine whether we
>    should update the chain file as opposed to reading the flags).
>
> As usual, a range-diff is included below as well for convenience. Thanks
> in advance for reviewing!
[...]
> Range-diff against v1:
[...]

This round addresses all the (admittedly minor) pieces of feedback I
had on v1, and I didn't spot anything new to comment on in looking
over the changes in this round.

Re: [PATCH v2 14/16] repack: implement incremental MIDX repacking

[Jeff King]

On Tue, Apr 21, 2026 at 04:37:54PM -0400, Taylor Blau wrote:

>  static int repack_fill_midx_stdin_packs(struct child_process *cmd,
> -					struct string_list *include)
> +					struct string_list *include,
> +					struct string_list *out)
>  {
>  	struct string_list_item *item;
>  	FILE *in;
>  	int ret;
>  
>  	cmd->in = -1;
> +	if (out)
> +		cmd->out = -1;

We are already piping to the sub-command, and now we will read the
output. Looks like we do them sequentially:

> @@ -312,6 +319,17 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
>  		fprintf(in, "%s\n", item->string);
>  	fclose(in);
>  
> +	if (out) {
> +		struct strbuf buf = STRBUF_INIT;
> +		FILE *outf = xfdopen(cmd->out, "r");
> +
> +		while (strbuf_getline(&buf, outf) != EOF)
> +			string_list_append(out, buf.buf);
> +		strbuf_release(&buf);
> +
> +		fclose(outf);
> +	}

Is it possible to deadlock here where we block writing to the child, but
the child is blocked trying to write back to us. It's probably quite
unlikely as it implies both pipe buffers are filled up (and we are
counting packs and midx hashes here, neither of which we'd expect to be
too numerous).

Using pipe_command() would solve this, but it also might be impossible
to trigger if the child reads all input before generating any output. I
_think_ that's the case looking at cmd_multi_pack_index_write(). So
we're OK, but you might want to double check.

> +static int midx_compaction_step_exec_compact(struct midx_compaction_step *step,
> +					     struct repack_write_midx_opts *opts)
> +{
> [...]
> +	ret = start_command(&cmd);
> +	if (ret)
> +		goto out;

OK, if we get an error here we'll jump to "out", but run-command.c will
have cleaned up everything about "cmd" that we need.

But later...

> +	out = xfdopen(cmd.out, "r");
> +	while (strbuf_getline_lf(&buf, out) != EOF) {
> +		if (step->csum) {
> +			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
> +			goto out;
> +		}

...here we jump to "out" but the command is left running. I guess it
will eventually get SIGPIPE when we fclose(out), but of course we won't
wait() for it and we're probably leaking some memory, too.

> +	}
> +
> +	ret = finish_command(&cmd);
> +
> +out:
> +	if (out)
> +		fclose(out);
> +	strbuf_release(&buf);

It would be nice if we had an idempotent version of finish_command(),
that would do nothing when the program was already done. But I don't
think we do. We don't even clear cmd->pid when we finish the command, so
you can't even do something like:

  out:
  if (cmd->pid)
	finish_command(&cmd);

So probably that "goto out" in the loop needs to call fclose() and
finish_command() itself.

-Peff

Re: [PATCH v2 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Jeff King]

On Tue, Apr 21, 2026 at 04:37:42PM -0400, Taylor Blau wrote:

> This simplifies `write_midx_included_packs()` and prepares for a
> subsequent commit where the same helper is called with `cmd->out = -1`
> to capture the MIDX's checksum from the command's standard output,
> which is needed when writing MIDX layers with `--checksum-only`.

This should be --no-write-chain-file now, right? It's not used in the
code here, so it's just a commit message fixup.

-Peff

Re: [PATCH v2 14/16] repack: implement incremental MIDX repacking

[Jeff King]

On Tue, Apr 21, 2026 at 04:37:54PM -0400, Taylor Blau wrote:

> Unlike the default mode which writes a single flat MIDX, the incremental
> mode constructs a compaction plan that determines which MIDX layers to
> write, compact, or copy, and then executes each step using `git
> multi-pack-index` subcommands with the --checksum-only flag.

This should be --no-write-chain-file, I think.

Ditto here:

> After writing the new layer, the strategy is evaluated among the
> existing MIDX layers in order from oldest to newest. Each step that
> writes a new MIDX layer uses "--checksum-only" to avoid updating the
> multi-pack-index-chain file. After all steps are complete, the new chain
> file is written and then atomically moved into place.

In the code I think it is all good, though:

> +	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);

-Peff

Re: [PATCH v2 00/16] repack: incremental MIDX/bitmap-based repacking

[Jeff King]

On Tue, Apr 21, 2026 at 04:37:08PM -0400, Taylor Blau wrote:

> The main changes since last time are described in [1], which I sent to
> the list last week just before the 2.54 release was tagged. For
> convenience, a copy of the main changes are below:
> 
>  - Use a strset instead of a string_list for keeping track of the MIDX
>    layers to retain when calling `either clear_midx_files_ext()` or
>    `clear_incremental_midx_files_ext()`.
> 
>  - A new patch to rewrite the logic for determining which MIDX layers
>    comprise the new chain via keep_hashes to build the array in order.
>    The subsequent patch converts that into a strvec, which no longer
>    requires direct manipulation.
> 
>  - The new "--checksum-only" option has been renamed to
>    "--no-write-chain-file", and various small implementation tweaks
>    (e.g., relying on `is_lock_file_locked()` to determine whether we
>    should update the chain file as opposed to reading the flags).

Thanks, I think that addresses my review from the previous round.

I looked over the final few patches. Aside from a small nit about a
possible leak, I didn't notice anything obviously wrong. There's enough
new and tricky concepts that I wouldn't guarantee there's not a subtle
corner case lurking, but I don't think me staring at it any longer is
going to yield any further insight.

-Peff

Re: [PATCH v2 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Taylor Blau]

On Wed, Apr 29, 2026 at 04:08:21AM -0400, Jeff King wrote:
> On Tue, Apr 21, 2026 at 04:37:42PM -0400, Taylor Blau wrote:
>
> > This simplifies `write_midx_included_packs()` and prepares for a
> > subsequent commit where the same helper is called with `cmd->out = -1`
> > to capture the MIDX's checksum from the command's standard output,
> > which is needed when writing MIDX layers with `--checksum-only`.
>
> This should be --no-write-chain-file now, right? It's not used in the
> code here, so it's just a commit message fixup.

Yup, good spotting. Fixed.

Thanks,
Taylor

Re: [PATCH v2 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

On Wed, Apr 29, 2026 at 03:51:50AM -0400, Jeff King wrote:
> > @@ -312,6 +319,17 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
> >  		fprintf(in, "%s\n", item->string);
> >  	fclose(in);
> >
> > +	if (out) {
> > +		struct strbuf buf = STRBUF_INIT;
> > +		FILE *outf = xfdopen(cmd->out, "r");
> > +
> > +		while (strbuf_getline(&buf, outf) != EOF)
> > +			string_list_append(out, buf.buf);
> > +		strbuf_release(&buf);
> > +
> > +		fclose(outf);
> > +	}
>
> Is it possible to deadlock here where we block writing to the child, but
> the child is blocked trying to write back to us. It's probably quite
> unlikely as it implies both pipe buffers are filled up (and we are
> counting packs and midx hashes here, neither of which we'd expect to be
> too numerous).
>
> Using pipe_command() would solve this, but it also might be impossible
> to trigger if the child reads all input before generating any output. I
> _think_ that's the case looking at cmd_multi_pack_index_write(). So
> we're OK, but you might want to double check.

I think in theory this could deadlock, though in practice it is highly
unlikely. Regardless, using pipe_command() here is straightforward, and
guarantees that we'll avoid a nasty deadlock, so let's do that.

The resulting code is a lot easier to read, too, which is nice:

--- 8< ---
diff --git a/repack-midx.c b/repack-midx.c
index 8f3720772b8..9db59b18334 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -300,37 +300,27 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 					struct string_list *include,
 					struct string_list *out)
 {
+	struct strbuf in_buf = STRBUF_INIT;
+	struct strbuf out_buf = STRBUF_INIT;
 	struct string_list_item *item;
-	FILE *in;
 	int ret;

-	cmd->in = -1;
-	if (out)
-		cmd->out = -1;
-
 	strvec_push(&cmd->args, "--stdin-packs");

-	ret = start_command(cmd);
-	if (ret)
-		return ret;
-
-	in = xfdopen(cmd->in, "w");
 	for_each_string_list_item(item, include)
-		fprintf(in, "%s\n", item->string);
-	fclose(in);
+		strbuf_addf(&in_buf, "%s\n", item->string);

-	if (out) {
-		struct strbuf buf = STRBUF_INIT;
-		FILE *outf = xfdopen(cmd->out, "r");
+	ret = pipe_command(cmd, in_buf.buf, in_buf.len,
+			   out ? &out_buf : NULL, 0, NULL, 0);

-		while (strbuf_getline(&buf, outf) != EOF)
-			string_list_append(out, buf.buf);
-		strbuf_release(&buf);
+	if (out)
+		string_list_split_f(out, out_buf.buf, "\n", -1,
+				    STRING_LIST_SPLIT_NONEMPTY);

-		fclose(outf);
-	}
+	strbuf_release(&in_buf);
+	strbuf_release(&out_buf);

-	return finish_command(cmd);
+	return ret;
 }

 static int write_midx_included_packs(struct repack_write_midx_opts *opts)
--- >8 ---

> > +static int midx_compaction_step_exec_compact(struct midx_compaction_step *step,
> > +					     struct repack_write_midx_opts *opts)
> > +{
> > [...]
> > +	ret = start_command(&cmd);
> > +	if (ret)
> > +		goto out;
>
> OK, if we get an error here we'll jump to "out", but run-command.c will
> have cleaned up everything about "cmd" that we need.
>
> But later...
>
> > +	out = xfdopen(cmd.out, "r");
> > +	while (strbuf_getline_lf(&buf, out) != EOF) {
> > +		if (step->csum) {
> > +			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
> > +			goto out;
> > +		}
>
> ...here we jump to "out" but the command is left running. I guess it
> will eventually get SIGPIPE when we fclose(out), but of course we won't
> wait() for it and we're probably leaking some memory, too.

Good catch, I fixed this up in the way you suggested to call
`fclose(out)` and `finish_command()` within this loop before jumping
out.

Thanks,
Taylor

Re: [PATCH v2 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

On Wed, Apr 29, 2026 at 04:10:17AM -0400, Jeff King wrote:
> On Tue, Apr 21, 2026 at 04:37:54PM -0400, Taylor Blau wrote:
>
> > Unlike the default mode which writes a single flat MIDX, the incremental
> > mode constructs a compaction plan that determines which MIDX layers to
> > write, compact, or copy, and then executes each step using `git
> > multi-pack-index` subcommands with the --checksum-only flag.
>
> This should be --no-write-chain-file, I think.
>
> Ditto here:

Yup, thanks for spotting.

> > After writing the new layer, the strategy is evaluated among the
> > existing MIDX layers in order from oldest to newest. Each step that
> > writes a new MIDX layer uses "--checksum-only" to avoid updating the
> > multi-pack-index-chain file. After all steps are complete, the new chain
> > file is written and then atomically moved into place.
>
> In the code I think it is all good, though:
>
> > +	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);

Heh. Clearly I 'git grep'd through the code, but didn't adjust the
commit messages. These should all be fixed, and now by the end of the
series we have:

    $ git log -p @{u}.. | grep -c ..checksum.only
    0

Thanks,
Taylor

[PATCH v3 00/16] repack: incremental MIDX/bitmap-based repacking

[Taylor Blau]

This a small reroll of my series to implement the last remaining
component of the incremental MIDX/bitmap-based repacking strategy that I
have been working on.

I expect that this should be the final reroll absent any late-breaking
feedback. The only changes since last time are the following:

 - Various stale references to "--checksum-only" have been eradicated
   from commit message(s).

 - `repack_prepare_midx_command()` now uses `pipe_command()` to
   eliminate the possibility of a deadlock.

 - `write_midx_included_packs()` now cleans up after itself properly
   when receiving multiple lines of output.

 - The errant test in t7705 was moved to the final commit (where it
   belongs) instead of the penultimate one.

As usual, a range-diff is included below as well for convenience. Thanks
in advance for reviewing!

Taylor Blau (16):
  midx-write: handle noop writes when converting incremental chains
  midx: use `strset` for retained MIDX files
  midx: build `keep_hashes` array in order
  midx: use `strvec` for `keep_hashes`
  midx: introduce `--no-write-chain-file` for incremental MIDX writes
  midx: support custom `--base` for incremental MIDX writes
  repack: track the ODB source via existing_packs
  midx: expose `midx_layer_contains_pack()`
  repack-midx: factor out `repack_prepare_midx_command()`
  repack-midx: extract `repack_fill_midx_stdin_packs()`
  repack-geometry: prepare for incremental MIDX repacking
  builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
  packfile: ensure `close_pack_revindex()` frees in-memory revindex
  repack: implement incremental MIDX repacking
  repack: introduce `--write-midx=incremental`
  repack: allow `--write-midx=incremental` without `--geometric`

 Documentation/config/repack.adoc        |  18 +
 Documentation/git-multi-pack-index.adoc |  32 +-
 Documentation/git-repack.adoc           |  44 +-
 builtin/multi-pack-index.c              |  48 +-
 builtin/repack.c                        | 102 +++-
 midx-write.c                            | 206 ++++---
 midx.c                                  | 104 ++--
 midx.h                                  |  11 +-
 packfile.c                              |   2 +
 repack-geometry.c                       |  48 +-
 repack-midx.c                           | 710 +++++++++++++++++++++++-
 repack.c                                |  58 +-
 repack.h                                |  26 +-
 t/meson.build                           |   1 +
 t/t5334-incremental-multi-pack-index.sh |  63 +++
 t/t5335-compact-multi-pack-index.sh     | 113 ++++
 t/t7705-repack-incremental-midx.sh      | 525 ++++++++++++++++++
 17 files changed, 1911 insertions(+), 200 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh

Range-diff against v2:
 -:  ----------- >  1:  d6c27317c25 midx-write: handle noop writes when converting incremental chains
 -:  ----------- >  2:  629c8d23116 midx: use `strset` for retained MIDX files
 -:  ----------- >  3:  e303bf6a4ac midx: build `keep_hashes` array in order
 -:  ----------- >  4:  42d76c70060 midx: use `strvec` for `keep_hashes`
 -:  ----------- >  5:  2c80aa34fac midx: introduce `--no-write-chain-file` for incremental MIDX writes
 7:  d9acef1334a !  6:  2a05f4b86f3 repack: allow `--write-midx=incremental` without `--geometric`
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>
     
      ## Commit message ##
    -    repack: allow `--write-midx=incremental` without `--geometric`
    +    midx: support custom `--base` for incremental MIDX writes
     
    -    Previously, `--write-midx=incremental` required `--geometric` and would
    -    die() without it. Relax this restriction so that incremental MIDX
    -    repacking can be used independently.
    +    Both `compact` and `write --incremental` fix the base of the resulting
    +    MIDX layer: `compact` always places the compacted result on top of
    +    "from's" immediate parent in the chain, and `write --incremental` always
    +    appends a new layer to the existing tip. In both cases the base is not
    +    configurable.
     
    -    Without `--geometric`, the behavior is append-only: a single new MIDX
    -    layer is created containing whatever packs were written by the repack
    -    and appended to the existing chain (or a new chain is started). Existing
    -    layers are preserved as-is with no compaction or merging.
    +    Future callers need additional flexibility. For instance, the incremental
    +    MIDX-based repacking code may wish to write a layer based on some
    +    intermediate ancestor rather than the current tip, or produce a root
    +    layer when replacing the bottommost entries in the chain.
     
    -    Implement this via a new repack_make_midx_append_plan() that builds a
    -    plan consisting of a WRITE step for the freshly written packs followed
    -    by COPY steps for every existing MIDX layer. The existing compaction
    -    plan (repack_make_midx_compaction_plan) is used only when `--geometric`
    -    is active.
    +    Introduce a new `--base` option for both subcommands to specify the
    +    checksum of the MIDX layer to use as the base. The given checksum must
    +    refer to a valid layer in the MIDX chain that is an ancestor of the
    +    topmost layer being written or compacted.
     
    -    Update the documentation to describe the behavior with and without
    -    `--geometric`, and replace the test that enforced the old restriction
    -    with one exercising append-only incremental MIDX repacking.
    +    The special value "none" is accepted to produce a root layer with no
    +    parent. This will be needed when the incremental repacking machinery
    +    determines that the bottommost layers of the chain should be replaced.
    +
    +    If no `--base` is given, behavior is unchanged: `compact` uses "from's"
    +    immediate parent in the chain, and `write` appends to the existing tip.
    +
    +    For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
    +    `write --incremental` appends a new layer to the live chain tip with no
    +    mechanism to atomically replace it; overriding the base would produce a
    +    layer that does not extend the tip, breaking chain invariants. With
    +    `--no-write-chain-file` the chain is left unmodified and the caller is
    +    responsible for assembling a valid chain.
    +
    +    For `compact`, no such restriction applies. The compaction operation
    +    atomically replaces the compacted range in the chain file, so writing
    +    the result on top of any valid ancestor preserves chain invariants.
     
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
    - ## Documentation/git-repack.adoc ##
    -@@ Documentation/git-repack.adoc: linkgit:git-multi-pack-index[1]).
    + ## Documentation/git-multi-pack-index.adoc ##
    +@@ Documentation/git-multi-pack-index.adoc: SYNOPSIS
    + 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
    + 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
    + 		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
    ++			 [--base=<checksum>]
    + 'git multi-pack-index' [<options>] compact [--[no-]incremental]
    +-		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
    ++		         [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]
    ++			 <from> <to>
    + 'git multi-pack-index' [<options>] verify
    + 'git multi-pack-index' [<options>] expire
    + 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
    +@@ Documentation/git-multi-pack-index.adoc: marker).
    + 		The checksum of the new layer is printed to standard
    + 		output, allowing the caller to assemble and write the
    + 		chain itself. Requires `--incremental`.
    ++
    ++	--base=<checksum>::
    ++		Specify the checksum of an existing MIDX layer to use
    ++		as the base when writing a new incremental layer.
    ++		The special value `none` indicates that the new layer
    ++		should have no base (i.e., it becomes a root layer).
    ++		Requires `--no-write-chain-file`.
    + --
      
    - 	`incremental`;;
    - 		Write an incremental MIDX chain instead of a single
    --		flat MIDX. This mode requires `--geometric`.
    -+		flat MIDX.
    - +
    --The incremental mode maintains a chain of MIDX layers that is compacted
    --over time using a geometric merging strategy. Each repack creates a new
    --tip layer containing the newly written pack(s). Adjacent layers are then
    --merged whenever the newer layer's object count exceeds
    --`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
    --that do not meet this condition are retained as-is.
    -+Without `--geometric`, a new MIDX layer is appended to the existing
    -+chain (or a new chain is started) containing whatever packs were written
    -+by the repack. Existing layers are preserved as-is.
    -++
    -+When combined with `--geometric`, the incremental mode maintains a chain
    -+of MIDX layers that is compacted over time using a geometric merging
    -+strategy. Each repack creates a new tip layer containing the newly
    -+written pack(s). Adjacent layers are then merged whenever the newer
    -+layer's object count exceeds `1/repack.midxSplitFactor` of the next
    -+deeper layer's count. Layers that do not meet this condition are
    -+retained as-is.
    + compact::
    +@@ Documentation/git-multi-pack-index.adoc: compact::
    + 		MIDX layer but do not update the multi-pack-index-chain
    + 		file. The checksum of the new layer is printed to
    + 		standard output. Requires `--incremental`.
    ++
    ++	--base=<checksum>::
    ++		Specify the checksum of an existing MIDX layer to use
    ++		as the base for the compacted result, instead of using
    ++		the immediate parent of `<from>`. The special value
    ++		`none` indicates that the result should have no base.
    + --
      +
    - The result is that newer (tip) layers tend to contain many small packs
    - with relatively few objects, while older (deeper) layers contain fewer,
    + Note that the compact command requires writing a version-2 midx that
     
    - ## builtin/repack.c ##
    -@@ builtin/repack.c: int cmd_repack(int argc,
    - 	if (pack_everything & PACK_CRUFT)
    - 		pack_everything |= ALL_INTO_ONE;
    - 
    --	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
    --		die(_("--write-midx=incremental requires --geometric"));
    --
    - 	if (write_bitmaps < 0) {
    - 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
    - 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
    -
    - ## repack-midx.c ##
    -@@ repack-midx.c: static void midx_compaction_step_release(struct midx_compaction_step *step)
    - 	free(step->csum);
    - }
    + ## builtin/multi-pack-index.c ##
    +@@
    + #define BUILTIN_MIDX_WRITE_USAGE \
    + 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
    + 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
    +-	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
    ++	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]\n" \
    ++	   "  [--base=<checksum>]")
      
    -+/*
    -+ * Build an append-only MIDX plan: a single WRITE step for the freshly
    -+ * written packs, plus COPY steps for every existing layer.  No
    -+ * compaction or merging is performed.
    -+ */
    -+static void repack_make_midx_append_plan(struct repack_write_midx_opts *opts,
    -+					 struct midx_compaction_step **steps_p,
    -+					 size_t *steps_nr_p)
    -+{
    -+	struct multi_pack_index *m;
    -+	struct midx_compaction_step *steps = NULL;
    -+	struct midx_compaction_step *step;
    -+	size_t steps_nr = 0, steps_alloc = 0;
    -+
    -+	odb_reprepare(opts->existing->repo->objects);
    -+	m = get_multi_pack_index(opts->existing->source);
    + #define BUILTIN_MIDX_COMPACT_USAGE \
    + 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
    +-	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
    ++	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]\n" \
    ++	   "  <from> <to>")
    + 
    + #define BUILTIN_MIDX_VERIFY_USAGE \
    + 	N_("git multi-pack-index [<options>] verify")
    +@@ builtin/multi-pack-index.c: static char const * const builtin_multi_pack_index_usage[] = {
    + static struct opts_multi_pack_index {
    + 	char *object_dir;
    + 	const char *preferred_pack;
    ++	const char *incremental_base;
    + 	char *refs_snapshot;
    + 	unsigned long batch_size;
    + 	unsigned flags;
    +@@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, const char **argv,
    + 			   N_("pack for reuse when computing a multi-pack bitmap")),
    + 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
    + 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
    ++		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
    ++			   N_("base MIDX for incremental writes")),
    + 		OPT_BIT(0, "incremental", &opts.flags,
    + 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
    + 		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
    +@@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, const char **argv,
    + 				   options);
    + 	}
    + 
    ++	if (opts.incremental_base &&
    ++	    !(opts.flags & MIDX_WRITE_NO_CHAIN)) {
    ++		error(_("cannot use --base without --no-write-chain-file"));
    ++		usage_with_options(builtin_multi_pack_index_write_usage,
    ++				   options);
    ++	}
     +
    -+	if (opts->names->nr) {
    -+		struct strbuf buf = STRBUF_INIT;
    -+		uint32_t i;
    + 	source = handle_object_dir_option(repo);
    + 
    + 	FREE_AND_NULL(options);
    +@@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_write(int argc, const char **argv,
    + 
    + 		ret = write_midx_file_only(source, &packs,
    + 					   opts.preferred_pack,
    +-					   opts.refs_snapshot, opts.flags);
    ++					   opts.refs_snapshot,
    ++					   opts.incremental_base, opts.flags);
    + 
    + 		string_list_clear(&packs, 0);
    + 		free(opts.refs_snapshot);
    +@@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, const char **argv,
    + 
    + 	struct option *options;
    + 	static struct option builtin_multi_pack_index_compact_options[] = {
    ++		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
    ++			   N_("base MIDX for incremental writes")),
    + 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
    + 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
    + 		OPT_BIT(0, "incremental", &opts.flags,
    +@@ builtin/multi-pack-index.c: static int cmd_multi_pack_index_compact(int argc, const char **argv,
    + 			die(_("MIDX %s must be an ancestor of %s"), argv[0], argv[1]);
    + 	}
    + 
    +-	ret = write_midx_file_compact(source, from_midx, to_midx, opts.flags);
    ++	ret = write_midx_file_compact(source, from_midx, to_midx,
    ++				      opts.incremental_base, opts.flags);
    + 
    + 	return ret;
    + }
    +
    + ## midx-write.c ##
    +@@ midx-write.c: struct write_midx_opts {
    + 
    + 	const char *preferred_pack_name;
    + 	const char *refs_snapshot;
    ++	const char *incremental_base;
    + 	unsigned flags;
    + };
    + 
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 
    + 	/*
    + 	 * If compacting MIDX layer(s) in the range [from, to], then the
    +-	 * compacted MIDX will share the same base MIDX as 'from'.
    ++	 * compacted MIDX will share the same base MIDX as 'from',
    ++	 * unless a custom --base is specified (see below).
    + 	 */
    + 	if (ctx.compact)
    + 		ctx.base_midx = ctx.compact_from->base_midx;
    + 
    ++	if (opts->incremental_base) {
    ++		if (!strcmp(opts->incremental_base, "none")) {
    ++			ctx.base_midx = NULL;
    ++		} else {
    ++			while (ctx.base_midx) {
    ++				const char *cmp = midx_get_checksum_hex(ctx.base_midx);
    ++				if (!strcmp(opts->incremental_base, cmp))
    ++					break;
     +
    -+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
    ++				ctx.base_midx = ctx.base_midx->base_midx;
    ++			}
     +
    -+		step = &steps[steps_nr++];
    -+		memset(step, 0, sizeof(*step));
    ++			if (!ctx.base_midx) {
    ++				error(_("could not find base MIDX '%s'"),
    ++				      opts->incremental_base);
    ++				goto cleanup;
    ++			}
    ++		}
    ++	}
     +
    -+		step->type = MIDX_COMPACTION_STEP_WRITE;
    -+		string_list_init_dup(&step->u.write);
    + 	ctx.nr = 0;
    + 	ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
    + 	ctx.info = NULL;
    +@@ midx-write.c: static int write_midx_internal(struct write_midx_opts *opts)
    + 
    + int write_midx_file(struct odb_source *source,
    + 		    const char *preferred_pack_name,
    +-		    const char *refs_snapshot, unsigned flags)
    ++		    const char *refs_snapshot,
    ++		    unsigned flags)
    + {
    + 	struct write_midx_opts opts = {
    + 		.source = source,
    +@@ midx-write.c: int write_midx_file(struct odb_source *source,
    + int write_midx_file_only(struct odb_source *source,
    + 			 struct string_list *packs_to_include,
    + 			 const char *preferred_pack_name,
    +-			 const char *refs_snapshot, unsigned flags)
    ++			 const char *refs_snapshot,
    ++			 const char *incremental_base,
    ++			 unsigned flags)
    + {
    + 	struct write_midx_opts opts = {
    + 		.source = source,
    + 		.packs_to_include = packs_to_include,
    + 		.preferred_pack_name = preferred_pack_name,
    + 		.refs_snapshot = refs_snapshot,
    ++		.incremental_base = incremental_base,
    + 		.flags = flags,
    + 	};
    + 
    +@@ midx-write.c: int write_midx_file_only(struct odb_source *source,
    + int write_midx_file_compact(struct odb_source *source,
    + 			    struct multi_pack_index *from,
    + 			    struct multi_pack_index *to,
    ++			    const char *incremental_base,
    + 			    unsigned flags)
    + {
    + 	struct write_midx_opts opts = {
    + 		.source = source,
    + 		.compact_from = from,
    + 		.compact_to = to,
    ++		.incremental_base = incremental_base,
    + 		.flags = flags | MIDX_WRITE_COMPACT,
    + 	};
    + 
    +
    + ## midx.h ##
    +@@ midx.h: int write_midx_file(struct odb_source *source,
    + int write_midx_file_only(struct odb_source *source,
    + 			 struct string_list *packs_to_include,
    + 			 const char *preferred_pack_name,
    +-			 const char *refs_snapshot, unsigned flags);
    ++			 const char *refs_snapshot,
    ++			 const char *incremental_base,
    ++			 unsigned flags);
    + int write_midx_file_compact(struct odb_source *source,
    + 			    struct multi_pack_index *from,
    + 			    struct multi_pack_index *to,
    ++			    const char *incremental_base,
    + 			    unsigned flags);
    + void clear_midx_file(struct repository *r);
    + int verify_midx_file(struct odb_source *source, unsigned flags);
    +
    + ## t/t5334-incremental-multi-pack-index.sh ##
    +@@ t/t5334-incremental-multi-pack-index.sh: test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file
    + 	test_grep "cannot use --no-write-chain-file without --incremental" err
    + '
    + 
    ++test_expect_success 'write MIDX layer with --base without --no-write-chain-file' '
    ++	test_must_fail git multi-pack-index write --bitmap --incremental \
    ++		--base=none 2>err &&
    ++	test_grep "cannot use --base without --no-write-chain-file" err
    ++'
     +
    -+		for (i = 0; i < opts->names->nr; i++) {
    -+			strbuf_reset(&buf);
    -+			strbuf_addf(&buf, "pack-%s.idx",
    -+				    opts->names->items[i].string);
    -+			string_list_append(&step->u.write, buf.buf);
    -+		}
    ++test_expect_success 'write MIDX layer with --base=none and --no-write-chain-file' '
    ++	test_commit base-none &&
    ++	git repack -d &&
     +
    -+		strbuf_release(&buf);
    -+	}
    ++	cp "$midx_chain" "$midx_chain.bak" &&
    ++	layer="$(git multi-pack-index write --bitmap --incremental \
    ++		--no-write-chain-file --base=none)" &&
     +
    -+	for (; m; m = m->base_midx) {
    -+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
    ++	test_cmp "$midx_chain.bak" "$midx_chain" &&
    ++	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
    ++'
     +
    -+		step = &steps[steps_nr++];
    -+		memset(step, 0, sizeof(*step));
    ++test_expect_success 'write MIDX layer with --base=<hash> and --no-write-chain-file' '
    ++	test_commit base-hash &&
    ++	git repack -d &&
     +
    -+		step->type = MIDX_COMPACTION_STEP_COPY;
    -+		step->u.copy = m;
    -+		step->objects_nr = m->num_objects;
    -+	}
    ++	cp "$midx_chain" "$midx_chain.bak" &&
    ++	layer="$(git multi-pack-index write --bitmap --incremental \
    ++		--no-write-chain-file --base="$(nth_line 1 "$midx_chain")")" &&
     +
    -+	*steps_p = steps;
    -+	*steps_nr_p = steps_nr;
    -+}
    ++	test_cmp "$midx_chain.bak" "$midx_chain" &&
    ++	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
    ++'
     +
    - static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
    - 					    struct midx_compaction_step **steps_p,
    - 					    size_t *steps_nr_p)
    -@@ repack-midx.c: static int write_midx_incremental(struct repack_write_midx_opts *opts)
    - 		goto done;
    - 	}
    + for reuse in false single multi
    + do
    + 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
    +
    + ## t/t5335-compact-multi-pack-index.sh ##
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '
      
    --	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
    --		ret = error(_("unable to generate compaction plan"));
    --		goto done;
    -+	if (opts->geometry->split_factor) {
    -+		if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
    -+			ret = error(_("unable to generate compaction plan"));
    -+			goto done;
    -+		}
    -+	} else {
    -+		repack_make_midx_append_plan(opts, &steps, &steps_nr);
    - 	}
    + 		layer="$(git multi-pack-index compact --incremental \
    + 			--no-write-chain-file \
    ++			--base="$(nth_line 1 "$midx_chain")" \
    + 			"$(nth_line 2 "$midx_chain")" \
    + 			"$(nth_line 3 "$midx_chain")")" &&
      
    - 	for (i = 0; i < steps_nr; i++) {
    -
    - ## t/t7705-repack-incremental-midx.sh ##
    -@@ t/t7705-repack-incremental-midx.sh: create_layers () {
    - 	done
    - }
    +@@ t/t5335-compact-multi-pack-index.sh: test_expect_success 'MIDX compaction with --no-write-chain-file' '
    + 	)
    + '
      
    --test_expect_success '--write-midx=incremental requires --geometric' '
    --	test_must_fail git repack --write-midx=incremental 2>err &&
    -+test_expect_success '--write-midx=incremental without --geometric' '
    -+	git init incremental-without-geometric &&
    ++test_expect_success 'MIDX compaction with --base' '
    ++	git init midx-compact-with--base &&
     +	(
    -+		cd incremental-without-geometric &&
    - 
    --	test_grep -- "--write-midx=incremental requires --geometric" err
    ++		cd midx-compact-with--base &&
    ++
     +		git config maintenance.auto false &&
     +
    -+		test_commit first &&
    -+		git repack -d &&
    ++		write_packs A B C D &&
     +
    -+		test_commit second &&
    -+		git repack --write-midx=incremental &&
    ++		test_line_count = 4 "$midx_chain" &&
     +
    -+		git multi-pack-index verify &&
    -+		test_line_count = 1 $midx_chain &&
    -+		cp $midx_chain $midx_chain.before &&
    ++		cp "$midx_chain" "$midx_chain.bak" &&
     +
    -+		# A second repack appends a new layer without
    -+		# disturbing the existing one.
    -+		test_commit third &&
    -+		git repack --write-midx=incremental &&
    -+
    -+		git multi-pack-index verify &&
    ++		git multi-pack-index compact --incremental \
    ++			--base="$(nth_line 1 "$midx_chain")" \
    ++			"$(nth_line 3 "$midx_chain")" \
    ++			"$(nth_line 4 "$midx_chain")" &&
     +		test_line_count = 2 $midx_chain &&
    -+		head -n 1 $midx_chain.before >expect &&
    -+		head -n 1 $midx_chain >actual &&
    -+		test_cmp expect actual &&
     +
    -+		git fsck
    ++		nth_line 1 "$midx_chain.bak" >expect &&
    ++		nth_line 1 "$midx_chain" >actual &&
    ++
    ++		test_cmp expect actual
     +	)
    - '
    - 
    - test_expect_success 'below layer threshold, tip packs excluded' '
    -@@ t/t7705-repack-incremental-midx.sh: test_expect_success 'kept packs are excluded from repack' '
    - 		# entirely, so no rollup occurs as there is only one
    - 		# non-kept pack. A new MIDX layer is written containing
    - 		# that pack.
    --		git repack --geometric=2 -d --write-midx=incremental \
    --			--write-bitmap-index &&
    -+		git repack --geometric=2 -d --write-midx=incremental &&
    - 
    - 		test-tool read-midx $objdir >actual &&
    - 		grep "^pack-.*\.idx$" actual >actual.packs &&
    ++'
    ++
    ++test_expect_success 'MIDX compaction with --base=none' '
    ++	git init midx-compact-base-none &&
    ++	(
    ++		cd midx-compact-base-none &&
    ++
    ++		git config maintenance.auto false &&
    ++
    ++		write_packs A B C D &&
    ++
    ++		test_line_count = 4 $midx_chain &&
    ++
    ++		cp "$midx_chain" "$midx_chain".bak &&
    ++
    ++		# Compact the two bottommost layers (A and B) into a new
    ++		# root layer with no parent.
    ++		git multi-pack-index compact --incremental \
    ++			--base=none \
    ++			"$(nth_line 1 "$midx_chain")" \
    ++			"$(nth_line 2 "$midx_chain")" &&
    ++
    ++		test_line_count = 3 $midx_chain &&
    ++
    ++		# The upper layers (C and D) should be preserved
    ++		# unchanged.
    ++		nth_line 3 "$midx_chain.bak" >expect &&
    ++		nth_line 4 "$midx_chain.bak" >>expect &&
    ++		nth_line 2 "$midx_chain" >actual &&
    ++		nth_line 3 "$midx_chain" >>actual &&
    ++
    ++		test_cmp expect actual
    ++	)
    ++'
    ++
    ++test_expect_success 'MIDX compaction with bogus --base checksum' '
    ++	git init midx-compact-bogus-base &&
    ++	(
    ++		cd midx-compact-bogus-base &&
    ++
    ++		git config maintenance.auto false &&
    ++
    ++		write_packs A B C &&
    ++
    ++		test_must_fail git multi-pack-index compact --incremental \
    ++			--base=deadbeef \
    ++			"$(nth_line 2 "$midx_chain")" \
    ++			"$(nth_line 3 "$midx_chain")" 2>err &&
    ++		test_grep "could not find base MIDX" err
    ++	)
    ++'
    ++
    + test_done
 -:  ----------- >  7:  92aba3d366f repack: track the ODB source via existing_packs
 -:  ----------- >  8:  d3ac65c1f11 midx: expose `midx_layer_contains_pack()`
 -:  ----------- >  9:  1bd2f194c6f repack-midx: factor out `repack_prepare_midx_command()`
 1:  44f522ea04d ! 10:  2a87a1e4561 repack-midx: extract `repack_fill_midx_stdin_packs()`
    @@ Commit message
         This simplifies `write_midx_included_packs()` and prepares for a
         subsequent commit where the same helper is called with `cmd->out = -1`
         to capture the MIDX's checksum from the command's standard output,
    -    which is needed when writing MIDX layers with `--checksum-only`.
    +    which is needed when writing MIDX layers with `--no-write-chain-file`.
     
         No functional changes are included in this patch.
     
 2:  f5642a46bbd = 11:  3d32b9c88da repack-geometry: prepare for incremental MIDX repacking
 3:  9fdcb253a96 = 12:  1f7a5479bb8 builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
 4:  1e1b957bf12 = 13:  b155f25d53c packfile: ensure `close_pack_revindex()` frees in-memory revindex
 5:  93e152fb6aa ! 14:  ef012314930 repack: implement incremental MIDX repacking
    @@ Commit message
         Unlike the default mode which writes a single flat MIDX, the incremental
         mode constructs a compaction plan that determines which MIDX layers to
         write, compact, or copy, and then executes each step using `git
    -    multi-pack-index` subcommands with the --checksum-only flag.
    +    multi-pack-index` subcommands with the --no-write-chain-file flag.
     
         The repacking strategy works as follows:
     
    @@ Commit message
     
         After writing the new layer, the strategy is evaluated among the
         existing MIDX layers in order from oldest to newest. Each step that
    -    writes a new MIDX layer uses "--checksum-only" to avoid updating the
    -    multi-pack-index-chain file. After all steps are complete, the new chain
    -    file is written and then atomically moved into place.
    +    writes a new MIDX layer uses "--no-write-chain-file" to avoid updating
    +    the multi-pack-index-chain file. After all steps are complete, the new
    +    chain file is written and then atomically moved into place.
     
         At present, this functionality is exposed behind a new enum value,
         `REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
    @@ repack-midx.c: static void repack_prepare_midx_command(struct child_process *cmd
     +					struct string_list *include,
     +					struct string_list *out)
      {
    ++	struct strbuf in_buf = STRBUF_INIT;
    ++	struct strbuf out_buf = STRBUF_INIT;
      	struct string_list_item *item;
    - 	FILE *in;
    +-	FILE *in;
      	int ret;
      
    - 	cmd->in = -1;
    -+	if (out)
    -+		cmd->out = -1;
    - 
    +-	cmd->in = -1;
    +-
      	strvec_push(&cmd->args, "--stdin-packs");
      
    -@@ repack-midx.c: static int repack_fill_midx_stdin_packs(struct child_process *cmd,
    - 		fprintf(in, "%s\n", item->string);
    - 	fclose(in);
    +-	ret = start_command(cmd);
    +-	if (ret)
    +-		return ret;
    +-
    +-	in = xfdopen(cmd->in, "w");
    + 	for_each_string_list_item(item, include)
    +-		fprintf(in, "%s\n", item->string);
    +-	fclose(in);
    ++		strbuf_addf(&in_buf, "%s\n", item->string);
      
    -+	if (out) {
    -+		struct strbuf buf = STRBUF_INIT;
    -+		FILE *outf = xfdopen(cmd->out, "r");
    +-	return finish_command(cmd);
    ++	ret = pipe_command(cmd, in_buf.buf, in_buf.len,
    ++			   out ? &out_buf : NULL, 0, NULL, 0);
     +
    -+		while (strbuf_getline(&buf, outf) != EOF)
    -+			string_list_append(out, buf.buf);
    -+		strbuf_release(&buf);
    ++	if (out)
    ++		string_list_split_f(out, out_buf.buf, "\n", -1,
    ++				    STRING_LIST_SPLIT_NONEMPTY);
     +
    -+		fclose(outf);
    -+	}
    ++	strbuf_release(&in_buf);
    ++	strbuf_release(&out_buf);
     +
    - 	return finish_command(cmd);
    ++	return ret;
      }
      
    + static int write_midx_included_packs(struct repack_write_midx_opts *opts)
     @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opts *opts)
      		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
      			     opts->refs_snapshot);
    @@ repack-midx.c: static int write_midx_included_packs(struct repack_write_midx_opt
     +	while (strbuf_getline_lf(&buf, out) != EOF) {
     +		if (step->csum) {
     +			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
    ++			fclose(out);
    ++			out = NULL;
    ++			finish_command(&cmd);
     +			goto out;
     +		}
     +		step->csum = strbuf_detach(&buf, NULL);
 6:  6119f15d3e8 ! 15:  04cfecd5136 repack: introduce `--write-midx=incremental`
    @@ t/t7705-repack-incremental-midx.sh (new)
     +	)
     +'
     +
    -+test_expect_success 'repack -ad --write-midx=incremental is safe' '
    -+	git init ad-incremental-midx &&
    -+	(
    -+		cd ad-incremental-midx &&
    -+
    -+		git config maintenance.auto false &&
    -+
    -+		# Build a MIDX chain with multiple layers referencing
    -+		# distinct packs.
    -+		test_commit first &&
    -+		git repack -d &&
    -+
    -+		test_commit second &&
    -+		git repack -d --write-midx=incremental &&
    -+
    -+		git multi-pack-index verify &&
    -+		test_line_count = 1 $midx_chain &&
    -+
    -+		# Now do a full -ad repack. The new pack contains all
    -+		# objects, but any retained MIDX layers still reference
    -+		# the now-deleted packs.
    -+		test_commit third &&
    -+		git repack -ad --write-midx=incremental &&
    -+
    -+		git multi-pack-index verify &&
    -+		git fsck &&
    -+		git rev-list --all --objects >/dev/null
    -+	)
    -+'
    -+
     +test_expect_success 'repack rejects invalid midxSplitFactor' '
     +	test_when_finished "rm -fr bad-split-factor" &&
     +	git init bad-split-factor &&
 -:  ----------- > 16:  1c05dfce579 repack: allow `--write-midx=incremental` without `--geometric`

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 01/16] midx-write: handle noop writes when converting incremental chains

[Taylor Blau]

When updating a MIDX, we optimize out writes that will result in an
identical MIDX as the one we already have on disk. See b3bab9d2729
(midx-write: extract function to test whether MIDX needs updating,
2025-12-10) for more details on exactly which writes are optimized out.

If `midx_needs_update()` can't rule out any of the obvious cases (e.g.,
the checksum is invalid, we're requesting a different version, or
performing compaction which always requires an update), then we compare
the packs we're writing to the packs we already know about. If there are
an equal number of packs being written as there are in any existing
MIDX layer(s), then we compare the packs by their name.

This comparison fails when we have an incremental MIDX chain with
at least two layers, since we do not recursively peel through earlier
layers, instead treating the `->pack_names` array of the tip MIDX layer
as containing all `m->num_packs + m->num_packs_in_base` packs.

Adjust this to instead look through the MIDX layers one by one when
comparing pack names. While we're at it, fix a typo above in the same
function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c                            | 18 ++++++++++--------
 t/t5334-incremental-multi-pack-index.sh | 16 ++++++++++++++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index a25cab75aba..9328f65a201 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1152,7 +1152,7 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 
 	/*
 	 * Ensure that we have a valid checksum before consulting the
-	 * exisiting MIDX in order to determine if we can avoid an
+	 * existing MIDX in order to determine if we can avoid an
 	 * update.
 	 *
 	 * This is necessary because the given MIDX is loaded directly
@@ -1208,14 +1208,16 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 			BUG("same pack added twice?");
 	}
 
-	for (uint32_t i = 0; i < ctx->nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addstr(&buf, midx->pack_names[i]);
-		strbuf_strip_suffix(&buf, ".idx");
+	for (struct multi_pack_index *m = midx; m; m = m->base_midx) {
+		for (uint32_t i = 0; i < m->num_packs; i++) {
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, m->pack_names[i]);
+			strbuf_strip_suffix(&buf, ".idx");
 
-		if (!strset_contains(&packs, buf.buf))
-			goto out;
-		strset_remove(&packs, buf.buf);
+			if (!strset_contains(&packs, buf.buf))
+				goto out;
+			strset_remove(&packs, buf.buf);
+		}
 	}
 
 	needed = false;
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index 99c7d44d8e9..c9f5b4e87aa 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -132,4 +132,20 @@ test_expect_success 'relink existing MIDX layer' '
 
 '
 
+test_expect_success 'non-incremental write with existing incremental chain' '
+	git init non-incremental-write-with-existing &&
+	test_when_finished "rm -fr non-incremental-write-with-existing" &&
+
+	(
+		cd non-incremental-write-with-existing &&
+
+		git config set maintenance.auto false &&
+
+		write_midx_layer &&
+		write_midx_layer &&
+
+		git multi-pack-index write
+	)
+'
+
 test_done
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 02/16] midx: use `strset` for retained MIDX files

[Taylor Blau]

Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
build a list of filenames to keep while pruning stale MIDX files. Today
they hand-roll an array instead of using a `strset`, thus requiring us
to pass an additional length parameter, and makes lookups linear.

Replace the bare array with a `strset` which can be passed around as a
single parameter. Though it improves lookup performance, the difference
is likely immeasurable given how small the keep_hashes array typically
is.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 57 +++++++++++++++++++++++++++------------------------------
 1 file changed, 27 insertions(+), 30 deletions(-)

diff --git a/midx.c b/midx.c
index 81d6ab11e6e..f75e3c9fa6d 100644
--- a/midx.c
+++ b/midx.c
@@ -758,8 +758,7 @@ int midx_checksum_valid(struct multi_pack_index *m)
 }
 
 struct clear_midx_data {
-	char **keep;
-	uint32_t keep_nr;
+	struct strset keep;
 	const char *ext;
 };
 
@@ -767,15 +766,12 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 				const char *file_name, void *_data)
 {
 	struct clear_midx_data *data = _data;
-	uint32_t i;
 
 	if (!(starts_with(file_name, "multi-pack-index-") &&
 	      ends_with(file_name, data->ext)))
 		return;
-	for (i = 0; i < data->keep_nr; i++) {
-		if (!strcmp(data->keep[i], file_name))
-			return;
-	}
+	if (strset_contains(&data->keep, file_name))
+		return;
 	if (unlink(full_path))
 		die_errno(_("failed to remove %s"), full_path);
 }
@@ -783,48 +779,49 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash)
 {
-	struct clear_midx_data data;
-	memset(&data, 0, sizeof(struct clear_midx_data));
+	struct clear_midx_data data = {
+		.keep = STRSET_INIT,
+		.ext = ext,
+	};
 
 	if (keep_hash) {
-		ALLOC_ARRAY(data.keep, 1);
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hash, ext);
 
-		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
-		data.keep_nr = 1;
+		strset_add(&data.keep, buf.buf);
+
+		strbuf_release(&buf);
 	}
-	data.ext = ext;
 
-	for_each_file_in_pack_dir(source->path,
-				  clear_midx_file_ext,
-				  &data);
+	for_each_file_in_pack_dir(source->path, clear_midx_file_ext, &data);
 
-	if (keep_hash)
-		free(data.keep[0]);
-	free(data.keep);
+	strset_clear(&data.keep);
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
 				      char **keep_hashes,
 				      uint32_t hashes_nr)
 {
-	struct clear_midx_data data;
+	struct clear_midx_data data = {
+		.keep = STRSET_INIT,
+		.ext = ext,
+	};
+	struct strbuf buf = STRBUF_INIT;
 	uint32_t i;
 
-	memset(&data, 0, sizeof(struct clear_midx_data));
+	for (i = 0; i < hashes_nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
+			    ext);
 
-	ALLOC_ARRAY(data.keep, hashes_nr);
-	for (i = 0; i < hashes_nr; i++)
-		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
-				       ext);
-	data.keep_nr = hashes_nr;
-	data.ext = ext;
+		strset_add(&data.keep, buf.buf);
+	}
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
 				     clear_midx_file_ext, &data);
 
-	for (i = 0; i < hashes_nr; i++)
-		free(data.keep[i]);
-	free(data.keep);
+	strbuf_release(&buf);
+	strset_clear(&data.keep);
 }
 
 void clear_midx_file(struct repository *r)
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 03/16] midx: build `keep_hashes` array in order

[Taylor Blau]

Instead of filling the keep_hashes array using reverse indexing (e.g.,
`keep_hashes[count - i - 1]`) while traversing linked lists forward,
collect linked list nodes into a temporary `layers` array and then
iterate it backwards to fill `keep_hashes` sequentially.

This makes the filling logic easier to follow, since each segment of the
array is filled with a simple forward-marching index. Moreover, this
change prepares us for a subsequent commit that will switch to using a
`strvec`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 66 ++++++++++++++++++++++++++++------------------------
 1 file changed, 36 insertions(+), 30 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 9328f65a201..55c778a97cb 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1731,6 +1731,9 @@ static int write_midx_internal(struct write_midx_opts *opts)
 		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
+		struct multi_pack_index **layers = NULL;
+		size_t layers_nr = 0, layers_alloc = 0;
+		size_t j = 0;
 
 		if (!chainf) {
 			error_errno(_("unable to open multi-pack-index chain file"));
@@ -1751,46 +1754,49 @@ static int write_midx_internal(struct write_midx_opts *opts)
 		strbuf_release(&final_midx_name);
 
 		if (ctx.compact) {
-			struct multi_pack_index *m;
-			uint32_t num_layers_before_from = 0;
-			uint32_t i;
+			struct multi_pack_index *mp;
 
-			for (m = ctx.base_midx; m; m = m->base_midx)
-				num_layers_before_from++;
-
-			m = ctx.base_midx;
-			for (i = 0; i < num_layers_before_from; i++) {
-				uint32_t j = num_layers_before_from - i - 1;
-
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
-				m = m->base_midx;
+			for (mp = ctx.base_midx; mp; mp = mp->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = mp;
 			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[i] = xstrdup(hash_to_hex_algop(midx_hash,
-								   r->hash_algo));
+			keep_hashes[j++] =
+				xstrdup(hash_to_hex_algop(midx_hash,
+							  r->hash_algo));
 
-			i = 0;
-			for (m = ctx.m;
-			     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
-			     m = m->base_midx) {
-				keep_hashes[keep_hashes_nr - i - 1] =
-					xstrdup(midx_get_checksum_hex(m));
-				i++;
+			for (mp = ctx.m;
+			     mp && midx_hashcmp(mp, ctx.compact_to,
+						r->hash_algo);
+			     mp = mp->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = mp;
 			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
 		} else {
-			keep_hashes[ctx.num_multi_pack_indexes_before] =
+			for (; m; m = m->base_midx) {
+				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
+				layers[layers_nr++] = m;
+			}
+			while (layers_nr)
+				keep_hashes[j++] =
+					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+
+			keep_hashes[j++] =
 				xstrdup(hash_to_hex_algop(midx_hash,
 							  r->hash_algo));
-
-			for (uint32_t i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
-				uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
-
-				keep_hashes[j] = xstrdup(midx_get_checksum_hex(m));
-				m = m->base_midx;
-			}
 		}
 
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
+		ASSERT(j == keep_hashes_nr);
+
+		free(layers);
+
+		for (uint32_t i = 0; i < j; i++)
 			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
 	} else {
 		keep_hashes[ctx.num_multi_pack_indexes_before] =
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 04/16] midx: use `strvec` for `keep_hashes`

[Taylor Blau]

The `keep_hashes` array in `write_midx_internal()` accumulates the
checksums of MIDX files that should be retained when pruning stale
entries from the MIDX chain. For similar reasons as in a previous
commit, rewrite this using a strvec, requiring us to pass one fewer
parameter.

Unlike the aforementioned previous commit, use a `strvec` instead of a
`string_list`, which provides a more ergonomic interface to adjust the
values at a particular index. The ordering is important here, as this
value is used to determine the contents of the resulting
`multi-pack-index-chain` file when writing with "--incremental".

Since the previous commit already builds the array in forward order, the
conversion is straightforward: replace indexed assignments with
`strvec_push()`, drop the pre-counting and `CALLOC_ARRAY()`, and
simplify cleanup via `strvec_clear()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 84 ++++++++++++++++++----------------------------------
 midx.c       | 20 ++++++-------
 2 files changed, 38 insertions(+), 66 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 55c778a97cb..5d9409a9741 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -29,8 +29,7 @@ extern void clear_midx_files_ext(struct odb_source *source, const char *ext,
 				 const char *keep_hash);
 extern void clear_incremental_midx_files_ext(struct odb_source *source,
 					     const char *ext,
-					     const char **keep_hashes,
-					     uint32_t hashes_nr);
+					     const struct strvec *keep_hashes);
 extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 				const char *idx_name);
 
@@ -1109,8 +1108,7 @@ static int link_midx_to_chain(struct multi_pack_index *m)
 }
 
 static void clear_midx_files(struct odb_source *source,
-			     const char **hashes, uint32_t hashes_nr,
-			     unsigned incremental)
+			     const struct strvec *hashes, unsigned incremental)
 {
 	/*
 	 * if incremental:
@@ -1124,13 +1122,15 @@ static void clear_midx_files(struct odb_source *source,
 	 */
 	struct strbuf buf = STRBUF_INIT;
 	const char *exts[] = { MIDX_EXT_BITMAP, MIDX_EXT_REV, MIDX_EXT_MIDX };
-	uint32_t i, j;
+	uint32_t i;
 
 	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		clear_incremental_midx_files_ext(source, exts[i],
-						 hashes, hashes_nr);
-		for (j = 0; j < hashes_nr; j++)
-			clear_midx_files_ext(source, exts[i], hashes[j]);
+		clear_incremental_midx_files_ext(source, exts[i], hashes);
+		if (hashes) {
+			for (size_t j = 0; j < hashes->nr; j++)
+				clear_midx_files_ext(source, exts[i],
+						     hashes->v[j]);
+		}
 	}
 
 	if (incremental)
@@ -1267,8 +1267,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = -1;
-	const char **keep_hashes = NULL;
-	size_t keep_hashes_nr = 0;
+	struct strvec keep_hashes = STRVEC_INIT;
 	struct chunkfile *cf;
 
 	trace2_region_enter("midx", "write_midx_internal", r);
@@ -1708,32 +1707,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
 		die(_("too many multi-pack-indexes"));
 
-	if (ctx.compact) {
-		struct multi_pack_index *m;
-
-		/*
-		 * Keep all MIDX layers excluding those in the range [from, to].
-		 */
-		for (m = ctx.base_midx; m; m = m->base_midx)
-			keep_hashes_nr++;
-		for (m = ctx.m;
-		     m && midx_hashcmp(m, ctx.compact_to, r->hash_algo);
-		     m = m->base_midx)
-			keep_hashes_nr++;
-
-		keep_hashes_nr++; /* include the compacted layer */
-	} else {
-		keep_hashes_nr = ctx.num_multi_pack_indexes_before + 1;
-	}
-	CALLOC_ARRAY(keep_hashes, keep_hashes_nr);
-
 	if (ctx.incremental) {
 		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
 		struct multi_pack_index **layers = NULL;
 		size_t layers_nr = 0, layers_alloc = 0;
-		size_t j = 0;
 
 		if (!chainf) {
 			error_errno(_("unable to open multi-pack-index chain file"));
@@ -1761,12 +1740,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 				layers[layers_nr++] = mp;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[j++] =
-				xstrdup(hash_to_hex_algop(midx_hash,
-							  r->hash_algo));
+			strvec_push(&keep_hashes,
+				    hash_to_hex_algop(midx_hash,
+						     r->hash_algo));
 
 			for (mp = ctx.m;
 			     mp && midx_hashcmp(mp, ctx.compact_to,
@@ -1776,31 +1755,29 @@ static int write_midx_internal(struct write_midx_opts *opts)
 				layers[layers_nr++] = mp;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 		} else {
 			for (; m; m = m->base_midx) {
 				ALLOC_GROW(layers, layers_nr + 1, layers_alloc);
 				layers[layers_nr++] = m;
 			}
 			while (layers_nr)
-				keep_hashes[j++] =
-					xstrdup(midx_get_checksum_hex(layers[--layers_nr]));
+				strvec_push(&keep_hashes,
+					    midx_get_checksum_hex(layers[--layers_nr]));
 
-			keep_hashes[j++] =
-				xstrdup(hash_to_hex_algop(midx_hash,
-							  r->hash_algo));
+			strvec_push(&keep_hashes,
+				    hash_to_hex_algop(midx_hash,
+						     r->hash_algo));
 		}
 
-		ASSERT(j == keep_hashes_nr);
-
 		free(layers);
 
-		for (uint32_t i = 0; i < j; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
+		for (size_t i = 0; i < keep_hashes.nr; i++)
+			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
 	} else {
-		keep_hashes[ctx.num_multi_pack_indexes_before] =
-			xstrdup(hash_to_hex_algop(midx_hash, r->hash_algo));
+		strvec_push(&keep_hashes,
+			    hash_to_hex_algop(midx_hash, r->hash_algo));
 	}
 
 	if (ctx.m || ctx.base_midx)
@@ -1809,8 +1786,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (commit_lock_file(&lk) < 0)
 		die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, keep_hashes, keep_hashes_nr,
-			 ctx.incremental);
+	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
 	result = 0;
 
 cleanup:
@@ -1826,11 +1802,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	free(ctx.entries);
 	free(ctx.pack_perm);
 	free(ctx.pack_order);
-	if (keep_hashes) {
-		for (uint32_t i = 0; i < keep_hashes_nr; i++)
-			free((char *)keep_hashes[i]);
-		free(keep_hashes);
-	}
+	strvec_clear(&keep_hashes);
 	strbuf_release(&midx_name);
 	close_midx(midx_to_free);
 
diff --git a/midx.c b/midx.c
index f75e3c9fa6d..bcb8c999015 100644
--- a/midx.c
+++ b/midx.c
@@ -12,6 +12,7 @@
 #include "chunk-format.h"
 #include "pack-bitmap.h"
 #include "pack-revindex.h"
+#include "strvec.h"
 
 #define MIDX_PACK_ERROR ((void *)(intptr_t)-1)
 
@@ -19,8 +20,7 @@ int midx_checksum_valid(struct multi_pack_index *m);
 void clear_midx_files_ext(struct odb_source *source, const char *ext,
 			  const char *keep_hash);
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr);
+				      const struct strvec *keep_hashes);
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
@@ -799,22 +799,22 @@ void clear_midx_files_ext(struct odb_source *source, const char *ext,
 }
 
 void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext,
-				      char **keep_hashes,
-				      uint32_t hashes_nr)
+				      const struct strvec *keep_hashes)
 {
 	struct clear_midx_data data = {
 		.keep = STRSET_INIT,
 		.ext = ext,
 	};
 	struct strbuf buf = STRBUF_INIT;
-	uint32_t i;
 
-	for (i = 0; i < hashes_nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addf(&buf, "multi-pack-index-%s.%s", keep_hashes[i],
-			    ext);
+	if (keep_hashes) {
+		for (size_t i = 0; i < keep_hashes->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "multi-pack-index-%s.%s",
+				    keep_hashes->v[i], ext);
 
-		strset_add(&data.keep, buf.buf);
+			strset_add(&data.keep, buf.buf);
+		}
 	}
 
 	for_each_file_in_pack_subdir(source->path, "multi-pack-index.d",
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 05/16] midx: introduce `--no-write-chain-file` for incremental MIDX writes

[Taylor Blau]

When writing an incremental MIDX layer, the MIDX machinery writes the
new layer into the multi-pack-index.d directory and then updates the
multi-pack-index-chain file to include the freshly written layer.

Future callers however may not wish to immediately update the MIDX chain
itself, preferring instead to write out new layer(s) themselves before
atomically updating the chain. Concretely, the new incremental
MIDX-based repacking strategy will want to do exactly this (that is,
assemble the new MIDX chain itself before writing a new chain file and
atomically linking it into place).

Introduce a `--no-write-chain-file` flag that:

 * writes the new MIDX layer into the multi-pack-index.d directory

 * prints its checksum

 * does not update the multi-pack-index-chain file.

The MIDX chain file (and thus, the lock protecting it) remain untouched,
allowing callers to assemble the chain themselves. This flag requires
`--incremental`, since the notion of a separate layer only makes sense
for incremental MIDXs.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc | 17 ++++++++--
 builtin/multi-pack-index.c              | 28 +++++++++++++++--
 midx-write.c                            | 42 ++++++++++++++++---------
 midx.h                                  |  1 +
 t/t5334-incremental-multi-pack-index.sh | 17 ++++++++++
 t/t5335-compact-multi-pack-index.sh     | 36 +++++++++++++++++++++
 6 files changed, 123 insertions(+), 18 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index 3a5aa227784..c26196815e2 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -11,9 +11,9 @@ SYNOPSIS
 [verse]
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
-		         [--refs-snapshot=<path>]
+		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] <from> <to>
+		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
@@ -83,6 +83,13 @@ marker).
 		and packs not present in an existing MIDX layer.
 		Migrates non-incremental MIDXs to incremental ones when
 		necessary.
+
+	--[no-]write-chain-file::
+		When used with `--incremental`, write a new MIDX layer
+		but do not update the multi-pack-index-chain file.
+		The checksum of the new layer is printed to standard
+		output, allowing the caller to assemble and write the
+		chain itself. Requires `--incremental`.
 --
 
 compact::
@@ -97,6 +104,12 @@ compact::
 
 	--[no-]bitmap::
 		Control whether or not a multi-pack bitmap is written.
+
+	--[no-]write-chain-file::
+		When used with `--incremental`, write a new compacted
+		MIDX layer but do not update the multi-pack-index-chain
+		file. The checksum of the new layer is printed to
+		standard output. Requires `--incremental`.
 --
 +
 Note that the compact command requires writing a version-2 midx that
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 0f72d96c02d..f861b4b8394 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,11 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>]")
+	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] <from> <to>")
+	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -153,6 +153,9 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
+			N_("write the multi-pack-index chain file"),
+			MIDX_WRITE_NO_CHAIN),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			 N_("write multi-pack index containing only given indexes")),
 		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
@@ -178,6 +181,15 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 	if (argc)
 		usage_with_options(builtin_multi_pack_index_write_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--no-write-chain-file", "--incremental");
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -221,6 +233,9 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
+		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
+			N_("write the multi-pack-index chain file"),
+			MIDX_WRITE_NO_CHAIN),
 		OPT_END(),
 	};
 
@@ -239,6 +254,15 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 	if (argc != 2)
 		usage_with_options(builtin_multi_pack_index_compact_usage,
 				   options);
+
+	if (opts.flags & MIDX_WRITE_NO_CHAIN &&
+	    !(opts.flags & MIDX_WRITE_INCREMENTAL)) {
+		error(_("cannot use %s without %s"),
+		      "--no-write-chain-file", "--incremental");
+		usage_with_options(builtin_multi_pack_index_compact_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(the_repository);
 
 	FREE_AND_NULL(options);
diff --git a/midx-write.c b/midx-write.c
index 5d9409a9741..38c898e5ff5 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1257,7 +1257,7 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	unsigned char midx_hash[GIT_MAX_RAWSZ];
 	uint32_t start_pack;
 	struct hashfile *f = NULL;
-	struct lock_file lk;
+	struct lock_file lk = LOCK_INIT;
 	struct tempfile *incr;
 	struct write_midx_context ctx = {
 		.preferred_pack_idx = NO_PREFERRED_PACK,
@@ -1601,11 +1601,14 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	}
 
 	if (ctx.incremental) {
-		struct strbuf lock_name = STRBUF_INIT;
+		if (!(opts->flags & MIDX_WRITE_NO_CHAIN)) {
+			struct strbuf lock_name = STRBUF_INIT;
 
-		get_midx_chain_filename(opts->source, &lock_name);
-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
-		strbuf_release(&lock_name);
+			get_midx_chain_filename(opts->source, &lock_name);
+			hold_lock_file_for_update(&lk, lock_name.buf,
+						  LOCK_DIE_ON_ERROR);
+			strbuf_release(&lock_name);
+		}
 
 		incr = mks_tempfile_m(midx_name.buf, 0444);
 		if (!incr) {
@@ -1707,16 +1710,23 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.num_multi_pack_indexes_before == UINT32_MAX)
 		die(_("too many multi-pack-indexes"));
 
+	if (!is_lock_file_locked(&lk))
+		printf("%s\n", hash_to_hex_algop(midx_hash, r->hash_algo));
+	else if (opts->flags & MIDX_WRITE_NO_CHAIN)
+		BUG("lockfile held with MIDX_WRITE_NO_CHAIN set?");
+
 	if (ctx.incremental) {
-		FILE *chainf = fdopen_lock_file(&lk, "w");
 		struct strbuf final_midx_name = STRBUF_INIT;
 		struct multi_pack_index *m = ctx.base_midx;
 		struct multi_pack_index **layers = NULL;
 		size_t layers_nr = 0, layers_alloc = 0;
 
-		if (!chainf) {
-			error_errno(_("unable to open multi-pack-index chain file"));
-			goto cleanup;
+		if (is_lock_file_locked(&lk)){
+			FILE *chainf = fdopen_lock_file(&lk, "w");
+			if (!chainf) {
+				error_errno(_("unable to open multi-pack-index chain file"));
+				goto cleanup;
+			}
 		}
 
 		if (link_midx_to_chain(ctx.base_midx) < 0)
@@ -1773,8 +1783,10 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 		free(layers);
 
-		for (size_t i = 0; i < keep_hashes.nr; i++)
-			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes.v[i]);
+		if (is_lock_file_locked(&lk))
+			for (size_t i = 0; i < keep_hashes.nr; i++)
+				fprintf(get_lock_file_fp(&lk), "%s\n",
+					keep_hashes.v[i]);
 	} else {
 		strvec_push(&keep_hashes,
 			    hash_to_hex_algop(midx_hash, r->hash_algo));
@@ -1783,10 +1795,12 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	if (ctx.m || ctx.base_midx)
 		odb_close(ctx.repo->objects);
 
-	if (commit_lock_file(&lk) < 0)
-		die_errno(_("could not write multi-pack-index"));
+	if (is_lock_file_locked(&lk)) {
+		if (commit_lock_file(&lk) < 0)
+			die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+		clear_midx_files(opts->source, &keep_hashes, ctx.incremental);
+	}
 	result = 0;
 
 cleanup:
diff --git a/midx.h b/midx.h
index 08f3728e520..5b193882dcf 100644
--- a/midx.h
+++ b/midx.h
@@ -83,6 +83,7 @@ struct multi_pack_index {
 #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
 #define MIDX_WRITE_INCREMENTAL (1 << 5)
 #define MIDX_WRITE_COMPACT (1 << 6)
+#define MIDX_WRITE_NO_CHAIN (1 << 7)
 
 #define MIDX_EXT_REV "rev"
 #define MIDX_EXT_BITMAP "bitmap"
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index c9f5b4e87aa..66d6894761b 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -96,6 +96,23 @@ test_expect_success 'show object from second pack' '
 	git cat-file -p 2.2
 '
 
+test_expect_success 'write MIDX layer with --no-write-chain-file' '
+	test_commit no-write-chain-file &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file' '
+	test_must_fail git multi-pack-index write --bitmap --no-write-chain-file 2>err &&
+	test_grep "cannot use --no-write-chain-file without --incremental" err
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 40f3844282f..1a65d48b62b 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -290,4 +290,40 @@ test_expect_success 'MIDX compaction with bitmaps (non-trivial)' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --no-write-chain-file' '
+	git init midx-compact-with--no-write-chain-file &&
+	(
+		cd midx-compact-with--no-write-chain-file &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		layer="$(git multi-pack-index compact --incremental \
+			--no-write-chain-file \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")")" &&
+
+		test_cmp "$midx_chain.bak" "$midx_chain" &&
+
+		# After writing the new layer, insert it into the chain
+		# manually. This is done in order to make $layer visible
+		# to the read-midx test helper below, and matches what
+		# the MIDX command would do without --no-write-chain-file.
+		{
+			nth_line 1 "$midx_chain.bak" &&
+			echo $layer &&
+			nth_line 4 "$midx_chain.bak"
+		} >$midx_chain &&
+
+		test-tool read-midx $objdir $layer >midx.data &&
+		grep "^pack-B-.*\.idx" midx.data &&
+		grep "^pack-C-.*\.idx" midx.data
+
+	)
+'
+
 test_done
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 06/16] midx: support custom `--base` for incremental MIDX writes

[Taylor Blau]

Both `compact` and `write --incremental` fix the base of the resulting
MIDX layer: `compact` always places the compacted result on top of
"from's" immediate parent in the chain, and `write --incremental` always
appends a new layer to the existing tip. In both cases the base is not
configurable.

Future callers need additional flexibility. For instance, the incremental
MIDX-based repacking code may wish to write a layer based on some
intermediate ancestor rather than the current tip, or produce a root
layer when replacing the bottommost entries in the chain.

Introduce a new `--base` option for both subcommands to specify the
checksum of the MIDX layer to use as the base. The given checksum must
refer to a valid layer in the MIDX chain that is an ancestor of the
topmost layer being written or compacted.

The special value "none" is accepted to produce a root layer with no
parent. This will be needed when the incremental repacking machinery
determines that the bottommost layers of the chain should be replaced.

If no `--base` is given, behavior is unchanged: `compact` uses "from's"
immediate parent in the chain, and `write` appends to the existing tip.

For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
`write --incremental` appends a new layer to the live chain tip with no
mechanism to atomically replace it; overriding the base would produce a
layer that does not extend the tip, breaking chain invariants. With
`--no-write-chain-file` the chain is left unmodified and the caller is
responsible for assembling a valid chain.

For `compact`, no such restriction applies. The compaction operation
atomically replaces the compacted range in the chain file, so writing
the result on top of any valid ancestor preserves chain invariants.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.adoc | 17 +++++-
 builtin/multi-pack-index.c              | 24 ++++++--
 midx-write.c                            | 34 ++++++++++-
 midx.h                                  |  5 +-
 t/t5334-incremental-multi-pack-index.sh | 30 ++++++++++
 t/t5335-compact-multi-pack-index.sh     | 77 +++++++++++++++++++++++++
 6 files changed, 178 insertions(+), 9 deletions(-)

diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index c26196815e2..c6d23aeeb9a 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -12,8 +12,10 @@ SYNOPSIS
 'git multi-pack-index' [<options>] write [--preferred-pack=<pack>]
 		         [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]
 		         [--refs-snapshot=<path>] [--[no-]write-chain-file]
+			 [--base=<checksum>]
 'git multi-pack-index' [<options>] compact [--[no-]incremental]
-		         [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>
+		         [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]
+			 <from> <to>
 'git multi-pack-index' [<options>] verify
 'git multi-pack-index' [<options>] expire
 'git multi-pack-index' [<options>] repack [--batch-size=<size>]
@@ -90,6 +92,13 @@ marker).
 		The checksum of the new layer is printed to standard
 		output, allowing the caller to assemble and write the
 		chain itself. Requires `--incremental`.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base when writing a new incremental layer.
+		The special value `none` indicates that the new layer
+		should have no base (i.e., it becomes a root layer).
+		Requires `--no-write-chain-file`.
 --
 
 compact::
@@ -110,6 +119,12 @@ compact::
 		MIDX layer but do not update the multi-pack-index-chain
 		file. The checksum of the new layer is printed to
 		standard output. Requires `--incremental`.
+
+	--base=<checksum>::
+		Specify the checksum of an existing MIDX layer to use
+		as the base for the compacted result, instead of using
+		the immediate parent of `<from>`. The special value
+		`none` indicates that the result should have no base.
 --
 +
 Note that the compact command requires writing a version-2 midx that
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index f861b4b8394..00ffb36394d 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -16,11 +16,13 @@
 #define BUILTIN_MIDX_WRITE_USAGE \
 	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]\n" \
 	   "  [--[no-]bitmap] [--[no-]incremental] [--[no-]stdin-packs]\n" \
-	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]")
+	   "  [--refs-snapshot=<path>] [--[no-]write-chain-file]\n" \
+	   "  [--base=<checksum>]")
 
 #define BUILTIN_MIDX_COMPACT_USAGE \
 	N_("git multi-pack-index [<options>] compact [--[no-]incremental]\n" \
-	   "  [--[no-]bitmap] [--[no-]write-chain-file] <from> <to>")
+	   "  [--[no-]bitmap] [--base=<checksum>] [--[no-]write-chain-file]\n" \
+	   "  <from> <to>")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -63,6 +65,7 @@ static char const * const builtin_multi_pack_index_usage[] = {
 static struct opts_multi_pack_index {
 	char *object_dir;
 	const char *preferred_pack;
+	const char *incremental_base;
 	char *refs_snapshot;
 	unsigned long batch_size;
 	unsigned flags;
@@ -151,6 +154,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			   N_("pack for reuse when computing a multi-pack bitmap")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "incremental", &opts.flags,
 			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
 		OPT_NEGBIT(0, "write-chain-file", &opts.flags,
@@ -190,6 +195,13 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 				   options);
 	}
 
+	if (opts.incremental_base &&
+	    !(opts.flags & MIDX_WRITE_NO_CHAIN)) {
+		error(_("cannot use --base without --no-write-chain-file"));
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+	}
+
 	source = handle_object_dir_option(repo);
 
 	FREE_AND_NULL(options);
@@ -201,7 +213,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 
 		ret = write_midx_file_only(source, &packs,
 					   opts.preferred_pack,
-					   opts.refs_snapshot, opts.flags);
+					   opts.refs_snapshot,
+					   opts.incremental_base, opts.flags);
 
 		string_list_clear(&packs, 0);
 		free(opts.refs_snapshot);
@@ -229,6 +242,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 
 	struct option *options;
 	static struct option builtin_multi_pack_index_compact_options[] = {
+		OPT_STRING(0, "base", &opts.incremental_base, N_("checksum"),
+			   N_("base MIDX for incremental writes")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "incremental", &opts.flags,
@@ -290,7 +305,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
 			die(_("MIDX %s must be an ancestor of %s"), argv[0], argv[1]);
 	}
 
-	ret = write_midx_file_compact(source, from_midx, to_midx, opts.flags);
+	ret = write_midx_file_compact(source, from_midx, to_midx,
+				      opts.incremental_base, opts.flags);
 
 	return ret;
 }
diff --git a/midx-write.c b/midx-write.c
index 38c898e5ff5..561e9eedc0e 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1247,6 +1247,7 @@ struct write_midx_opts {
 
 	const char *preferred_pack_name;
 	const char *refs_snapshot;
+	const char *incremental_base;
 	unsigned flags;
 };
 
@@ -1330,11 +1331,32 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 	/*
 	 * If compacting MIDX layer(s) in the range [from, to], then the
-	 * compacted MIDX will share the same base MIDX as 'from'.
+	 * compacted MIDX will share the same base MIDX as 'from',
+	 * unless a custom --base is specified (see below).
 	 */
 	if (ctx.compact)
 		ctx.base_midx = ctx.compact_from->base_midx;
 
+	if (opts->incremental_base) {
+		if (!strcmp(opts->incremental_base, "none")) {
+			ctx.base_midx = NULL;
+		} else {
+			while (ctx.base_midx) {
+				const char *cmp = midx_get_checksum_hex(ctx.base_midx);
+				if (!strcmp(opts->incremental_base, cmp))
+					break;
+
+				ctx.base_midx = ctx.base_midx->base_midx;
+			}
+
+			if (!ctx.base_midx) {
+				error(_("could not find base MIDX '%s'"),
+				      opts->incremental_base);
+				goto cleanup;
+			}
+		}
+	}
+
 	ctx.nr = 0;
 	ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
 	ctx.info = NULL;
@@ -1827,7 +1849,8 @@ static int write_midx_internal(struct write_midx_opts *opts)
 
 int write_midx_file(struct odb_source *source,
 		    const char *preferred_pack_name,
-		    const char *refs_snapshot, unsigned flags)
+		    const char *refs_snapshot,
+		    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
@@ -1842,13 +1865,16 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags)
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.packs_to_include = packs_to_include,
 		.preferred_pack_name = preferred_pack_name,
 		.refs_snapshot = refs_snapshot,
+		.incremental_base = incremental_base,
 		.flags = flags,
 	};
 
@@ -1858,12 +1884,14 @@ int write_midx_file_only(struct odb_source *source,
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags)
 {
 	struct write_midx_opts opts = {
 		.source = source,
 		.compact_from = from,
 		.compact_to = to,
+		.incremental_base = incremental_base,
 		.flags = flags | MIDX_WRITE_COMPACT,
 	};
 
diff --git a/midx.h b/midx.h
index 5b193882dcf..77dd66de02b 100644
--- a/midx.h
+++ b/midx.h
@@ -132,10 +132,13 @@ int write_midx_file(struct odb_source *source,
 int write_midx_file_only(struct odb_source *source,
 			 struct string_list *packs_to_include,
 			 const char *preferred_pack_name,
-			 const char *refs_snapshot, unsigned flags);
+			 const char *refs_snapshot,
+			 const char *incremental_base,
+			 unsigned flags);
 int write_midx_file_compact(struct odb_source *source,
 			    struct multi_pack_index *from,
 			    struct multi_pack_index *to,
+			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct odb_source *source, unsigned flags);
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
index 66d6894761b..68a103d13d2 100755
--- a/t/t5334-incremental-multi-pack-index.sh
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -113,6 +113,36 @@ test_expect_success 'write non-incremental MIDX layer with --no-write-chain-file
 	test_grep "cannot use --no-write-chain-file without --incremental" err
 '
 
+test_expect_success 'write MIDX layer with --base without --no-write-chain-file' '
+	test_must_fail git multi-pack-index write --bitmap --incremental \
+		--base=none 2>err &&
+	test_grep "cannot use --base without --no-write-chain-file" err
+'
+
+test_expect_success 'write MIDX layer with --base=none and --no-write-chain-file' '
+	test_commit base-none &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file --base=none)" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
+test_expect_success 'write MIDX layer with --base=<hash> and --no-write-chain-file' '
+	test_commit base-hash &&
+	git repack -d &&
+
+	cp "$midx_chain" "$midx_chain.bak" &&
+	layer="$(git multi-pack-index write --bitmap --incremental \
+		--no-write-chain-file --base="$(nth_line 1 "$midx_chain")")" &&
+
+	test_cmp "$midx_chain.bak" "$midx_chain" &&
+	test_path_is_file "$midxdir/multi-pack-index-$layer.midx"
+'
+
 for reuse in false single multi
 do
 	test_expect_success "full clone (pack.allowPackReuse=$reuse)" '
diff --git a/t/t5335-compact-multi-pack-index.sh b/t/t5335-compact-multi-pack-index.sh
index 1a65d48b62b..ec1dafe89fc 100755
--- a/t/t5335-compact-multi-pack-index.sh
+++ b/t/t5335-compact-multi-pack-index.sh
@@ -304,6 +304,7 @@ test_expect_success 'MIDX compaction with --no-write-chain-file' '
 
 		layer="$(git multi-pack-index compact --incremental \
 			--no-write-chain-file \
+			--base="$(nth_line 1 "$midx_chain")" \
 			"$(nth_line 2 "$midx_chain")" \
 			"$(nth_line 3 "$midx_chain")")" &&
 
@@ -326,4 +327,80 @@ test_expect_success 'MIDX compaction with --no-write-chain-file' '
 	)
 '
 
+test_expect_success 'MIDX compaction with --base' '
+	git init midx-compact-with--base &&
+	(
+		cd midx-compact-with--base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 "$midx_chain" &&
+
+		cp "$midx_chain" "$midx_chain.bak" &&
+
+		git multi-pack-index compact --incremental \
+			--base="$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" \
+			"$(nth_line 4 "$midx_chain")" &&
+		test_line_count = 2 $midx_chain &&
+
+		nth_line 1 "$midx_chain.bak" >expect &&
+		nth_line 1 "$midx_chain" >actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with --base=none' '
+	git init midx-compact-base-none &&
+	(
+		cd midx-compact-base-none &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C D &&
+
+		test_line_count = 4 $midx_chain &&
+
+		cp "$midx_chain" "$midx_chain".bak &&
+
+		# Compact the two bottommost layers (A and B) into a new
+		# root layer with no parent.
+		git multi-pack-index compact --incremental \
+			--base=none \
+			"$(nth_line 1 "$midx_chain")" \
+			"$(nth_line 2 "$midx_chain")" &&
+
+		test_line_count = 3 $midx_chain &&
+
+		# The upper layers (C and D) should be preserved
+		# unchanged.
+		nth_line 3 "$midx_chain.bak" >expect &&
+		nth_line 4 "$midx_chain.bak" >>expect &&
+		nth_line 2 "$midx_chain" >actual &&
+		nth_line 3 "$midx_chain" >>actual &&
+
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'MIDX compaction with bogus --base checksum' '
+	git init midx-compact-bogus-base &&
+	(
+		cd midx-compact-bogus-base &&
+
+		git config maintenance.auto false &&
+
+		write_packs A B C &&
+
+		test_must_fail git multi-pack-index compact --incremental \
+			--base=deadbeef \
+			"$(nth_line 2 "$midx_chain")" \
+			"$(nth_line 3 "$midx_chain")" 2>err &&
+		test_grep "could not find base MIDX" err
+	)
+'
+
 test_done
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 07/16] repack: track the ODB source via existing_packs

[Taylor Blau]

Store the ODB source in the `existing_packs` struct and use that in
place of the raw `repo->objects->sources` access within `cmd_repack()`.

The source used is still assigned from the first source in the list, so
there are no functional changes in this commit. The changes instead
serve two purposes (one immediate, one not):

 - The incremental MIDX-based repacking machinery will need to know what
   source is being used to read the existing MIDX/chain (should one
   exist).

 - In the future, if "git repack" is taught how to operate on other
   object sources, this field will serve as the authoritative value for
   that source.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 5 ++---
 repack.c         | 2 ++
 repack.h         | 1 +
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 4c5a82c2c8d..24be147d39a 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -417,7 +417,7 @@ int cmd_repack(int argc,
 		 * midx_has_unknown_packs() will make the decision for
 		 * us.
 		 */
-		if (!get_multi_pack_index(repo->objects->sources))
+		if (!get_multi_pack_index(existing.source))
 			midx_must_contain_cruft = 1;
 	}
 
@@ -564,8 +564,7 @@ int cmd_repack(int argc,
 		unsigned flags = 0;
 		if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL, 0))
 			flags |= MIDX_WRITE_INCREMENTAL;
-		write_midx_file(repo->objects->sources,
-				NULL, NULL, flags);
+		write_midx_file(existing.source, NULL, NULL, flags);
 	}
 
 cleanup:
diff --git a/repack.c b/repack.c
index 596841027af..2ee6b51420a 100644
--- a/repack.c
+++ b/repack.c
@@ -154,6 +154,8 @@ void existing_packs_collect(struct existing_packs *existing,
 			string_list_append(&existing->non_kept_packs, buf.buf);
 	}
 
+	existing->source = existing->repo->objects->sources;
+
 	string_list_sort(&existing->kept_packs);
 	string_list_sort(&existing->non_kept_packs);
 	string_list_sort(&existing->cruft_packs);
diff --git a/repack.h b/repack.h
index bc9f2e1a5de..c0e9f0ca647 100644
--- a/repack.h
+++ b/repack.h
@@ -56,6 +56,7 @@ struct packed_git;
 
 struct existing_packs {
 	struct repository *repo;
+	struct odb_source *source;
 	struct string_list kept_packs;
 	struct string_list non_kept_packs;
 	struct string_list cruft_packs;
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 08/16] midx: expose `midx_layer_contains_pack()`

[Taylor Blau]

Rename the function `midx_contains_pack_1()` to instead be called
`midx_layer_contains_pack()` and make it accessible. Unlike
`midx_contains_pack()` (which recurses through the entire chain), this
function checks only a single MIDX layer.

This will be used by a subsequent commit to determine whether a given
pack belongs to the tip MIDX layer specifically, rather than to any
layer in the chain.

No functional changes are present in this commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 6 +++---
 midx.h | 2 ++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index bcb8c999015..dc86c8e7fee 100644
--- a/midx.c
+++ b/midx.c
@@ -667,8 +667,8 @@ static int midx_pack_names_cmp(const void *a, const void *b, void *m_)
 		      m->pack_names[*(const size_t *)b]);
 }
 
-static int midx_contains_pack_1(struct multi_pack_index *m,
-				const char *idx_or_pack_name)
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name)
 {
 	uint32_t first = 0, last = m->num_packs;
 
@@ -709,7 +709,7 @@ static int midx_contains_pack_1(struct multi_pack_index *m,
 int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 {
 	for (; m; m = m->base_midx)
-		if (midx_contains_pack_1(m, idx_or_pack_name))
+		if (midx_layer_contains_pack(m, idx_or_pack_name))
 			return 1;
 	return 0;
 }
diff --git a/midx.h b/midx.h
index 77dd66de02b..3ee12dd08ec 100644
--- a/midx.h
+++ b/midx.h
@@ -119,6 +119,8 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 int fill_midx_entry(struct multi_pack_index *m, const struct object_id *oid, struct pack_entry *e);
 int midx_contains_pack(struct multi_pack_index *m,
 		       const char *idx_or_pack_name);
+int midx_layer_contains_pack(struct multi_pack_index *m,
+			     const char *idx_or_pack_name);
 int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id);
 int prepare_multi_pack_index_one(struct odb_source *source);
 
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 09/16] repack-midx: factor out `repack_prepare_midx_command()`

[Taylor Blau]

The `write_midx_included_packs()` function assembles and executes a
`git multi-pack-index write` command, constructing the argument list
inline.

Future commits will introduce additional callers that need to construct
similar `git multi-pack-index` commands (for both `write` and `compact`
subcommands), so extract the common portions of the command setup into a
reusable `repack_prepare_midx_command()` helper.

The extracted helper sets `git_cmd`, pushes the `multi-pack-index`
subcommand and verb, and handles `--progress`/`--no-progress` and
`--bitmap` flags. The remaining arguments that are specific to the
`write` subcommand (such as `--stdin-packs`) are left to the caller.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 0682b80c427..83151d4734a 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -275,6 +275,23 @@ static void remove_redundant_bitmaps(struct string_list *include,
 	strbuf_release(&path);
 }
 
+static void repack_prepare_midx_command(struct child_process *cmd,
+					struct repack_write_midx_opts *opts,
+					const char *verb)
+{
+	cmd->git_cmd = 1;
+
+	strvec_pushl(&cmd->args, "multi-pack-index", verb, NULL);
+
+	if (opts->show_progress)
+		strvec_push(&cmd->args, "--progress");
+	else
+		strvec_push(&cmd->args, "--no-progress");
+
+	if (opts->write_bitmaps)
+		strvec_push(&cmd->args, "--bitmap");
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
@@ -289,18 +306,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		goto done;
 
 	cmd.in = -1;
-	cmd.git_cmd = 1;
 
-	strvec_push(&cmd.args, "multi-pack-index");
-	strvec_pushl(&cmd.args, "write", "--stdin-packs", NULL);
-
-	if (opts->show_progress)
-		strvec_push(&cmd.args, "--progress");
-	else
-		strvec_push(&cmd.args, "--no-progress");
-
-	if (opts->write_bitmaps)
-		strvec_push(&cmd.args, "--bitmap");
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 10/16] repack-midx: extract `repack_fill_midx_stdin_packs()`

[Taylor Blau]

The function `write_midx_included_packs()` manages the lifecycle of
writing packs to stdin when running `git multi-pack-index write` as a
child process.

Extract a standalone `repack_fill_midx_stdin_packs()` helper, which
handles `--stdin-packs` argument setup, starting the command, writing
pack names to its standard input, and finishing the command.

This simplifies `write_midx_included_packs()` and prepares for a
subsequent commit where the same helper is called with `cmd->out = -1`
to capture the MIDX's checksum from the command's standard output,
which is needed when writing MIDX layers with `--no-write-chain-file`.

No functional changes are included in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-midx.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/repack-midx.c b/repack-midx.c
index 83151d4734a..78f069c2151 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -292,23 +292,42 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 		strvec_push(&cmd->args, "--bitmap");
 }
 
+static int repack_fill_midx_stdin_packs(struct child_process *cmd,
+					struct string_list *include)
+{
+	struct string_list_item *item;
+	FILE *in;
+	int ret;
+
+	cmd->in = -1;
+
+	strvec_push(&cmd->args, "--stdin-packs");
+
+	ret = start_command(cmd);
+	if (ret)
+		return ret;
+
+	in = xfdopen(cmd->in, "w");
+	for_each_string_list_item(item, include)
+		fprintf(in, "%s\n", item->string);
+	fclose(in);
+
+	return finish_command(cmd);
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
 	struct string_list_item *item;
 	struct packed_git *preferred = pack_geometry_preferred_pack(opts->geometry);
-	FILE *in;
 	int ret = 0;
 
 	midx_included_packs(&include, opts);
 	if (!include.nr)
 		goto done;
 
-	cmd.in = -1;
-
 	repack_prepare_midx_command(&cmd, opts, "write");
-	strvec_push(&cmd.args, "--stdin-packs");
 
 	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
@@ -350,16 +369,7 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = start_command(&cmd);
-	if (ret)
-		goto done;
-
-	in = xfdopen(cmd.in, "w");
-	for_each_string_list_item(item, &include)
-		fprintf(in, "%s\n", item->string);
-	fclose(in);
-
-	ret = finish_command(&cmd);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 11/16] repack-geometry: prepare for incremental MIDX repacking

[Taylor Blau]

Teach `pack_geometry_init()` to optionally restrict the set of
repacking candidates to only packs in the tip MIDX layer when a
`midx_layer_threshold` is configured. If the tip layer has fewer packs
than the threshold, those packs are excluded entirely; otherwise only
packs in that layer participate in the geometric repack.

Also track whether any tip-layer packs were included in the rollup
(`midx_tip_rewritten`), which a subsequent commit will use to decide
how to update the MIDX chain after repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 repack-geometry.c | 35 +++++++++++++++++++++++++++++++++++
 repack.h          |  4 ++++
 2 files changed, 39 insertions(+)

diff --git a/repack-geometry.c b/repack-geometry.c
index 7cebd0cb45f..2408b8a3cc2 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -4,6 +4,7 @@
 #include "repack.h"
 #include "repository.h"
 #include "hex.h"
+#include "midx.h"
 #include "packfile.h"
 
 static uint32_t pack_geometry_weight(struct packed_git *p)
@@ -31,8 +32,28 @@ void pack_geometry_init(struct pack_geometry *geometry,
 {
 	struct packed_git *p;
 	struct strbuf buf = STRBUF_INIT;
+	struct multi_pack_index *m = get_multi_pack_index(existing->source);
 
 	repo_for_each_pack(existing->repo, p) {
+		if (geometry->midx_layer_threshold_set && m &&
+		    p->multi_pack_index) {
+			/*
+			 * When writing MIDX layers incrementally,
+			 * ignore packs unless they are in the most
+			 * recent MIDX layer *and* there are at least
+			 * 'midx_layer_threshold' packs in that layer.
+			 *
+			 * Otherwise 'p' is either in an older layer, or
+			 * the youngest layer does not have enough packs
+			 * to consider its packs as candidates for
+			 * repacking. In either of those cases we want
+			 * to ignore the pack.
+			 */
+			if (m->num_packs < geometry->midx_layer_threshold ||
+			    !midx_layer_contains_pack(m, pack_basename(p)))
+				continue;
+		}
+
 		if (args->local && !p->pack_local)
 			/*
 			 * When asked to only repack local packfiles we skip
@@ -173,6 +194,20 @@ void pack_geometry_split(struct pack_geometry *geometry)
 	geometry->promisor_split = compute_pack_geometry_split(geometry->promisor_pack,
 							       geometry->promisor_pack_nr,
 							       geometry->split_factor);
+	for (uint32_t i = 0; i < geometry->split; i++) {
+		struct packed_git *p = geometry->pack[i];
+		/*
+		 * During incremental MIDX/bitmap repacking, any packs
+		 * included in the rollup are either (a) not MIDX'd, or
+		 * (b) contained in the tip layer iff it has at least
+		 * the threshold number of packs.
+		 *
+		 * In the latter case, we can safely conclude that the
+		 * tip of the MIDX chain will be rewritten.
+		 */
+		if (p->multi_pack_index)
+			geometry->midx_tip_rewritten = true;
+	}
 }
 
 struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry)
diff --git a/repack.h b/repack.h
index c0e9f0ca647..77d24ee45fb 100644
--- a/repack.h
+++ b/repack.h
@@ -108,6 +108,10 @@ struct pack_geometry {
 	uint32_t promisor_pack_nr, promisor_pack_alloc;
 	uint32_t promisor_split;
 
+	uint32_t midx_layer_threshold;
+	bool midx_layer_threshold_set;
+	bool midx_tip_rewritten;
+
 	int split_factor;
 };
 
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 12/16] builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

[Taylor Blau]

Change the --write-midx (-m) flag from an OPT_BOOL to an OPT_CALLBACK
that accepts an optional mode argument. Introduce an enum with
REPACK_WRITE_MIDX_NONE and REPACK_WRITE_MIDX_DEFAULT to distinguish
between the two states, and update all existing boolean checks
accordingly.

For now, passing no argument (or just `-m`) selects the default mode,
preserving existing behavior. A subsequent commit will add a new mode
for writing incremental MIDXs.

Extract repack_write_midx() as a dispatcher that selects the
appropriate MIDX-writing implementation based on the mode.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c | 50 ++++++++++++++++++++++++++++++++++++------------
 repack-midx.c    | 14 +++++++++++++-
 repack.h         |  8 +++++++-
 3 files changed, 58 insertions(+), 14 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 24be147d39a..5d366340c34 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -97,6 +97,24 @@ static int repack_config(const char *var, const char *value,
 	return git_default_config(var, value, ctx, cb);
 }
 
+static int option_parse_write_midx(const struct option *opt, const char *arg,
+				   int unset)
+{
+	enum repack_write_midx_mode *cfg = opt->value;
+
+	if (unset) {
+		*cfg = REPACK_WRITE_MIDX_NONE;
+		return 0;
+	}
+
+	if (!arg || !*arg)
+		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else
+		return error(_("unknown value for %s: %s"), opt->long_name, arg);
+
+	return 0;
+}
+
 int cmd_repack(int argc,
 	       const char **argv,
 	       const char *prefix,
@@ -119,7 +137,7 @@ int cmd_repack(int argc,
 	struct string_list keep_pack_list = STRING_LIST_INIT_NODUP;
 	struct pack_objects_args po_args = PACK_OBJECTS_ARGS_INIT;
 	struct pack_objects_args cruft_po_args = PACK_OBJECTS_ARGS_INIT;
-	int write_midx = 0;
+	enum repack_write_midx_mode write_midx = REPACK_WRITE_MIDX_NONE;
 	const char *cruft_expiration = NULL;
 	const char *expire_to = NULL;
 	const char *filter_to = NULL;
@@ -185,8 +203,14 @@ int cmd_repack(int argc,
 				N_("do not repack this pack")),
 		OPT_INTEGER('g', "geometric", &geometry.split_factor,
 			    N_("find a geometric progression with factor <N>")),
-		OPT_BOOL('m', "write-midx", &write_midx,
-			   N_("write a multi-pack index of the resulting packs")),
+		OPT_CALLBACK_F(0, "write-midx", &write_midx,
+			   N_("mode"),
+			   N_("write a multi-pack index of the resulting packs"),
+			   PARSE_OPT_OPTARG, option_parse_write_midx),
+		OPT_SET_INT_F('m', NULL, &write_midx,
+			   N_("write a multi-pack index of the resulting packs"),
+			   REPACK_WRITE_MIDX_DEFAULT,
+			   PARSE_OPT_HIDDEN),
 		OPT_STRING(0, "expire-to", &expire_to, N_("dir"),
 			   N_("pack prefix to store a pack containing pruned objects")),
 		OPT_STRING(0, "filter-to", &filter_to, N_("dir"),
@@ -221,14 +245,16 @@ int cmd_repack(int argc,
 		pack_everything |= ALL_INTO_ONE;
 
 	if (write_bitmaps < 0) {
-		if (!write_midx &&
+		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
 			write_bitmaps = 0;
 	}
 	if (po_args.pack_kept_objects < 0)
-		po_args.pack_kept_objects = write_bitmaps > 0 && !write_midx;
+		po_args.pack_kept_objects = write_bitmaps > 0 &&
+			write_midx == REPACK_WRITE_MIDX_NONE;
 
-	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
+	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) &&
+	    write_midx == REPACK_WRITE_MIDX_NONE)
 		die(_(incremental_bitmap_conflict_error));
 
 	if (write_bitmaps && po_args.local &&
@@ -244,7 +270,7 @@ int cmd_repack(int argc,
 		write_bitmaps = 0;
 	}
 
-	if (write_midx && write_bitmaps) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
 		strbuf_addf(&path, "%s/%s_XXXXXX",
@@ -297,7 +323,7 @@ int cmd_repack(int argc,
 	}
 	if (repo_has_promisor_remote(repo))
 		strvec_push(&cmd.args, "--exclude-promisor-objects");
-	if (!write_midx) {
+	if (write_midx == REPACK_WRITE_MIDX_NONE) {
 		if (write_bitmaps > 0)
 			strvec_push(&cmd.args, "--write-bitmap-index");
 		else if (write_bitmaps < 0)
@@ -519,7 +545,7 @@ int cmd_repack(int argc,
 	if (delete_redundant && pack_everything & ALL_INTO_ONE)
 		existing_packs_mark_for_deletion(&existing, &names);
 
-	if (write_midx) {
+	if (write_midx != REPACK_WRITE_MIDX_NONE) {
 		struct repack_write_midx_opts opts = {
 			.existing = &existing,
 			.geometry = &geometry,
@@ -528,11 +554,11 @@ int cmd_repack(int argc,
 			.packdir = packdir,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
-			.midx_must_contain_cruft = midx_must_contain_cruft
+			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.mode = write_midx,
 		};
 
-		ret = write_midx_included_packs(&opts);
-
+		ret = repack_write_midx(&opts);
 		if (ret)
 			goto cleanup;
 	}
diff --git a/repack-midx.c b/repack-midx.c
index 78f069c2151..4a568a2a9b8 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -315,7 +315,7 @@ static int repack_fill_midx_stdin_packs(struct child_process *cmd,
 	return finish_command(cmd);
 }
 
-int write_midx_included_packs(struct repack_write_midx_opts *opts)
+static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list include = STRING_LIST_INIT_DUP;
@@ -378,3 +378,15 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 
 	return ret;
 }
+
+int repack_write_midx(struct repack_write_midx_opts *opts)
+{
+	switch (opts->mode) {
+	case REPACK_WRITE_MIDX_NONE:
+		BUG("write_midx mode is NONE?");
+	case REPACK_WRITE_MIDX_DEFAULT:
+		return write_midx_included_packs(opts);
+	default:
+		BUG("unhandled write_midx mode: %d", opts->mode);
+	}
+}
diff --git a/repack.h b/repack.h
index 77d24ee45fb..81907fcce7f 100644
--- a/repack.h
+++ b/repack.h
@@ -134,6 +134,11 @@ void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
 
+enum repack_write_midx_mode {
+	REPACK_WRITE_MIDX_NONE,
+	REPACK_WRITE_MIDX_DEFAULT,
+};
+
 struct repack_write_midx_opts {
 	struct existing_packs *existing;
 	struct pack_geometry *geometry;
@@ -143,10 +148,11 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	enum repack_write_midx_mode mode;
 };
 
 void midx_snapshot_refs(struct repository *repo, struct tempfile *f);
-int write_midx_included_packs(struct repack_write_midx_opts *opts);
+int repack_write_midx(struct repack_write_midx_opts *opts);
 
 int write_filtered_pack(const struct write_pack_opts *opts,
 			struct existing_packs *existing,
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 13/16] packfile: ensure `close_pack_revindex()` frees in-memory revindex

[Taylor Blau]

The following commit will introduce a case where we write a MIDX bitmap
over packs that do not themselves have on-disk *.rev files.

This case is supported within Git, and we will simply fall back to
generating the revindex in memory. But we don't ever release that
memory, causing a leak that is exposed by a test introduced in the
following commit.

(As far as I could find, we never free()'d memory allocated as a
byproduct of creating an in-memory revindex, likely because that code
predates the leak-checking niceties we have in the test suite now.)

Rectify this by calling `FREE_AND_NULL()` on the `p->revindex` field
when calling `close_pack_revindex()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 packfile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packfile.c b/packfile.c
index b012d648ada..a1e88fdb223 100644
--- a/packfile.c
+++ b/packfile.c
@@ -420,6 +420,8 @@ void close_pack_index(struct packed_git *p)
 
 static void close_pack_revindex(struct packed_git *p)
 {
+	FREE_AND_NULL(p->revindex);
+
 	if (!p->revindex_map)
 		return;
 
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 14/16] repack: implement incremental MIDX repacking

[Taylor Blau]

Implement the `write_midx_incremental()` function, which builds and
maintains an incremental MIDX chain as part of the geometric repacking
process.

Unlike the default mode which writes a single flat MIDX, the incremental
mode constructs a compaction plan that determines which MIDX layers to
write, compact, or copy, and then executes each step using `git
multi-pack-index` subcommands with the --no-write-chain-file flag.

The repacking strategy works as follows:

 * Acquire the lock guarding the multi-pack-index-chain.

 * A new MIDX layer is always written containing the newly created
   pack(s). If the tip MIDX layer was rewritten during geometric
   repacking, any surviving packs from that layer are also included.

 * Starting from the new layer, adjacent MIDX layers are merged together
   as long as the accumulated object count exceeds half the object count
   of the next deeper layer (controlled by 'repack.midxSplitFactor').

 * Remaining layers in the chain are evaluated pairwise and either
   compacted or copied as-is, following the same merging condition.

 * Write the contents of the new multi-pack-index chain, atomically move
   it into place, and then release the lock.

 * Delete any now-unused MIDX layers.

After writing the new layer, the strategy is evaluated among the
existing MIDX layers in order from oldest to newest. Each step that
writes a new MIDX layer uses "--no-write-chain-file" to avoid updating
the multi-pack-index-chain file. After all steps are complete, the new
chain file is written and then atomically moved into place.

At present, this functionality is exposed behind a new enum value,
`REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
subsequent commit will expose this mode via `git repack
--write-midx=incremental`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c |   5 +
 repack-midx.c    | 593 +++++++++++++++++++++++++++++++++++++++++++++--
 repack.h         |   3 +
 3 files changed, 588 insertions(+), 13 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 5d366340c34..75c57736780 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -42,6 +42,9 @@ static const char incremental_bitmap_conflict_error[] = N_(
 "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
 );
 
+#define DEFAULT_MIDX_SPLIT_FACTOR 2
+#define DEFAULT_MIDX_NEW_LAYER_THRESHOLD 8
+
 struct repack_config_ctx {
 	struct pack_objects_args *po_args;
 	struct pack_objects_args *cruft_po_args;
@@ -555,6 +558,8 @@ int cmd_repack(int argc,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
 			.midx_must_contain_cruft = midx_must_contain_cruft,
+			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
+			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
 			.mode = write_midx,
 		};
 
diff --git a/repack-midx.c b/repack-midx.c
index 4a568a2a9b8..405f4662174 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -2,12 +2,16 @@
 #include "repack.h"
 #include "hash.h"
 #include "hex.h"
+#include "lockfile.h"
+#include "midx.h"
 #include "odb.h"
 #include "oidset.h"
 #include "pack-bitmap.h"
+#include "path.h"
 #include "refs.h"
 #include "run-command.h"
 #include "tempfile.h"
+#include "trace2.h"
 
 struct midx_snapshot_ref_data {
 	struct repository *repo;
@@ -293,26 +297,30 @@ static void repack_prepare_midx_command(struct child_process *cmd,
 }
 
 static int repack_fill_midx_stdin_packs(struct child_process *cmd,
-					struct string_list *include)
+					struct string_list *include,
+					struct string_list *out)
 {
+	struct strbuf in_buf = STRBUF_INIT;
+	struct strbuf out_buf = STRBUF_INIT;
 	struct string_list_item *item;
-	FILE *in;
 	int ret;
 
-	cmd->in = -1;
-
 	strvec_push(&cmd->args, "--stdin-packs");
 
-	ret = start_command(cmd);
-	if (ret)
-		return ret;
-
-	in = xfdopen(cmd->in, "w");
 	for_each_string_list_item(item, include)
-		fprintf(in, "%s\n", item->string);
-	fclose(in);
+		strbuf_addf(&in_buf, "%s\n", item->string);
 
-	return finish_command(cmd);
+	ret = pipe_command(cmd, in_buf.buf, in_buf.len,
+			   out ? &out_buf : NULL, 0, NULL, 0);
+
+	if (out)
+		string_list_split_f(out, out_buf.buf, "\n", -1,
+				    STRING_LIST_SPLIT_NONEMPTY);
+
+	strbuf_release(&in_buf);
+	strbuf_release(&out_buf);
+
+	return ret;
 }
 
 static int write_midx_included_packs(struct repack_write_midx_opts *opts)
@@ -369,7 +377,7 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s",
 			     opts->refs_snapshot);
 
-	ret = repack_fill_midx_stdin_packs(&cmd, &include);
+	ret = repack_fill_midx_stdin_packs(&cmd, &include, NULL);
 done:
 	if (!ret && opts->write_bitmaps)
 		remove_redundant_bitmaps(&include, opts->packdir);
@@ -379,6 +387,563 @@ static int write_midx_included_packs(struct repack_write_midx_opts *opts)
 	return ret;
 }
 
+struct midx_compaction_step {
+	union {
+		struct multi_pack_index *copy;
+		struct string_list write;
+		struct {
+			struct multi_pack_index *from;
+			struct multi_pack_index *to;
+		} compact;
+	} u;
+
+	uint32_t objects_nr;
+	char *csum;
+
+	enum {
+		MIDX_COMPACTION_STEP_UNKNOWN,
+		MIDX_COMPACTION_STEP_COPY,
+		MIDX_COMPACTION_STEP_WRITE,
+		MIDX_COMPACTION_STEP_COMPACT,
+	} type;
+};
+
+static const char *midx_compaction_step_base(const struct midx_compaction_step *step)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot use UNKNOWN step as a base");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_get_checksum_hex(step->u.copy);
+	case MIDX_COMPACTION_STEP_WRITE:
+		BUG("cannot use WRITE step as a base");
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_get_checksum_hex(step->u.compact.to);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static int midx_compaction_step_exec_copy(struct midx_compaction_step *step)
+{
+	step->csum = xstrdup(midx_get_checksum_hex(step->u.copy));
+	return 0;
+}
+
+static int midx_compaction_step_exec_write(struct midx_compaction_step *step,
+					   struct repack_write_midx_opts *opts,
+					   const char *base)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct string_list hash = STRING_LIST_INIT_DUP;
+	struct string_list_item *item;
+	const char *preferred_pack = NULL;
+	int ret = 0;
+
+	if (!step->u.write.nr) {
+		ret = error(_("no packs to write MIDX during compaction"));
+		goto out;
+	}
+
+	for_each_string_list_item(item, &step->u.write) {
+		if (item->util)
+			preferred_pack = item->string;
+	}
+
+	repack_prepare_midx_command(&cmd, opts, "write");
+	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file", NULL);
+	strvec_pushf(&cmd.args, "--base=%s", base ? base : "none");
+
+	if (preferred_pack) {
+		struct strbuf buf = STRBUF_INIT;
+
+		strbuf_addstr(&buf, preferred_pack);
+		strbuf_strip_suffix(&buf, ".idx");
+		strbuf_addstr(&buf, ".pack");
+
+		strvec_pushf(&cmd.args, "--preferred-pack=%s", buf.buf);
+
+		strbuf_release(&buf);
+	}
+
+	ret = repack_fill_midx_stdin_packs(&cmd, &step->u.write, &hash);
+	if (hash.nr != 1) {
+		ret = error(_("expected exactly one line during MIDX write, "
+			      "got: %"PRIuMAX),
+			    (uintmax_t)hash.nr);
+		goto out;
+	}
+
+	step->csum = xstrdup(hash.items[0].string);
+
+out:
+	string_list_clear(&hash, 0);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec_compact(struct midx_compaction_step *step,
+					     struct repack_write_midx_opts *opts)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf buf = STRBUF_INIT;
+	FILE *out = NULL;
+	int ret;
+
+	repack_prepare_midx_command(&cmd, opts, "compact");
+	strvec_pushl(&cmd.args, "--incremental", "--no-write-chain-file",
+		     midx_get_checksum_hex(step->u.compact.from),
+		     midx_get_checksum_hex(step->u.compact.to), NULL);
+
+	cmd.out = -1;
+
+	ret = start_command(&cmd);
+	if (ret)
+		goto out;
+
+	out = xfdopen(cmd.out, "r");
+	while (strbuf_getline_lf(&buf, out) != EOF) {
+		if (step->csum) {
+			ret = error(_("unexpected MIDX output: '%s'"), buf.buf);
+			fclose(out);
+			out = NULL;
+			finish_command(&cmd);
+			goto out;
+		}
+		step->csum = strbuf_detach(&buf, NULL);
+	}
+
+	ret = finish_command(&cmd);
+
+out:
+	if (out)
+		fclose(out);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+static int midx_compaction_step_exec(struct midx_compaction_step *step,
+				     struct repack_write_midx_opts *opts,
+				     const char *base)
+{
+	switch (step->type) {
+	case MIDX_COMPACTION_STEP_UNKNOWN:
+		BUG("cannot execute UNKNOWN midx compaction step");
+	case MIDX_COMPACTION_STEP_COPY:
+		return midx_compaction_step_exec_copy(step);
+	case MIDX_COMPACTION_STEP_WRITE:
+		return midx_compaction_step_exec_write(step, opts, base);
+	case MIDX_COMPACTION_STEP_COMPACT:
+		return midx_compaction_step_exec_compact(step, opts);
+	default:
+		BUG("unhandled midx compaction step type %d", step->type);
+	}
+}
+
+static void midx_compaction_step_release(struct midx_compaction_step *step)
+{
+	if (step->type == MIDX_COMPACTION_STEP_WRITE)
+		string_list_clear(&step->u.write, 0);
+	free(step->csum);
+}
+
+static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
+					    struct midx_compaction_step **steps_p,
+					    size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step step = { 0 };
+	struct strbuf buf = STRBUF_INIT;
+	size_t steps_nr = 0, steps_alloc = 0;
+	uint32_t i;
+	int ret = 0;
+
+	trace2_region_enter("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	for (i = 0; m && i < m->num_packs + m->num_packs_in_base; i++) {
+		if (prepare_midx_pack(m, i)) {
+			ret = error(_("could not load pack %"PRIu32" from MIDX"),
+				    i);
+			goto out;
+		}
+	}
+
+	trace2_region_enter("repack", "steps:write", opts->existing->repo);
+
+	/*
+	 * The first MIDX in the resulting chain is always going to be
+	 * new.
+	 *
+	 * At a minimum, it will include all of the newly written packs.
+	 * If there is an existing MIDX whose tip layer contains packs
+	 * that were repacked, it will also include any of its packs
+	 * which were *not* rolled up as part of the geometric repack
+	 * (if any), and the previous tip will be replaced.
+	 *
+	 * It may grow to include the packs from zero or more MIDXs from
+	 * the old chain, beginning either at the old tip (if the MIDX
+	 * was *not* rewritten) or the old tip's base MIDX layer
+	 * (otherwise).
+	 */
+	step.type = MIDX_COMPACTION_STEP_WRITE;
+	string_list_init_dup(&step.u.write);
+
+	for (i = 0; i < opts->names->nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "pack-%s.idx", opts->names->items[i].string);
+		string_list_append(&step.u.write, buf.buf);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:fresh",
+				   step.u.write.items[step.u.write.nr - 1].string);
+	}
+	for (i = 0; i < opts->geometry->split; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:fresh:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * Now handle any existing packs which were *not* rewritten.
+	 *
+	 * The list of packs in opts->geometry only contains MIDX'd
+	 * packs from the newest layer when that layer has more than
+	 * 'repack.midxNewLayerThreshold' number of packs.
+	 *
+	 * If the MIDX tip was rewritten (that is, one or more of those
+	 * packs appear below the split line), then add all packs above
+	 * the split line to the new layer, as the old one is no longer
+	 * usable.
+	 *
+	 * If the MIDX tip was not rewritten (that is, all MIDX'd packs
+	 * from the youngest layer appear below the split line, or were
+	 * not included in the geometric repack at all because there
+	 * were too few of them), ignore them since we'll retain the
+	 * existing layer as-is.
+	 */
+	for (i = opts->geometry->split; i < opts->geometry->pack_nr; i++) {
+		struct packed_git *p = opts->geometry->pack[i];
+		struct string_list_item *item;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, pack_basename(p));
+		strbuf_strip_suffix(&buf, ".pack");
+		strbuf_addstr(&buf, ".idx");
+
+		if (p->multi_pack_index &&
+		    !opts->geometry->midx_tip_rewritten) {
+			trace2_data_string("repack", opts->existing->repo,
+					   "exclude:unmodified", buf.buf);
+			continue;
+		}
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified", buf.buf);
+		trace2_data_string("repack", opts->existing->repo,
+				   "include:unmodified:midx",
+				   p->multi_pack_index ? "true" : "false");
+
+		item = string_list_append(&step.u.write, buf.buf);
+		if (p->multi_pack_index || i == opts->geometry->pack_nr - 1)
+			item->util = (void *)1; /* mark as preferred */
+
+		if (unsigned_add_overflows(step.objects_nr, p->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+
+		step.objects_nr += p->num_objects;
+	}
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "include:unmodified:objects_nr",
+			   (uintmax_t)step.objects_nr);
+
+	/*
+	 * If the MIDX tip was rewritten, then we no longer consider it
+	 * a candidate for compaction, since it will not exist in the
+	 * MIDX chain being built.
+	 */
+	if (opts->geometry->midx_tip_rewritten)
+		m = m->base_midx;
+
+	trace2_data_string("repack", opts->existing->repo, "midx:rewrote-tip",
+			   opts->geometry->midx_tip_rewritten ? "true" : "false");
+
+	trace2_region_enter("repack", "compact", opts->existing->repo);
+
+	/*
+	 * Compact additional MIDX layers into this proposed one until
+	 * the merging condition is violated.
+	 */
+	while (m) {
+		uint32_t preferred_pack_idx;
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "candidate", midx_get_checksum_hex(m));
+
+		if (step.objects_nr < m->num_objects / opts->midx_split_factor) {
+			/*
+			 * Stop compacting MIDX layer as soon as the
+			 * merged size is less than half the size of the
+			 * next layer in the chain.
+			 */
+			trace2_data_string("repack", opts->existing->repo,
+					   "compact", "violated");
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "objects_nr",
+					   (uintmax_t)step.objects_nr);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "next_objects_nr",
+					   (uintmax_t)m->num_objects);
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "split_factor",
+					   (uintmax_t)opts->midx_split_factor);
+
+			break;
+		}
+
+		if (midx_preferred_pack(m, &preferred_pack_idx) < 0) {
+			ret = error(_("could not find preferred pack for MIDX "
+				      "%s"), midx_get_checksum_hex(m));
+			goto out;
+		}
+
+		for (i = 0; i < m->num_packs; i++) {
+			struct string_list_item *item;
+			uint32_t pack_int_id = i + m->num_packs_in_base;
+			struct packed_git *p = nth_midxed_pack(m, pack_int_id);
+
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, pack_basename(p));
+			strbuf_strip_suffix(&buf, ".pack");
+			strbuf_addstr(&buf, ".idx");
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "midx:pack", buf.buf);
+
+			item = string_list_append(&step.u.write, buf.buf);
+			if (pack_int_id == preferred_pack_idx)
+				item->util = (void *)1; /* mark as preferred */
+		}
+
+		if (unsigned_add_overflows(step.objects_nr, m->num_objects)) {
+			ret = error(_("too many objects in MIDX compaction step"));
+			goto out;
+		}
+		step.objects_nr += m->num_objects;
+
+		m = m->base_midx;
+	}
+
+	if (step.u.write.nr > 0) {
+		/*
+		 * As long as there is at least one new pack to write
+		 * (and thus the MIDX is non-empty), add it to the plan.
+		 */
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+		steps[steps_nr++] = step;
+	}
+
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:objects_nr", (uintmax_t)step.objects_nr);
+	trace2_data_intmax("repack", opts->existing->repo,
+			   "step:packs_nr", (uintmax_t)step.u.write.nr);
+
+	trace2_region_leave("repack", "compact", opts->existing->repo);
+	trace2_region_leave("repack", "steps:write", opts->existing->repo);
+
+	trace2_region_enter("repack", "steps:rest", opts->existing->repo);
+
+	/*
+	 * Then start over, repeat, and either compact or keep as-is
+	 * each MIDX layer until we have exhausted the chain.
+	 *
+	 * Finally, evaluate the remainder of the chain (if any) and
+	 * either compact a sequence of adjacent layers, or keep
+	 * individual layers as-is according to the same merging
+	 * condition as above.
+	 */
+	while (m) {
+		struct multi_pack_index *next = m;
+
+		ALLOC_GROW(steps, steps_nr + 1, steps_alloc);
+
+		memset(&step, 0, sizeof(step));
+		step.type = MIDX_COMPACTION_STEP_UNKNOWN;
+
+		trace2_region_enter("repack", "step", opts->existing->repo);
+
+		trace2_data_string("repack", opts->existing->repo,
+				   "from", midx_get_checksum_hex(m));
+
+		while (next) {
+			uint32_t proposed_objects_nr;
+			if (unsigned_add_overflows(step.objects_nr, next->num_objects)) {
+				ret = error(_("too many objects in MIDX compaction step"));
+				trace2_region_leave("repack", "step", opts->existing->repo);
+				goto out;
+			}
+
+			proposed_objects_nr = step.objects_nr + next->num_objects;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "proposed",
+					   midx_get_checksum_hex(next));
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "proposed:objects_nr",
+					   (uintmax_t)next->num_objects);
+
+			if (!next->base_midx) {
+				/*
+				 * If we are at the end of the MIDX
+				 * chain, there is nothing to compact,
+				 * so mark it and stop.
+				 */
+				step.objects_nr = proposed_objects_nr;
+				break;
+			}
+
+			if (proposed_objects_nr < next->base_midx->num_objects / opts->midx_split_factor) {
+				/*
+				 * If there is a MIDX following this
+				 * one, but our accumulated size is less
+				 * than half of its size, compacting
+				 * them would violate the merging
+				 * condition, so stop here.
+				 */
+
+				trace2_data_string("repack", opts->existing->repo,
+						   "compact:violated:at",
+						   midx_get_checksum_hex(next->base_midx));
+				trace2_data_intmax("repack", opts->existing->repo,
+						   "compact:violated:at:objects_nr",
+						   (uintmax_t)next->base_midx->num_objects);
+				break;
+			}
+
+			/*
+			 * Otherwise, it is OK to compact the next layer
+			 * into this one. Do so, and then continue
+			 * through the remainder of the chain.
+			 */
+			step.objects_nr = proposed_objects_nr;
+			trace2_data_intmax("repack", opts->existing->repo,
+					   "step:objects_nr",
+					   (uintmax_t)step.objects_nr);
+			next = next->base_midx;
+		}
+
+		if (m == next) {
+			step.type = MIDX_COMPACTION_STEP_COPY;
+			step.u.copy = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "copy");
+		} else {
+			step.type = MIDX_COMPACTION_STEP_COMPACT;
+			step.u.compact.from = next;
+			step.u.compact.to = m;
+
+			trace2_data_string("repack", opts->existing->repo,
+					   "to", midx_get_checksum_hex(m));
+			trace2_data_string("repack", opts->existing->repo,
+					   "type", "compact");
+		}
+
+		m = next->base_midx;
+		steps[steps_nr++] = step;
+		trace2_region_leave("repack", "step", opts->existing->repo);
+	}
+
+	trace2_region_leave("repack", "steps:rest", opts->existing->repo);
+
+out:
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+
+	strbuf_release(&buf);
+
+	trace2_region_leave("repack", "make_midx_compaction_plan",
+			    opts->existing->repo);
+
+	return ret;
+}
+
+static int write_midx_incremental(struct repack_write_midx_opts *opts)
+{
+	struct midx_compaction_step *steps = NULL;
+	struct strbuf lock_name = STRBUF_INIT;
+	struct lock_file lf;
+	size_t steps_nr = 0;
+	size_t i;
+	int ret = 0;
+
+	get_midx_chain_filename(opts->existing->source, &lock_name);
+	if (safe_create_leading_directories(opts->existing->repo,
+					    lock_name.buf))
+		die_errno(_("unable to create leading directories of %s"),
+			  lock_name.buf);
+	hold_lock_file_for_update(&lf, lock_name.buf, LOCK_DIE_ON_ERROR);
+
+	if (!fdopen_lock_file(&lf, "w")) {
+		ret = error_errno(_("unable to open multi-pack-index chain file"));
+		goto done;
+	}
+
+	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+		ret = error(_("unable to generate compaction plan"));
+		goto done;
+	}
+
+	for (i = 0; i < steps_nr; i++) {
+		struct midx_compaction_step *step = &steps[i];
+		char *base = NULL;
+
+		if (i + 1 < steps_nr)
+			base = xstrdup(midx_compaction_step_base(&steps[i + 1]));
+
+		if (midx_compaction_step_exec(step, opts, base) < 0) {
+			ret = error(_("unable to execute compaction step %"PRIuMAX),
+				    (uintmax_t)i);
+			free(base);
+			goto done;
+		}
+
+		free(base);
+	}
+
+	i = steps_nr;
+	while (i--) {
+		struct midx_compaction_step *step = &steps[i];
+		if (!step->csum)
+			BUG("missing result for compaction step %"PRIuMAX,
+			    (uintmax_t)i);
+		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+	}
+
+	commit_lock_file(&lf);
+
+done:
+	strbuf_release(&lock_name);
+	for (i = 0; i < steps_nr; i++)
+		midx_compaction_step_release(&steps[i]);
+	free(steps);
+	return ret;
+}
+
 int repack_write_midx(struct repack_write_midx_opts *opts)
 {
 	switch (opts->mode) {
@@ -386,6 +951,8 @@ int repack_write_midx(struct repack_write_midx_opts *opts)
 		BUG("write_midx mode is NONE?");
 	case REPACK_WRITE_MIDX_DEFAULT:
 		return write_midx_included_packs(opts);
+	case REPACK_WRITE_MIDX_INCREMENTAL:
+		return write_midx_incremental(opts);
 	default:
 		BUG("unhandled write_midx mode: %d", opts->mode);
 	}
diff --git a/repack.h b/repack.h
index 81907fcce7f..831ccfb1c6c 100644
--- a/repack.h
+++ b/repack.h
@@ -137,6 +137,7 @@ struct tempfile;
 enum repack_write_midx_mode {
 	REPACK_WRITE_MIDX_NONE,
 	REPACK_WRITE_MIDX_DEFAULT,
+	REPACK_WRITE_MIDX_INCREMENTAL,
 };
 
 struct repack_write_midx_opts {
@@ -148,6 +149,8 @@ struct repack_write_midx_opts {
 	int show_progress;
 	int write_bitmaps;
 	int midx_must_contain_cruft;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 	enum repack_write_midx_mode mode;
 };
 
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 15/16] repack: introduce `--write-midx=incremental`

[Taylor Blau]

Expose the incremental MIDX repacking mode (implemented in an earlier
commit) via a new --write-midx=incremental option for `git repack`.

Add "incremental" as a recognized argument to the --write-midx
OPT_CALLBACK, mapping it to REPACK_WRITE_MIDX_INCREMENTAL. When this
mode is active and --geometric is in use, set the midx_layer_threshold
on the pack geometry so that only packs in sufficiently large tip layers
are considered for repacking.

Two new configuration options control the compaction behavior:

 - repack.midxSplitFactor (default: 2): the factor used in the
   geometric merging condition for MIDX layers.

 - repack.midxNewLayerThreshold (default: 8): the minimum number of
   packs in the tip MIDX layer before its packs are considered as
   candidates for geometric repacking.

Add tests exercising the new mode across a variety of scenarios
including basic geometric violations, multi-round chain integrity,
branching and merging histories, cross-layer object uniqueness, and
threshold-based compaction.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/config/repack.adoc   |  18 ++
 Documentation/git-repack.adoc      |  39 ++-
 builtin/repack.c                   |  49 ++-
 midx.c                             |  31 ++
 midx.h                             |   3 +
 repack-geometry.c                  |  13 +-
 repack-midx.c                      |   5 +
 repack.c                           |  56 +++-
 repack.h                           |  10 +-
 t/meson.build                      |   1 +
 t/t7705-repack-incremental-midx.sh | 470 +++++++++++++++++++++++++++++
 11 files changed, 671 insertions(+), 24 deletions(-)
 create mode 100755 t/t7705-repack-incremental-midx.sh

diff --git a/Documentation/config/repack.adoc b/Documentation/config/repack.adoc
index e9e78dcb198..4c22a499f62 100644
--- a/Documentation/config/repack.adoc
+++ b/Documentation/config/repack.adoc
@@ -46,3 +46,21 @@ repack.midxMustContainCruft::
 	`--write-midx`. When false, cruft packs are only included in the MIDX
 	when necessary (e.g., because they might be required to form a
 	reachability closure with MIDX bitmaps). Defaults to true.
+
+repack.midxSplitFactor::
+	The factor used in the geometric merging condition when
+	compacting incremental MIDX layers during `git repack` when
+	invoked with the `--write-midx=incremental` option.
++
+Adjacent layers are merged when the accumulated object count of the
+newer layer exceeds `1/<N>` of the object count of the next deeper
+layer. Must be at least 2. Defaults to 2.
+
+repack.midxNewLayerThreshold::
+	The minimum number of packs in the tip MIDX layer before those
+	packs are considered as candidates for geometric repacking
+	during `git repack --write-midx=incremental`.
++
+When the tip layer has fewer packs than this threshold, those packs are
+excluded from the geometric repack entirely, and are thus left
+unmodified. Must be at least 1. Defaults to 8.
diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 673ce910837..27a99cc46f4 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
-	[--write-midx] [--name-hash-version=<n>] [--path-walk]
+	[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]
 
 DESCRIPTION
 -----------
@@ -250,9 +250,42 @@ pack as the preferred pack for object selection by the MIDX (see
 linkgit:git-multi-pack-index[1]).
 
 -m::
---write-midx::
+--write-midx[=<mode>]::
 	Write a multi-pack index (see linkgit:git-multi-pack-index[1])
-	containing the non-redundant packs.
+	containing the non-redundant packs. The following modes are
+	available:
++
+--
+	`default`;;
+		Write a single MIDX covering all packs. This is the
+		default when `--write-midx` is given without an
+		explicit mode.
+
+	`incremental`;;
+		Write an incremental MIDX chain instead of a single
+		flat MIDX. This mode requires `--geometric`.
++
+The incremental mode maintains a chain of MIDX layers that is compacted
+over time using a geometric merging strategy. Each repack creates a new
+tip layer containing the newly written pack(s). Adjacent layers are then
+merged whenever the newer layer's object count exceeds
+`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
+that do not meet this condition are retained as-is.
++
+The result is that newer (tip) layers tend to contain many small packs
+with relatively few objects, while older (deeper) layers contain fewer,
+larger packs covering more objects. Because compaction is driven by the
+tip of the chain, newer layers are also rewritten more frequently than
+older ones, which are only touched when enough objects have accumulated
+to justify merging into them. This keeps the total number of layers
+logarithmic relative to the total number of objects.
++
+Only packs in the tip MIDX layer are considered as candidates for the
+geometric repack; packs in deeper layers are left untouched. If the tip
+layer contains fewer packs than `repack.midxNewLayerThreshold`, those
+packs are excluded from the geometry entirely, and a new layer is
+created for any new pack(s) without disturbing the existing chain.
+--
 
 --name-hash-version=<n>::
 	Provide this argument to the underlying `git pack-objects` process.
diff --git a/builtin/repack.c b/builtin/repack.c
index 75c57736780..5ffa18e085e 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -33,7 +33,7 @@ static int midx_must_contain_cruft = 1;
 static const char *const git_repack_usage[] = {
 	N_("git repack [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]\n"
 	   "[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]\n"
-	   "[--write-midx] [--name-hash-version=<n>] [--path-walk]"),
+	   "[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]"),
 	NULL
 };
 
@@ -48,6 +48,8 @@ static const char incremental_bitmap_conflict_error[] = N_(
 struct repack_config_ctx {
 	struct pack_objects_args *po_args;
 	struct pack_objects_args *cruft_po_args;
+	int midx_split_factor;
+	int midx_new_layer_threshold;
 };
 
 static int repack_config(const char *var, const char *value,
@@ -97,6 +99,16 @@ static int repack_config(const char *var, const char *value,
 		midx_must_contain_cruft = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "repack.midxsplitfactor")) {
+		repack_ctx->midx_split_factor = git_config_int(var, value,
+							       ctx->kvi);
+		return 0;
+	}
+	if (!strcmp(var, "repack.midxnewlayerthreshold")) {
+		repack_ctx->midx_new_layer_threshold = git_config_int(var, value,
+								      ctx->kvi);
+		return 0;
+	}
 	return git_default_config(var, value, ctx, cb);
 }
 
@@ -112,6 +124,8 @@ static int option_parse_write_midx(const struct option *opt, const char *arg,
 
 	if (!arg || !*arg)
 		*cfg = REPACK_WRITE_MIDX_DEFAULT;
+	else if (!strcmp(arg, "incremental"))
+		*cfg = REPACK_WRITE_MIDX_INCREMENTAL;
 	else
 		return error(_("unknown value for %s: %s"), opt->long_name, arg);
 
@@ -226,6 +240,8 @@ int cmd_repack(int argc,
 	memset(&config_ctx, 0, sizeof(config_ctx));
 	config_ctx.po_args = &po_args;
 	config_ctx.cruft_po_args = &cruft_po_args;
+	config_ctx.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR;
+	config_ctx.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD;
 
 	repo_config(repo, repack_config, &config_ctx);
 
@@ -247,6 +263,9 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
+	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
+		die(_("--write-midx=incremental requires --geometric"));
+
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
@@ -273,6 +292,13 @@ int cmd_repack(int argc,
 		write_bitmaps = 0;
 	}
 
+	if (config_ctx.midx_split_factor < 2)
+		die(_("invalid value for %s: %d"), "--midx-split-factor",
+		    config_ctx.midx_split_factor);
+	if (config_ctx.midx_new_layer_threshold < 1)
+		die(_("invalid value for %s: %d"), "--midx-new-layer-threshold",
+		    config_ctx.midx_new_layer_threshold);
+
 	if (write_midx != REPACK_WRITE_MIDX_NONE && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
@@ -296,6 +322,10 @@ int cmd_repack(int argc,
 	if (geometry.split_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
+		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL) {
+			geometry.midx_layer_threshold = config_ctx.midx_new_layer_threshold;
+			geometry.midx_layer_threshold_set = true;
+		}
 		pack_geometry_init(&geometry, &existing, &po_args);
 		pack_geometry_split(&geometry);
 	}
@@ -545,8 +575,11 @@ int cmd_repack(int argc,
 				       packtmp);
 	/* End of pack replacement. */
 
-	if (delete_redundant && pack_everything & ALL_INTO_ONE)
+	if (delete_redundant && pack_everything & ALL_INTO_ONE) {
+		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL)
+			existing_packs_retain_midx_packs(&existing);
 		existing_packs_mark_for_deletion(&existing, &names);
+	}
 
 	if (write_midx != REPACK_WRITE_MIDX_NONE) {
 		struct repack_write_midx_opts opts = {
@@ -558,8 +591,8 @@ int cmd_repack(int argc,
 			.show_progress = show_progress,
 			.write_bitmaps = write_bitmaps > 0,
 			.midx_must_contain_cruft = midx_must_contain_cruft,
-			.midx_split_factor = DEFAULT_MIDX_SPLIT_FACTOR,
-			.midx_new_layer_threshold = DEFAULT_MIDX_NEW_LAYER_THRESHOLD,
+			.midx_split_factor = config_ctx.midx_split_factor,
+			.midx_new_layer_threshold = config_ctx.midx_new_layer_threshold,
 			.mode = write_midx,
 		};
 
@@ -572,11 +605,15 @@ int cmd_repack(int argc,
 
 	if (delete_redundant) {
 		int opts = 0;
-		existing_packs_remove_redundant(&existing, packdir);
+		bool wrote_incremental_midx = write_midx == REPACK_WRITE_MIDX_INCREMENTAL;
+
+		existing_packs_remove_redundant(&existing, packdir,
+						wrote_incremental_midx);
 
 		if (geometry.split_factor)
 			pack_geometry_remove_redundant(&geometry, &names,
-						       &existing, packdir);
+						       &existing, packdir,
+						       wrote_incremental_midx);
 		if (show_progress)
 			opts |= PRUNE_PACKED_VERBOSE;
 		prune_packed_objects(opts);
diff --git a/midx.c b/midx.c
index dc86c8e7fee..cd31fa20788 100644
--- a/midx.c
+++ b/midx.c
@@ -850,6 +850,37 @@ void clear_midx_file(struct repository *r)
 	strbuf_release(&midx);
 }
 
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes)
+{
+	struct strbuf chain = STRBUF_INIT;
+
+	get_midx_chain_filename(r->objects->sources, &chain);
+
+	if (r->objects) {
+		struct odb_source *source = r->objects->sources;
+		for (source = r->objects->sources; source; source = source->next) {
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			if (files->packed->midx)
+				close_midx(files->packed->midx);
+			files->packed->midx = NULL;
+		}
+	}
+
+	if (!keep_hashes && remove_path(chain.buf))
+		die(_("failed to clear multi-pack-index chain at %s"),
+		    chain.buf);
+
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_BITMAP,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_REV,
+					 keep_hashes);
+	clear_incremental_midx_files_ext(r->objects->sources, MIDX_EXT_MIDX,
+					 keep_hashes);
+
+	strbuf_release(&chain);
+}
+
 static int verify_midx_error;
 
 __attribute__((format (printf, 1, 2)))
diff --git a/midx.h b/midx.h
index 3ee12dd08ec..63853a03a47 100644
--- a/midx.h
+++ b/midx.h
@@ -9,6 +9,7 @@ struct repository;
 struct bitmapped_pack;
 struct git_hash_algo;
 struct odb_source;
+struct strvec;
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION_V1 1
@@ -143,6 +144,8 @@ int write_midx_file_compact(struct odb_source *source,
 			    const char *incremental_base,
 			    unsigned flags);
 void clear_midx_file(struct repository *r);
+void clear_incremental_midx_files(struct repository *r,
+				  const struct strvec *keep_hashes);
 int verify_midx_file(struct odb_source *source, unsigned flags);
 int expire_midx_packs(struct odb_source *source, unsigned flags);
 int midx_repack(struct odb_source *source, size_t batch_size, unsigned flags);
diff --git a/repack-geometry.c b/repack-geometry.c
index 2408b8a3cc2..2064683dcfe 100644
--- a/repack-geometry.c
+++ b/repack-geometry.c
@@ -249,7 +249,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 				   uint32_t pack_nr,
 				   struct string_list *names,
 				   struct existing_packs *existing,
-				   const char *packdir)
+				   const char *packdir,
+				   bool wrote_incremental_midx)
 {
 	const struct git_hash_algo *algop = existing->repo->hash_algo;
 	struct strbuf buf = STRBUF_INIT;
@@ -269,7 +270,8 @@ static void remove_redundant_packs(struct packed_git **pack,
 		    (string_list_has_string(&existing->kept_packs, buf.buf)))
 			continue;
 
-		repack_remove_redundant_pack(existing->repo, packdir, buf.buf);
+		repack_remove_redundant_pack(existing->repo, packdir, buf.buf,
+					     wrote_incremental_midx);
 	}
 
 	strbuf_release(&buf);
@@ -278,12 +280,13 @@ static void remove_redundant_packs(struct packed_git **pack,
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir)
+				    const char *packdir,
+				    bool wrote_incremental_midx)
 {
 	remove_redundant_packs(geometry->pack, geometry->split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 	remove_redundant_packs(geometry->promisor_pack, geometry->promisor_split,
-			       names, existing, packdir);
+			       names, existing, packdir, wrote_incremental_midx);
 }
 
 void pack_geometry_release(struct pack_geometry *geometry)
diff --git a/repack-midx.c b/repack-midx.c
index 405f4662174..bb3bee03ace 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -887,6 +887,7 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 	struct midx_compaction_step *steps = NULL;
 	struct strbuf lock_name = STRBUF_INIT;
 	struct lock_file lf;
+	struct strvec keep_hashes = STRVEC_INIT;
 	size_t steps_nr = 0;
 	size_t i;
 	int ret = 0;
@@ -932,11 +933,15 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 			BUG("missing result for compaction step %"PRIuMAX,
 			    (uintmax_t)i);
 		fprintf(get_lock_file_fp(&lf), "%s\n", step->csum);
+		strvec_push(&keep_hashes, step->csum);
 	}
 
 	commit_lock_file(&lf);
 
+	clear_incremental_midx_files(opts->existing->repo, &keep_hashes);
+
 done:
+	strvec_clear(&keep_hashes);
 	strbuf_release(&lock_name);
 	for (i = 0; i < steps_nr; i++)
 		midx_compaction_step_release(&steps[i]);
diff --git a/repack.c b/repack.c
index 2ee6b51420a..571dabb665e 100644
--- a/repack.c
+++ b/repack.c
@@ -55,14 +55,18 @@ void pack_objects_args_release(struct pack_objects_args *args)
 }
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name)
+				  const char *base_name,
+				  bool wrote_incremental_midx)
 {
 	struct strbuf buf = STRBUF_INIT;
 	struct odb_source *source = repo->objects->sources;
 	struct multi_pack_index *m = get_multi_pack_index(source);
 	strbuf_addf(&buf, "%s.pack", base_name);
-	if (m && source->local && midx_contains_pack(m, buf.buf))
+	if (m && source->local && midx_contains_pack(m, buf.buf)) {
 		clear_midx_file(repo);
+		if (!wrote_incremental_midx)
+			clear_incremental_midx_files(repo, NULL);
+	}
 	strbuf_insertf(&buf, 0, "%s/", dir_name);
 	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
@@ -250,25 +254,63 @@ void existing_packs_mark_for_deletion(struct existing_packs *existing,
 					   &existing->cruft_packs);
 }
 
+/*
+ * Mark every pack that is referenced by the existing MIDX chain as
+ * retained, so that a subsequent call to
+ * existing_packs_mark_for_deletion() will not mark them for deletion.
+ *
+ * This is used when writing an incremental MIDX layer on top of an
+ * existing chain: retained layers continue to reference the same
+ * packs on disk, so those packs must not be unlinked even if the
+ * freshly-written pack supersedes them.
+ */
+void existing_packs_retain_midx_packs(struct existing_packs *existing)
+{
+	struct string_list_item *item;
+	struct strbuf buf = STRBUF_INIT;
+
+	for_each_string_list_item(item, &existing->midx_packs) {
+		struct string_list_item *found;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, item->string);
+		strbuf_strip_suffix(&buf, ".pack");
+		strbuf_strip_suffix(&buf, ".idx");
+
+		found = string_list_lookup(&existing->non_kept_packs, buf.buf);
+		if (found)
+			existing_packs_mark_retained(found);
+
+		found = string_list_lookup(&existing->cruft_packs, buf.buf);
+		if (found)
+			existing_packs_mark_retained(found);
+	}
+
+	strbuf_release(&buf);
+}
+
 static void remove_redundant_packs_1(struct repository *repo,
 				     struct string_list *packs,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	struct string_list_item *item;
 	for_each_string_list_item(item, packs) {
 		if (!existing_pack_is_marked_for_deletion(item))
 			continue;
-		repack_remove_redundant_pack(repo, packdir, item->string);
+		repack_remove_redundant_pack(repo, packdir, item->string,
+					     wrote_incremental_midx);
 	}
 }
 
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir)
+				     const char *packdir,
+				     bool wrote_incremental_midx)
 {
 	remove_redundant_packs_1(existing->repo, &existing->non_kept_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 	remove_redundant_packs_1(existing->repo, &existing->cruft_packs,
-				 packdir);
+				 packdir, wrote_incremental_midx);
 }
 
 void existing_packs_release(struct existing_packs *existing)
diff --git a/repack.h b/repack.h
index 831ccfb1c6c..f9fbc895f02 100644
--- a/repack.h
+++ b/repack.h
@@ -34,7 +34,8 @@ void prepare_pack_objects(struct child_process *cmd,
 void pack_objects_args_release(struct pack_objects_args *args);
 
 void repack_remove_redundant_pack(struct repository *repo, const char *dir_name,
-				  const char *base_name);
+				  const char *base_name,
+				  bool wrote_incremental_midx);
 
 struct write_pack_opts {
 	struct pack_objects_args *po_args;
@@ -83,8 +84,10 @@ void existing_packs_retain_cruft(struct existing_packs *existing,
 				 struct packed_git *cruft);
 void existing_packs_mark_for_deletion(struct existing_packs *existing,
 				      struct string_list *names);
+void existing_packs_retain_midx_packs(struct existing_packs *existing);
 void existing_packs_remove_redundant(struct existing_packs *existing,
-				     const char *packdir);
+				     const char *packdir,
+				     bool wrote_incremental_midx);
 void existing_packs_release(struct existing_packs *existing);
 
 struct generated_pack;
@@ -129,7 +132,8 @@ struct packed_git *pack_geometry_preferred_pack(struct pack_geometry *geometry);
 void pack_geometry_remove_redundant(struct pack_geometry *geometry,
 				    struct string_list *names,
 				    struct existing_packs *existing,
-				    const char *packdir);
+				    const char *packdir,
+				    bool wrote_incremental_midx);
 void pack_geometry_release(struct pack_geometry *geometry);
 
 struct tempfile;
diff --git a/t/meson.build b/t/meson.build
index 7528e5cda5f..25f0d823d8e 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -951,6 +951,7 @@ integration_tests = [
   't7702-repack-cyclic-alternate.sh',
   't7703-repack-geometric.sh',
   't7704-repack-cruft.sh',
+  't7705-repack-incremental-midx.sh',
   't7800-difftool.sh',
   't7810-grep.sh',
   't7811-grep-open.sh',
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
new file mode 100755
index 00000000000..9e317ff6e8f
--- /dev/null
+++ b/t/t7705-repack-incremental-midx.sh
@@ -0,0 +1,470 @@
+#!/bin/sh
+
+test_description='git repack --write-midx=incremental'
+
+. ./test-lib.sh
+
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
+
+objdir=.git/objects
+packdir=$objdir/pack
+midxdir=$packdir/multi-pack-index.d
+midx_chain=$midxdir/multi-pack-index-chain
+
+# incrementally_repack N
+#
+# Make "N" new commits, each stored in their own pack, and then repacked
+# with the --write-midx=incremental strategy.
+incrementally_repack () {
+	for i in $(test_seq 1 "$1")
+	do
+		test_commit "$i" &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify || return 1
+	done
+}
+
+# Create packs with geometrically increasing sizes so that they
+# satisfy the geometric progression and survive a --geometric=2
+# repack without being rolled up. Creates 3 packs containing 1,
+# 2, and 6 commits (3, 6, and 18 objects) respectively.
+create_geometric_packs () {
+	test_commit "small" &&
+	git repack -d &&
+
+	test_commit_bulk --message="medium" 2 &&
+	test_commit_bulk --message="large" 6 &&
+
+	git repack --geometric=2 -d --write-midx=incremental \
+		--write-bitmap-index
+}
+
+# create_layer <test_commit_bulk args>
+#
+# Creates a new MIDX layer with the contents of "test_commit_bulk $@".
+create_layer () {
+	test_commit_bulk "$@" &&
+
+	git multi-pack-index write --incremental --bitmap
+}
+
+# create_layers
+#
+# Reads lines of "<message> <nr>" from stdin and creates a new MIDX
+# layer for each line. See create_layer above for more.
+create_layers () {
+	while read msg nr
+	do
+		create_layer --message="$msg" "$nr" || return 1
+	done
+}
+
+test_expect_success '--write-midx=incremental requires --geometric' '
+	test_must_fail git repack --write-midx=incremental 2>err &&
+
+	test_grep -- "--write-midx=incremental requires --geometric" err
+'
+
+test_expect_success 'below layer threshold, tip packs excluded' '
+	git init below-layer-threshold-tip-packs-excluded &&
+	(
+		cd below-layer-threshold-tip-packs-excluded &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 4 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create 3 packs forming a geometric progression by
+		# object count such that they are unmodified by the
+		# initial repack. The MIDX chain thusly contains a
+		# single layer with three packs.
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a new commit. Since the layer threshold is
+		# unmet, a new MIDX layer is added on top of the
+		# existing one.
+		test_commit extra &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+		git multi-pack-index verify &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+		test_line_count = 1 packs.new &&
+
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs repacked' '
+	git init above-layer-threshold-tip-packs-repacked &&
+	(
+		cd above-layer-threshold-tip-packs-repacked &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Same setup, but with the layer threshold set to 2.
+		# Since the tip MIDX layer meets that threshold, its
+		# packs are considered repack candidates.
+		create_geometric_packs &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Perturb the existing progression such that it is
+		# rolled up into a single new pack, invalidating the
+		# existing MIDX layer and replacing it with a new one.
+		test_commit extra &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip layer preserved' '
+	git init above-layer-threshold-tip-layer-preserved &&
+	(
+		cd above-layer-threshold-tip-layer-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		test_commit_bulk --message="medium" 2 &&
+		test_commit_bulk --message="large" 6 &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 1 "$midx_chain" &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Create objects to form a pack satisfying the geometric
+		# progression (thus preserving the tip layer), but not
+		# so large that it meets the layer merging condition.
+		test_commit_bulk --message="small" 1 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		test_line_count = 1 packs.new &&
+		test_line_count = 3 packs.after &&
+		test_line_count = 2 "$midx_chain" &&
+		head -n 1 "$midx_chain.before" >expect &&
+		head -n 1 "$midx_chain" >actual &&
+		test_cmp expect actual &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'above layer threshold, tip packs preserved' '
+	git init above-layer-threshold-tip-packs-preserved &&
+	(
+		cd above-layer-threshold-tip-packs-preserved &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 2 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_geometric_packs &&
+		ls $packdir/pack-*.idx | sort >packs.before &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Same setup as above, but this time the new objects do
+		# not satisfy the new layer merging condition, resulting
+		# in a new tip layer.
+		test_commit_bulk --message="huge" 18 &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		ls $packdir/pack-*.idx | sort >packs.after &&
+		comm -13 packs.before packs.after >packs.new &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 1 $midx_chain &&
+		test_line_count = 1 packs.new &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'new tip absorbs multiple layers' '
+	git init new-tip-absorbs-multiple-layers &&
+	(
+		cd new-tip-absorbs-multiple-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a 4-layer chain where each layer is too small to
+		# absorb the one below it. The sizes must satisfy L(n) <
+		# L(n-1)/2 for each adjacent pair:
+		#
+		#   L0 (oldest): 75 obj (25 commits)
+		#   L1:          21 obj  (7 commits, 21 < 75/2)
+		#   L2:           9 obj  (3 commits,  9 < 21/2)
+		#   L3 (tip):     3 obj  (1 commit,   3 <  9/2)
+		create_layers <<-\EOF &&
+		L0 25
+		L1 7
+		L2 3
+		L3 1
+		EOF
+
+		test_line_count = 4 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Now add a new commit. The merging condition is
+		# satisfied between L3-L1, but violated at L0, which is
+		# too large relative to the accumulated size.
+		#
+		# As a result, the chain shrinks from 4 to 2 layers.
+		test_commit new &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'compaction of older layers' '
+	git init compaction-of-older-layers &&
+	(
+		cd compaction-of-older-layers &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Build a chain with two small layers at the bottom
+		# and a larger barrier layer on top, producing a
+		# chain that violates the compaction invariant, since
+		# the two small layers would normally have been merged.
+		create_layers <<-\EOF &&
+		one 2
+		two 4
+		barrier 54
+		EOF
+
+		cp $midx_chain $midx_chain.before &&
+
+		# Running an incremental repack compacts the two
+		# small layers at the bottom of the chain as a
+		# separate step in the compaction plan.
+		test_commit another &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'geometric rollup with surviving tip packs' '
+	git init geometric-rollup-with-surviving-tip-packs &&
+	(
+		cd geometric-rollup-with-surviving-tip-packs &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create a pack large enough to anchor the geometric
+		# progression when small packs are added alongside it.
+		create_layer --message="big" 5 &&
+
+		test_line_count = 1 "$midx_chain" &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Repack a small number of objects such that the
+		# progression is unbothered. Note that the existing pack
+		# is considered a repack candidate as the new layer
+		# threshold is set to 1.
+		test_commit small-1 &&
+		git repack -d &&
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		! test_cmp $midx_chain.before $midx_chain &&
+		cp $midx_chain $midx_chain.before
+	)
+'
+
+test_expect_success 'kept packs are excluded from repack' '
+	git init kept-packs-excluded-from-repack &&
+	(
+		cd kept-packs-excluded-from-repack &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		# Create two equal-sized packs, marking one as kept.
+		for i in A B
+		do
+			test_commit "$i" && git repack -d || return 1
+		done &&
+
+		keep=$(ls $packdir/pack-*.idx | head -n 1) &&
+		touch "${keep%.idx}.keep" &&
+
+		# The kept pack is excluded as a repacking candidate
+		# entirely, so no rollup occurs as there is only one
+		# non-kept pack. A new MIDX layer is written containing
+		# that pack.
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count = 1 actual.packs &&
+		test_grep ! "$keep" actual.packs &&
+
+		git multi-pack-index verify &&
+
+		# All objects (from both kept and non-kept packs)
+		# must still be accessible.
+		git fsck
+	)
+'
+
+test_expect_success 'incremental MIDX with --max-pack-size' '
+	git init incremental-midx-with--max-pack-size &&
+	(
+		cd incremental-midx-with--max-pack-size &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		# Now add enough data that a small --max-pack-size will
+		# cause pack-objects to split its output. Create objects
+		# large enough to fill multiple packs.
+		test-tool genrandom foo 1M >big1 &&
+		test-tool genrandom bar 1M >big2 &&
+		git add big1 big2 &&
+		test_tick &&
+		git commit -a -m "big blobs" &&
+		git repack -d &&
+
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index --max-pack-size=1M &&
+
+		test_line_count = 1 "$midx_chain" &&
+		test-tool read-midx $objdir >actual &&
+		grep "^pack-.*\.idx$" actual >actual.packs &&
+		test_line_count -gt 1 actual.packs &&
+
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'noop repack preserves valid MIDX chain' '
+	git init noop-repack-preserves-valid-midx-chain &&
+	(
+		cd noop-repack-preserves-valid-midx-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layer --message="base" 1 &&
+
+		git multi-pack-index verify &&
+		cp $midx_chain $midx_chain.before &&
+
+		# Running again with no new objects should not break
+		# the MIDX chain. It produces "Nothing new to pack."
+		git repack --geometric=2 -d --write-midx=incremental \
+			--write-bitmap-index &&
+
+		test_cmp $midx_chain.before $midx_chain &&
+
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
+test_expect_success 'repack -ad removes stale incremental chain' '
+	git init repack--ad-removes-stale-incremental-chain &&
+	(
+		cd repack--ad-removes-stale-incremental-chain &&
+
+		git config maintenance.auto false &&
+		git config repack.midxnewlayerthreshold 1 &&
+		git config repack.midxsplitfactor 2 &&
+
+		create_layers <<-\EOF &&
+		one 1
+		two 1
+		EOF
+
+		test_path_is_file $midx_chain &&
+		test_line_count = 2 $midx_chain &&
+
+		git repack -ad &&
+
+		test_path_is_missing $packdir/multi-pack-index &&
+		test_dir_is_empty $midxdir
+	)
+'
+
+test_expect_success 'repack rejects invalid midxSplitFactor' '
+	test_when_finished "rm -fr bad-split-factor" &&
+	git init bad-split-factor &&
+	(
+		cd bad-split-factor &&
+		test_commit base &&
+
+		for v in 0 1 -1
+		do
+			test_must_fail git -c repack.midxSplitFactor=$v \
+				repack -d --geometric=2 --write-midx=incremental 2>err &&
+			test_grep "invalid value for --midx-split-factor" err ||
+			return 1
+		done
+	)
+'
+
+test_expect_success 'repack rejects invalid midxNewLayerThreshold' '
+	test_when_finished "rm -fr bad-layer-threshold" &&
+	git init bad-layer-threshold &&
+	(
+		cd bad-layer-threshold &&
+		test_commit base &&
+
+		for v in 0 -1
+		do
+			test_must_fail git -c repack.midxNewLayerThreshold=$v \
+				repack -d --geometric=2 --write-midx=incremental 2>err &&
+			test_grep "invalid value for --midx-new-layer-threshold" err ||
+			return 1
+		done
+	)
+'
+
+test_done
-- 
2.54.0.16.g1c05dfce579

[PATCH v3 16/16] repack: allow `--write-midx=incremental` without `--geometric`

[Taylor Blau]

Previously, `--write-midx=incremental` required `--geometric` and would
die() without it. Relax this restriction so that incremental MIDX
repacking can be used independently.

Without `--geometric`, the behavior is append-only: a single new MIDX
layer is created containing whatever packs were written by the repack
and appended to the existing chain (or a new chain is started). Existing
layers are preserved as-is with no compaction or merging.

Implement this via a new repack_make_midx_append_plan() that builds a
plan consisting of a WRITE step for the freshly written packs followed
by COPY steps for every existing MIDX layer. The existing compaction
plan (repack_make_midx_compaction_plan) is used only when `--geometric`
is active.

Update the documentation to describe the behavior with and without
`--geometric`, and replace the test that enforced the old restriction
with one exercising append-only incremental MIDX repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-repack.adoc      | 19 +++++----
 builtin/repack.c                   |  3 --
 repack-midx.c                      | 64 +++++++++++++++++++++++++++--
 t/t7705-repack-incremental-midx.sh | 65 +++++++++++++++++++++++++++---
 4 files changed, 133 insertions(+), 18 deletions(-)

diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 27a99cc46f4..72c42015e23 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -263,14 +263,19 @@ linkgit:git-multi-pack-index[1]).
 
 	`incremental`;;
 		Write an incremental MIDX chain instead of a single
-		flat MIDX. This mode requires `--geometric`.
+		flat MIDX.
 +
-The incremental mode maintains a chain of MIDX layers that is compacted
-over time using a geometric merging strategy. Each repack creates a new
-tip layer containing the newly written pack(s). Adjacent layers are then
-merged whenever the newer layer's object count exceeds
-`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
-that do not meet this condition are retained as-is.
+Without `--geometric`, a new MIDX layer is appended to the existing
+chain (or a new chain is started) containing whatever packs were written
+by the repack. Existing layers are preserved as-is.
++
+When combined with `--geometric`, the incremental mode maintains a chain
+of MIDX layers that is compacted over time using a geometric merging
+strategy. Each repack creates a new tip layer containing the newly
+written pack(s). Adjacent layers are then merged whenever the newer
+layer's object count exceeds `1/repack.midxSplitFactor` of the next
+deeper layer's count. Layers that do not meet this condition are
+retained as-is.
 +
 The result is that newer (tip) layers tend to contain many small packs
 with relatively few objects, while older (deeper) layers contain fewer,
diff --git a/builtin/repack.c b/builtin/repack.c
index 5ffa18e085e..1524a9c13ad 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -263,9 +263,6 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
-	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
-		die(_("--write-midx=incremental requires --geometric"));
-
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))
diff --git a/repack-midx.c b/repack-midx.c
index bb3bee03ace..799f2b03697 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -548,6 +548,60 @@ static void midx_compaction_step_release(struct midx_compaction_step *step)
 	free(step->csum);
 }
 
+/*
+ * Build an append-only MIDX plan: a single WRITE step for the freshly
+ * written packs, plus COPY steps for every existing layer.  No
+ * compaction or merging is performed.
+ */
+static void repack_make_midx_append_plan(struct repack_write_midx_opts *opts,
+					 struct midx_compaction_step **steps_p,
+					 size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step *step;
+	size_t steps_nr = 0, steps_alloc = 0;
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	if (opts->names->nr) {
+		struct strbuf buf = STRBUF_INIT;
+		uint32_t i;
+
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_WRITE;
+		string_list_init_dup(&step->u.write);
+
+		for (i = 0; i < opts->names->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "pack-%s.idx",
+				    opts->names->items[i].string);
+			string_list_append(&step->u.write, buf.buf);
+		}
+
+		strbuf_release(&buf);
+	}
+
+	for (; m; m = m->base_midx) {
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_COPY;
+		step->u.copy = m;
+		step->objects_nr = m->num_objects;
+	}
+
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+}
+
 static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
 					    struct midx_compaction_step **steps_p,
 					    size_t *steps_nr_p)
@@ -904,9 +958,13 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 		goto done;
 	}
 
-	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
-		ret = error(_("unable to generate compaction plan"));
-		goto done;
+	if (opts->geometry->split_factor) {
+		if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+			ret = error(_("unable to generate compaction plan"));
+			goto done;
+		}
+	} else {
+		repack_make_midx_append_plan(opts, &steps, &steps_nr);
 	}
 
 	for (i = 0; i < steps_nr; i++) {
diff --git a/t/t7705-repack-incremental-midx.sh b/t/t7705-repack-incremental-midx.sh
index 9e317ff6e8f..25a8c40e8ee 100755
--- a/t/t7705-repack-incremental-midx.sh
+++ b/t/t7705-repack-incremental-midx.sh
@@ -63,10 +63,36 @@ create_layers () {
 	done
 }
 
-test_expect_success '--write-midx=incremental requires --geometric' '
-	test_must_fail git repack --write-midx=incremental 2>err &&
+test_expect_success '--write-midx=incremental without --geometric' '
+	git init incremental-without-geometric &&
+	(
+		cd incremental-without-geometric &&
 
-	test_grep -- "--write-midx=incremental requires --geometric" err
+		git config maintenance.auto false &&
+
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# A second repack appends a new layer without
+		# disturbing the existing one.
+		test_commit third &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 2 $midx_chain &&
+		head -n 1 $midx_chain.before >expect &&
+		head -n 1 $midx_chain >actual &&
+		test_cmp expect actual &&
+
+		git fsck
+	)
 '
 
 test_expect_success 'below layer threshold, tip packs excluded' '
@@ -334,8 +360,7 @@ test_expect_success 'kept packs are excluded from repack' '
 		# entirely, so no rollup occurs as there is only one
 		# non-kept pack. A new MIDX layer is written containing
 		# that pack.
-		git repack --geometric=2 -d --write-midx=incremental \
-			--write-bitmap-index &&
+		git repack --geometric=2 -d --write-midx=incremental &&
 
 		test-tool read-midx $objdir >actual &&
 		grep "^pack-.*\.idx$" actual >actual.packs &&
@@ -433,6 +458,36 @@ test_expect_success 'repack -ad removes stale incremental chain' '
 	)
 '
 
+test_expect_success 'repack -ad --write-midx=incremental is safe' '
+	git init ad-incremental-midx &&
+	(
+		cd ad-incremental-midx &&
+
+		git config maintenance.auto false &&
+
+		# Build a MIDX chain with multiple layers referencing
+		# distinct packs.
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack -d --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+
+		# Now do a full -ad repack. The new pack contains all
+		# objects, but any retained MIDX layers still reference
+		# the now-deleted packs.
+		test_commit third &&
+		git repack -ad --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		git fsck &&
+		git rev-list --all --objects >/dev/null
+	)
+'
+
 test_expect_success 'repack rejects invalid midxSplitFactor' '
 	test_when_finished "rm -fr bad-split-factor" &&
 	git init bad-split-factor &&
-- 
2.54.0.16.g1c05dfce579

Re: [PATCH v3 00/16] repack: incremental MIDX/bitmap-based repacking

[Jeff King]

On Wed, Apr 29, 2026 at 08:13:04PM -0400, Taylor Blau wrote:

> I expect that this should be the final reroll absent any late-breaking
> feedback. The only changes since last time are the following:
> 
>  - Various stale references to "--checksum-only" have been eradicated
>    from commit message(s).
> 
>  - `repack_prepare_midx_command()` now uses `pipe_command()` to
>    eliminate the possibility of a deadlock.
> 
>  - `write_midx_included_packs()` now cleans up after itself properly
>    when receiving multiple lines of output.
> 
>  - The errant test in t7705 was moved to the final commit (where it
>    belongs) instead of the penultimate one.

Thanks, the new pipe_command() and leak-fix for patch 14 look good to
me.

> As usual, a range-diff is included below as well for convenience. Thanks
> in advance for reviewing!

Something funny going on here:

> Range-diff against v2:
>  -:  ----------- >  1:  d6c27317c25 midx-write: handle noop writes when converting incremental chains
>  -:  ----------- >  2:  629c8d23116 midx: use `strset` for retained MIDX files
>  -:  ----------- >  3:  e303bf6a4ac midx: build `keep_hashes` array in order
>  -:  ----------- >  4:  42d76c70060 midx: use `strvec` for `keep_hashes`
>  -:  ----------- >  5:  2c80aa34fac midx: introduce `--no-write-chain-file` for incremental MIDX writes
>  7:  d9acef1334a !  6:  2a05f4b86f3 repack: allow `--write-midx=incremental` without `--geometric`
>     @@ Metadata
>      Author: Taylor Blau <me@ttaylorr.com>
>      
>       ## Commit message ##
>     -    repack: allow `--write-midx=incremental` without `--geometric`
>     +    midx: support custom `--base` for incremental MIDX writes
> [...gigantic range-diff for this commit...]

I'm not sure what's happening here. The 6th patch in v2 was "midx:
support custom --base...". The series you sent looks right, I think, but
maybe your range-diff endpoints were wrong or something?

-Peff

Re: [PATCH v3 09/16] repack-midx: factor out `repack_prepare_midx_command()`

[SZEDER Gábor]

On Wed, Apr 29, 2026 at 08:13:31PM -0400, Taylor Blau wrote:
> The `write_midx_included_packs()` function assembles and executes a
> `git multi-pack-index write` command, constructing the argument list
> inline.
> 
> Future commits will introduce additional callers that need to construct
> similar `git multi-pack-index` commands (for both `write` and `compact`
> subcommands), so extract the common portions of the command setup into a
> reusable `repack_prepare_midx_command()` helper.
> 
> The extracted helper sets `git_cmd`, pushes the `multi-pack-index`
> subcommand and verb,

We don't have "verbs" in Git, "multi-pack-index" is the name of the
git command, and "write"/"compact"/etc. are the subcommands.

> and handles `--progress`/`--no-progress` and
> `--bitmap` flags. The remaining arguments that are specific to the
> `write` subcommand (such as `--stdin-packs`) are left to the caller.
> 
> No functional changes are included in this patch.
> 
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  repack-midx.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/repack-midx.c b/repack-midx.c
> index 0682b80c427..83151d4734a 100644
> --- a/repack-midx.c
> +++ b/repack-midx.c
> @@ -275,6 +275,23 @@ static void remove_redundant_bitmaps(struct string_list *include,
>  	strbuf_release(&path);
>  }
>  
> +static void repack_prepare_midx_command(struct child_process *cmd,
> +					struct repack_write_midx_opts *opts,
> +					const char *verb)
> +{
> +	cmd->git_cmd = 1;
> +
> +	strvec_pushl(&cmd->args, "multi-pack-index", verb, NULL);
> +
> +	if (opts->show_progress)
> +		strvec_push(&cmd->args, "--progress");
> +	else
> +		strvec_push(&cmd->args, "--no-progress");
> +
> +	if (opts->write_bitmaps)
> +		strvec_push(&cmd->args, "--bitmap");
> +}
> +
>  int write_midx_included_packs(struct repack_write_midx_opts *opts)
>  {
>  	struct child_process cmd = CHILD_PROCESS_INIT;
> @@ -289,18 +306,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
>  		goto done;
>  
>  	cmd.in = -1;
> -	cmd.git_cmd = 1;
>  
> -	strvec_push(&cmd.args, "multi-pack-index");
> -	strvec_pushl(&cmd.args, "write", "--stdin-packs", NULL);
> -
> -	if (opts->show_progress)
> -		strvec_push(&cmd.args, "--progress");
> -	else
> -		strvec_push(&cmd.args, "--no-progress");
> -
> -	if (opts->write_bitmaps)
> -		strvec_push(&cmd.args, "--bitmap");
> +	repack_prepare_midx_command(&cmd, opts, "write");
> +	strvec_push(&cmd.args, "--stdin-packs");
>  
>  	if (preferred)
>  		strvec_pushf(&cmd.args, "--preferred-pack=%s",
> -- 
> 2.54.0.16.g1c05dfce579
> 

Re: [PATCH v3 15/16] repack: introduce `--write-midx=incremental`

[Jeff King]

On Wed, Apr 29, 2026 at 08:13:49PM -0400, Taylor Blau wrote:

> +void clear_incremental_midx_files(struct repository *r,
> +				  const struct strvec *keep_hashes)
> +{
> +	struct strbuf chain = STRBUF_INIT;
> +
> +	get_midx_chain_filename(r->objects->sources, &chain);
> +
> +	if (r->objects) {
> +		struct odb_source *source = r->objects->sources;
> +		for (source = r->objects->sources; source; source = source->next) {
> +			struct odb_source_files *files = odb_source_files_downcast(source);
> +			if (files->packed->midx)
> +				close_midx(files->packed->midx);
> +			files->packed->midx = NULL;
> +		}
> +	}

Can r->objects be NULL here? If so, then the get_midx_chain_filename()
call will segfault, since we look at r->objects->sources. If not, then
this conditional is tautological and can be dropped.

Looking at the two callers, the one in repack_remove_redundant_pack()
unconditionally looks at repo->objects->sources itself, so we know it is
not NULL there.

The other is in write_midx_incremental(), but the flow there is less
clear.

I'd guess it is probably the case that it is always non-NULL so there is
no bug, but dropping the conditional would make the code more clear.

-Peff