[GSoC PATCH v5 5/6] builtin/repack: actually drop filtered promisor blobs

[Siddharth Shrimali <r.siddharth.shrimali@gmail.com>]

Make --drop-filtered remove the enumerated promisor blobs instead of
only listing them.

The drop set is computed before repack_promisor_objects() runs, and on
a real run it is passed in so the rebuilt promisor pack omits those
blobs. --drop-filtered implies -d so the old promisor packs, which
still contain the dropped blobs, are removed. Without this the blobs
would survive in the redundant packs. The existing repack machinery
performs the write-before-delete and fsync, so the drop is crash-safe.

The dropped blobs become absent locally but remain recoverable from the
promisor remote, so a later access lazy-fetches them back
transparently. --dry-run keeps its previous behavior, i.e. it lists the
candidates and changes nothing.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
---
 Documentation/git-repack.adoc   | 28 ++++++++++++++++++++++++++++
 builtin/repack.c                | 14 ++++++++++----
 t/t7706-repack-drop-filtered.sh | 12 ++++++++++++
 3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc
index 72c42015e2..130249a139 100644
--- a/Documentation/git-repack.adoc
+++ b/Documentation/git-repack.adoc
@@ -12,6 +12,7 @@ SYNOPSIS
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
 	[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]
+	[--filter=<filter-spec>] [--drop-filtered [--dry-run]]
 
 DESCRIPTION
 -----------
@@ -182,6 +183,33 @@ depth is 4095.
 	`objects` and `objects/info/alternates` sections of
 	linkgit:gitrepository-layout[5].
 
+--drop-filtered::
+	Delete the local objects that match the `--filter` specification
+	instead of keeping them in a separate packfile, reclaiming the
+	disk space they occupy. This is intended for partial clones,
+	where the filtered objects are promisor objects that remain
+	recoverable from the promisor remote and are lazily re-fetched
+	on demand when they are next needed.
++
+Only large blobs are supported for now, so `--filter=blob:limit=<n>`
+is currently the only accepted filter. Because dropped objects must be
+recoverable, this option requires a promisor remote to be configured
+and refuses to run otherwise.
++
+This option requires `-a`, and implies `-d`: the objects are dropped by
+rebuilding the promisor pack without them and then removing the now
+redundant old packs, so the redundant packs must be deleted for the
+space to actually be reclaimed. It is incompatible with `--filter-to`
+and with bitmap writing (`-b`/`--write-bitmap-index`), since filtering
+breaks the single-pack closure that bitmaps require. A bitmap setting
+coming from configuration is silently disabled for the duration of the
+command.
+
+--dry-run::
+	Only meaningful with `--drop-filtered`. List the objects that
+	would be dropped, one object ID per line, without rebuilding any
+	pack or deleting anything.
+
 -b::
 --write-bitmap-index::
 	Write a reachability bitmap index as part of the repack. This
diff --git a/builtin/repack.c b/builtin/repack.c
index 3633b17ce8..a5f13fdd87 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -287,9 +287,6 @@ int cmd_repack(int argc,
 		die(_("--dry-run only takes effect with --drop-filtered"));
 
 	if (drop_filtered) {
-		if (!dry_run)
-			die(_("--drop-filtered does not work without --dry-run yet"));
-
 		if (!po_args.filter_options.choice)
 			die(_("--drop-filtered requires --filter"));
 
@@ -322,6 +319,14 @@ int cmd_repack(int argc,
 
 		write_bitmaps = 0;
 
+		/*
+		 * Dropping objects means rebuilding the promisor packs
+		 * without them and then removing the old packs, so the
+		 * redundant packs must be deleted. Imply -d on a real run.
+		 */
+		if (!dry_run)
+			delete_redundant = 1;
+
 		ret = enumerate_promisor_blobs(repo, &po_args.filter_options, &drop_oids);
 
 		if (ret)
@@ -447,7 +452,8 @@ int cmd_repack(int argc,
 		strvec_push(&cmd.args, "--delta-islands");
 
 	if (pack_everything & ALL_INTO_ONE) {
-		repack_promisor_objects(repo, &po_args, &names, packtmp, NULL);
+		repack_promisor_objects(repo, &po_args, &names, packtmp,
+			(drop_filtered && !dry_run) ? &drop_oids : NULL);
 
 		if (existing_packs_has_non_kept(&existing) &&
 		    delete_redundant &&
diff --git a/t/t7706-repack-drop-filtered.sh b/t/t7706-repack-drop-filtered.sh
index 6352f1fdce..80c695742f 100755
--- a/t/t7706-repack-drop-filtered.sh
+++ b/t/t7706-repack-drop-filtered.sh
@@ -135,4 +135,16 @@ test_expect_success '--dry-run does not remove the filtered objects' '
 	git -C repo cat-file -e "$BIG"
 '
 
+test_expect_success '--drop-filtered removes the promisor blob locally' '
+	BIG=$(cat big_oid) &&
+	SMALL=$(cat small_oid) &&
+
+	git -C repo -c repack.writeBitmaps=false \
+		repack --drop-filtered --filter=blob:limit=1k -a &&
+
+	git -C repo cat-file --batch-all-objects --batch-check="%(objectname)" >present &&
+	test_grep ! "$BIG" present &&
+	test_grep "$SMALL" present
+'
+
 test_done
-- 
2.54.0