[PATCH 4/4] repack: retain cruft packs in MIDXs containing kept packs

[Taylor Blau <ttaylorr@openai.com>]

When performing a geometric repack with 'repack.midxMustContainCruft'
set to "false", Git uses '--stdin-packs=follow' to copy (once-cruft)
objects needed for reachability closure out of cruft packs. .keep packs
do not need to participate in that walk, though they *are* included in
the resulting MIDX.

A .keep pack can contain a commit that reaches an object whose only copy
is in a cruft pack. When there is no previous MIDX and the repack writes
a new pack, neither `midx_has_unknown_packs()` nor the `!names.nr`
fallback require that cruft pack to be included. If the kept commit (or
a descendant of it) is selected for bitmap coverage, the bitmap writer
fails because the MIDX does not contain all of its reachable objects.

Include cruft packs whenever the MIDX contains kept packs. This also
retains cruft when the kept packs happen to have full closure, or when
'--pack-kept-objects' lets the repack walk them. It avoids having to
establish their closure before deciding which packs the MIDX needs.

Add a test that packs the tip commit and its tree into a kept pack,
leaving its parent in the cruft pack. The new commit's blob remains
loose, making the geometric repack write a new pack and bypass the
no-new-packs fallback. Verify that the repack succeeds and that we are
able to successfully write a bitmap.

Signed-off-by: Taylor Blau <ttaylorr@openai.com>
---
 repack-midx.c           |  5 +++++
 t/t7704-repack-cruft.sh | 20 ++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/repack-midx.c b/repack-midx.c
index 64c7f8d0f42..622c3c9d236 100644
--- a/repack-midx.c
+++ b/repack-midx.c
@@ -197,6 +197,7 @@ static void midx_included_packs(struct string_list *include,
 	}
 
 	if (opts->midx_must_contain_cruft ||
+	    existing->kept_packs.nr ||
 	    midx_has_unknown_packs(include, geometry, existing)) {
 		/*
 		 * If there are one or more unknown pack(s) present (see
@@ -209,6 +210,10 @@ static void midx_included_packs(struct string_list *include,
 		 * reachability closure if the MIDX is bitmapped and one
 		 * or more of the bitmap's selected commits reaches a
 		 * once-cruft object that was later made reachable.
+		 *
+		 * Kept packs may also depend on cruft objects, since
+		 * they are included above without necessarily being
+		 * traversed by the repack.
 		 */
 		for_each_string_list_item(item, &existing->cruft_packs) {
 			/*
diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index f7f83e70ffe..02db2a06d9e 100755
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -798,6 +798,26 @@ test_expect_success 'incremental repack includes cruft for MIDX bitmaps' '
 	)
 '
 
+test_expect_success 'geometric repack includes cruft for kept packs' '
+	setup_cruft_exclude_tests kept-cruft &&
+	(
+		cd kept-cruft &&
+
+		# Keep HEAD and its tree outside the geometric repack. Its
+		# parent is reachable again, but still in the cruft pack.
+		git rev-parse HEAD HEAD^{tree} >objects &&
+		pack=$(git pack-objects $packdir/pack <objects) &&
+		touch $packdir/pack-$pack.keep &&
+		git prune-packed &&
+
+		# The new blob is still loose, so this writes a pack instead
+		# of taking the no-new-packs fallback.
+		GIT_TEST_MULTI_PACK_INDEX=0 \
+		git repack -d --geometric=2 --write-midx --write-bitmap-index &&
+		git rev-list --test-bitmap HEAD
+	)
+'
+
 test_expect_success 'repack --write-midx includes cruft when instructed' '
 	setup_cruft_exclude_tests exclude-cruft-when-instructed &&
 	(
-- 
2.56.0.4.gbee41d2fc68