pack-objects: support excluded-open packs with --stdin-packs

In cd846bacc7d (pack-objects: introduce '--stdin-packs=follow',
2025-06-23), pack-objects learned to traverse through commits in
included packs when using '--stdin-packs=follow', rescuing reachable
objects from unlisted packs into the output.

When we encounter a commit in an excluded pack during this rescuing
phase we will traverse through its parents. But because we set
`revs.no_kept_objects = 1`, commit simplification will prevent us from
showing it via `get_revision()`. (In practice, `--stdin-packs=follow`
walks commits down to the roots, but only opens up trees for ones that
do not appear in an excluded pack.)

But there are certain cases where we *do* need to see the parents of an
object in an excluded pack. Namely, if an object is rescue-able, but
only reachable from object(s) which appear in excluded packs, then
commit simplification will exclude those commits from the object
traversal, and we will never see a copy of that object, and thus not
rescue it.

This is what causes the failure in the previous commit during repacking.
When performing a geometric repack, packs above the geometric split that
weren't part of the previous MIDX (e.g., packs pushed directly into
`$GIT_DIR/objects/pack`) may not have full object closure.  When those
packs are listed as excluded via the '^' marker, the reachability
traversal encounters the sequence described above, and may miss objects
which we expect to rescue with `--stdin-packs=follow`.

Introduce a new "excluded-open" pack prefix, '!'. Like '^'-prefixed
packs, objects from '!'-prefixed packs are excluded from the resulting
pack. But unlike '^', commits in '!'-prefixed packs *are* used as
starting points for the follow traversal, and the traversal does not
treat them as a closure boundary.

In order to distinguish excluded-closed from excluded-open packs during
the traversal, introduce a new `pack_keep_in_core_open` bit on
`struct packed_git`, along with a corresponding `KEPT_PACK_IN_CORE_OPEN`
flag for the kept-pack cache.

In `add_object_entry_from_pack()`, move the `want_object_in_pack()`
check to *after* `add_pending_oid()`. This is necessary so that commits
from excluded-open packs are added as traversal tips even though their
objects won't appear in the output. As a consequence, the caller
`for_each_object_in_pack()` will always provide a non-NULL 'p', hence we
are able to drop the "if (p)" conditional.

The `include_check` and `include_check_obj` callbacks on `rev_info` are
used to halt the walk at closed-excluded packs, since objects behind a
'^' boundary are guaranteed to have closure and need not be rescued.

The following commit will make use of this new functionality within the
repack layer to resolve the test failure demonstrated in the previous
commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 105 insertions, 0 deletions

diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index 7eb79bc2cd..c74b5861af 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -415,4 +415,109 @@ test_expect_success '--stdin-packs=follow tolerates missing commits' '
 	stdin_packs__follow_with_only HEAD HEAD^{tree}
 '
 
+test_expect_success '--stdin-packs=follow with open-excluded packs' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		git config set maintenance.auto false &&
+
+		git branch -M main &&
+
+		# Create the following commit structure:
+		#
+		#   A <-- B <-- D     (main)
+		#         ^
+		#          \
+		#           C        (other)
+		test_commit A &&
+		test_commit B &&
+		git checkout -B other &&
+		test_commit C &&
+		git checkout main &&
+		test_commit D &&
+
+		A="$(echo A | git pack-objects --revs $packdir/pack)" &&
+		B="$(echo A..B | git pack-objects --revs $packdir/pack)" &&
+		C="$(echo B..C | git pack-objects --revs $packdir/pack)" &&
+		D="$(echo B..D | git pack-objects --revs $packdir/pack)" &&
+
+		C_ONLY="$(git rev-parse other | git pack-objects $packdir/pack)" &&
+
+		git prune-packed &&
+
+		# Create a pack using --stdin-packs=follow where:
+		#
+		#  - pack D is included,
+		#  - pack C_ONLY is excluded, but open,
+		#  - pack B is excluded, but closed, and
+		#  - packs A and C are unknown
+		#
+		# The resulting pack should therefore contain:
+		#
+		#  - objects from the included pack D,
+		#  - A.t (rescued via D^{tree}), and
+		#  - C^{tree} and C.t (rescued via pack C_ONLY)
+		#
+		# , but should omit:
+		#
+		#  - C (excluded via C_ONLY),
+		#  - objects from pack B (trivially excluded-closed)
+		#  - A and A^{tree} (ancestors of B)
+		P=$(git pack-objects --stdin-packs=follow $packdir/pack <<-EOF
+		pack-$D.pack
+		!pack-$C_ONLY.pack
+		^pack-$B.pack
+		EOF
+		) &&
+
+		{
+			objects_in_packs $D &&
+			git rev-parse A:A.t "C^{tree}" C:C.t
+		} >expect.raw &&
+		sort expect.raw >expect &&
+
+		objects_in_packs $P >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--stdin-packs with !-delimited pack without follow' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		test_commit A &&
+		test_commit B &&
+		test_commit C &&
+
+		A="$(echo A | git pack-objects --revs $packdir/pack)" &&
+		B="$(echo A..B | git pack-objects --revs $packdir/pack)" &&
+		C="$(echo B..C | git pack-objects --revs $packdir/pack)" &&
+
+		cat >in <<-EOF &&
+		!pack-$A.pack
+		pack-$B.pack
+		pack-$C.pack
+		EOF
+
+		# Without --stdin-packs=follow, we treat the first
+		# line of input as a literal packfile name, and thus
+		# expect pack-objects to complain of a missing pack
+		test_must_fail git pack-objects --stdin-packs --stdout \
+			>/dev/null <in 2>err &&
+		test_grep "could not find pack .!pack-$A.pack." err &&
+
+		# With --stdin-packs=follow, we treat the second line
+		# of input as indicating pack-$A.pack is an excluded
+		# open pack, and thus expect pack-objects to succeed
+		P=$(git pack-objects --stdin-packs=follow $packdir/pack <in) &&
+
+		objects_in_packs $B $C >expect &&
+		objects_in_packs $P >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done