midx.c: consider annotated tags during bitmap selection

When generating a multi-pack bitmap without a `--refs-snapshot` (e.g.,
by running `git multi-pack-index write --bitmap` directly), we determine
the set of bitmap-able commits by enumerating each reference, and adding
the referrent as the tip of a reachability traversal when it appears
somewhere in the MIDX. (Any commit we encounter during the reachability
traversal then becomes a candidate for bitmap selection).

But we incorrectly avoid peeling the object at the tip of each
reference. So if we see some reference that points at an annotated tag
(which in turn points through zero or more additional annotated tags at
a commit), that we will not add it as a tip for the reachability
traversal. This means that if some commit C is only referenced through
one or more annotated tag(s), then C won't become a bitmap candidate.

Correct this by peeling the reference tips as we enumerate them to
ensure that we consider commits which are the targets of annotated tags,
in addition to commits which are referenced directly.

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

2 files changed, 28 insertions, 0 deletions

diff --git a/midx.c b/midx.c
index ea24e30b6e..a8d2111e96 100644
--- a/midx.c
+++ b/midx.c
@@ -980,6 +980,7 @@ static int add_ref_to_pending(const char *refname,
 			      int flag, void *cb_data)
 {
 	struct rev_info *revs = (struct rev_info*)cb_data;
+	struct object_id peeled;
 	struct object *object;
 
 	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
@@ -987,6 +988,9 @@ static int add_ref_to_pending(const char *refname,
 		return 0;
 	}
 
+	if (!peel_iterated_oid(oid, &peeled))
+		oid = &peeled;
+
 	object = parse_object_or_die(oid, refname);
 	if (object->type != OBJ_COMMIT)
 		return 0;
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index ad6eea5fa0..0882cbb6e4 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -410,4 +410,28 @@ test_expect_success 'preferred pack change with existing MIDX bitmap' '
 	)
 '
 
+test_expect_success 'tagged commits are selected for bitmapping' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit --annotate base &&
+		git repack -d &&
+
+		# Remove refs/heads/main which points at the commit directly,
+		# leaving only a reference to the annotated tag.
+		git branch -M main &&
+		git checkout base &&
+		git branch -d main &&
+
+		git multi-pack-index write --bitmap &&
+
+		git rev-parse HEAD >want &&
+		test-tool bitmap list-commits >actual &&
+		grep $(cat want) actual
+	)
+'
+
 test_done