In order to write a bitmap, we need to have full coverage of all objects
that are about to be packed. In the traditional non-multi-pack-index
world this meant we need to do a full repack of all objects into a
single packfile. But in the new multi-pack-index world we can get away
with writing bitmaps when we have multiple packfiles as long as the
multi-pack-index covers all objects.

This is not always the case though. When asked to perform a repack of
local objects, only, then we cannot guarantee to have full coverage of
all objects regardless of whether we do a full repack or a repack with a
multi-pack-index. The end result is that writing the bitmap will fail in
both worlds:

    $ git multi-pack-index write --stdin-packs --bitmap <packfiles
    warning: Failed to write bitmap index. Packfile doesn't have full closure (object 1529341d78cf45377407369acb0f4ff2b5cdae42 is missing)
    error: could not write multi-pack bitmap

Now there are two different ways to fix this. The first one would be to
amend git-multi-pack-index(1) to disable writing bitmaps when we notice
that we don't have full object coverage.

    - We don't have enough information in git-multi-pack-index(1) in
      order to tell whether the local repository _should_ have full
      coverage. Because even when connected to an alternate object
      directory, it may be the case that we still have all objects
      around in the main object database.

    - git-multi-pack-index(1) is quite a low-level tool. Automatically
      disabling functionality that it was asked to provide does not feel
      like the right thing to do.

We can easily fix it at a higher level in git-repack(1) though. When
asked to only include local objects via `-l` and when connected to an
alternate object directory then we will override the user's ask and
disable writing bitmaps with a warning. This is similar to what we do in
git-pack-objects(1), where we also disable writing bitmaps in case we
omit an object from the pack.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/repack.c            | 12 ++++++++++++
 object-file.c               |  6 ++++++
 object-store.h              |  1 +
 t/t7700-repack.sh           | 17 +++++++++++++++++
 t/t7703-repack-geometric.sh | 27 +++++++++++++++++++++++++++
 5 files changed, 63 insertions(+)

diff --git a/builtin/repack.c b/builtin/repack.c
index 5768aeb1f3..e2abcea2ee 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -887,6 +887,18 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));
 
+	if (write_bitmaps && po_args.local && has_alt_odb(the_repository)) {
+		/*
+		 * When asked to do a local repack, but we have
+		 * packfiles that are inherited from an alternate, then
+		 * we cannot guarantee that the multi-pack-index would
+		 * have full coverage of all objects. We thus disable
+		 * writing bitmaps in that case.
+		 */
+		warning(_("disabling bitmap writing, as some objects are not being packed"));
+		write_bitmaps = 0;
+	}
+
 	if (write_midx && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
diff --git a/object-file.c b/object-file.c
index 8fab8dbe80..3ad11c683b 100644
--- a/object-file.c
+++ b/object-file.c
@@ -946,6 +946,12 @@ void prepare_alt_odb(struct repository *r)
 	r->objects->loaded_alternates = 1;
 }
 
+int has_alt_odb(struct repository *r)
+{
+	prepare_alt_odb(r);
+	return !!r->objects->odb->next;
+}
+
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */
 static int freshen_file(const char *fn)
 {
diff --git a/object-store.h b/object-store.h
index 82201ec3e7..574f16140f 100644
--- a/object-store.h
+++ b/object-store.h
@@ -56,6 +56,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct object_directory *, 1, fspathhash, fspatheq)
 
 void prepare_alt_odb(struct repository *r);
+int has_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index 4aabe98139..93c5763e7d 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -106,6 +106,23 @@ test_expect_success SYMLINKS '--local keeps packs when alternate is objectdir '
 	test_cmp expect actual
 '
 
+test_expect_success '--local disables writing bitmaps when connected to alternate ODB' '
+	test_when_finished "rm -rf shared member" &&
+
+	git init shared &&
+	git clone --shared shared member &&
+	(
+		cd member &&
+		test_commit "object" &&
+		git repack -Adl --write-bitmap-index 2>err &&
+		cat >expect <<-EOF &&
+		warning: disabling bitmap writing, as some objects are not being packed
+		EOF
+		test_cmp expect err &&
+		test ! -f .git/objects/pack-*.bitmap
+	)
+'
+
 test_expect_success 'packed obs in alt ODB are repacked even when local repo is packless' '
 	mkdir alt_objects/pack &&
 	mv .git/objects/pack/* alt_objects/pack &&
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index c440956ad5..33d7977fca 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -419,4 +419,31 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	test_cmp expected-objects actual-objects.sorted
 '
 
+test_expect_success '--geometric -l disables writing bitmaps with non-local packfiles' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# When performing a geometric repack with `-l` while connecting to an
+	# alternate object database that has a packfile we do not have full
+	# coverage of objects. As a result, we expect that writing the bitmap
+	# will be disabled.
+	git -C member repack -l --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	cat >expect <<-EOF &&
+	warning: disabling bitmap writing, as some objects are not being packed
+	EOF
+	test_cmp expect err &&
+	test ! -f member/.git/objects/pack/multi-pack-index-*.bitmap &&
+
+	# On the other hand, when we repack without `-l`, we should see that
+	# the bitmap gets created.
+	git -C member repack --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	test_must_be_empty err &&
+	test -f member/.git/objects/pack/multi-pack-index-*.bitmap
+'
+
 test_done
-- 
2.40.0