The function `get_multi_pack_index()` loads multi-pack indices via
`prepare_packed_git()` and then returns the linked list of multi-pack
indices that is stored in `struct object_database`. That list is in the
process of being removed though in favor of storing the MIDX as part of
the object database source it belongs to.

Refactor `get_multi_pack_index()` so that it returns the multi-pack
index for a single object source. Callers are now expected to call this
function for each source they are interested in. This requires them to
iterate through alternates, so we have to prepare alternate object
sources before doing so.

Signed-off-by: Patrick Steinhardt <redacted>
---
 builtin/pack-objects.c |  9 ++++++---
 builtin/repack.c       |  4 ++--
 midx-write.c           | 22 ++--------------------
 object-name.c          | 21 ++++++++++++++-------
 pack-bitmap.c          | 20 ++++++++++++++------
 packfile.c             | 30 +++++++++++-------------------
 packfile.h             |  3 +--
 7 files changed, 50 insertions(+), 59 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 5781dec9808..f889e69e07d 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c

@@ -1701,7 +1701,6 @@ static int want_object_in_pack_mtime(const struct object_id *oid,
 {
 	int want;
 	struct list_head *pos;
-	struct multi_pack_index *m;

 	if (!exclude && local && has_loose_object_nonlocal(oid))
 		return 0;

@@ -1721,9 +1720,13 @@ static int want_object_in_pack_mtime(const struct object_id *oid,
 		*found_offset = 0;
 	}

-	for (m = get_multi_pack_index(the_repository); m; m = m->next) {
+	odb_prepare_alternates(the_repository->objects);
+
+	for (struct odb_source *source = the_repository->objects->sources; source; source = source->next) {
+		struct multi_pack_index *m = get_multi_pack_index(source);

 			want = want_object_in_pack_one(e.p, oid, exclude, found_pack, found_offset, found_mtime);
 			if (want != -1)
 				return want;

diff --git a/builtin/repack.c b/builtin/repack.c
index 9bbf032b6dd..5956df5d927 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c

@@ -218,9 +218,9 @@ static void mark_packs_for_deletion(struct existing_packs *existing,
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
 	struct strbuf buf = STRBUF_INIT;
-	struct multi_pack_index *m = get_local_multi_pack_index(the_repository);
+	struct multi_pack_index *m = get_multi_pack_index(the_repository->objects->sources);

 		clear_midx_file(the_repository);
 	strbuf_insertf(&buf, 0, "%s/", dir_name);
 	unlink_pack_path(buf.buf, 1);

diff --git a/midx-write.c b/midx-write.c
index f2cfb85476e..c1ae62d3549 100644
--- a/midx-write.c
+++ b/midx-write.c

@@ -916,26 +916,8 @@ static int write_midx_bitmap(struct write_midx_context *ctx,
 static struct multi_pack_index *lookup_multi_pack_index(struct repository *r,
 							const char *object_dir)
 {
-	struct multi_pack_index *result = NULL;
-	struct multi_pack_index *cur;
-	char *obj_dir_real = real_pathdup(object_dir, 1);
-	struct strbuf cur_path_real = STRBUF_INIT;
-
-	/* Ensure the given object_dir is local, or a known alternate. */
-	odb_find_source(r->objects, obj_dir_real);
-
-	for (cur = get_multi_pack_index(r); cur; cur = cur->next) {
-		strbuf_realpath(&cur_path_real, cur->object_dir, 1);
-		if (!strcmp(obj_dir_real, cur_path_real.buf)) {
-			result = cur;
-			goto cleanup;
-		}
-	}
-
-cleanup:
-	free(obj_dir_real);
-	strbuf_release(&cur_path_real);
-	return result;
+	struct odb_source *source = odb_find_source(r->objects, object_dir);
+	return get_multi_pack_index(source);

 }

 static int fill_packs_from_midx(struct write_midx_context *ctx,

diff --git a/object-name.c b/object-name.c
index ddafe7f9b13..1e7fdcb90a8 100644
--- a/object-name.c
+++ b/object-name.c

@@ -198,16 +198,19 @@ static void unique_in_pack(struct packed_git *p,

 static void find_short_packed_object(struct disambiguate_state *ds)
 {
-	struct multi_pack_index *m;
 	struct packed_git *p;

 	/* Skip, unless oids from the storage hash algorithm are wanted */
 	if (ds->bin_pfx.algo && (&hash_algos[ds->bin_pfx.algo] != ds->repo->hash_algo))
 		return;

-	for (m = get_multi_pack_index(ds->repo); m && !ds->ambiguous;
-	     m = m->next)
-		unique_in_midx(m, ds);
+	odb_prepare_alternates(ds->repo->objects);
+	for (struct odb_source *source = ds->repo->objects->sources; source && !ds->ambiguous; source = source->next) {
+		struct multi_pack_index *m = get_multi_pack_index(source);
+		if (m)
+			unique_in_midx(m, ds);
+	}
+

diff --git a/pack-bitmap.c b/pack-bitmap.c
index 0a4af199c05..7b51d381837 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c

@@ -692,14 +692,16 @@ static int open_midx_bitmap(struct repository *r,
 			    struct bitmap_index *bitmap_git)
 {
 	int ret = -1;
-	struct multi_pack_index *midx;

 	assert(!bitmap_git->map);

-	for (midx = get_multi_pack_index(r); midx; midx = midx->next) {
-		if (!open_midx_bitmap_1(bitmap_git, midx))
+	odb_prepare_alternates(r->objects);
+	for (struct odb_source *source = r->objects->sources; source; source = source->next) {
+		struct multi_pack_index *midx = get_multi_pack_index(source);
+		if (midx && !open_midx_bitmap_1(bitmap_git, midx))
 			ret = 0;
 	}

@@ -3307,9 +3309,15 @@ int verify_bitmap_files(struct repository *r)
 {
 	int res = 0;

-	for (struct multi_pack_index *m = get_multi_pack_index(r);
-	     m; m = m->next) {
-		char *midx_bitmap_name = midx_bitmap_filename(m);
+	odb_prepare_alternates(r->objects);
+	for (struct odb_source *source = r->objects->sources; source; source = source->next) {
+		struct multi_pack_index *m = get_multi_pack_index(source);
+		char *midx_bitmap_name;
+
+		if (!m)
+			continue;
+
+		midx_bitmap_name = midx_bitmap_filename(m);

diff --git a/packfile.c b/packfile.c
index e5d9d7ac8bc..e1ced050451 100644
--- a/packfile.c
+++ b/packfile.c

@@ -963,14 +963,17 @@ static void prepare_packed_git(struct repository *r);
 unsigned long repo_approximate_object_count(struct repository *r)
 {
 	if (!r->objects->approximate_object_count_valid) {
-		unsigned long count;
-		struct multi_pack_index *m;
+		unsigned long count = 0;
 		struct packed_git *p;

 		prepare_packed_git(r);