midx: start tracking per object database source

Multi-pack indices are tracked via `struct multi_pack_index`. This data
structure is stored as a linked list inside `struct object_database`,
which is the global database that spans across all of the object
sources.

This layout causes two problems:

  - Object databases consist of multiple object sources (e.g. one source
    per alternate object directory), where each multi-pack index is
    specific to one of those sources. Regardless of that though, the
    MIDX is not tracked per source, but tracked globally for the whole
    object database. This creates a mismatch between the on-disk layout
    and how things are organized in the object database subsystems and
    makes some parts, like figuring out whether a source has an MIDX,
    quite awkward.

  - Multi-pack indices are an implementation detail of how efficient
    access for packfiles work. As such, they are neither relevant in the
    context of loose objects, nor in a potential future where we have
    pluggable backends.

Refactor `prepare_multi_pack_index_one()` so that it works on a specific
source, which allows us to easily store a pointer to the multi-pack
index inside of it. For now, this pointer exists next to the existing
linked list we have in the object database. Users will be adjusted in
subsequent patches to instead use the per-source pointers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -724,28 +724,29 @@ int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id)
 	return 0;
 }
 
-int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
+int prepare_multi_pack_index_one(struct odb_source *source, int local)
 {
+	struct repository *r = source->odb->repo;
 	struct multi_pack_index *m;
-	struct multi_pack_index *m_search;
 
 	prepare_repo_settings(r);
 	if (!r->settings.core_multi_pack_index)
 		return 0;
 
-	for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)
+	if (source->midx)
-		if (!strcmp(object_dir, m_search->object_dir))
+		return 1;
-			return 1;
-
-	m = load_multi_pack_index(r, object_dir, local);
 
+	m = load_multi_pack_index(r, source->path, local);
 	if (m) {
 		struct multi_pack_index *mp = r->objects->multi_pack_index;
 		if (mp) {
 			m->next = mp->next;
 			mp->next = m;
-		} else
+		} else {
 			r->objects->multi_pack_index = m;
+		}
+		source->midx = m;
+
 		return 1;
 	}
 
@@ -837,6 +838,8 @@ void clear_midx_file(struct repository *r)
 	if (r->objects && r->objects->multi_pack_index) {
 		close_midx(r->objects->multi_pack_index);
 		r->objects->multi_pack_index = NULL;
+		for (struct odb_source *source = r->objects->sources; source; source = source->next)
+			source->midx = NULL;
 	}
 
 	if (remove_path(midx.buf))

midx.h

@@ -8,6 +8,7 @@ struct pack_entry;
 struct repository;
 struct bitmapped_pack;
 struct git_hash_algo;
+struct odb_source;
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -123,7 +124,7 @@ int fill_midx_entry(struct repository *r, const struct object_id *oid, struct pa
 int midx_contains_pack(struct multi_pack_index *m,
 		       const char *idx_or_pack_name);
 int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id);
-int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);
+int prepare_multi_pack_index_one(struct odb_source *source, int local);
 
 /*
  * Variant of write_midx_file which writes a MIDX containing only the packs

odb.h

@@ -13,6 +13,7 @@ struct oidmap;
 struct oidtree;
 struct strbuf;
 struct repository;
+struct multi_pack_index;
 
 /*
  * Compute the exact path an alternate is at and returns it. In case of
@@ -55,6 +56,13 @@ struct odb_source {
 	/* Map between object IDs for loose objects. */
 	struct loose_object_map *loose_map;
 
+	/*
+	 * private data
+	 *
+	 * should only be accessed directly by packfile.c and midx.c
+	 */
+	struct multi_pack_index *midx;
+
 	/*
 	 * This is a temporary object store created by the tmp_objdir
 	 * facility. Disable ref updates since the objects in the store
@@ -75,7 +83,6 @@ struct odb_source {
 };
 
 struct packed_git;
-struct multi_pack_index;
 struct cached_object_entry;
 
 /*

packfile.c

@@ -372,6 +372,8 @@ void close_object_store(struct object_database *o)
 	if (o->multi_pack_index) {
 		close_midx(o->multi_pack_index);
 		o->multi_pack_index = NULL;
+		for (struct odb_source *source = o->sources; source; source = source->next)
+			source->midx = NULL;
 	}
 
 	close_commit_graph(o);
@@ -1037,7 +1039,7 @@ static void prepare_packed_git(struct repository *r)
 	odb_prepare_alternates(r->objects);
 	for (source = r->objects->sources; source; source = source->next) {
 		int local = (source == r->objects->sources);
-		prepare_multi_pack_index_one(r, source->path, local);
+		prepare_multi_pack_index_one(source, local);
 		prepare_packed_git_one(r, source->path, local);
 	}
 	rearrange_packed_git(r);