odb: move commit-graph into the object sources

Commit graphs are inherently tied to one specific object source.
Furthermore, with the upcoming pluggable object sources, it is not even
guaranteed that an object source may even have a commit graph as these
are specific to the actual on-disk data format.

Prepare for this future by moving the commit-graph pointer from `struct
object_database` to `struct odb_source`. Eventually, this will allow us
to make commit graphs an implementation detail of an object source's
backend.

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

commit-graph.c

@@ -721,11 +721,15 @@ static struct commit_graph *load_commit_graph_chain(struct odb_source *source)
 
 struct commit_graph *read_commit_graph_one(struct odb_source *source)
 {
-	struct commit_graph *g = load_commit_graph_v1(source);
+	struct commit_graph *g;
 
+	if (source->commit_graph_attempted)
+		return NULL;
+	source->commit_graph_attempted = true;
+
+	g = load_commit_graph_v1(source);
 	if (!g)
 		g = load_commit_graph_chain(source);
-
 	return g;
 }
 
@@ -737,6 +741,7 @@ struct commit_graph *read_commit_graph_one(struct odb_source *source)
  */
 static struct commit_graph *prepare_commit_graph(struct repository *r)
 {
+	bool all_attempted = true;
 	struct odb_source *source;
 
 	/*
@@ -749,9 +754,19 @@ static struct commit_graph *prepare_commit_graph(struct repository *r)
 	if (!r->gitdir || r->commit_graph_disabled)
 		return NULL;
 
-	if (r->objects->commit_graph_attempted)
-		return r->objects->commit_graph;
-	r->objects->commit_graph_attempted = 1;
+	odb_prepare_alternates(r->objects);
+	for (source = r->objects->sources; source; source = source->next) {
+		all_attempted &= source->commit_graph_attempted;
+		if (source->commit_graph)
+			return source->commit_graph;
+	}
+
+	/*
+	 * There is no point in re-trying to load commit graphs if we already
+	 * tried loading all of them beforehand.
+	 */
+	if (all_attempted)
+		return NULL;
 
 	prepare_repo_settings(r);
 
@@ -768,14 +783,16 @@ static struct commit_graph *prepare_commit_graph(struct repository *r)
 	if (!commit_graph_compatible(r))
 		return NULL;
 
-	odb_prepare_alternates(r->objects);
 	for (source = r->objects->sources; source; source = source->next) {
-		r->objects->commit_graph = read_commit_graph_one(source);
-		if (r->objects->commit_graph)
-			break;
+		if (source->commit_graph_attempted)
+			continue;
+
+		source->commit_graph = read_commit_graph_one(source);
+		if (source->commit_graph)
+			return source->commit_graph;
 	}
 
-	return r->objects->commit_graph;
+	return NULL;
 }
 
 int generation_numbers_enabled(struct repository *r)
@@ -806,7 +823,7 @@ int corrected_commit_dates_enabled(struct repository *r)
 
 struct bloom_filter_settings *get_bloom_filter_settings(struct repository *r)
 {
-	struct commit_graph *g = r->objects->commit_graph;
+	struct commit_graph *g = prepare_commit_graph(r);
 	while (g) {
 		if (g->bloom_filter_settings)
 			return g->bloom_filter_settings;
@@ -815,15 +832,16 @@ struct bloom_filter_settings *get_bloom_filter_settings(struct repository *r)
 	return NULL;
 }
 
-void close_commit_graph(struct object_database *o)
+void close_commit_graph(struct odb_source *source)
 {
-	if (!o->commit_graph)
+	if (!source->commit_graph)
 		return;
 
 	clear_commit_graph_data_slab(&commit_graph_data_slab);
 	deinit_bloom_filters();
-	free_commit_graph(o->commit_graph);
-	o->commit_graph = NULL;
+	free_commit_graph(source->commit_graph);
+	source->commit_graph = NULL;
+	source->commit_graph_attempted = 0;
 }
 
 static int bsearch_graph(struct commit_graph *g, const struct object_id *oid, uint32_t *pos)
@@ -1119,7 +1137,15 @@ static struct tree *get_commit_tree_in_graph_one(struct commit_graph *g,
 
 struct tree *get_commit_tree_in_graph(struct repository *r, const struct commit *c)
 {
-	return get_commit_tree_in_graph_one(r->objects->commit_graph, c);
+	struct odb_source *source;
+
+	for (source = r->objects->sources; source; source = source->next) {
+		if (!source->commit_graph)
+			continue;
+		return get_commit_tree_in_graph_one(source->commit_graph, c);
+	}
+
+	return NULL;
 }
 
 struct packed_commit_list {
@@ -2165,7 +2191,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 2] = new_base_hash;
 	}
 
-	close_commit_graph(ctx->r->objects);
+	for (struct odb_source *s = ctx->r->objects->sources; s; s = s->next)
+		close_commit_graph(s);
 	finalize_hashfile(f, file_hash, FSYNC_COMPONENT_COMMIT_GRAPH,
 			  CSUM_HASH_IN_STREAM | CSUM_FSYNC);
 	free_chunkfile(cf);
@@ -2667,8 +2694,8 @@ cleanup:
 	oid_array_clear(&ctx.oids);
 	clear_topo_level_slab(&topo_levels);
 
-	if (ctx.r->objects->commit_graph) {
-		struct commit_graph *g = ctx.r->objects->commit_graph;
+	if (source->commit_graph) {
+		struct commit_graph *g = source->commit_graph;
 
 		while (g) {
 			g->topo_levels = NULL;

commit-graph.h

@@ -185,7 +185,7 @@ int write_commit_graph(struct odb_source *source,
 
 int verify_commit_graph(struct commit_graph *g, int flags);
 
-void close_commit_graph(struct object_database *);
+void close_commit_graph(struct odb_source *);
 void free_commit_graph(struct commit_graph *);
 
 /*

odb.c

@@ -363,6 +363,11 @@ static void free_object_directory(struct odb_source *source)
 	free(source->path);
 	odb_clear_loose_cache(source);
 	loose_object_map_clear(&source->loose_map);
+
+	free_commit_graph(source->commit_graph);
+	source->commit_graph = NULL;
+	source->commit_graph_attempted = 0;
+
 	free(source);
 }
 
@@ -1023,10 +1028,6 @@ void odb_clear(struct object_database *o)
 	oidmap_clear(&o->replace_map, 1);
 	pthread_mutex_destroy(&o->replace_mutex);
 
-	free_commit_graph(o->commit_graph);
-	o->commit_graph = NULL;
-	o->commit_graph_attempted = 0;
-
 	free_object_directories(o);
 	o->sources_tail = NULL;
 	o->loaded_alternates = 0;

odb.h

@@ -63,6 +63,9 @@ struct odb_source {
 	 */
 	struct multi_pack_index *midx;
 
+	struct commit_graph *commit_graph;
+	bool commit_graph_attempted; /* if loading has been attempted */
+
 	/*
 	 * This is a temporary object store created by the tmp_objdir
 	 * facility. Disable ref updates since the objects in the store
@@ -120,9 +123,6 @@ struct object_database {
 	unsigned replace_map_initialized : 1;
 	pthread_mutex_t replace_mutex; /* protect object replace functions */
 
-	struct commit_graph *commit_graph;
-	unsigned commit_graph_attempted : 1; /* if loading has been attempted */
-
 	/*
 	 * private data
 	 *

packfile.c

@@ -374,9 +374,8 @@ void close_object_store(struct object_database *o)
 		if (source->midx)
 			close_midx(source->midx);
 		source->midx = NULL;
+		close_commit_graph(source);
 	}
-
-	close_commit_graph(o);
 }
 
 void unlink_pack_path(const char *pack_name, int force_delete)