commit-graph: return the prepared commit graph from prepare_commit_graph()

When making use of commit graphs, one needs to first prepare them by
calling `prepare_commit_graph()`. Once that function was called and the
commit graph was prepared successfully, the caller is now expected to
access the graph directly via `struct object_database::commit_graph`.

In a subsequent change, we're going to move the commit graph pointer
from `struct object_database` into `struct odb_source`. With this
change, semantics will change so that we use the commit graph of the
first source that has one. Consequently, all callers that currently
deference the `commit_graph` pointer would now have to loop around the
list of sources to find the commit graph.

This would become quite unwieldy. So instead of shifting the burden onto
such callers, adapt `prepare_commit_graph()` to return the prepared
commit graph, if any. Like this, callers are expected to call that
function and then use the returned commit graph.

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

Author: pks-t

commit-graph.c

@@ -735,7 +735,7 @@ struct commit_graph *read_commit_graph_one(struct odb_source *source)
  * On the first invocation, this function attempts to load the commit
  * graph if the repository is configured to have one.
  */
-static int prepare_commit_graph(struct repository *r)
+static struct commit_graph *prepare_commit_graph(struct repository *r)
 {
 	struct odb_source *source;
 
@@ -747,10 +747,10 @@ static int prepare_commit_graph(struct repository *r)
 	 * we want to disable even an already-loaded graph file.
 	 */
 	if (!r->gitdir || r->commit_graph_disabled)
-		return 0;
+		return NULL;
 
 	if (r->objects->commit_graph_attempted)
-		return !!r->objects->commit_graph;
+		return r->objects->commit_graph;
 	r->objects->commit_graph_attempted = 1;
 
 	prepare_repo_settings(r);
@@ -763,10 +763,10 @@ static int prepare_commit_graph(struct repository *r)
 		 * so that commit graph loading is not attempted again for this
 		 * repository.)
 		 */
-		return 0;
+		return NULL;
 
 	if (!commit_graph_compatible(r))
-		return 0;
+		return NULL;
 
 	odb_prepare_alternates(r->objects);
 	for (source = r->objects->sources; source; source = source->next) {
@@ -775,20 +775,17 @@ static int prepare_commit_graph(struct repository *r)
 			break;
 	}
 
-	return !!r->objects->commit_graph;
+	return r->objects->commit_graph;
 }
 
 int generation_numbers_enabled(struct repository *r)
 {
 	uint32_t first_generation;
 	struct commit_graph *g;
-	if (!prepare_commit_graph(r))
-	       return 0;
 
-	g = r->objects->commit_graph;
-
-	if (!g->num_commits)
-		return 0;
+	g = prepare_commit_graph(r);
+	if (!g || !g->num_commits)
+	       return 0;
 
 	first_generation = get_be32(g->chunk_commit_data +
 				    g->hash_algo->rawsz + 8) >> 2;
@@ -799,12 +796,9 @@ int generation_numbers_enabled(struct repository *r)
 int corrected_commit_dates_enabled(struct repository *r)
 {
 	struct commit_graph *g;
-	if (!prepare_commit_graph(r))
-		return 0;
 
-	g = r->objects->commit_graph;
-
-	if (!g->num_commits)
+	g = prepare_commit_graph(r);
+	if (!g || !g->num_commits)
 		return 0;
 
 	return g->read_generation_data;
@@ -1012,23 +1006,26 @@ static int find_commit_pos_in_graph(struct commit *item, struct commit_graph *g,
 int repo_find_commit_pos_in_graph(struct repository *r, struct commit *c,
 				  uint32_t *pos)
 {
-	if (!prepare_commit_graph(r))
+	struct commit_graph *g = prepare_commit_graph(r);
+	if (!g)
 		return 0;
-	return find_commit_pos_in_graph(c, r->objects->commit_graph, pos);
+	return find_commit_pos_in_graph(c, g, pos);
 }
 
 struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
 {
 	static int commit_graph_paranoia = -1;
+	struct commit_graph *g;
 	struct commit *commit;
 	uint32_t pos;
 
 	if (commit_graph_paranoia == -1)
 		commit_graph_paranoia = git_env_bool(GIT_COMMIT_GRAPH_PARANOIA, 0);
 
-	if (!prepare_commit_graph(repo))
+	g = prepare_commit_graph(repo);
+	if (!g)
 		return NULL;
-	if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
+	if (!search_commit_pos_in_graph(id, g, &pos))
 		return NULL;
 	if (commit_graph_paranoia && !odb_has_object(repo->objects, id, 0))
 		return NULL;
@@ -1039,7 +1036,7 @@ struct commit *lookup_commit_in_graph(struct repository *repo, const struct obje
 	if (commit->object.parsed)
 		return commit;
 
-	if (!fill_commit_in_graph(commit, repo->objects->commit_graph, pos))
+	if (!fill_commit_in_graph(commit, g, pos))
 		return NULL;
 
 	return commit;
@@ -1062,16 +1059,18 @@ static int parse_commit_in_graph_one(struct commit_graph *g,
 int parse_commit_in_graph(struct repository *r, struct commit *item)
 {
 	static int checked_env = 0;
+	struct commit_graph *g;
 
 	if (!checked_env &&
 	    git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE, 0))
 		die("dying as requested by the '%s' variable on commit-graph parse!",
 		    GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE);
 	checked_env = 1;
 
-	if (!prepare_commit_graph(r))
+	g = prepare_commit_graph(r);
+	if (!g)
 		return 0;
-	return parse_commit_in_graph_one(r->objects->commit_graph, item);
+	return parse_commit_in_graph_one(g, item);
 }
 
 void load_commit_graph_info(struct repository *r, struct commit *item)
@@ -2519,6 +2518,7 @@ int write_commit_graph(struct odb_source *source,
 	int replace = 0;
 	struct bloom_filter_settings bloom_settings = DEFAULT_BLOOM_FILTER_SETTINGS;
 	struct topo_level_slab topo_levels;
+	struct commit_graph *g;
 
 	prepare_repo_settings(r);
 	if (!r->settings.core_commit_graph) {
@@ -2547,23 +2547,13 @@ int write_commit_graph(struct odb_source *source,
 	init_topo_level_slab(&topo_levels);
 	ctx.topo_levels = &topo_levels;
 
-	prepare_commit_graph(ctx.r);
-	if (ctx.r->objects->commit_graph) {
-		struct commit_graph *g = ctx.r->objects->commit_graph;
-
-		while (g) {
-			g->topo_levels = &topo_levels;
-			g = g->base_graph;
-		}
-	}
+	g = prepare_commit_graph(ctx.r);
+	for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
+		g->topo_levels = &topo_levels;
 
 	if (flags & COMMIT_GRAPH_WRITE_BLOOM_FILTERS)
 		ctx.changed_paths = 1;
 	if (!(flags & COMMIT_GRAPH_NO_WRITE_BLOOM_FILTERS)) {
-		struct commit_graph *g;
-
-		g = ctx.r->objects->commit_graph;
-
 		/* We have changed-paths already. Keep them in the next graph */
 		if (g && g->bloom_filter_settings) {
 			ctx.changed_paths = 1;
@@ -2580,22 +2570,15 @@ int write_commit_graph(struct odb_source *source,
 	bloom_settings.hash_version = bloom_settings.hash_version == 2 ? 2 : 1;
 
 	if (ctx.split) {
-		struct commit_graph *g = ctx.r->objects->commit_graph;
-
-		while (g) {
+		for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
 			ctx.num_commit_graphs_before++;
-			g = g->base_graph;
-		}
 
 		if (ctx.num_commit_graphs_before) {
 			ALLOC_ARRAY(ctx.commit_graph_filenames_before, ctx.num_commit_graphs_before);
 			i = ctx.num_commit_graphs_before;
-			g = ctx.r->objects->commit_graph;
 
-			while (g) {
-				ctx.commit_graph_filenames_before[--i] = xstrdup(g->filename);
-				g = g->base_graph;
-			}
+			for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
+				ctx.commit_graph_filenames_before[--i] = xstrdup(chain->filename);
 		}
 
 		if (ctx.opts)
@@ -2604,8 +2587,7 @@ int write_commit_graph(struct odb_source *source,
 
 	ctx.approx_nr_objects = repo_approximate_object_count(r);
 
-	if (ctx.append && ctx.r->objects->commit_graph) {
-		struct commit_graph *g = ctx.r->objects->commit_graph;
+	if (ctx.append && g) {
 		for (i = 0; i < g->num_commits; i++) {
 			struct object_id oid;
 			oidread(&oid, g->chunk_oid_lookup + st_mult(g->hash_algo->rawsz, i),
@@ -2651,7 +2633,7 @@ int write_commit_graph(struct odb_source *source,
 	} else
 		ctx.num_commit_graphs_after = 1;
 
-	ctx.trust_generation_numbers = validate_mixed_generation_chain(ctx.r->objects->commit_graph);
+	ctx.trust_generation_numbers = validate_mixed_generation_chain(g);
 
 	compute_topological_levels(&ctx);
 	if (ctx.write_generation_data)