commit-graph: allow cross-alternate chains

In an environment like a fork network, it is helpful to have a
commit-graph chain that spans both the base repo and the fork repo. The
fork is usually a small set of data on top of the large repo, but
sometimes the fork is much larger. For example, git-for-windows/git has
almost double the number of commits as git/git because it rebases its
commits on every major version update.

To allow cross-alternate commit-graph chains, we need a few pieces:

1. When looking for a graph-{hash}.graph file, check all alternates.

2. When merging commit-graph chains, do not merge across alternates.

3. When writing a new commit-graph chain based on a commit-graph file
   in another object directory, do not allow success if the base file
   has of the name "commit-graph" instead of
   "commit-graphs/graph-{hash}.graph".

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -320,6 +320,9 @@ static struct commit_graph *load_commit_graph_v1(struct repository *r, const cha
 	struct commit_graph *g = load_commit_graph_one(graph_name);
 	free(graph_name);
 
+	if (g)
+		g->obj_dir = obj_dir;
+
 	return g;
 }
 
@@ -379,9 +382,10 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
 	count = st.st_size / (the_hash_algo->hexsz + 1);
 	oids = xcalloc(count, sizeof(struct object_id));
 
-	for (i = 0; i < count && valid; i++) {
-		char *graph_name;
-		struct commit_graph *g;
+	prepare_alt_odb(r);
+
+	for (i = 0; i < count; i++) {
+		struct object_directory *odb;
 
 		if (strbuf_getline_lf(&line, fp) == EOF)
 			break;
@@ -393,14 +397,29 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
 			break;
 		}
 
-		graph_name = get_split_graph_filename(obj_dir, line.buf);
-		g = load_commit_graph_one(graph_name);
-		free(graph_name);
+		valid = 0;
+		for (odb = r->objects->odb; odb; odb = odb->next) {
+			char *graph_name = get_split_graph_filename(odb->path, line.buf);
+			struct commit_graph *g = load_commit_graph_one(graph_name);
 
-		if (g && add_graph_to_chain(g, graph_chain, oids, i))
-			graph_chain = g;
-		else
-			valid = 0;
+			free(graph_name);
+
+			if (g) {
+				g->obj_dir = odb->path;
+
+				if (add_graph_to_chain(g, graph_chain, oids, i)) {
+					graph_chain = g;
+					valid = 1;
+				}
+
+				break;
+			}
+		}
+
+		if (!valid) {
+			warning(_("unable to find all commit-graph files"));
+			break;
+		}
 	}
 
 	free(oids);
@@ -1418,7 +1437,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 
 	if (ctx->split && ctx->base_graph_name && ctx->num_commit_graphs_after > 1) {
 		char *new_base_hash = xstrdup(oid_to_hex(&ctx->new_base_graph->oid));
-		char *new_base_name = get_split_graph_filename(ctx->obj_dir, new_base_hash);
+		char *new_base_name = get_split_graph_filename(ctx->new_base_graph->obj_dir, new_base_hash);
 
 		free(ctx->commit_graph_filenames_after[ctx->num_commit_graphs_after - 2]);
 		free(ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 2]);
@@ -1493,6 +1512,9 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
 	while (g && (g->num_commits <= split_strategy_size_mult * num_commits ||
 		     num_commits > split_strategy_max_commits)) {
+		if (strcmp(g->obj_dir, ctx->obj_dir))
+			break;
+
 		num_commits += g->num_commits;
 		g = g->base_graph;
 
@@ -1501,6 +1523,18 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
 	ctx->new_base_graph = g;
 
+	if (ctx->num_commit_graphs_after == 2) {
+		char *old_graph_name = get_commit_graph_filename(g->obj_dir);
+
+		if (!strcmp(g->filename, old_graph_name) &&
+		    strcmp(g->obj_dir, ctx->obj_dir)) {
+			ctx->num_commit_graphs_after = 1;
+			ctx->new_base_graph = NULL;
+		}
+
+		free(old_graph_name);
+	}
+
 	ALLOC_ARRAY(ctx->commit_graph_filenames_after, ctx->num_commit_graphs_after);
 	ALLOC_ARRAY(ctx->commit_graph_hash_after, ctx->num_commit_graphs_after);