commit-graph: build graph from starting commits

Teach git-commit-graph to read commits from stdin when the
--stdin-commits flag is specified. Commits reachable from these
commits are added to the graph. This is a much faster way to construct
the graph than inspecting all packed objects, but is restricted to
known tips.

For the Linux repository, 700,000+ commits were added to the graph
file starting from 'master' in 7-9 seconds, depending on the number
of packfiles in the repo (1, 24, or 120).

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

1 files changed, 25 insertions, 2 deletions

diff --git a/commit-graph.c b/commit-graph.c
index 70472840a3..a59d1e387b 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -551,7 +551,9 @@ static void close_reachable(struct packed_oid_list *oids)
 
 void write_commit_graph(const char *obj_dir,
 			const char **pack_indexes,
-			int nr_packs)
+			int nr_packs,
+			const char **commit_hex,
+			int nr_commits)
 {
 	struct packed_oid_list oids;
 	struct packed_commit_list commits;
@@ -591,7 +593,28 @@ void write_commit_graph(const char *obj_dir,
 			close_pack(p);
 		}
 		strbuf_release(&packname);
-	} else
+	}
+
+	if (commit_hex) {
+		for (i = 0; i < nr_commits; i++) {
+			const char *end;
+			struct object_id oid;
+			struct commit *result;
+
+			if (commit_hex[i] && parse_oid_hex(commit_hex[i], &oid, &end))
+				continue;
+
+			result = lookup_commit_reference_gently(&oid, 1);
+
+			if (result) {
+				ALLOC_GROW(oids.list, oids.nr + 1, oids.alloc);
+				oidcpy(&oids.list[oids.nr], &(result->object.oid));
+				oids.nr++;
+			}
+		}
+	}
+
+	if (!pack_indexes && !commit_hex)
 		for_each_packed_object(add_packed_commits, &oids, 0);
 
 	close_reachable(&oids);