[PATCH v2 14/14] commit-graph: build graph from starting commits

[Derrick Stolee <stolee@gmail.com>]

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>
---
 Documentation/git-commit-graph.txt |  7 ++++++-
 builtin/commit-graph.c             | 34 +++++++++++++++++++++++++---------
 commit-graph.c                     | 26 +++++++++++++++++++++++---
 commit-graph.h                     |  4 +++-
 t/t5318-commit-graph.sh            | 18 ++++++++++++++++++
 5 files changed, 75 insertions(+), 14 deletions(-)

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index d0571cd896..3357c0cf8f 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -46,7 +46,12 @@ OPTIONS
 --stdin-packs::
 	When used with --write, generate the new graph by walking objects
 	only in the specified packfiles and any commits in the
-	existing graph-head.
+	existing graph-head. (Cannot be combined with --stdin-commits.)
+
+--stdin-commits::
+	When used with --write, generate the new graph by walking commits
+	starting at the commits specified in stdin as a list of OIDs in
+	hex, one OID per line. (Cannot be combined with --stdin-packs.)
 
 EXAMPLES
 --------
diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 80a409e784..adc05f0582 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -12,7 +12,7 @@ static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph [--pack-dir <packdir>]"),
 	N_("git commit-graph --clear [--pack-dir <packdir>]"),
 	N_("git commit-graph --read [--graph-hash=<hash>]"),
-	N_("git commit-graph --write [--pack-dir <packdir>] [--update-head] [--delete-expired] [--stdin-packs]"),
+	N_("git commit-graph --write [--pack-dir <packdir>] [--update-head] [--delete-expired] [--stdin-packs|--stdin-commits]"),
 	NULL
 };
 
@@ -25,6 +25,7 @@ static struct opts_commit_graph {
 	int update_head;
 	int delete_expired;
 	int stdin_packs;
+	int stdin_commits;
 	int has_existing;
 	struct object_id old_graph_hash;
 } opts;
@@ -117,23 +118,36 @@ static int graph_write(void)
 {
 	struct object_id *graph_hash;
 	char **pack_indexes = NULL;
+	char **commits = NULL;
 	int num_packs = 0;
-	int size_packs = 0;
+	int num_commits = 0;
+	char **lines = NULL;
+	int num_lines = 0;
+	int size_lines = 0;
 
-	if (opts.stdin_packs) {
+	if (opts.stdin_packs || opts.stdin_commits) {
 		struct strbuf buf = STRBUF_INIT;
-		size_packs = 128;
-		ALLOC_ARRAY(pack_indexes, size_packs);
+		size_lines = 128;
+		ALLOC_ARRAY(lines, size_lines);
 
 		while (strbuf_getline(&buf, stdin) != EOF) {
-			ALLOC_GROW(pack_indexes, num_packs + 1, size_packs);
-			pack_indexes[num_packs++] = buf.buf;
+			ALLOC_GROW(lines, num_lines + 1, size_lines);
+			lines[num_lines++] = buf.buf;
 			strbuf_detach(&buf, NULL);
 		}
-	}
 
-	graph_hash = construct_commit_graph(opts.pack_dir, pack_indexes, num_packs);
+		if (opts.stdin_packs) {
+			pack_indexes = lines;
+			num_packs = num_lines;
+		}
+		if (opts.stdin_commits) {
+			commits = lines;
+			num_commits = num_lines;
+		}
+	}
 
+	graph_hash = construct_commit_graph(opts.pack_dir, pack_indexes, num_packs,
+					    commits, num_commits);
 	if (opts.update_head)
 		update_head_file(opts.pack_dir, graph_hash);
 
@@ -172,6 +186,8 @@ int cmd_commit_graph(int argc, const char **argv, const char *prefix)
 			N_("delete expired head graph file")),
 		OPT_BOOL('s', "stdin-packs", &opts.stdin_packs,
 			N_("only scan packfiles listed by stdin")),
+		OPT_BOOL('C', "stdin-commits", &opts.stdin_commits,
+			N_("start walk at commits listed by stdin")),
 		{ OPTION_STRING, 'H', "graph-hash", &opts.graph_hash,
 			N_("hash"),
 			N_("A hash for a specific graph file in the pack-dir."),
diff --git a/commit-graph.c b/commit-graph.c
index cfa0415a21..7f31a6c795 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -664,7 +664,9 @@ static void close_reachable(struct packed_oid_list *oids)
 
 struct object_id *construct_commit_graph(const char *pack_dir,
 					 char **pack_indexes,
-					 int nr_packs)
+					 int nr_packs,
+					 char **commit_hex,
+					 int nr_commits)
 {
 	struct packed_oid_list oids;
 	struct packed_commit_list commits;
@@ -717,10 +719,28 @@ struct object_id *construct_commit_graph(const char *pack_dir,
 			for_each_object_in_pack(p, if_packed_commit_add_to_list, &oids);
 			close_pack(p);
 		}
-	} else {
-		for_each_packed_object(if_packed_commit_add_to_list, &oids, 0);
 	}
 
+	if (commit_hex) {
+		for (i = 0; i < nr_commits; i++) {
+			const char *end;
+			ALLOC_GROW(oids.list, oids.num + 1, oids.size);
+
+			oids.list[oids.num] = malloc(sizeof(struct object_id));
+
+			if (parse_oid_hex(commit_hex[i], oids.list[oids.num], &end)) {
+				free(oids.list[oids.num]);
+				continue;
+			}
+
+			if (lookup_commit(oids.list[oids.num]))
+				oids.num++;
+		}
+	}
+
+	if (!pack_indexes && !commit_hex)
+		for_each_packed_object(if_packed_commit_add_to_list, &oids, 0);
+
 	close_reachable(&oids);
 	QSORT(oids.list, oids.num, commit_compare);
 
diff --git a/commit-graph.h b/commit-graph.h
index 3ae1eadce0..619b1f6def 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -62,6 +62,8 @@ extern struct object_id *get_nth_commit_oid(struct commit_graph *g,
 
 extern struct object_id *construct_commit_graph(const char *pack_dir,
 						char **pack_indexes,
-						int nr_packs);
+						int nr_packs,
+						char **commits,
+						int nr_commits);
 
 #endif
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 2001b0b5b5..0bf27a2e7c 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -227,6 +227,24 @@ test_expect_success 'build graph from latest pack with closure' \
 _graph_git_behavior commits/20 merge/1
 _graph_git_behavior commits/20 merge/2
 
+test_expect_success 'build graph from commits with closure' \
+    'git rev-parse commits/20 >commits-in &&
+     git rev-parse merge/1 >>commits-in &&
+     git rev-parse merge/2 >>commits-in &&
+     graph6=$(cat commits-in | git commit-graph --write --update-head --delete-expired --stdin-commits) &&
+     test_path_is_file ${packdir}/graph-${graph6}.graph &&
+     test_path_is_missing ${packdir}/graph-${graph5}.graph &&
+     test_path_is_file ${packdir}/graph-${graph1}.graph &&
+     test_path_is_file ${packdir}/graph-head &&
+     echo ${graph6} >expect &&
+     cmp -n 40 expect ${packdir}/graph-head &&
+     git commit-graph --read --graph-hash=${graph6} >output &&
+     _graph_read_expect "23" "${packdir}" &&
+     cmp expect output'
+
+_graph_git_behavior commits/20 merge/1
+_graph_git_behavior commits/20 merge/2
+
 test_expect_success 'setup bare repo' \
     'cd .. &&
      git clone --bare full bare &&
-- 
2.16.0.15.g9c3cf44.dirty