commit-graph: add '--reachable' option

When writing commit-graph files, it can be convenient to ask for all
reachable commits (starting at the ref set) in the resulting file. This
is particularly helpful when writing to stdin is complicated, such as a
future integration with 'git gc'.

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

Documentation/git-commit-graph.txt

@@ -38,12 +38,16 @@ Write a commit graph file based on the commits found in packfiles.
 +
 With the `--stdin-packs` option, generate the new commit graph by
 walking objects only in the specified pack-indexes. (Cannot be combined
-with --stdin-commits.)
+with `--stdin-commits` or `--reachable`.)
 +
 With the `--stdin-commits` option, generate the new commit 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.)
+`--stdin-packs` or `--reachable`.)
++
+With the `--reachable` option, generate the new commit graph by walking
+commits starting at all refs. (Cannot be combined with `--stdin-commits`
+or `--stdin-packs`.)
 +
 With the `--append` option, include all commits that are present in the
 existing commit-graph file.

builtin/commit-graph.c

@@ -10,7 +10,7 @@ static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph [--object-dir <objdir>]"),
 	N_("git commit-graph read [--object-dir <objdir>]"),
 	N_("git commit-graph verify [--object-dir <objdir>]"),
-	N_("git commit-graph write [--object-dir <objdir>] [--append] [--stdin-packs|--stdin-commits]"),
+	N_("git commit-graph write [--object-dir <objdir>] [--append] [--reachable|--stdin-packs|--stdin-commits]"),
 	NULL
 };
 
@@ -25,12 +25,13 @@ static const char * const builtin_commit_graph_read_usage[] = {
 };
 
 static const char * const builtin_commit_graph_write_usage[] = {
-	N_("git commit-graph write [--object-dir <objdir>] [--append] [--stdin-packs|--stdin-commits]"),
+	N_("git commit-graph write [--object-dir <objdir>] [--append] [--reachable|--stdin-packs|--stdin-commits]"),
 	NULL
 };
 
 static struct opts_commit_graph {
 	const char *obj_dir;
+	int reachable;
 	int stdin_packs;
 	int stdin_commits;
 	int append;
@@ -127,6 +128,8 @@ static int graph_write(int argc, const char **argv)
 		OPT_STRING(0, "object-dir", &opts.obj_dir,
 			N_("dir"),
 			N_("The object directory to store the graph")),
+		OPT_BOOL(0, "reachable", &opts.reachable,
+			N_("start walk at all refs")),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			N_("scan pack-indexes listed by stdin for commits")),
 		OPT_BOOL(0, "stdin-commits", &opts.stdin_commits,
@@ -140,11 +143,16 @@ static int graph_write(int argc, const char **argv)
 			     builtin_commit_graph_write_options,
 			     builtin_commit_graph_write_usage, 0);
 
-	if (opts.stdin_packs && opts.stdin_commits)
-		die(_("cannot use both --stdin-commits and --stdin-packs"));
+	if (opts.reachable + opts.stdin_packs + opts.stdin_commits > 1)
+		die(_("use at most one of --reachable, --stdin-commits, or --stdin-packs"));
 	if (!opts.obj_dir)
 		opts.obj_dir = get_object_directory();
 
+	if (opts.reachable) {
+		write_commit_graph_reachable(opts.obj_dir, opts.append);
+		return 0;
+	}
+
 	string_list_init(&lines, 0);
 	if (opts.stdin_packs || opts.stdin_commits) {
 		struct strbuf buf = STRBUF_INIT;

commit-graph.c

@@ -7,6 +7,7 @@
 #include "packfile.h"
 #include "commit.h"
 #include "object.h"
+#include "refs.h"
 #include "revision.h"
 #include "sha1-lookup.h"
 #include "commit-graph.h"
@@ -656,6 +657,25 @@ static void compute_generation_numbers(struct packed_commit_list* commits)
 	}
 }
 
+static int add_ref_to_list(const char *refname,
+			   const struct object_id *oid,
+			   int flags, void *cb_data)
+{
+	struct string_list *list = (struct string_list *)cb_data;
+
+	string_list_append(list, oid_to_hex(oid));
+	return 0;
+}
+
+void write_commit_graph_reachable(const char *obj_dir, int append)
+{
+	struct string_list list;
+
+	string_list_init(&list, 1);
+	for_each_ref(add_ref_to_list, &list);
+	write_commit_graph(obj_dir, NULL, &list, append);
+}
+
 void write_commit_graph(const char *obj_dir,
 			struct string_list *pack_indexes,
 			struct string_list *commit_hex,

commit-graph.h

@@ -48,6 +48,7 @@ struct commit_graph {
 
 struct commit_graph *load_commit_graph_one(const char *graph_file);
 
+void write_commit_graph_reachable(const char *obj_dir, int append);
 void write_commit_graph(const char *obj_dir,
 			struct string_list *pack_indexes,
 			struct string_list *commit_hex,

t/t5318-commit-graph.sh

@@ -205,6 +205,16 @@ test_expect_success 'build graph from commits with append' '
 graph_git_behavior 'append graph, commit 8 vs merge 1' full commits/8 merge/1
 graph_git_behavior 'append graph, commit 8 vs merge 2' full commits/8 merge/2
 
+test_expect_success 'build graph using --reachable' '
+	cd "$TRASH_DIRECTORY/full" &&
+	git commit-graph write --reachable &&
+	test_path_is_file $objdir/info/commit-graph &&
+	graph_read_expect "11" "large_edges"
+'
+
+graph_git_behavior 'append graph, commit 8 vs merge 1' full commits/8 merge/1
+graph_git_behavior 'append graph, commit 8 vs merge 2' full commits/8 merge/2
+
 test_expect_success 'setup bare repo' '
 	cd "$TRASH_DIRECTORY" &&
 	git clone --bare --no-local full bare &&