@@ -33,7 +33,9 @@ COMMANDS
 Write a commit graph file based on the commits found in packfiles.
 Includes all commits from the existing commit graph file. Outputs the
 checksum hash of the written file.
-
++
+With `--update-head` option, update the graph-head file to point
+to the written graph file.
 
 'read'::
 

@@ -53,6 +55,13 @@ EXAMPLES
 $ git commit-graph write
 ------------------------------------------------
 
+* Write a graph file for the packed commits in your local .git folder
+* and update graph-head.
++
+------------------------------------------------
+$ git commit-graph write --update-head
+------------------------------------------------
+
 * Read basic information from a graph file.
 +
 ------------------------------------------------

@@ -1,12 +1,13 @@
 #include "builtin.h"
 #include "config.h"
+#include "lockfile.h"
 #include "parse-options.h"
 #include "commit-graph.h"
 
 static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph [--pack-dir <packdir>]"),
 	N_("git commit-graph read [--graph-hash=<hash>]"),
-	N_("git commit-graph write [--pack-dir <packdir>]"),
+	N_("git commit-graph write [--pack-dir <packdir>] [--update-head]"),
 	NULL
 };
 

@@ -16,13 +17,14 @@ static const char * const builtin_commit_graph_read_usage[] = {
 };
 
 static const char * const builtin_commit_graph_write_usage[] = {
-	N_("git commit-graph write [--pack-dir <packdir>]"),
+	N_("git commit-graph write [--pack-dir <packdir>] [--update-head]"),
 	NULL
 };
 
 static struct opts_commit_graph {
 	const char *pack_dir;
 	const char *graph_hash;
+	int update_head;
 } opts;
 
 static int graph_read(int argc, const char **argv)

@@ -87,6 +89,22 @@ static int graph_read(int argc, const char **argv)
 	return 0;
 }
 
+static void update_head_file(const char *pack_dir, const struct object_id *graph_hash)
+{
+	int fd;
+	struct lock_file lk = LOCK_INIT;
+	char *head_fname = get_graph_head_filename(pack_dir);
+
+	fd = hold_lock_file_for_update(&lk, head_fname, LOCK_DIE_ON_ERROR);
+	FREE_AND_NULL(head_fname);
+
+	if (fd < 0)
+		die_errno("unable to open graph-head");
+
+	write_in_full(fd, oid_to_hex(graph_hash), GIT_MAX_HEXSZ);
+	commit_lock_file(&lk);
+}
+
 static int graph_write(int argc, const char **argv)
 {
 	struct object_id *graph_hash;

@@ -95,6 +113,8 @@ static int graph_write(int argc, const char **argv)
 		{ OPTION_STRING, 'p', "pack-dir", &opts.pack_dir,
 			N_("dir"),
 			N_("The pack directory to store the graph") },
+		OPT_BOOL('u', "update-head", &opts.update_head,
+			N_("update graph-head to written graph file")),
 		OPT_END(),
 	};
 

@@ -111,6 +131,9 @@ static int graph_write(int argc, const char **argv)
 
 	graph_hash = write_commit_graph(opts.pack_dir);
 
+	if (opts.update_head)
+		update_head_file(opts.pack_dir, graph_hash);
+
 	if (graph_hash) {
 		printf("%s\n", oid_to_hex(graph_hash));
 		FREE_AND_NULL(graph_hash);

@@ -38,6 +38,14 @@
 #define GRAPH_MIN_SIZE (GRAPH_CHUNKLOOKUP_SIZE + GRAPH_FANOUT_SIZE + \
 			GRAPH_OID_LEN + 8)
 
+char *get_graph_head_filename(const char *pack_dir)
+{
+	struct strbuf fname = STRBUF_INIT;
+	strbuf_addstr(&fname, pack_dir);
+	strbuf_addstr(&fname, "/graph-head");
+	return strbuf_detach(&fname, 0);
+}
+
 char* get_commit_graph_filename_hash(const char *pack_dir,
 				     struct object_id *hash)
 {

@@ -4,6 +4,7 @@
 #include "git-compat-util.h"
 #include "commit.h"
 
+extern char *get_graph_head_filename(const char *pack_dir);
 extern char* get_commit_graph_filename_hash(const char *pack_dir,
 					    struct object_id *hash);
 

@@ -11,7 +11,8 @@ test_expect_success 'setup full repo' '
 	packdir=".git/objects/pack"'
 
 test_expect_success 'write graph with no packs' '
-	git commit-graph write --pack-dir .'
+	git commit-graph write --pack-dir . &&
+	test_path_is_missing graph-head'
 
 test_expect_success 'create commits and repack' '
 	for i in $(test_seq 3)

@@ -32,6 +33,7 @@ graph_read_expect() {
 
 test_expect_success 'write graph' '
 	graph1=$(git commit-graph write) &&
+	test_path_is_missing graph-head &&
 	test_path_is_file $packdir/graph-$graph1.graph &&
 	git commit-graph read --graph-hash=$graph1 >output &&
 	graph_read_expect "3" "$packdir" &&

@@ -73,8 +75,11 @@ test_expect_success 'Add more commits' '
 
 
 test_expect_success 'write graph with merges' '
-	graph2=$(git commit-graph write)&&
+	graph2=$(git commit-graph write --update-head)&&
 	test_path_is_file $packdir/graph-$graph2.graph &&
+	test_path_is_file $packdir/graph-head &&
+	printf $graph2 >expect &&
+	test_cmp expect $packdir/graph-head &&
 	git commit-graph read --graph-hash=$graph2 >output &&
 	graph_read_expect "10" "$packdir" &&
 	test_cmp expect output'

@@ -99,19 +104,24 @@ test_expect_success 'Add one more commit' '
 # 1
 
 test_expect_success 'write graph with new commit' '
-	graph3=$(git commit-graph write) &&
-	test_path_is_file $packdir/graph-$graph3.graph &&
+	graph3=$(git commit-graph write --update-head) &&
 	test_path_is_file $packdir/graph-$graph3.graph &&
+        test_path_is_file $packdir/graph-head &&
+        printf $graph3 >expect &&
+        test_cmp expect $packdir/graph-head &&
 	git commit-graph read --graph-hash=$graph3 >output &&
 	graph_read_expect "11" "$packdir" &&
 	test_cmp expect output'
 
 test_expect_success 'write graph with nothing new' '
-	graph4=$(git commit-graph write) &&
+	graph4=$(git commit-graph write --update-head) &&
 	test_path_is_file $packdir/graph-$graph4.graph &&
 	printf $graph3 >expect &&
 	printf $graph4 >output &&
 	test_cmp expect output &&
+        test_path_is_file $packdir/graph-head &&
+        printf $graph4 >expect &&
+        test_cmp expect $packdir/graph-head &&
 	git commit-graph read --graph-hash=$graph4 >output &&
 	graph_read_expect "11" "$packdir" &&
 	test_cmp expect output'

@@ -123,8 +133,11 @@ test_expect_success 'setup bare repo' '
 	baredir="./objects/pack"'
 
 test_expect_success 'write graph in bare repo' '
-	graphbare=$(git commit-graph write) &&
+	graphbare=$(git commit-graph write --update-head) &&
 	test_path_is_file $baredir/graph-$graphbare.graph &&
+        test_path_is_file $baredir/graph-head &&
+        printf $graphbare >expect &&
+        test_cmp expect $baredir/graph-head &&
 	git commit-graph read --graph-hash=$graphbare >output &&
 	graph_read_expect "11" "$baredir" &&
 	test_cmp expect output'