Merge branch 'ps/gc-stale-lock-warning' into next

Give a bit of advice/hint message when "git gc" stops finding a
lock file left by another instance of "git gc" that still is
potentially running.

* ps/gc-stale-lock-warning:
  builtin/gc: provide hint when maintenance hits a stale schedule lock

builtin/gc.c

@@ -2890,8 +2890,17 @@ static int update_background_schedule(const struct maintenance_start_opts *opts,
 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->odb->path);
 
 	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+		if (errno == EEXIST)
+			error(_("unable to create '%s.lock': %s.\n\n"
+			    "Another scheduled git-maintenance(1) process seems to be running in this\n"
+			    "repository. Please make sure no other maintenance processes are running and\n"
+			    "then try again. If it still fails, a git-maintenance(1) process may have\n"
+			    "crashed in this repository earlier: remove the file manually to continue."),
+			    absolute_path(lock_path), strerror(errno));
+		else
+			error_errno(_("cannot acquire lock for scheduled background maintenance"));
 		free(lock_path);
-		return error(_("another process is scheduling background maintenance"));
+		return -1;
 	}
 
 	for (i = 1; i < ARRAY_SIZE(scheduler_fn); i++) {

t/t7900-maintenance.sh

@@ -1011,4 +1011,12 @@ test_expect_success 'repacking loose objects is quiet' '
 	)
 '
 
+test_expect_success 'maintenance aborts with existing lock file' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	: >repo/.git/objects/schedule.lock &&
+	test_must_fail git -C repo maintenance start 2>err &&
+	test_grep "Another scheduled git-maintenance(1) process seems to be running" err
+'
+
 test_done