Merge branch 'lh/systemd-timers'

"git maintenance" scheduler learned to use systemd timers as a
possible backend.

* lh/systemd-timers:
  maintenance: add support for systemd timers on Linux
  maintenance: `git maintenance run` learned `--scheduler=<scheduler>`
  cache.h: Introduce a generic "xdg_config_home_for(…)" function
diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
index 1e738ad..e2cfb68 100644
--- a/Documentation/git-maintenance.txt
+++ b/Documentation/git-maintenance.txt
@@ -179,6 +179,17 @@
 	`maintenance.<task>.enabled` configured as `true` are considered.
 	See the 'TASKS' section for the list of accepted `<task>` values.
 
+--scheduler=auto|crontab|systemd-timer|launchctl|schtasks::
+	When combined with the `start` subcommand, specify the scheduler
+	for running the hourly, daily and weekly executions of
+	`git maintenance run`.
+	Possible values for `<scheduler>` are `auto`, `crontab`
+	(POSIX), `systemd-timer` (Linux), `launchctl` (macOS), and
+	`schtasks` (Windows). When `auto` is specified, the
+	appropriate platform-specific scheduler is used; on Linux,
+	`systemd-timer` is used if available, otherwise
+	`crontab`. Default is `auto`.
+
 
 TROUBLESHOOTING
 ---------------
@@ -277,6 +288,52 @@
 schedule.
 
 
+BACKGROUND MAINTENANCE ON LINUX SYSTEMD SYSTEMS
+-----------------------------------------------
+
+While Linux supports `cron`, depending on the distribution, `cron` may
+be an optional package not necessarily installed. On modern Linux
+distributions, systemd timers are superseding it.
+
+If user systemd timers are available, they will be used as a replacement
+of `cron`.
+
+In this case, `git maintenance start` will create user systemd timer units
+and start the timers. The current list of user-scheduled tasks can be found
+by running `systemctl --user list-timers`. The timers written by `git
+maintenance start` are similar to this:
+
+-----------------------------------------------------------------------
+$ systemctl --user list-timers
+NEXT                         LEFT          LAST                         PASSED     UNIT                         ACTIVATES
+Thu 2021-04-29 19:00:00 CEST 42min left    Thu 2021-04-29 18:00:11 CEST 17min ago  git-maintenance@hourly.timer git-maintenance@hourly.service
+Fri 2021-04-30 00:00:00 CEST 5h 42min left Thu 2021-04-29 00:00:11 CEST 18h ago    git-maintenance@daily.timer  git-maintenance@daily.service
+Mon 2021-05-03 00:00:00 CEST 3 days left   Mon 2021-04-26 00:00:11 CEST 3 days ago git-maintenance@weekly.timer git-maintenance@weekly.service
+-----------------------------------------------------------------------
+
+One timer is registered for each `--schedule=<frequency>` option.
+
+The definition of the systemd units can be inspected in the following files:
+
+-----------------------------------------------------------------------
+~/.config/systemd/user/git-maintenance@.timer
+~/.config/systemd/user/git-maintenance@.service
+~/.config/systemd/user/timers.target.wants/git-maintenance@hourly.timer
+~/.config/systemd/user/timers.target.wants/git-maintenance@daily.timer
+~/.config/systemd/user/timers.target.wants/git-maintenance@weekly.timer
+-----------------------------------------------------------------------
+
+`git maintenance start` will overwrite these files and start the timer
+again with `systemctl --user`, so any customization should be done by
+creating a drop-in file, i.e. a `.conf` suffixed file in the
+`~/.config/systemd/user/git-maintenance@.service.d` directory.
+
+`git maintenance stop` will stop the user systemd timers and delete
+the above mentioned files.
+
+For more details, see `systemd.timer(5)`.
+
+
 BACKGROUND MAINTENANCE ON MACOS SYSTEMS
 ---------------------------------------
 
diff --git a/builtin/gc.c b/builtin/gc.c
index 43c3602..84d7710 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -1529,6 +1529,93 @@ static const char *get_frequency(enum schedule_priority schedule)
 	}
 }
 
+/*
+ * get_schedule_cmd` reads the GIT_TEST_MAINT_SCHEDULER environment variable
+ * to mock the schedulers that `git maintenance start` rely on.
+ *
+ * For test purpose, GIT_TEST_MAINT_SCHEDULER can be set to a comma-separated
+ * list of colon-separated key/value pairs where each pair contains a scheduler
+ * and its corresponding mock.
+ *
+ * * If $GIT_TEST_MAINT_SCHEDULER is not set, return false and leave the
+ *   arguments unmodified.
+ *
+ * * If $GIT_TEST_MAINT_SCHEDULER is set, return true.
+ *   In this case, the *cmd value is read as input.
+ *
+ *   * if the input value *cmd is the key of one of the comma-separated list
+ *     item, then *is_available is set to true and *cmd is modified and becomes
+ *     the mock command.
+ *
+ *   * if the input value *cmd isn’t the key of any of the comma-separated list
+ *     item, then *is_available is set to false.
+ *
+ * Ex.:
+ *   GIT_TEST_MAINT_SCHEDULER not set
+ *     +-------+-------------------------------------------------+
+ *     | Input |                     Output                      |
+ *     | *cmd  | return code |       *cmd        | *is_available |
+ *     +-------+-------------+-------------------+---------------+
+ *     | "foo" |    false    | "foo" (unchanged) |  (unchanged)  |
+ *     +-------+-------------+-------------------+---------------+
+ *
+ *   GIT_TEST_MAINT_SCHEDULER set to “foo:./mock_foo.sh,bar:./mock_bar.sh”
+ *     +-------+-------------------------------------------------+
+ *     | Input |                     Output                      |
+ *     | *cmd  | return code |       *cmd        | *is_available |
+ *     +-------+-------------+-------------------+---------------+
+ *     | "foo" |    true     |  "./mock.foo.sh"  |     true      |
+ *     | "qux" |    true     | "qux" (unchanged) |     false     |
+ *     +-------+-------------+-------------------+---------------+
+ */
+static int get_schedule_cmd(const char **cmd, int *is_available)
+{
+	char *testing = xstrdup_or_null(getenv("GIT_TEST_MAINT_SCHEDULER"));
+	struct string_list_item *item;
+	struct string_list list = STRING_LIST_INIT_NODUP;
+
+	if (!testing)
+		return 0;
+
+	if (is_available)
+		*is_available = 0;
+
+	string_list_split_in_place(&list, testing, ',', -1);
+	for_each_string_list_item(item, &list) {
+		struct string_list pair = STRING_LIST_INIT_NODUP;
+
+		if (string_list_split_in_place(&pair, item->string, ':', 2) != 2)
+			continue;
+
+		if (!strcmp(*cmd, pair.items[0].string)) {
+			*cmd = pair.items[1].string;
+			if (is_available)
+				*is_available = 1;
+			string_list_clear(&list, 0);
+			UNLEAK(testing);
+			return 1;
+		}
+	}
+
+	string_list_clear(&list, 0);
+	free(testing);
+	return 1;
+}
+
+static int is_launchctl_available(void)
+{
+	const char *cmd = "launchctl";
+	int is_available;
+	if (get_schedule_cmd(&cmd, &is_available))
+		return is_available;
+
+#ifdef __APPLE__
+	return 1;
+#else
+	return 0;
+#endif
+}
+
 static char *launchctl_service_name(const char *frequency)
 {
 	struct strbuf label = STRBUF_INIT;
@@ -1555,19 +1642,17 @@ static char *launchctl_get_uid(void)
 	return xstrfmt("gui/%d", getuid());
 }
 
-static int launchctl_boot_plist(int enable, const char *filename, const char *cmd)
+static int launchctl_boot_plist(int enable, const char *filename)
 {
+	const char *cmd = "launchctl";
 	int result;
 	struct child_process child = CHILD_PROCESS_INIT;
 	char *uid = launchctl_get_uid();
 
+	get_schedule_cmd(&cmd, NULL);
 	strvec_split(&child.args, cmd);
-	if (enable)
-		strvec_push(&child.args, "bootstrap");
-	else
-		strvec_push(&child.args, "bootout");
-	strvec_push(&child.args, uid);
-	strvec_push(&child.args, filename);
+	strvec_pushl(&child.args, enable ? "bootstrap" : "bootout", uid,
+		     filename, NULL);
 
 	child.no_stderr = 1;
 	child.no_stdout = 1;
@@ -1581,23 +1666,23 @@ static int launchctl_boot_plist(int enable, const char *filename, const char *cm
 	return result;
 }
 
-static int launchctl_remove_plist(enum schedule_priority schedule, const char *cmd)
+static int launchctl_remove_plist(enum schedule_priority schedule)
 {
 	const char *frequency = get_frequency(schedule);
 	char *name = launchctl_service_name(frequency);
 	char *filename = launchctl_service_filename(name);
-	int result = launchctl_boot_plist(0, filename, cmd);
+	int result = launchctl_boot_plist(0, filename);
 	unlink(filename);
 	free(filename);
 	free(name);
 	return result;
 }
 
-static int launchctl_remove_plists(const char *cmd)
+static int launchctl_remove_plists(void)
 {
-	return launchctl_remove_plist(SCHEDULE_HOURLY, cmd) ||
-		launchctl_remove_plist(SCHEDULE_DAILY, cmd) ||
-		launchctl_remove_plist(SCHEDULE_WEEKLY, cmd);
+	return launchctl_remove_plist(SCHEDULE_HOURLY) ||
+	       launchctl_remove_plist(SCHEDULE_DAILY) ||
+	       launchctl_remove_plist(SCHEDULE_WEEKLY);
 }
 
 static int launchctl_list_contains_plist(const char *name, const char *cmd)
@@ -1623,7 +1708,7 @@ static int launchctl_list_contains_plist(const char *name, const char *cmd)
 	return !result;
 }
 
-static int launchctl_schedule_plist(const char *exec_path, enum schedule_priority schedule, const char *cmd)
+static int launchctl_schedule_plist(const char *exec_path, enum schedule_priority schedule)
 {
 	int i, fd;
 	const char *preamble, *repeat;
@@ -1634,7 +1719,9 @@ static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
 	static unsigned long lock_file_timeout_ms = ULONG_MAX;
 	struct strbuf plist = STRBUF_INIT, plist2 = STRBUF_INIT;
 	struct stat st;
+	const char *cmd = "launchctl";
 
+	get_schedule_cmd(&cmd, NULL);
 	preamble = "<?xml version=\"1.0\"?>\n"
 		   "<!DOCTYPE plist PUBLIC \"-//Apple//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">\n"
 		   "<plist version=\"1.0\">"
@@ -1715,8 +1802,8 @@ static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
 			die_errno(_("could not write '%s'"), filename);
 
 		/* bootout might fail if not already running, so ignore */
-		launchctl_boot_plist(0, filename, cmd);
-		if (launchctl_boot_plist(1, filename, cmd))
+		launchctl_boot_plist(0, filename);
+		if (launchctl_boot_plist(1, filename))
 			die(_("failed to bootstrap service %s"), filename);
 	}
 
@@ -1727,21 +1814,35 @@ static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
 	return 0;
 }
 
-static int launchctl_add_plists(const char *cmd)
+static int launchctl_add_plists(void)
 {
 	const char *exec_path = git_exec_path();
 
-	return launchctl_schedule_plist(exec_path, SCHEDULE_HOURLY, cmd) ||
-		launchctl_schedule_plist(exec_path, SCHEDULE_DAILY, cmd) ||
-		launchctl_schedule_plist(exec_path, SCHEDULE_WEEKLY, cmd);
+	return launchctl_schedule_plist(exec_path, SCHEDULE_HOURLY) ||
+	       launchctl_schedule_plist(exec_path, SCHEDULE_DAILY) ||
+	       launchctl_schedule_plist(exec_path, SCHEDULE_WEEKLY);
 }
 
-static int launchctl_update_schedule(int run_maintenance, int fd, const char *cmd)
+static int launchctl_update_schedule(int run_maintenance, int fd)
 {
 	if (run_maintenance)
-		return launchctl_add_plists(cmd);
+		return launchctl_add_plists();
 	else
-		return launchctl_remove_plists(cmd);
+		return launchctl_remove_plists();
+}
+
+static int is_schtasks_available(void)
+{
+	const char *cmd = "schtasks";
+	int is_available;
+	if (get_schedule_cmd(&cmd, &is_available))
+		return is_available;
+
+#ifdef GIT_WINDOWS_NATIVE
+	return 1;
+#else
+	return 0;
+#endif
 }
 
 static char *schtasks_task_name(const char *frequency)
@@ -1751,13 +1852,15 @@ static char *schtasks_task_name(const char *frequency)
 	return strbuf_detach(&label, NULL);
 }
 
-static int schtasks_remove_task(enum schedule_priority schedule, const char *cmd)
+static int schtasks_remove_task(enum schedule_priority schedule)
 {
+	const char *cmd = "schtasks";
 	int result;
 	struct strvec args = STRVEC_INIT;
 	const char *frequency = get_frequency(schedule);
 	char *name = schtasks_task_name(frequency);
 
+	get_schedule_cmd(&cmd, NULL);
 	strvec_split(&args, cmd);
 	strvec_pushl(&args, "/delete", "/tn", name, "/f", NULL);
 
@@ -1768,15 +1871,16 @@ static int schtasks_remove_task(enum schedule_priority schedule, const char *cmd
 	return result;
 }
 
-static int schtasks_remove_tasks(const char *cmd)
+static int schtasks_remove_tasks(void)
 {
-	return schtasks_remove_task(SCHEDULE_HOURLY, cmd) ||
-		schtasks_remove_task(SCHEDULE_DAILY, cmd) ||
-		schtasks_remove_task(SCHEDULE_WEEKLY, cmd);
+	return schtasks_remove_task(SCHEDULE_HOURLY) ||
+	       schtasks_remove_task(SCHEDULE_DAILY) ||
+	       schtasks_remove_task(SCHEDULE_WEEKLY);
 }
 
-static int schtasks_schedule_task(const char *exec_path, enum schedule_priority schedule, const char *cmd)
+static int schtasks_schedule_task(const char *exec_path, enum schedule_priority schedule)
 {
+	const char *cmd = "schtasks";
 	int result;
 	struct child_process child = CHILD_PROCESS_INIT;
 	const char *xml;
@@ -1785,6 +1889,8 @@ static int schtasks_schedule_task(const char *exec_path, enum schedule_priority
 	char *name = schtasks_task_name(frequency);
 	struct strbuf tfilename = STRBUF_INIT;
 
+	get_schedule_cmd(&cmd, NULL);
+
 	strbuf_addf(&tfilename, "%s/schedule_%s_XXXXXX",
 		    get_git_common_dir(), frequency);
 	tfile = xmks_tempfile(tfilename.buf);
@@ -1889,28 +1995,52 @@ static int schtasks_schedule_task(const char *exec_path, enum schedule_priority
 	return result;
 }
 
-static int schtasks_schedule_tasks(const char *cmd)
+static int schtasks_schedule_tasks(void)
 {
 	const char *exec_path = git_exec_path();
 
-	return schtasks_schedule_task(exec_path, SCHEDULE_HOURLY, cmd) ||
-		schtasks_schedule_task(exec_path, SCHEDULE_DAILY, cmd) ||
-		schtasks_schedule_task(exec_path, SCHEDULE_WEEKLY, cmd);
+	return schtasks_schedule_task(exec_path, SCHEDULE_HOURLY) ||
+	       schtasks_schedule_task(exec_path, SCHEDULE_DAILY) ||
+	       schtasks_schedule_task(exec_path, SCHEDULE_WEEKLY);
 }
 
-static int schtasks_update_schedule(int run_maintenance, int fd, const char *cmd)
+static int schtasks_update_schedule(int run_maintenance, int fd)
 {
 	if (run_maintenance)
-		return schtasks_schedule_tasks(cmd);
+		return schtasks_schedule_tasks();
 	else
-		return schtasks_remove_tasks(cmd);
+		return schtasks_remove_tasks();
+}
+
+static int is_crontab_available(void)
+{
+	const char *cmd = "crontab";
+	int is_available;
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	if (get_schedule_cmd(&cmd, &is_available))
+		return is_available;
+
+	strvec_split(&child.args, cmd);
+	strvec_push(&child.args, "-l");
+	child.no_stdin = 1;
+	child.no_stdout = 1;
+	child.no_stderr = 1;
+	child.silent_exec_failure = 1;
+
+	if (start_command(&child))
+		return 0;
+	/* Ignore exit code, as an empty crontab will return error. */
+	finish_command(&child);
+	return 1;
 }
 
 #define BEGIN_LINE "# BEGIN GIT MAINTENANCE SCHEDULE"
 #define END_LINE "# END GIT MAINTENANCE SCHEDULE"
 
-static int crontab_update_schedule(int run_maintenance, int fd, const char *cmd)
+static int crontab_update_schedule(int run_maintenance, int fd)
 {
+	const char *cmd = "crontab";
 	int result = 0;
 	int in_old_region = 0;
 	struct child_process crontab_list = CHILD_PROCESS_INIT;
@@ -1918,6 +2048,7 @@ static int crontab_update_schedule(int run_maintenance, int fd, const char *cmd)
 	FILE *cron_list, *cron_in;
 	struct strbuf line = STRBUF_INIT;
 
+	get_schedule_cmd(&cmd, NULL);
 	strvec_split(&crontab_list.args, cmd);
 	strvec_push(&crontab_list.args, "-l");
 	crontab_list.in = -1;
@@ -1994,66 +2125,376 @@ static int crontab_update_schedule(int run_maintenance, int fd, const char *cmd)
 	return result;
 }
 
-#if defined(__APPLE__)
-static const char platform_scheduler[] = "launchctl";
-#elif defined(GIT_WINDOWS_NATIVE)
-static const char platform_scheduler[] = "schtasks";
-#else
-static const char platform_scheduler[] = "crontab";
-#endif
-
-static int update_background_schedule(int enable)
+static int real_is_systemd_timer_available(void)
 {
-	int result;
-	const char *scheduler = platform_scheduler;
-	const char *cmd = scheduler;
-	char *testing;
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	strvec_pushl(&child.args, "systemctl", "--user", "list-timers", NULL);
+	child.no_stdin = 1;
+	child.no_stdout = 1;
+	child.no_stderr = 1;
+	child.silent_exec_failure = 1;
+
+	if (start_command(&child))
+		return 0;
+	if (finish_command(&child))
+		return 0;
+	return 1;
+}
+
+static int is_systemd_timer_available(void)
+{
+	const char *cmd = "systemctl";
+	int is_available;
+
+	if (get_schedule_cmd(&cmd, &is_available))
+		return is_available;
+
+	return real_is_systemd_timer_available();
+}
+
+static char *xdg_config_home_systemd(const char *filename)
+{
+	return xdg_config_home_for("systemd/user", filename);
+}
+
+static int systemd_timer_enable_unit(int enable,
+				     enum schedule_priority schedule)
+{
+	const char *cmd = "systemctl";
+	struct child_process child = CHILD_PROCESS_INIT;
+	const char *frequency = get_frequency(schedule);
+
+	/*
+	 * Disabling the systemd unit while it is already disabled makes
+	 * systemctl print an error.
+	 * Let's ignore it since it means we already are in the expected state:
+	 * the unit is disabled.
+	 *
+	 * On the other hand, enabling a systemd unit which is already enabled
+	 * produces no error.
+	 */
+	if (!enable)
+		child.no_stderr = 1;
+
+	get_schedule_cmd(&cmd, NULL);
+	strvec_split(&child.args, cmd);
+	strvec_pushl(&child.args, "--user", enable ? "enable" : "disable",
+		     "--now", NULL);
+	strvec_pushf(&child.args, "git-maintenance@%s.timer", frequency);
+
+	if (start_command(&child))
+		return error(_("failed to start systemctl"));
+	if (finish_command(&child))
+		/*
+		 * Disabling an already disabled systemd unit makes
+		 * systemctl fail.
+		 * Let's ignore this failure.
+		 *
+		 * Enabling an enabled systemd unit doesn't fail.
+		 */
+		if (enable)
+			return error(_("failed to run systemctl"));
+	return 0;
+}
+
+static int systemd_timer_delete_unit_templates(void)
+{
+	int ret = 0;
+	char *filename = xdg_config_home_systemd("git-maintenance@.timer");
+	if (unlink(filename) && !is_missing_file_error(errno))
+		ret = error_errno(_("failed to delete '%s'"), filename);
+	FREE_AND_NULL(filename);
+
+	filename = xdg_config_home_systemd("git-maintenance@.service");
+	if (unlink(filename) && !is_missing_file_error(errno))
+		ret = error_errno(_("failed to delete '%s'"), filename);
+
+	free(filename);
+	return ret;
+}
+
+static int systemd_timer_delete_units(void)
+{
+	return systemd_timer_enable_unit(0, SCHEDULE_HOURLY) ||
+	       systemd_timer_enable_unit(0, SCHEDULE_DAILY) ||
+	       systemd_timer_enable_unit(0, SCHEDULE_WEEKLY) ||
+	       systemd_timer_delete_unit_templates();
+}
+
+static int systemd_timer_write_unit_templates(const char *exec_path)
+{
+	char *filename;
+	FILE *file;
+	const char *unit;
+
+	filename = xdg_config_home_systemd("git-maintenance@.timer");
+	if (safe_create_leading_directories(filename)) {
+		error(_("failed to create directories for '%s'"), filename);
+		goto error;
+	}
+	file = fopen_or_warn(filename, "w");
+	if (file == NULL)
+		goto error;
+
+	unit = "# This file was created and is maintained by Git.\n"
+	       "# Any edits made in this file might be replaced in the future\n"
+	       "# by a Git command.\n"
+	       "\n"
+	       "[Unit]\n"
+	       "Description=Optimize Git repositories data\n"
+	       "\n"
+	       "[Timer]\n"
+	       "OnCalendar=%i\n"
+	       "Persistent=true\n"
+	       "\n"
+	       "[Install]\n"
+	       "WantedBy=timers.target\n";
+	if (fputs(unit, file) == EOF) {
+		error(_("failed to write to '%s'"), filename);
+		fclose(file);
+		goto error;
+	}
+	if (fclose(file) == EOF) {
+		error_errno(_("failed to flush '%s'"), filename);
+		goto error;
+	}
+	free(filename);
+
+	filename = xdg_config_home_systemd("git-maintenance@.service");
+	file = fopen_or_warn(filename, "w");
+	if (file == NULL)
+		goto error;
+
+	unit = "# This file was created and is maintained by Git.\n"
+	       "# Any edits made in this file might be replaced in the future\n"
+	       "# by a Git command.\n"
+	       "\n"
+	       "[Unit]\n"
+	       "Description=Optimize Git repositories data\n"
+	       "\n"
+	       "[Service]\n"
+	       "Type=oneshot\n"
+	       "ExecStart=\"%s/git\" --exec-path=\"%s\" for-each-repo --config=maintenance.repo maintenance run --schedule=%%i\n"
+	       "LockPersonality=yes\n"
+	       "MemoryDenyWriteExecute=yes\n"
+	       "NoNewPrivileges=yes\n"
+	       "RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6\n"
+	       "RestrictNamespaces=yes\n"
+	       "RestrictRealtime=yes\n"
+	       "RestrictSUIDSGID=yes\n"
+	       "SystemCallArchitectures=native\n"
+	       "SystemCallFilter=@system-service\n";
+	if (fprintf(file, unit, exec_path, exec_path) < 0) {
+		error(_("failed to write to '%s'"), filename);
+		fclose(file);
+		goto error;
+	}
+	if (fclose(file) == EOF) {
+		error_errno(_("failed to flush '%s'"), filename);
+		goto error;
+	}
+	free(filename);
+	return 0;
+
+error:
+	free(filename);
+	systemd_timer_delete_unit_templates();
+	return -1;
+}
+
+static int systemd_timer_setup_units(void)
+{
+	const char *exec_path = git_exec_path();
+
+	int ret = systemd_timer_write_unit_templates(exec_path) ||
+		  systemd_timer_enable_unit(1, SCHEDULE_HOURLY) ||
+		  systemd_timer_enable_unit(1, SCHEDULE_DAILY) ||
+		  systemd_timer_enable_unit(1, SCHEDULE_WEEKLY);
+	if (ret)
+		systemd_timer_delete_units();
+	return ret;
+}
+
+static int systemd_timer_update_schedule(int run_maintenance, int fd)
+{
+	if (run_maintenance)
+		return systemd_timer_setup_units();
+	else
+		return systemd_timer_delete_units();
+}
+
+enum scheduler {
+	SCHEDULER_INVALID = -1,
+	SCHEDULER_AUTO,
+	SCHEDULER_CRON,
+	SCHEDULER_SYSTEMD,
+	SCHEDULER_LAUNCHCTL,
+	SCHEDULER_SCHTASKS,
+};
+
+static const struct {
+	const char *name;
+	int (*is_available)(void);
+	int (*update_schedule)(int run_maintenance, int fd);
+} scheduler_fn[] = {
+	[SCHEDULER_CRON] = {
+		.name = "crontab",
+		.is_available = is_crontab_available,
+		.update_schedule = crontab_update_schedule,
+	},
+	[SCHEDULER_SYSTEMD] = {
+		.name = "systemctl",
+		.is_available = is_systemd_timer_available,
+		.update_schedule = systemd_timer_update_schedule,
+	},
+	[SCHEDULER_LAUNCHCTL] = {
+		.name = "launchctl",
+		.is_available = is_launchctl_available,
+		.update_schedule = launchctl_update_schedule,
+	},
+	[SCHEDULER_SCHTASKS] = {
+		.name = "schtasks",
+		.is_available = is_schtasks_available,
+		.update_schedule = schtasks_update_schedule,
+	},
+};
+
+static enum scheduler parse_scheduler(const char *value)
+{
+	if (!value)
+		return SCHEDULER_INVALID;
+	else if (!strcasecmp(value, "auto"))
+		return SCHEDULER_AUTO;
+	else if (!strcasecmp(value, "cron") || !strcasecmp(value, "crontab"))
+		return SCHEDULER_CRON;
+	else if (!strcasecmp(value, "systemd") ||
+		 !strcasecmp(value, "systemd-timer"))
+		return SCHEDULER_SYSTEMD;
+	else if (!strcasecmp(value, "launchctl"))
+		return SCHEDULER_LAUNCHCTL;
+	else if (!strcasecmp(value, "schtasks"))
+		return SCHEDULER_SCHTASKS;
+	else
+		return SCHEDULER_INVALID;
+}
+
+static int maintenance_opt_scheduler(const struct option *opt, const char *arg,
+				     int unset)
+{
+	enum scheduler *scheduler = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+
+	*scheduler = parse_scheduler(arg);
+	if (*scheduler == SCHEDULER_INVALID)
+		return error(_("unrecognized --scheduler argument '%s'"), arg);
+	return 0;
+}
+
+struct maintenance_start_opts {
+	enum scheduler scheduler;
+};
+
+static enum scheduler resolve_scheduler(enum scheduler scheduler)
+{
+	if (scheduler != SCHEDULER_AUTO)
+		return scheduler;
+
+#if defined(__APPLE__)
+	return SCHEDULER_LAUNCHCTL;
+
+#elif defined(GIT_WINDOWS_NATIVE)
+	return SCHEDULER_SCHTASKS;
+
+#elif defined(__linux__)
+	if (is_systemd_timer_available())
+		return SCHEDULER_SYSTEMD;
+	else if (is_crontab_available())
+		return SCHEDULER_CRON;
+	else
+		die(_("neither systemd timers nor crontab are available"));
+
+#else
+	return SCHEDULER_CRON;
+#endif
+}
+
+static void validate_scheduler(enum scheduler scheduler)
+{
+	if (scheduler == SCHEDULER_INVALID)
+		BUG("invalid scheduler");
+	if (scheduler == SCHEDULER_AUTO)
+		BUG("resolve_scheduler should have been called before");
+
+	if (!scheduler_fn[scheduler].is_available())
+		die(_("%s scheduler is not available"),
+		    scheduler_fn[scheduler].name);
+}
+
+static int update_background_schedule(const struct maintenance_start_opts *opts,
+				      int enable)
+{
+	unsigned int i;
+	int result = 0;
 	struct lock_file lk;
 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->odb->path);
 
-	testing = xstrdup_or_null(getenv("GIT_TEST_MAINT_SCHEDULER"));
-	if (testing) {
-		char *sep = strchr(testing, ':');
-		if (!sep)
-			die("GIT_TEST_MAINT_SCHEDULER unparseable: %s", testing);
-		*sep = '\0';
-		scheduler = testing;
-		cmd = sep + 1;
-	}
-
 	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
-		result = error(_("another process is scheduling background maintenance"));
-		goto cleanup;
+		free(lock_path);
+		return error(_("another process is scheduling background maintenance"));
 	}
 
-	if (!strcmp(scheduler, "launchctl"))
-		result = launchctl_update_schedule(enable, get_lock_file_fd(&lk), cmd);
-	else if (!strcmp(scheduler, "schtasks"))
-		result = schtasks_update_schedule(enable, get_lock_file_fd(&lk), cmd);
-	else if (!strcmp(scheduler, "crontab"))
-		result = crontab_update_schedule(enable, get_lock_file_fd(&lk), cmd);
-	else
-		die("unknown background scheduler: %s", scheduler);
+	for (i = 1; i < ARRAY_SIZE(scheduler_fn); i++) {
+		if (enable && opts->scheduler == i)
+			continue;
+		if (!scheduler_fn[i].is_available())
+			continue;
+		scheduler_fn[i].update_schedule(0, get_lock_file_fd(&lk));
+	}
+
+	if (enable)
+		result = scheduler_fn[opts->scheduler].update_schedule(
+			1, get_lock_file_fd(&lk));
 
 	rollback_lock_file(&lk);
 
-cleanup:
 	free(lock_path);
-	free(testing);
 	return result;
 }
 
-static int maintenance_start(void)
+static const char *const builtin_maintenance_start_usage[] = {
+	N_("git maintenance start [--scheduler=<scheduler>]"),
+	NULL
+};
+
+static int maintenance_start(int argc, const char **argv, const char *prefix)
 {
+	struct maintenance_start_opts opts = { 0 };
+	struct option options[] = {
+		OPT_CALLBACK_F(
+			0, "scheduler", &opts.scheduler, N_("scheduler"),
+			N_("scheduler to trigger git maintenance run"),
+			PARSE_OPT_NONEG, maintenance_opt_scheduler),
+		OPT_END()
+	};
+
+	argc = parse_options(argc, argv, prefix, options,
+			     builtin_maintenance_start_usage, 0);
+	if (argc)
+		usage_with_options(builtin_maintenance_start_usage, options);
+
+	opts.scheduler = resolve_scheduler(opts.scheduler);
+	validate_scheduler(opts.scheduler);
+
 	if (maintenance_register())
 		warning(_("failed to add repo to global config"));
-
-	return update_background_schedule(1);
+	return update_background_schedule(&opts, 1);
 }
 
 static int maintenance_stop(void)
 {
-	return update_background_schedule(0);
+	return update_background_schedule(NULL, 0);
 }
 
 static const char builtin_maintenance_usage[] =	N_("git maintenance <subcommand> [<options>]");
@@ -2067,7 +2508,7 @@ int cmd_maintenance(int argc, const char **argv, const char *prefix)
 	if (!strcmp(argv[1], "run"))
 		return maintenance_run(argc - 1, argv + 1, prefix);
 	if (!strcmp(argv[1], "start"))
-		return maintenance_start();
+		return maintenance_start(argc - 1, argv + 1, prefix);
 	if (!strcmp(argv[1], "stop"))
 		return maintenance_stop();
 	if (!strcmp(argv[1], "register"))
diff --git a/cache.h b/cache.h
index d23de69..72410b8 100644
--- a/cache.h
+++ b/cache.h
@@ -1299,6 +1299,13 @@ int looks_like_command_line_option(const char *str);
 
 /**
  * Return a newly allocated string with the evaluation of
+ * "$XDG_CONFIG_HOME/$subdir/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
+ * "$HOME/.config/$subdir/$filename". Return NULL upon error.
+ */
+char *xdg_config_home_for(const char *subdir, const char *filename);
+
+/**
+ * Return a newly allocated string with the evaluation of
  * "$XDG_CONFIG_HOME/git/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
  * "$HOME/.config/git/$filename". Return NULL upon error.
  */
diff --git a/path.c b/path.c
index 0bc788e..2c89547 100644
--- a/path.c
+++ b/path.c
@@ -1510,21 +1510,28 @@ int looks_like_command_line_option(const char *str)
 	return str && str[0] == '-';
 }
 
-char *xdg_config_home(const char *filename)
+char *xdg_config_home_for(const char *subdir, const char *filename)
 {
 	const char *home, *config_home;
 
+	assert(subdir);
 	assert(filename);
 	config_home = getenv("XDG_CONFIG_HOME");
 	if (config_home && *config_home)
-		return mkpathdup("%s/git/%s", config_home, filename);
+		return mkpathdup("%s/%s/%s", config_home, subdir, filename);
 
 	home = getenv("HOME");
 	if (home)
-		return mkpathdup("%s/.config/git/%s", home, filename);
+		return mkpathdup("%s/.config/%s/%s", home, subdir, filename);
+
 	return NULL;
 }
 
+char *xdg_config_home(const char *filename)
+{
+	return xdg_config_home_for("git", filename);
+}
+
 char *xdg_cache_home(const char *filename)
 {
 	const char *home, *cache_home;
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index fc16ac2..36a4218 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -20,6 +20,18 @@
 	fi
 }
 
+test_lazy_prereq SYSTEMD_ANALYZE '
+	systemd-analyze --help >out &&
+	grep verify out
+'
+
+test_systemd_analyze_verify () {
+	if test_have_prereq SYSTEMD_ANALYZE
+	then
+		systemd-analyze verify "$@"
+	fi
+}
+
 test_expect_success 'help text' '
 	test_expect_code 129 git maintenance -h 2>err &&
 	test_i18ngrep "usage: git maintenance <subcommand>" err &&
@@ -492,8 +504,21 @@
 		maintenance.repo "$(pwd)/$META"
 '
 
+test_expect_success 'start --scheduler=<scheduler>' '
+	test_expect_code 129 git maintenance start --scheduler=foo 2>err &&
+	test_i18ngrep "unrecognized --scheduler argument" err &&
+
+	test_expect_code 129 git maintenance start --no-scheduler 2>err &&
+	test_i18ngrep "unknown option" err &&
+
+	test_expect_code 128 \
+		env GIT_TEST_MAINT_SCHEDULER="launchctl:true,schtasks:true" \
+		git maintenance start --scheduler=crontab 2>err &&
+	test_i18ngrep "fatal: crontab scheduler is not available" err
+'
+
 test_expect_success 'start from empty cron table' '
-	GIT_TEST_MAINT_SCHEDULER="crontab:test-tool crontab cron.txt" git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER="crontab:test-tool crontab cron.txt" git maintenance start --scheduler=crontab &&
 
 	# start registers the repo
 	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
@@ -516,7 +541,7 @@
 
 test_expect_success 'start preserves existing schedule' '
 	echo "Important information!" >cron.txt &&
-	GIT_TEST_MAINT_SCHEDULER="crontab:test-tool crontab cron.txt" git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER="crontab:test-tool crontab cron.txt" git maintenance start --scheduler=crontab &&
 	grep "Important information!" cron.txt
 '
 
@@ -545,7 +570,7 @@
 	EOF
 
 	rm -f args &&
-	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start --scheduler=launchctl &&
 
 	# start registers the repo
 	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
@@ -584,11 +609,11 @@
 
 test_expect_success 'use launchctl list to prevent extra work' '
 	# ensure we are registered
-	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start --scheduler=launchctl &&
 
 	# do it again on a fresh args file
 	rm -f args &&
-	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER=launchctl:./print-args git maintenance start --scheduler=launchctl &&
 
 	ls "$HOME/Library/LaunchAgents" >actual &&
 	cat >expect <<-\EOF &&
@@ -613,7 +638,7 @@
 	EOF
 
 	rm -f args &&
-	GIT_TEST_MAINT_SCHEDULER="schtasks:./print-args" git maintenance start &&
+	GIT_TEST_MAINT_SCHEDULER="schtasks:./print-args" git maintenance start --scheduler=schtasks &&
 
 	# start registers the repo
 	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
@@ -636,6 +661,83 @@
 	test_cmp expect args
 '
 
+test_expect_success 'start and stop Linux/systemd maintenance' '
+	write_script print-args <<-\EOF &&
+	printf "%s\n" "$*" >>args
+	EOF
+
+	XDG_CONFIG_HOME="$PWD" &&
+	export XDG_CONFIG_HOME &&
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args" git maintenance start --scheduler=systemd-timer &&
+
+	# start registers the repo
+	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
+
+	test_systemd_analyze_verify "systemd/user/git-maintenance@.service" &&
+
+	printf -- "--user enable --now git-maintenance@%s.timer\n" hourly daily weekly >expect &&
+	test_cmp expect args &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args" git maintenance stop &&
+
+	# stop does not unregister the repo
+	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
+
+	test_path_is_missing "systemd/user/git-maintenance@.timer" &&
+	test_path_is_missing "systemd/user/git-maintenance@.service" &&
+
+	printf -- "--user disable --now git-maintenance@%s.timer\n" hourly daily weekly >expect &&
+	test_cmp expect args
+'
+
+test_expect_success 'start and stop when several schedulers are available' '
+	write_script print-args <<-\EOF &&
+	printf "%s\n" "$*" | sed "s:gui/[0-9][0-9]*:gui/[UID]:; s:\(schtasks /create .* /xml\).*:\1:;" >>args
+	EOF
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args systemctl,launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance start --scheduler=systemd-timer &&
+	printf "launchctl bootout gui/[UID] $pfx/Library/LaunchAgents/org.git-scm.git.%s.plist\n" \
+		hourly daily weekly >expect &&
+	printf "schtasks /delete /tn Git Maintenance (%s) /f\n" \
+		hourly daily weekly >>expect &&
+	printf -- "systemctl --user enable --now git-maintenance@%s.timer\n" hourly daily weekly >>expect &&
+	test_cmp expect args &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args systemctl,launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance start --scheduler=launchctl &&
+	printf -- "systemctl --user disable --now git-maintenance@%s.timer\n" hourly daily weekly >expect &&
+	printf "schtasks /delete /tn Git Maintenance (%s) /f\n" \
+		hourly daily weekly >>expect &&
+	for frequency in hourly daily weekly
+	do
+		PLIST="$pfx/Library/LaunchAgents/org.git-scm.git.$frequency.plist" &&
+		echo "launchctl bootout gui/[UID] $PLIST" >>expect &&
+		echo "launchctl bootstrap gui/[UID] $PLIST" >>expect || return 1
+	done &&
+	test_cmp expect args &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args systemctl,launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance start --scheduler=schtasks &&
+	printf -- "systemctl --user disable --now git-maintenance@%s.timer\n" hourly daily weekly >expect &&
+	printf "launchctl bootout gui/[UID] $pfx/Library/LaunchAgents/org.git-scm.git.%s.plist\n" \
+		hourly daily weekly >>expect &&
+	printf "schtasks /create /tn Git Maintenance (%s) /f /xml\n" \
+		hourly daily weekly >>expect &&
+	test_cmp expect args &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemctl:./print-args systemctl,launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance stop &&
+	printf -- "systemctl --user disable --now git-maintenance@%s.timer\n" hourly daily weekly >expect &&
+	printf "launchctl bootout gui/[UID] $pfx/Library/LaunchAgents/org.git-scm.git.%s.plist\n" \
+		hourly daily weekly >>expect &&
+	printf "schtasks /delete /tn Git Maintenance (%s) /f\n" \
+		hourly daily weekly >>expect &&
+	test_cmp expect args
+'
+
 test_expect_success 'register preserves existing strategy' '
 	git config maintenance.strategy none &&
 	git maintenance register &&