[PATCH v2 1/1] maintenance: use systemd timers on Linux

From

Lénaïc Huard <lenaic@lhuard.fr>

Date

May 9, 2021, 21:32 UTC

Message-ID

<20210509213217.449489-2-lenaic@lhuard.fr>

In-Reply-To

<20210509213217.449489-1-lenaic@lhuard.fr>

The main motivations to implement systemd timers in addition to cron
are:
* cron is optional and Linux systems running systemd might not have it
  installed.
* The execution of `crontab -l` can tell us if cron is installed but not
  if the daemon is actually running.
* With systemd, each service is run in its own cgroup and its logs are
  tagged by the service inside journald. With cron, all scheduled tasks
  are running in the cron daemon cgroup and all the logs of the
  user-scheduled tasks are pretended to belong to the system cron
  service.
  Concretely, a user that doesn’t have access to the system logs won’t
  have access to the log of its own tasks scheduled by cron whereas he
  will have access to the log of its own tasks scheduled by systemd
  timer.
  Although `cron` attempts to send email, that email may go unseen by
  the user because these days, local mailboxes are not heavily used
  anymore.

In order to schedule git maintenance, we need two unit template files:
* ~/.config/systemd/user/git-maintenance@.service
  to define the command to be started by systemd and
* ~/.config/systemd/user/git-maintenance@.timer
  to define the schedule at which the command should be run.

Those units are templates that are parametrized by the frequency.

Based on those templates, 3 timers are started:
* git-maintenance@hourly.timer
* git-maintenance@daily.timer
* git-maintenance@weekly.timer

Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
---
 Documentation/git-maintenance.txt |  60 +++++
 builtin/gc.c                      | 375 ++++++++++++++++++++++++++++--
 t/t7900-maintenance.sh            |  51 ++++
 3 files changed, 462 insertions(+), 24 deletions(-)

diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
index 80ddd33ceb..f012923333 100644
--- a/Documentation/git-maintenance.txt
+++ b/Documentation/git-maintenance.txt
@@ -181,6 +181,20 @@ OPTIONS
 	`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
+	to use to run the hourly, daily and weekly executions of
+	`git maintenance run`.
+	The possible values for `<scheduler>` depend on the system: `crontab`
+	is available on POSIX systems, `systemd-timer` is available on Linux
+	systems, `launchctl` is available on MacOS and `schtasks` is available
+	on Windows.
+	By default or when `auto` is specified, the most appropriate scheduler
+	for the system is used. On MacOS, `launchctl` is used. On Windows,
+	`schtasks` is used. On Linux, `systemd-timers` is used if user systemd
+	timers are available, otherwise, `crontab` is used. On all other systems,
+	`crontab` is used.
+
 
 TROUBLESHOOTING
 ---------------
@@ -279,6 +293,52 @@ schedule to ensure you are executing the correct binaries in your
 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 ef7226d7bc..7c72aa3b99 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -1544,6 +1544,15 @@ static const char *get_frequency(enum schedule_priority schedule)
 	}
 }
 
+static int is_launchctl_available(const char *cmd)
+{
+#ifdef __APPLE__
+	return 1;
+#else
+	return 0;
+#endif
+}
+
 static char *launchctl_service_name(const char *frequency)
 {
 	struct strbuf label = STRBUF_INIT;
@@ -1710,6 +1719,15 @@ static int launchctl_update_schedule(int run_maintenance, int fd, const char *cm
 		return launchctl_remove_plists(cmd);
 }
 
+static int is_schtasks_available(const char *cmd)
+{
+#ifdef GIT_WINDOWS_NATIVE
+	return 1;
+#else
+	return 0;
+#endif
+}
+
 static char *schtasks_task_name(const char *frequency)
 {
 	struct strbuf label = STRBUF_INIT;
@@ -1872,6 +1890,28 @@ static int schtasks_update_schedule(int run_maintenance, int fd, const char *cmd
 		return schtasks_remove_tasks(cmd);
 }
 
+static int is_crontab_available(const char *cmd)
+{
+	static int cached_result = -1;
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	if (cached_result != -1)
+		return cached_result;
+
+	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 cached_result = 0;
+	/* Ignore exit code, as an empty crontab will return error. */
+	finish_command(&child);
+	return cached_result = 1;
+}
+
 #define BEGIN_LINE "# BEGIN GIT MAINTENANCE SCHEDULE"
 #define END_LINE "# END GIT MAINTENANCE SCHEDULE"
 
@@ -1959,61 +1999,348 @@ static int crontab_update_schedule(int run_maintenance, int fd, const char *cmd)
 	return result;
 }
 
+static int is_systemd_timer_available(const char *cmd)
+{
+#ifdef __linux__
+	static int cached_result = -1;
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	if (cached_result != -1)
+		return cached_result;
+
+	strvec_split(&child.args, cmd);
+	strvec_pushl(&child.args, "--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 cached_result = 0;
+	if (finish_command(&child))
+		return cached_result = 0;
+	return cached_result = 1;
+#else
+	return 0;
+#endif
+}
+
+static char *xdg_config_home_systemd(const char *filename)
+{
+	const char *home, *config_home;
+
+	assert(filename);
+	config_home = getenv("XDG_CONFIG_HOME");
+	if (config_home && *config_home)
+		return mkpathdup("%s/systemd/user/%s", config_home, filename);
+
+	home = getenv("HOME");
+	if (home)
+		return mkpathdup("%s/.config/systemd/user/%s", home, filename);
+
+	die(_("failed to get $XDG_CONFIG_HOME and $HOME"));
+}
+
+static int systemd_timer_enable_unit(int enable,
+				     enum schedule_priority schedule,
+				     const char *cmd)
+{
+	struct child_process child = CHILD_PROCESS_INIT;
+	const char *frequency = get_frequency(schedule);
+
+	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))
+		die(_("failed to run systemctl"));
+	return finish_command(&child);
+}
+
+static int systemd_timer_delete_unit_templates(void)
+{
+	char *filename = xdg_config_home_systemd("git-maintenance@.timer");
+	unlink(filename);
+	free(filename);
+
+	filename = xdg_config_home_systemd("git-maintenance@.service");
+	unlink(filename);
+	free(filename);
+
+	return 0;
+}
+
+static int systemd_timer_delete_units(const char *cmd)
+{
+	return systemd_timer_enable_unit(0, SCHEDULE_HOURLY, cmd) ||
+	       systemd_timer_enable_unit(0, SCHEDULE_DAILY, cmd) ||
+	       systemd_timer_enable_unit(0, SCHEDULE_WEEKLY, cmd) ||
+	       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))
+		die(_("failed to create directories for '%s'"), filename);
+	file = xfopen(filename, "w");
+	free(filename);
+
+	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";
+	fputs(unit, file);
+	fclose(file);
+
+	filename = xdg_config_home_systemd("git-maintenance@.service");
+	if (safe_create_leading_directories(filename))
+		die(_("failed to create directories for '%s'"), filename);
+	file = xfopen(filename, "w");
+	free(filename);
+
+	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=\"%1$s/git\" --exec-path=\"%1$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";
+	fprintf(file, unit, exec_path);
+	fclose(file);
+
+	return 0;
+}
+
+static int systemd_timer_setup_units(const char *cmd)
+{
+	const char *exec_path = git_exec_path();
+
+	return systemd_timer_write_unit_templates(exec_path) ||
+	       systemd_timer_enable_unit(1, SCHEDULE_HOURLY, cmd) ||
+	       systemd_timer_enable_unit(1, SCHEDULE_DAILY, cmd) ||
+	       systemd_timer_enable_unit(1, SCHEDULE_WEEKLY, cmd);
+}
+
+static int systemd_timer_update_schedule(int run_maintenance, int fd,
+					 const char *cmd)
+{
+	if (run_maintenance)
+		return systemd_timer_setup_units(cmd);
+	else
+		return systemd_timer_delete_units(cmd);
+}
+
+enum scheduler {
+	SCHEDULER_INVALID = -1,
+	SCHEDULER_AUTO = 0,
+	SCHEDULER_CRON = 1,
+	SCHEDULER_SYSTEMD = 2,
+	SCHEDULER_LAUNCHCTL = 3,
+	SCHEDULER_SCHTASKS = 4,
+};
+
+static const struct {
+	int (*is_available)(const char *cmd);
+	int (*update_schedule)(int run_maintenance, int fd, const char *cmd);
+	const char *cmd;
+} scheduler_fn[] = {
+	[SCHEDULER_CRON] = { is_crontab_available, crontab_update_schedule,
+			     "crontab" },
+	[SCHEDULER_SYSTEMD] = { is_systemd_timer_available,
+				systemd_timer_update_schedule, "systemctl" },
+	[SCHEDULER_LAUNCHCTL] = { is_launchctl_available,
+				  launchctl_update_schedule, "launchctl" },
+	[SCHEDULER_SCHTASKS] = { is_schtasks_available,
+				 schtasks_update_schedule, "schtasks" },
+};
+
+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;
+
+	if (unset)
+		die(_("--no-scheduler is not allowed"));
+
+	*scheduler = parse_scheduler(arg);
+
+	if (*scheduler == SCHEDULER_INVALID)
+		die(_("unrecognized --scheduler argument '%s'"), arg);
+
+	return 0;
+}
+
+struct maintenance_start_opts {
+	enum scheduler scheduler;
+};
+
+static void resolve_auto_scheduler(enum scheduler *scheduler)
+{
+	if (*scheduler != SCHEDULER_AUTO)
+		return;
+
 #if defined(__APPLE__)
-static const char platform_scheduler[] = "launchctl";
+	*scheduler = SCHEDULER_LAUNCHCTL;
+	return;
+
 #elif defined(GIT_WINDOWS_NATIVE)
-static const char platform_scheduler[] = "schtasks";
+	*scheduler = SCHEDULER_SCHTASKS;
+	return;
+
+#elif defined(__linux__)
+	if (is_systemd_timer_available("systemctl"))
+		*scheduler = SCHEDULER_SYSTEMD;
+	else if (is_crontab_available("crontab"))
+		*scheduler = SCHEDULER_CRON;
+	else
+		die(_("neither systemd timers nor crontab are available"));
+	return;
+
 #else
-static const char platform_scheduler[] = "crontab";
+	*scheduler = SCHEDULER_CRON;
+	return;
 #endif
+}
 
-static int update_background_schedule(int enable)
+static void validate_scheduler(enum scheduler scheduler)
 {
-	int result;
-	const char *scheduler = platform_scheduler;
-	const char *cmd = scheduler;
+	const char *cmd;
+
+	if (scheduler == SCHEDULER_INVALID)
+		BUG("invalid scheduler");
+	if (scheduler == SCHEDULER_AUTO)
+		BUG("resolve_auto_scheduler should have been called before");
+
+	cmd = scheduler_fn[scheduler].cmd;
+	if (!scheduler_fn[scheduler].is_available(cmd))
+		die(_("%s scheduler is not available"), cmd);
+}
+
+static int update_background_schedule(const struct maintenance_start_opts *opts,
+				      int enable)
+{
+	unsigned int i;
+	int res, result = 0;
+	enum scheduler scheduler;
+	const char *cmd = NULL;
 	char *testing;
 	struct lock_file lk;
 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->odb->path);
 
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0)
+		return error(_("another process is scheduling background maintenance"));
+
 	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;
+		scheduler = parse_scheduler(testing);
 		cmd = sep + 1;
+		result = scheduler_fn[scheduler].update_schedule(
+			enable, get_lock_file_fd(&lk), cmd);
+		goto done;
 	}
 
-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0)
-		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++) {
+		int enable_scheduler = enable && (opts->scheduler == i);
+		cmd = scheduler_fn[i].cmd;
+		if (!scheduler_fn[i].is_available(cmd))
+			continue;
+		res = scheduler_fn[i].update_schedule(
+			enable_scheduler, get_lock_file_fd(&lk), cmd);
+		if (enable_scheduler)
+			result = res;
+	}
 
+done:
 	rollback_lock_file(&lk);
 	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;
+	struct option builtin_maintenance_start_options[] = {
+		OPT_CALLBACK(
+			0, "scheduler", &opts.scheduler, N_("scheduler"),
+			N_("scheduler to use to trigger git maintenance run"),
+			maintenance_opt_scheduler),
+		OPT_END()
+	};
+	memset(&opts, 0, sizeof(opts));
+
+	argc = parse_options(argc, argv, prefix,
+			     builtin_maintenance_start_options,
+			     builtin_maintenance_start_usage, 0);
+
+	resolve_auto_scheduler(&opts.scheduler);
+	validate_scheduler(opts.scheduler);
+
+	if (argc > 0)
+		usage_with_options(builtin_maintenance_start_usage,
+				   builtin_maintenance_start_options);
+
 	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>]");
@@ -2027,7 +2354,7 @@ 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/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index 2412d8c5c0..6e6316cd90 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -20,6 +20,20 @@ test_xmllint () {
 	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 "$@"
+	else
+		true
+	fi
+}
+
 test_expect_success 'help text' '
 	test_expect_code 129 git maintenance -h 2>err &&
 	test_i18ngrep "usage: git maintenance <subcommand>" err &&
@@ -615,6 +629,43 @@ test_expect_success 'start and stop Windows maintenance' '
 	test_cmp expect args
 '
 
+test_expect_success 'start and stop Linux/systemd maintenance' '
+	write_script print-args <<-\EOF &&
+	echo $* >>args
+	EOF
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemd:./print-args" git maintenance start &&
+
+	# start registers the repo
+	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
+
+	test_systemd_analyze_verify "$HOME/.config/systemd/user/git-maintenance@.service" &&
+
+	rm -f expect &&
+	for frequency in hourly daily weekly
+	do
+		echo "--user enable --now git-maintenance@${frequency}.timer" >>expect || return 1
+	done &&
+	test_cmp expect args &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="systemd:./print-args" git maintenance stop &&
+
+	# stop does not unregister the repo
+	git config --get --global --fixed-value maintenance.repo "$(pwd)" &&
+
+	test_path_is_missing "$HOME/.config/systemd/user/git-maintenance@.timer" &&
+	test_path_is_missing "$HOME/.config/systemd/user/git-maintenance@.service" &&
+
+	rm -f expect &&
+	for frequency in hourly daily weekly
+	do
+		echo "--user disable --now git-maintenance@${frequency}.timer" >>expect || return 1
+	done &&
+	test_cmp expect args
+'
+
 test_expect_success 'register preserves existing strategy' '
 	git config maintenance.strategy none &&
 	git maintenance register &&

-- 
2.31.1