[PATCH v7 3/3] maintenance: add support for systemd timers on Linux

["Lénaïc Huard" <lenaic@lhuard.fr>]

The existing mechanism for scheduling background maintenance is done
through cron. On Linux systems managed by systemd, systemd provides an
alternative to schedule recurring tasks: systemd timers.

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 their own tasks scheduled by cron whereas
  they will have access to the log of their 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 parameterized by the frequency.

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

The command launched by those three timers are the same as with the
other scheduling methods:

/path/to/git for-each-repo --exec-path=/path/to
--config=maintenance.repo maintenance run --schedule=%i

with the full path for git to ensure that the version of git launched
for the scheduled maintenance is the same as the one used to run
`maintenance start`.

The timer unit contains `Persistent=true` so that, if the computer is
powered down when a maintenance task should run, the task will be run
when the computer is back powered on.

Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
---
 Documentation/git-maintenance.txt |  58 +++++++-
 builtin/gc.c                      | 227 ++++++++++++++++++++++++++++++
 t/t7900-maintenance.sh            |  67 ++++++++-
 3 files changed, 341 insertions(+), 11 deletions(-)

diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
index 576290b5c6..e2cfb68ab5 100644
--- a/Documentation/git-maintenance.txt
+++ b/Documentation/git-maintenance.txt
@@ -179,14 +179,16 @@ OPTIONS
 	`maintenance.<task>.enabled` configured as `true` are considered.
 	See the 'TASKS' section for the list of accepted `<task>` values.
 
---scheduler=auto|crontab|launchctl|schtasks::
+--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),
-	`launchctl` (macOS), and `schtasks` (Windows).
-	When `auto` is specified, the appropriate platform-specific
-	scheduler is used. Default is `auto`.
+	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
@@ -286,6 +288,52 @@ 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 96a43f99b4..0cc02fa79e 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -2079,10 +2079,221 @@ static int crontab_update_schedule(int run_maintenance, int fd)
 	return result;
 }
 
+#ifdef __linux__
+
+static int real_is_systemd_timer_available(void)
+{
+	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;
+}
+
+#else
+
+static int real_is_systemd_timer_available(void)
+{
+	return 0;
+}
+
+#endif
+
+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,
 };
@@ -2097,6 +2308,11 @@ static const struct {
 		.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,
@@ -2117,6 +2333,9 @@ static enum scheduler parse_scheduler(const char *value)
 		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"))
@@ -2153,6 +2372,14 @@ static enum scheduler resolve_scheduler(enum scheduler scheduler)
 #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
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index b36b7f5fb0..b289cae6b9 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -20,6 +20,18 @@ 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 "$@"
+	fi
+}
+
 test_expect_success 'help text' '
 	test_expect_code 129 git maintenance -h 2>err &&
 	test_i18ngrep "usage: git maintenance <subcommand>" err &&
@@ -634,15 +646,56 @@ 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 &&
+	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="launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance start --scheduler=launchctl &&
-	printf "schtasks /delete /tn Git Maintenance (%s) /f\n" \
+	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" &&
@@ -652,17 +705,19 @@ test_expect_success 'start and stop when several schedulers are available' '
 	test_cmp expect args &&
 
 	rm -f args &&
-	GIT_TEST_MAINT_SCHEDULER="launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance start --scheduler=schtasks &&
+	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 &&
+		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="launchctl:./print-args launchctl,schtasks:./print-args schtasks" git maintenance stop &&
+	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 &&
+		hourly daily weekly >>expect &&
 	printf "schtasks /delete /tn Git Maintenance (%s) /f\n" \
 		hourly daily weekly >>expect &&
 	test_cmp expect args
-- 
2.32.0