Re: [PATCH 03/26] libfrog: add support code for starting systemd services programmatically

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Mar 03, 2026 at 07:45:37AM -0800, Christoph Hellwig wrote:
> On Mon, Mar 02, 2026 at 04:34:36PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Add some simple routines for computing the name of systemd service
> > instances and starting systemd services.  These will be used by the
> > xfs_healer_start service to start per-filesystem xfs_healer service
> > instances.
> > 
> > Note that we run systemd helper programs as subprocesses for a couple of
> > reasons.  First, the path-escaping functionality is not a part of any
> > library-accessible API, which means it can only be accessed via
> > systemd-escape(1).  Second, although the service startup functionality
> > can be reached via dbus, doing so would introduce a new library
> > dependency.  Systemd is also undergoing a dbus -> varlink RPC transition
> > so we avoid that mess by calling the cli systemctl(1) program.
> 
> Just curious: did you run this past the systemd folks?  Shelling out
> always feel a bit iffy, and they're usually happy to help on how to
> integrate with their services, so just asking might result in a better
> way.

I'll do that, though even if they add a dbus/varlink endpoint for path
escaping, it'll be a few releases more until we can depend on it
existing in the distros. :(

Service startup can be done fairly easily with something like this,
though obviously you'd want to do real error checking here:

static DBusConnection*
connect_to_system_bus(void)
{
	// Connect to the system bus (requires root or polkit permissions)
	DBusConnection *conn = dbus_bus_get(DBUS_BUS_SYSTEM, error);

	return conn;
}

int
systemd_stop_service(
	const char *service_name)
{
	DBusError error;
	DBusConnection *conn = connect_to_system_bus();

	const char *manager_path = "/org/freedesktop/systemd1";
	const char *manager_interface = "org.freedesktop.systemd1.Manager";
	const char *method = "StopUnit";

	DBusMessage *msg = dbus_message_new_method_call(
		"org.freedesktop.systemd1",
		manager_path,
		manager_interface,
		method
	);

	const char *mode = "replace"; // Stop and replace existing job
	dbus_message_append_args(msg, DBUS_TYPE_STRING, &service_name,
			DBUS_TYPE_STRING, &mode, DBUS_TYPE_INVALID);

	DBusMessage *reply =
			dbus_connection_send_with_reply_and_block(conn,
					msg, 5000, &error);

	dbus_message_unref(reply);
	dbus_message_unref(msg);
	dbus_connection_unref(conn);

	return 0;
}

with the previously mentioned problem that now xfsprogs grows build and
packaging dependencies on libdbus.  My guess is that it'll be a long
time till they deprecate starting services over dbus.  AFAICT systemd in
Trixie doesn't even expose varlink endpoints yet.

(Note that xfs_scrub_all already has a runtime dependency on
python3-dbus)

--D