engine: resources: mount: Fix the dbus call for reloading systemd

The Reload method cannot just be invoked on the administrative DBus
object. Just like the method for reloading specific units, it needs
to be invoked on the proper DBus service, addressing the proper object
and using the right interface.

Added an additional constant for the systemd DBus service. Even though
it shares the same value as the interface base name, this is
happenstance and it's technically incorrect to open a connection to an
interface name. The connection needs a service name.

Fixes #509

engine/resources/mount.go

@@ -75,6 +75,8 @@ const (
 	// diskByLabel is the location of symlinks for partitions by label.
 	diskByPartLabel = devDisk + "by-partlabel/"
 
+	// dbusSystemdService is the service to connect to systemd itself.
+	dbusSystemd1Service = "org.freedesktop.systemd1"
 	// dbusSystemd1Interface is the base systemd1 path.
 	dbusSystemd1Path = "/org/freedesktop/systemd1"
 	// dbusUnitPath is the dbus path where mount unit files are found.
@@ -88,6 +90,9 @@ const (
 	dbusManagerInterface = dbusSystemd1Interface + ".Manager"
 	// dbusRestartUnit is the dbus method for restarting systemd units.
 	dbusRestartUnit = dbusManagerInterface + ".RestartUnit"
+	// dbusReloadSystemd is the dbus method for reloading systemd settings.
+	// (i.e. systemctl daemon-reload)
+	dbusReloadSystemd = dbusManagerInterface + ".Reload"
 	// restartTimeout is the delay before restartUnit is assumed to have
 	// failed.
 	dbusRestartCtxTimeout = 10
@@ -576,7 +581,10 @@ func mountReload() error {
 	}
 	defer conn.Close()
 	// systemctl daemon-reload
-	conn.BusObject().Call("Reload", 0)
+	call := conn.Object(dbusSystemd1Service, dbusSystemd1Path).Call(dbusReloadSystemd, 0)
+	if call.Err != nil {
+		return errwrap.Wrapf(call.Err, "error reloading systemd")
+	}
 
 	// systemctl restart local-fs.target
 	if err := restartUnit(conn, "local-fs.target"); err != nil {
@@ -619,7 +627,7 @@ func restartUnit(conn *dbus.Conn, unit string) error {
 	defer conn.RemoveSignal(ch)
 
 	// restart the unit
-	sd1 := conn.Object(dbusSystemd1Interface, dbus.ObjectPath(dbusSystemd1Path))
+	sd1 := conn.Object(dbusSystemd1Service, dbus.ObjectPath(dbusSystemd1Path))
 	if call := sd1.Call(dbusRestartUnit, 0, unit, "fail"); call.Err != nil {
 		return errwrap.Wrapf(call.Err, "error restarting unit: %s", unit)
 	}