if (c->vtable->x.method.handler) {
r = c->vtable->x.method.handler(bus, m, u, &error);
return bus_maybe_reply_error(m, r, &error);
if (c->vtable->x.method.handler) {
r = c->vtable->x.method.handler(bus, m, u, &error);
return bus_maybe_reply_error(m, r, &error);