Handler will be invoked synchronously, when a new message has been injected into the bus.
This function is mostly used internally. Only one sync handler can be attached to a given bus.
If the handler returns GST_BUS_DROP, it should unref the message, else the message should not be unreffed by the sync handler.
the Bus that sent the message
user data that has been given, when registering the handler
BusSyncReply stating what to do with the message