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.
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.