Created attachment 187566 [details] [review]
multiqueue: ensure MT-safety when adding a pad

gst_single_queue_new creates a single queue and adds this to the multiqueue, but it seems that GST_MULTI_QUEUE_MUTEX_UNLOCK is performed too soon.  Specifically, the lock is dropped before various fields of the single queue are setup, most notably the actual ->queue DataQueue, although the single queue is then already added to the list of multiqueue's queues.  Particularly the latter field may get accessed in some underrun/overrun callback iterating over all GstSingleQueue (leading to a NULL data queue).