An application that triggers a state transition from PLAYING to PAUSED
needs to acquire the LIVE_LOCK. Consequently the LIVE_LOCK must not be
taken while pushing anything on the pads because this operation might
get blocked by something that cannot be unblocked without the
application being able to proceed with the state transitions for other
elements in the pipeline. This commit extends the previous behaviour
where the live lock was released before pushing buffers (indirectly
through the unlock before subclass->create) to now also include
unlocking before pushing events.
The issue was discovered in a case for WebRTC where the application
tried to shut down a pipeline but an event originating from a video
source element (based on basesrc) was in the process of being pushed
down the pipeline when it got stuck on the STREAM_LOCK for the pad after
the rtpgccbwe element. This lock in turn was held by the rtcpgccbwe
element as it was in the process of pushing data down the pipeline but
was stuck on the blocking probes installed on dtlssrtpenc to prevent
data from flowing before dtls keys had been negotiated. What should have
happened here is that the blocking probes should be removed, but that
can only happen if the application may continue driving the state
transitions.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6671>
a3f8f036fe