[Bug 696675] New: Possible fix required in gstbasesink

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=696675
  GStreamer | gstreamer (core) | 1.0.6

           Summary: Possible fix required in gstbasesink
    Classification: Platform
           Product: GStreamer
           Version: 1.0.6
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: visechelle at gmail.com
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Created an attachment (id=239930)
 --> (https://bugzilla.gnome.org/attachment.cgi?id=239930)
Trace

While using alsasink element in a bin in a dynamic pipeline, I faced an issue
when this bin selecting alsasink is already in PLAYING state. In such a case,
alsasink might block in GST_BASE_SINK_PREROLL_WAIT() in
gst_base_sink_wait_preroll().
Indeed, if the bin is already in PLAYING state and alsasink's state is
synchronized on its parent, the pending state will be playing. Then, if
gst_base_sink_do_preroll() is called before ASYNC set states have been
performed, it will commit the state to PLAYING. Doing this, will prevent from
calling gst_audio_base_sink_change_state() where important things are
performed. Indeed, in the transition from pause to playing,
gst_audio_ring_buffer_may_start() is called and set to true, and because it
does not pass through this step in the conditions described above, it will
block in preroll_wait.
Indeed, in gst_base_sink_chain_unlocked(), after committing to PLAYING state,
render function of gstaudiobasesink is called and there it tries to render the
buffer and write it in the ringbuffer which has not been started
(gst_audio_base_sink_change_state() not called), the condition written==samples
is not satisfied, gst_base_sink_wait_preroll() is called and it is stuck there.

In the patch in attachment, I added the call to gst_element_change_state() in
gst_base_sink_commit_state() when committing to PLAYING state. It is not
necessary to call it when committing to PAUSE state as the method is called
before returning GST_STATE_CHANGE_ASYNC from the function setting the state.
I am not sure so far if this is a correct solution and comments would be
greatly appreciated :)

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