Due to race condition it was previously possible that
gst_element_add_pad was not completed for each RTSP stream before
signal 'no-more-pads' was emitted.
Race condition explained:
Lets say two RTSP streams are created: Video and Audio.
1. Callback new_manager_pad is called for the Video stream =>
stream->added=TRUE.
all_added=FALSE because both streams are not yet added.
Call gst_element_add_pad and emit signal 'pad-added' for Video stream.
2. Callback new_manager_pad is called for Audio stream =>
stream->added=TRUE.
all_added=TRUE because both streams are added.
Call gst_element_add_pad and emit signal 'pad-added' for Audio stream.
3. Lets say gst_element_add_pad for the audio stream completes before
the video stream. Since the audio stream already has all_added==TRUE
this will result in the signal 'no-more-pads' to be emitted before
gst_element_add_pad for the video stream is completed.
Solution is to move the logic that sets added=True and checks if all
streams are added to after gst_element_add_pad. This will make sure
signal 'no-more-pads' is not emitted until all code in
gst_element_add_pad is completed for all streams.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8832>
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