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Seqnums (Sequence numbers)
Seqnums are integers associated to events and messages. They are used to identify a group of events and messages as being part of the same operation over the pipeline.
Whenever a new event or message is created, a seqnum is set into them. This seqnum is created from an ever increasing source (starting from 0 and it might wrap around), so each new event and message gets a new and hopefully unique seqnum.
Suppose an element receives an event A and, as part of the logic of handling the event A, creates a new event B. B should have its seqnum to the same as A, because they are part of the same operation. The same logic applies if this element had to create multiple events or messages, all of those should have the seqnum set to the value on the received event. For example, when a sink element receives an EOS event and creates a new EOS message to post, it should copy the seqnum from the event to the message because the EOS message is a consequence of the EOS event being received.
Preserving the seqnums accross related events and messages allows the elements and applications to identify a set of events/messages as being part of a single operation on the pipeline. For example, flushes, segments and EOS that are related to a seek event started by the application.
Seqnums are also useful for elements to discard duplicated events, avoiding handling them again.
Below are some scenarios as examples of how to handle seqnums when receving events:
Forcing EOS on the pipeline
The application has a pipeline running and does a
gst_element_send_event to the pipeline with an EOS event. All the
sources in the pipeline will have their send_event handlers called and
will receive the event from the application.
When handling this event, the sources will push either the same EOS downstream or create their own EOS event and push. In the later case, the source should copy the seqnum from the original EOS to the newly created. This same logic applies to all elements that receive the EOS downstream, either push the same event or, if creating a new one, copy the seqnum.
When the EOS reaches the sink, it will create an EOS message, copy the seqnum to the message and post to the bus. The application receives the message and can compare the seqnum of the message with the one from the original event sent to the pipeline. If they match, it knows that this EOS message was caused by the event it pushed and not from other reason (input finished or configured segment was over).
Seeking
A seek event sent to the pipeline is forwarded to all sinks in it. Those sinks, then, push the seek event upstream until they reach an element that is capable of handling it. If the element handling the seek has multiple source pads (tipically a demuxer is handling the seek) it might receive the same seek event on all pads. To prevent handling the same seek event multiple times, the seqnum can be used to identify those events as being the same and only handle the first received.
Also, when handling the seek, the element might push flush-start, flush-stop and a segment event. All those events should have the same seqnum of the seek event received. When this segment is over and an EOS/Segment-done event is going to be pushed, it also should have the same seqnum of the seek that originated the segment to be played.
Having the same seqnum as the seek on the segment-done or EOS events is important for the application to identify that the segment requested by its seek has finished playing.
Questions
What happens if the application has sent a seek to the pipeline and, while the segment relative to this seek is playing, it sends an EOS event? Should the EOS pushed by the source have the seqnum of the segment or the EOS from the application?
If the EOS was received from the application before the segment ended, it should have the EOS from the application event. If the segment ends before the application event is received/handled, it should have the seek/segment seqnum.