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5880c9c4c3
Hopefully clarifies how seqnums should be used and copied from events to events/messages when those are handled. https://bugzilla.gnome.org/show_bug.cgi?id=722791
91 lines
4 KiB
Text
91 lines
4 KiB
Text
Seqnums (Sequence numbers)
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--------------------------
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Seqnums are integers associated to events and messages. They are used to
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identify a group of events and messages as being part of the same 'operation'
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over the pipeline.
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Whenever a new event or message is created, a seqnum is set into them. This
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seqnum is created from an ever increasing source (starting from 0 and it
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might wrap around), so each new event and message gets a new and hopefully
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unique seqnum.
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Suppose an element receives an event A and, as part of the logic of handling
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the event A, creates a new event B. B should have its seqnum to the same as A,
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because they are part of the same operation. The same logic applies if this
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element had to create multiple events or messages, all of those should have
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the seqnum set to the value on the received event. For example, when a sink
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element receives an EOS event and creates a new EOS message to post, it
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should copy the seqnum from the event to the message because the EOS message
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is a consequence of the EOS event being received.
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Preserving the seqnums accross related events and messages allows the elements
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and applications to identify a set of events/messages as being part of a single
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operation on the pipeline. For example, flushes, segments and EOS that are
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related to a seek event started by the application.
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Seqnums are also useful for elements to discard duplicated events, avoiding
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handling them again.
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Below are some scenarios as examples of how to handle seqnums when receving
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events:
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Forcing EOS on the pipeline
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---------------------------
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The application has a pipeline running and does a gst_element_send_event
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to the pipeline with an EOS event. All the sources in the pipeline will
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have their send_event handlers called and will receive the event from
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the application.
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When handling this event, the sources will push either the same EOS downstream
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or create their own EOS event and push. In the later case, the source should
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copy the seqnum from the original EOS to the newly created. This same logic
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applies to all elements that receive the EOS downstream, either push the
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same event or, if creating a new one, copy the seqnum.
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When the EOS reaches the sink, it will create an EOS message, copy the
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seqnum to the message and post to the bus. The application receives the
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message and can compare the seqnum of the message with the one from the
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original event sent to the pipeline. If they match, it knows that this
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EOS message was caused by the event it pushed and not from other reason
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(input finished or configured segment was over).
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Seeking
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-------
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A seek event sent to the pipeline is forwarded to all sinks in it. Those
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sinks, then, push the seek event upstream until they reach an element
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that is capable of handling it. If the element handling the seek has
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multiple source pads (tipically a demuxer is handling the seek) it might
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receive the same seek event on all pads. To prevent handling the same
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seek event multiple times, the seqnum can be used to identify those
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events as being the same and only handle the first received.
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Also, when handling the seek, the element might push flush-start, flush-stop
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and a segment event. All those events should have the same seqnum of the seek
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event received. When this segment is over and an EOS/Segment-done event is
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going to be pushed, it also should have the same seqnum of the seek that
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originated the segment to be played.
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Having the same seqnum as the seek on the segment-done or EOS events is
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important for the application to identify that the segment requested
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by its seek has finished playing.
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Questions
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---------
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A) What happens if the application has sent a seek to the pipeline and,
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while the segment relative to this seek is playing, it sends an EOS
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event? Should the EOS pushed by the source have the seqnum of the
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segment or the EOS from the application?
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If the EOS was received from the application before the segment ended, it
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should have the EOS from the application event. If the segment ends before
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the application event is received/handled, it should have the seek/segment
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seqnum.
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