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87 lines
4 KiB
Markdown
87 lines
4 KiB
Markdown
# Seqnums (Sequence numbers)
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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
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*operation* over the pipeline.
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Whenever a new event or message is created, a seqnum is set into them.
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This seqnum is created from an ever increasing source (starting from 0
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but it might wrap around), so each new event and message gets a new and
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hopefully unique seqnum.
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Suppose an element receives an event A and, as part of the logic of
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handling the event A, creates a new event B. B should have its seqnum be
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the same as A, because they are part of the same operation. The same
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logic applies if this element had to create multiple events or messages,
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all of those should have the seqnum set to the value on the received
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event. For example, when a sink element receives an EOS event and
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creates a new EOS message to post, it should copy the seqnum from the
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event to the message because the EOS message is a consequence of the EOS
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event being received.
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Preserving the seqnums accross related events and messages allows the
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elements and applications to identify a set of events/messages as being
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part of a single operation on the pipeline. For example, flushes,
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segments and EOS that are related to a seek event started by the
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application.
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Seqnums are also useful for elements to discard duplicated events,
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avoiding handling them again.
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## Scenarios
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Below are some scenarios as examples of how to handle seqnums when
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receving events:
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### Forcing EOS on the pipeline
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The application has a pipeline running and does a
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`gst_element_send_event()` to the pipeline with an EOS event. All the
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sources in the pipeline will have their `send_event` handlers called and
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will receive the event from the application.
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When handling this event, the sources will push either the same EOS
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downstream or create their own EOS event and push. In the later case,
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the source should copy the seqnum from the original EOS to the newly
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created one. This same logic applies to all elements that receive the EOS
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downstream, either push the same event or, if creating a new one, copy
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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 its seqnum 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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A seek event sent to the pipeline is forwarded to all sinks in it. These
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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 one received first.
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Also, when handling the seek, the element might push `FLUSH_START`,
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`FLUSH_STOP` and a segment event. All these events should have the
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seqnum of the received seek event. When this segment is over and an
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`EOS/SEGMENT_DONE` event is going to be pushed, it should also have the
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seqnum of the seek that 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 by
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its seek has finished playing.
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## Questions
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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,
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it should have the EOS from the application event. If the segment ends
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before the application event is received/handled, it should have the
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seek/segment seqnum.
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