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playbin: Handle error message with redirection indication

Thibault Saunier requested to merge thiblahute/gst-plugins-base:qtdemux_redirect into master

There are in the wild (mp4) streams that basically contain no tracks but do have a redirect info0, in which case, qtdemux won't be able to expose any pad (there are no tracks) so can't post anything but an error on the bus, as:

  • it can't send EOS downstream, it has no pad,
  • posting an EOS message will be useless as PAUSED state can't be reached and there is no sink in the pipeline meaning GstBin will simply ignore it

In that case, currently the application could try to handle that but it is pretty complex as it will get the REDIRECT message on the bus at which point it could set the URL but playbin will ignore it, as it will only be for the next EOS, it thus need to set the pipeline to NULL (READY won't do as it is already in READY at that point). And it needs to figure out the following ERROR message on the bus needs to be ignored, which is not really simple.

The approach here is to allow element to add details to the ERROR message with a redirect-location field which elements like playbin handle and use right away.

We could also use the element 'redirect' message in playbin, but the issue with that approach is that the element will still emit the ERROR message on the bus, leading to wrong behaviour. That can't be avoided since in the case the app/parent pipeline is not handling the redirect instruction, the ERROR message is necessary (and there is no way to detect that the message has been "handled" from the element emitting the redirect).

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