[Webkit-unassigned] [Bug 211623] New: [GStreamer] Playbin3 track switch rework

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=211623

            Bug ID: 211623
           Summary: [GStreamer] Playbin3 track switch rework
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKitGTK
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: aboya at igalia.com
                CC: bugs-noreply at webkitgtk.org

This patch reworks how track selection and reporting of selected
tracks is done in the player.

The following found limitations and assumptions in current GStreamer
have informed this patch:

a) Although the API for playbin3 is designed to be able to accept any
number of tracks of any kind, this is not supported in practice.

b) The first track of each type is always selected. Even in playbin3
mode, looking for GST_STREAM_FLAG_SELECT is not a reliable method, as
in most cases the demuxer does not set it at all. [qtdemux never sets
it at all, and matroskademux only sets it in certain cases.]

c) Sending GST_EVENT_SELECT_STREAMS is only safe at certain moments.
It's not safe before pre-roll, after EOS or during the handling of
another SELECT_STREAMS.

d) Selecting text tracks with playbin APIs is not relevant. All text
tracks are already being picked by WebKitTextCombiner, unaffected by
playbin track selection.

e) Tracks requested in a GST_EVENT_SELECT_STREAMS are eventually
selected. On the other hand,  looking at
GST_MESSAGE_STREAMS_SELECTED's content is not reliable, as this has
been seen to miss tracks depending on thread luck.

This patch takes the points above into account to rework how track
selection is handled in MediaPlayerPrivateGStreamer and fix the
following issues:

1) In playbin3 mode, no track was marked as selected initially,
because of reliance on GST_STREAM_FLAG_SELECT.

2) In playbin2 mode, sometimes tracks would not be initially marked as
selected. This occurred because of reliance on the "selected" property
in inputselector sinkpads, whose initialization is racy -- it can
occur after the track has been added and picked up by WebKit.

3) In playbin3 mode, the limitations explained before has been honored
to make track selection stable, delaying SELECT_STREAMS events until
they are safe to send.

This patch doesn't introduce significative behavior changes, rather
aiming for improving the stabilitity of the player. Existing tests
should provide enough coverage.

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