[Webkit-unassigned] [Bug 208001] New: [GStreamer] Fix race in TextCombinerGStreamer

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Thu Feb 20 04:34:40 PST 2020


            Bug ID: 208001
           Summary: [GStreamer] Fix race in TextCombinerGStreamer
           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

TextCombinerGStreamer uses the CAPS event to determine whether adding
a webvttenc between the text track pad and the funnel element used to
be able to display several subtitles at the same time.

The way this was done previously had a race though: all text track
pads were preemptively linked directly to the funnel, only adding the
webvttenc element later in the middle when receiving the CAPS event.

When two or more text tracks were present, it wasn't infrequent that
one track had its CAPS event processed (causing the webvttenc element
to be added) and propagated (fixating the funnel caps) before another
track attempted caps negotiation. Because the pads were connected to
the funnel preemptively, and because without the webvttenc element the
caps of the text pad don't match the funnel's, this causes a caps
mismatch error, stopping playback completely. The CAPS event is
therefore never sent.

To avoid this race, we must avoid linking elements until we get the
CAPS events, when we actually know where we should link them to,
therefore avoiding early caps negotiation errors.

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