[GStreamer][MSE] Append pipeline is counting streams incorrectly with new init segments

[calvaris]

847e28d

[GStreamer][MSE] Append pipeline is counting streams incorrectly with new init segments
https://bugs.webkit.org/show_bug.cgi?id=254664

Reviewed by Alicia Boya Garcia.

When you append another init segment we match GStreamer pads against tracks but we are not counting properly as we are
skipping the pads that are already linked we can end up with less pads than tracks. Besides it is against the spec. We
need to match the number of tracks and track IDs if there are more than one for a kind.

* LayoutTests/media/media-source/media-source-seek-detach-crash.html: Fail the test earlier instead of waiting for a
timeout.
* LayoutTests/platform/glib/TestExpectations: Unmark
media/encrypted-media/encrypted-media-append-encrypted-unencrypted.html as failure. It works now.
* Source/WebCore/platform/graphics/gstreamer/mse/AppendPipeline.cpp:
(WebCore::AppendPipeline::didReceiveInitializationSegment):
(WebCore::AppendPipeline::recycleTrackForPad):

Canonical link: https://commits.webkit.org/264676@main

b483c02

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[webkit-early-warning-system]

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[calvaris]

Some tests fail. I need to investigate.

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[ntrrgc]

Thanks for this patch.

gst_pad_get_stream_id() is a great find. I have been able to confirm through looking at the source code that both qtdemux and matroskademux emit the track IDs found in the containers metadata (namely the tkhd box for MP4 and the TrackNumber:TrackUID pair for Matroska/WebM), and this is not a new feature, already being present in GStreamer 1.10 and earlier.

So far, for lack of a way to get these bits of information, AppendPipeline has been using pad indices as track IDs (see AppendPipeline::generateTrackId()), so the first video pad gets V1, the second V2 and so on...

Now, taking this into consideration, it makes sense to update the code and replace the autogenerated track IDs with actual track IDs from GStreamer. I also noticed that the MediaSource spec requires track IDs exposed to the user to be distinct but doesn't specify they need to match the exact values in tkhd (although the engine internally must match those).

Something I've been wondering about this patch is, how are we handling recycling pads? This is the current code for matching tracks and pads:

// Try to find a matching pre-existing track. Ideally, tracks should be matched by track ID, but matching by type
// is provided as a fallback -- which will be used, since we don't have a way to fetch those from GStreamer at the moment.
Track* matchingTrack = nullptr;
for (std::unique_ptr<Track>& track : m_tracks) {
    if (track->streamType != streamType)
        continue;
    matchingTrack = &*track;
    if (track->trackId == trackId)
        break;
}

Assuming pad indices are consistent, our code should still work. For that reason, this patch LGTM as it is. Feel free to rework the track IDs to use the data from GStreamer stream IDs if you want, but that can be a future patch as well, leaving this fix contained.

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[webkit-commit-queue]

Committed 264676@main (847e28d): https://commits.webkit.org/264676@main

Reviewed commits have been landed. Closing PR #12114 and removing active labels.