Bug 314723 – [MSE] Narrow processMediaSample's decode-key cleanup to true orphans and equal-key replacement

• Status: RESOLVED
• Resolution: FIXED
• Component: Media

[Jean-Yves Avenard [:jya]]

Address a latent gap in MSE "Coded Frame Processing" step 1.15 and tighten
    the B-frame DTS re-key guard it sits next to. The gap: samples whose decode
    key lies in
        [incoming.decodeKey, erasedSamples.decodeOrder().begin()->first)
    survive both step 1.14's sync cascade (which uses findSampleAfterDecodeKey
    = upper_bound, strictly greater) and step 1.15's main dependents sweep
    (which starts at erasedSamples.begin()). The block originally added in
    205636@main covered this gap, but its range was too wide (lower_bound
    inclusive on both ends), so it also erased legitimate next-GOP samples
    when the cascade had deliberately broken early on a sync whose pts was
    greater than the incoming sample's.

Spec issues were lodged as 
https://github.com/w3c/media-source/issues/374
and https://github.com/w3c/media-source/issues/375

[Radar WebKit Bug Importer]

<rdar://problem/176971800>

[Jean-Yves Avenard [:jya]]

Pull request: https://github.com/WebKit/WebKit/pull/64888

[EWS]

Committed 313296@main (9d7e16e79a52): <https://commits.webkit.org/313296@main>

Reviewed commits have been landed. Closing PR #64888 and removing active labels.