Attempting to ship "WebRTC-AllowDisablingLegacyScalability" revealed a
DCHECK that happens when negotiating 3 VP9 streams prior to the
setParameters() call:
1. By default, `scalability_mode` is missing, so those 3 streams
defaulted to legacy SVC, meaning only a single stream is used.
2. Then, setParameters() was called to make
`encodings[0].scalability_mode = "L2T2_KEY"` and
`encodings[1-2].active = false`. The inactive streams were just
dummies and never expected to exist.
Without simulcast support this is OK, because both 1) and 2) are
interpreted to have a single stream. But with simulcast support, 1) is
interpreted as single stream and 2) as three streams (1 active, 2
inactive). This should be roughly the same setup, but our code treats
them differently.
The DCHECK crash was a mismatch in number of streams in one of the
layers.
The fix is to re-create the streams when the number of streams change
for this reason. The new test revealed other issues and fixes too:
- Support for multiple spatial layers (e.g. "L2T2_KEY") when multiple
encodings exist but only one encoding is active.
- Allow inactive layers not to have a scalability mode set.
A laundry list (https://crbug.com/webrtc/15028) has been created to
update known places doing "if streams == 1" that need to do "if
active streams == 1" instead.
Credit:
The RecreateWebRtcStream() fix is based on eshr@'s POC from
https://webrtc-review.googlesource.com/c/src/+/298565.
Bug: webrtc:15016
Change-Id: I909a3f83a4ef53562894549ade0a870b208cec7e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/298443
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/main@{#39651}
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