Webrtcbin - recipients stop receiving video when resolution changes

Trey Hutcheson trey.hutcheson at gmail.com
Mon Aug 31 23:20:20 UTC 2020

I am currently testing an application that runs on 1.16 against 1.18 to see
what breaks. This scenario is as follows:
* a pipeline with four bins - one per conference participant
* one "sender" bin; this bin contains webrtcbin, and the remote peer
provides vp8 video, sendonly
* a "recorder" bin. This bin tees the vp8 stream from the video sender bin
to a webmmux container and filesink.
* two video receiver bins (these are the viewers). These bins also contain
a webrtcbin element. These bins also tee the original vp8 video stream
directly (re-payloading) to webrtcbin.

This composition works just fine and N number of receiving endpoints can
view the video, and the video is recorded to a file. Great.

However, sometimes the video resolution changes. The webmmux container
cannot handle changing caps, so we had to jump through a few hoops. I
monitor for a change in resolution, and when I see it, I install a blocking
pad probe on the upstream T. In the pad block, I pause the recorder bin,
remove the old webmmux and filesink elements, create new elements to a new
file, unpause the bin, then remove the block. That works just lovely in
1.16. A new recording file is created every time the video changes, and it
happens completely transparently to the viewers.

When tested against 1.18, the stuff with the blocking probe and the
redirecting to a new file continues to work. However, now the viewing peers
stop receiving video (audio continues to flow). I've cranked up GST_DEBUG
but I don't see anything helpful.

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