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* Prevent missing entry in pending tracks Problem: -------- A track received via signalling request `AddTrack` is stored in `pendingTracks` of participant. A MediaTrack is created when `onTrack` fires after `SetRemoteDescription`. At that time, pending tracks are searched to find a matching track and look up an already published MediaTrack. This is because `onTrack` fires once for every layer of Simulcast and MediaTrack abstraction is for a media track and not one for every layer of Simulcast track. To accomplish that, pending tracks are cleaned up 5 seconds after the MediaTrack is created. The theory there is that `onTrack` will fire on all layers within 5 seconds. But, have observed several instances on my slow machine of that firing after 5 seconds which results in the search failing and we end up creating a new MediaTrack. The above is probably the reason (I am guessing though) for subscriber PC having an extra m-line some times. Considered fix: --------------- One possible option is to increase that 5 seconds timeout to a very large value. But, it has another issue. `getPendingTrack` is given the track id which comes in the SDP. Entries are added to the pending tracks using track id received via the `AddTrack` signalling message. And those two need not be the same. Especially Firefox has different ids every time. Not sure if that is something we do on client side which causes that, but it does look like a real possibility. To handle that case, `getPendingTrack` looks up tracks by media kind (audio/video) if the look up by SDP client id fails. Here, it is possible that there are two pending tracks of type video (think camera and screen sharing as an example) and looking up by kind might end up picking the wrong one. Fix: ---- Store the signalled client id and SDP client id in the MediaTrack and look up the published tracks by SDP client id for a track match. If there is no match, create a new MediaTrack and add it to publishedTracks and delete the corresponding pending track all within the lock (yeah not great to have a lot of code within the lock, but this is probably worth it to have the correctness). This does solve the issue of deferred pending track removal causing issues. However, note that kind based look up may do some switching. In a scenario where there are two pending tracks of kind video and the look up has to rely on kind, it is possible that signalCid and sdpCid get cross matched (i. e. client might have sent a signalCid for a Simulcast track, but during kind based look up it gets assigned to a non-simulcast track). I think that is okay as there is no strong correlation between the two. Testing: -------- - Connect from Chrome, Firefox (both orders, Chrome joining first, Firefox joining first) and ensure that media subscriptions and publishing are correct - Ensure that DTX munging works properly too. * Fix tests Add back adding track to publishedTracks for testing purposes. * Add a test to check case of `AddTrack` rejecting already published track * Remove debug. * Address PR comments - do not need to return SDP cid from `getPendingTracks`.