WebRTC: Track.onOpen() is called, but track is not open

Question

Using libdatachannel, I establish a PeerConnection between two partners using some out-of-band signaling. I can create DataChannels and send data between them successfully (with peer_connection->createDataChannel()).

But I am struggling to do the same with a video track. Here is what I do:

• I create a track from one partner:

rtc::Description::Video media("myvideo", rtc::Description::Direction::RecvOnly);
media.addH264Codec(96);
media.setBitrate(3000);

auto track = peer_connection->addTrack(media);
peer_connection->setLocalDescription();

Note how I call setLocalDescription() after addTrack, so that libdatachannel will negotiate the track and I don't need to send an SDP out of band (at least that's my understanding).

• From the other partner, I check the onTrack() callback:

peer_connection->onTrack([this](const std::shared_ptr<rtc::Track>& track) {
    track->onClosed([]() {
        std::cout << "onClosed" << std::endl;
    });
    track->onOpen([]() {
        std::cout << "onOpen" << std::endl;

        if (track->isOpen()) {
            std::cout << "track is open" << std::endl;
        } else {
            std::cout << "track is not open" << std::endl;
        }
    });
}

What I observe is that onTrack is called, and the track has the mid I set from the sending side ("myvideo"). But right in the onOpen() callback, the call to track->isOpen() says that "track is not open".

If I try to use the track later (with e.g. track->send()), if fails with a SIGABRT:

terminate called after throwing an instance of 'std::runtime_error'
  what():  Track is closed
Signal: SIGABRT (Aborted)

So somehow it feels like my track was never properly open in the first place. Though I would expect track->onOpen() to be called... when the track is open.

Am I missing something?

Answer 1

That was a bug in libdatachannel.