Audio playout never starts after the first PeerConnection is closed when one AudioDeviceModule is reused

Summary

When a single PeerConnectionFactory + AudioDeviceModule is reused across connections, the
first connection works normally, but every subsequent connection waits exactly 10 seconds
inside setRemoteDescription and never plays audio:

ERROR (audio_device_pulse_linux.cc:1199): failed to activate playout

Environment

• Ubuntu 22.04 / ARM64 (aarch64), PulseAudio backend
• Java 25
• webrtc-java 0.18.0, 0.19.0 and 0.20.0 all reproduce
• webrtc-java 0.14.0 – 0.17.0 do not reproduce
• Media: audio only, RECV_ONLY transceiver, remote Answer over HTTPS signaling

Steps to reproduce

AudioDeviceModule adm = new AudioDeviceModule();   // platform default (PulseAudio)
adm.setPlayoutDevice(playoutDevice);
adm.initPlayout();
PeerConnectionFactory factory = new PeerConnectionFactory(adm);

for (int i = 0; i < 5; i++) {
    RTCPeerConnection pc = factory.createPeerConnection(new RTCConfiguration(), observer);

    RTCRtpTransceiverInit init = new RTCRtpTransceiverInit();
    init.direction = RTCRtpTransceiverDirection.RECV_ONLY;
    AudioTrackSource src = factory.createAudioSource(new AudioOptions());
    AudioTrack track = factory.createAudioTrack("dummy", src);
    pc.addTransceiver(track, init);

    // createOffer -> setLocalDescription -> exchange SDP via signaling
    // -> setRemoteDescription(answer)
    // The 1st iteration completes in a few ms, every later one takes 10 s.

    pc.close();
}

Observed timings (same process, same factory)

While the 10 s delay is pending, close() on that connection also blocks — it waits for the
remaining part of the timeout (measured 6239 ms and 7001 ms).

Root cause

1. Fix JNI memory-safety and thread-safety bugs in the native bridge #283 (0.18.0) made RTCPeerConnection.close() call pc->Release().
2. Destroying the last PeerConnection drops ConnectionContext::media_engine_reference_count_
   to 0, so ReleaseMediaEngine() calls media_engine_->Terminate() →
   WebRtcVoiceEngine::Terminate() → adm()->Terminate().
3. The next PeerConnection triggers AddRefMediaEngine() → media_engine_->Init() →
   WebRtcVoiceEngine::Init() → adm_helpers::Init(adm) → adm->Init().
4. AudioDeviceLinuxPulse::Init() re-spawns webrtc_audio_module_play_thread but never resets
   quit_, which Terminate() set to true. quit_ is written in only two places: the
   constructor (false) and Terminate() (true).
5. PlayThreadProcess() hits if (quit_) return false; and the thread exits. _playStartEvent is
   therefore never signalled, and StartPlayout() waits its full 10 s timeout, logs
   failed to activate playout and returns -1.

Before 0.18.0, close() never released the PeerConnection, so the media engine reference count
never reached 0 and the ADM was never terminated — which is why 0.14.0 – 0.17.0 accidentally avoid
the bug (at the cost of leaking the PeerConnection on every cycle).

Evidence

• audio_device_pulse_linux.cc:172: failed to open X display is logged from inside
  AudioDeviceLinuxPulse::Init(). It appears only from the 2nd connection on, proving the ADM
  was terminated and re-initialised between connections.
• Can't query latency is only emitted once a play stream exists. It appears only on the 1st
  connection.
• Line numbers in the native log (172, 1199, 1872) match branch-heads/7977 exactly, i.e. the
  library was built from the pinned WebRTC revision.

Workaround (verified on device, 5 consecutive connections)

Keep one idle PeerConnection alive per PeerConnectionFactory:

this.keepAlivePeerConnection =
        factory.createPeerConnection(new RTCConfiguration(), observer);  // never closed until shutdown

With the reference count pinned above 0, Terminate() never runs, the ADM stays initialised, and
every connection applies the Answer in 3–8 ms with playback starting normally.

Note: the keep-alive connection also prevents adm_helpers::Init() from resetting the playout
device to index 0 on every re-initialisation.

Suggested fix

• libwebrtc: reset quit_ = false in AudioDeviceLinuxPulse::Init().
• webrtc-java: keep the media engine referenced for the factory's lifetime, or avoid
  adm->Terminate() when the last PeerConnection goes away.

The above content was generated by AI based on AI analysis and hands-on testing. It is provided for reference only, and we hope it can be helpful to the project.