fix: avoid double RegisterObserver on DataChannel to prevent message loss

In M114, SctpDataChannel uses an ObserverAdapter that relays callbacks
through the signaling thread via SafeTask. When RegisterObserver was
called twice (first from DataChannelObserver's constructor, then from
RTCDataChannel's constructor), the adapter's SetDelegate call could
race with in-flight message delivery tasks on the signaling thread,
causing messages sent immediately after dc.onopen to be silently lost.

The fix removes the initial RegisterObserver from DataChannelObserver,
so registration only happens once in RTCDataChannel's constructor. The
SCTP layer queues any messages that arrive before an observer is
registered, and DeliverQueuedReceivedData() delivers them when
RegisterObserver is called.

Also dispatches the "open" state change event if the channel has
already transitioned to kOpen before the RTCDataChannel wrapper is
constructed, ensuring JS onopen handlers fire.

Author: kevindhanna

src/interfaces/rtc_data_channel.cc

@@ -30,7 +30,9 @@ DataChannelObserver::DataChannelObserver(
     PeerConnectionFactory *factory,
     rtc::scoped_refptr<webrtc::DataChannelInterface> jingleDataChannel)
     : _factory(factory), _jingleDataChannel(std::move(jingleDataChannel)) {
-  _jingleDataChannel->RegisterObserver(this);
+  // Don't register here. Registration happens in RTCDataChannel's constructor,
+  // which avoids a double RegisterObserver that causes message loss through
+  // M114's ObserverAdapter signaling thread relay.
 }
 
 void DataChannelObserver::OnStateChange() {
@@ -72,9 +74,20 @@ RTCDataChannel::RTCDataChannel(const Napi::CallbackInfo &info)
   _jingleDataChannel = observer->_jingleDataChannel;
   _jingleDataChannel->RegisterObserver(this);
 
-  // Re-queue cached observer events
+  // Re-queue any cached observer events (from the window between
+  // OnDataChannel and this constructor).
   requeue(*observer, *this);
 
+  // If the channel already transitioned to open before we registered,
+  // dispatch the open event so JS onopen handlers fire.
+  auto state = _jingleDataChannel->state();
+  if (state == webrtc::DataChannelInterface::kOpen) {
+    Dispatch(
+        Callback1<RTCDataChannel>::Create([state](RTCDataChannel &channel) {
+          RTCDataChannel::HandleStateChange(channel, state);
+        }));
+  }
+
   delete observer;
 
   // NOTE(mroberts): These doesn't actually matter yet.