Ensure that the Churn candidates' MID matches the SDP.

Fixes UWNetworksLab/uProxy-p2p#2143

Author: bemasc

src/churn/churn.ts

@@ -31,6 +31,20 @@ export var filterCandidatesFromSdp = (sdp:string) : string => {
   }).join('\n');
 }
 
+export var getMidFromSdp = (sdp:string) : string => {
+  var mids :string[] = [];
+  sdp.split('\n').forEach((s) => {
+    var match = s.match(/^a=mid:(\S+)\s*$/);
+    if (match) {
+      mids.push(match[1]);
+    }
+  });
+  if (mids.length !== 1) {
+    throw new Error('Expected 1 mid, but there are actually ' + mids.length);
+  }
+  return mids[0];
+}
+
 export interface NatPair {
   internal: net.Endpoint;
   external: net.Endpoint;
@@ -137,13 +151,17 @@ export class Connection implements peerconnection.PeerConnection<ChurnSignalling
   // Number of instances created, for logging purposes.
   private static id_ = 0;
 
+  // The "mid" 
+  private mid_ :string;
+
   // Maximum time to spend gathering ICE candidates.
   // We cap this so that slow STUN servers, in the absence
   // of trickle ICE, don't make the user wait unnecessarily.
   private static PROBE_TIMEOUT_MS_ = 3000;
 
   public peerOpenedChannelQueue :handler.QueueHandler<peerconnection.DataChannel, void>;
   public signalForPeerQueue :handler.Queue<ChurnSignallingMessage, void>;
+  private candidateQueue_ :handler.Queue<Candidate, void>;
 
   public onceConnected :Promise<void>;
   public onceClosed :Promise<void>;
@@ -196,6 +214,7 @@ export class Connection implements peerconnection.PeerConnection<ChurnSignalling
     Connection.id_++;
 
     this.signalForPeerQueue = new handler.Queue<ChurnSignallingMessage,void>();
+    this.candidateQueue_ = new handler.Queue<Candidate,void>();
 
     this.configureObfuscatedConnection_();
     // When the probe connection is complete, it will trigger the
@@ -246,13 +265,10 @@ export class Connection implements peerconnection.PeerConnection<ChurnSignalling
             }
           }
 
-          // It's immediately safe to send each candidate to the remote peer,
-          // because the remote peer will retry for several seconds, and the
-          // probe connection will not respond to any pings because it doesn't
-          // have a remote description.
-          this.signalForPeerQueue.handle({
-            webrtcMessage: message
-          });
+          // We enqueue this candidate for transmission to the remote peer.
+          // This queue will start draining once an Offer is available, so
+          // that we can make sure that the mid matches.
+          this.candidateQueue_.handle(c);
 
           // We cannot rebind any local ports until the probe connection has
           // released them, so we have to store them and wait until probing is
@@ -281,6 +297,20 @@ export class Connection implements peerconnection.PeerConnection<ChurnSignalling
     this.probeConnection_.negotiateConnection();
   }
 
+  private candidateQueueHandler_ = (candidate:Candidate) : Promise<void> => {
+    // The candidate was generated by the probe connection, which might have a
+    // different mid from the obfuscated connection.  Make sure that the mid
+    // matches before we transmit the candidate.
+    candidate.sdpMid = this.mid_;
+
+    return this.signalForPeerQueue.handle({
+      webrtcMessage: {
+        type: signals.Type.CANDIDATE,
+        candidate: candidate.toRTCIceCandidate()
+      }
+    });
+  }
+
   private processProbeCandidates_ = (candidates:Candidate[]) => {
     candidates.forEach((c) => {
       this.pipe_.bindLocal(c.getLocalEndpoint());
@@ -367,6 +397,11 @@ export class Connection implements peerconnection.PeerConnection<ChurnSignalling
           webrtcMessage: message
         };
         this.signalForPeerQueue.handle(churnSignal);
+
+        // We need to know the MID so that we can rewrite candidates to
+        // have the correct MID before transmitting them.
+        this.mid_ = getMidFromSdp(message.description.sdp);
+        this.candidateQueue_.setHandler(this.candidateQueueHandler_);
       } else if (message.type === signals.Type.CANDIDATE) {
         // This will tell us on which port webrtc is operating.
         // There's no need to send this to the peer because it can