fix(TPCUtils): Re-use the existing recv-only transceiver for secondary video tracks.

Fixes cases where SS fails in multi-stream mode with p2p->jvb->p2p switching.

modules/RTC/TPCUtils.js

@@ -376,7 +376,10 @@ export class TPCUtils {
             transceiver = this.pc.peerconnection.getTransceivers().find(
                 t => t.receiver.track.kind === mediaType
                 && t.direction === MediaDirection.RECVONLY
-                && t.currentDirection === MediaDirection.INACTIVE);
+
+                // Re-use any existing recvonly transceiver (if available) for p2p case.
+                && ((this.pc.isP2P && t.currentDirection === MediaDirection.RECVONLY)
+                    || t.currentDirection === MediaDirection.INACTIVE));
 
         // For mute/unmute operations, find the transceiver based on the track index in the source name if present,
         // otherwise it is assumed to be the first local track that was added to the peerconnection.
@@ -387,17 +390,20 @@ export class TPCUtils {
             if (sourceName) {
                 const trackIndex = Number(sourceName.split('-')[1].substring(1));
 
-                if (trackIndex) {
-                    transceiver = this.pc.isP2P
-                        ? this.pc.peerconnection.getTransceivers()
-                            .filter(t => t.receiver.track.kind === mediaType)[trackIndex]
-                        : this.pc.peerconnection.getTransceivers()
+                if (this.pc.isP2P) {
+                    transceiver = this.pc.peerconnection.getTransceivers()
+                        .filter(t => t.receiver.track.kind === mediaType)[trackIndex];
+                } else if (oldTrack) {
+                    const transceiverMid = this.pc._localTrackTransceiverMids.get(oldTrack.rtcId);
+
+                    transceiver = this.pc.peerconnection.getTransceivers().find(t => t.mid === transceiverMid);
+                } else if (trackIndex) {
+                    transceiver = this.pc.peerconnection.getTransceivers()
                             .filter(t => t.receiver.track.kind === mediaType
                                 && t.direction !== MediaDirection.RECVONLY)[trackIndex];
                 }
             }
         }
-
         if (!transceiver) {
             return Promise.reject(new Error('replace track failed'));
         }

modules/xmpp/JingleSessionPC.js

@@ -636,7 +636,6 @@ export default class JingleSessionPC extends JingleSession {
             const remoteDescription = this.peerconnection.remoteDescription;
 
             if (this.usesUnifiedPlan
-                && !this.isP2P
                 && state === 'stable'
                 && remoteDescription
                 && typeof remoteDescription.sdp === 'string') {
@@ -2122,10 +2121,18 @@ export default class JingleSessionPC extends JingleSession {
         const workFunction = finishedCallback => {
             const oldLocalSDP = new SDP(this.peerconnection.localDescription.sdp);
             const remoteSdp = new SDP(this.peerconnection.peerconnection.remoteDescription.sdp);
-
-            // Add transceivers by adding a new mline in the remote description for each track.
+            const recvOnlyTransceiver = this.peerconnection.peerconnection.getTransceivers()
+                    .find(t => t.receiver.track.kind === MediaType.VIDEO
+                        && t.direction === MediaDirection.RECVONLY
+                        && t.currentDirection === MediaDirection.RECVONLY);
+
+            // Add transceivers by adding a new mline in the remote description for each track. Do not create a new
+            // m-line if a recv-only transceiver exists in the p2p case. The new track will be attached to the
+            // existing one in that case.
             for (const track of localTracks) {
-                remoteSdp.addMlineForNewLocalSource(track.getType());
+                if (!this.isP2P || !recvOnlyTransceiver) {
+                    remoteSdp.addMlineForNewLocalSource(track.getType());
+                }
             }
 
             const remoteDescription = new RTCSessionDescription({