Safely mutate local description

adding candidates races with negotiation check

peerconnection.go

@@ -320,10 +320,11 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 	// To check if negotiation is needed for connection, perform the following checks:
 	// Skip 1, 2 steps
 	// Step 3
-	pc.mu.RLock()
+	pc.mu.Lock()
+	defer pc.mu.Unlock()
+
 	localDesc := pc.currentLocalDescription
 	remoteDesc := pc.currentRemoteDescription
-	pc.mu.RUnlock()
 
 	if localDesc == nil {
 		return true
@@ -337,9 +338,6 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 		return true
 	}
 
-	pc.mu.Lock()
-	defer pc.mu.Unlock()
-
 	for _, t := range pc.rtpTransceivers {
 		// https://www.w3.org/TR/webrtc/#dfn-update-the-negotiation-needed-flag
 		// Step 5.1
@@ -1843,6 +1841,8 @@ func (pc *PeerConnection) newRTPTransceiver(
 // into the stable state plus any local candidates that have been generated
 // by the ICEAgent since the offer or answer was created.
 func (pc *PeerConnection) CurrentLocalDescription() *SessionDescription {
+	pc.mu.Lock()
+	defer pc.mu.Unlock()
 	return populateLocalCandidates(pc.currentLocalDescription, pc.iceGatherer, pc.ICEGatheringState())
 }
 
@@ -1851,6 +1851,8 @@ func (pc *PeerConnection) CurrentLocalDescription() *SessionDescription {
 // generated by the ICEAgent since the offer or answer was created. If the
 // PeerConnection is in the stable state, the value is null.
 func (pc *PeerConnection) PendingLocalDescription() *SessionDescription {
+	pc.mu.Lock()
+	defer pc.mu.Unlock()
 	return populateLocalCandidates(pc.pendingLocalDescription, pc.iceGatherer, pc.ICEGatheringState())
 }
 

sdp.go

@@ -267,7 +267,8 @@ func populateLocalCandidates(sessionDescription *SessionDescription, i *ICEGathe
 	}
 
 	parsed := sessionDescription.parsed
-	for _, m := range parsed.MediaDescriptions {
+	if len(parsed.MediaDescriptions) > 0 {
+		m := parsed.MediaDescriptions[0]
 		if err = addCandidatesToMediaDescriptions(candidates, m, iceGatheringState); err != nil {
 			return sessionDescription
 		}