Fix track creation synchronization

src/impl/peerconnection.cpp

@@ -751,6 +751,8 @@ void PeerConnection::remoteCloseDataChannels() {
 }
 
 shared_ptr<Track> PeerConnection::emplaceTrack(Description::Media description) {
+	std::unique_lock lock(mTracksMutex); // we are going to emplace
+
 #if !RTC_ENABLE_MEDIA
 	// No media support, mark as removed
 	PLOG_WARNING << "Tracks are disabled (not compiled with media support)";
@@ -759,10 +761,12 @@ shared_ptr<Track> PeerConnection::emplaceTrack(Description::Media description) {
 
 	shared_ptr<Track> track;
 	if (auto it = mTracks.find(description.mid()); it != mTracks.end())
-		if (track = it->second.lock(); track)
-			track->setDescription(std::move(description));
+		if (auto t = it->second.lock(); t && !t->isClosed())
+			track = std::move(t);
 
-	if (!track) {
+	if (track) {
+		track->setDescription(std::move(description));
+	} else {
 		track = std::make_shared<Track>(weak_from_this(), std::move(description));
 		mTracks.emplace(std::make_pair(track->mid(), track));
 		mTrackLines.emplace_back(track);
@@ -896,7 +900,7 @@ void PeerConnection::processLocalDescription(Description description) {
 				        description.addMedia(std::move(reciprocated));
 			        },
 			        [&](Description::Media *remoteMedia) {
-				        std::shared_lock lock(mTracksMutex);
+				        std::unique_lock lock(mTracksMutex); // we may emplace a track
 				        if (auto it = mTracks.find(remoteMedia->mid()); it != mTracks.end()) {
 					        // Prefer local description
 					        if (auto track = it->second.lock()) {