Populate RTPTransceiver stopped flag

There is use of `t.stopped` in `peerconnection.go`, but that
flag is not set/used in `RTPTransceiver` at all. This PR
sets that when a transceiver is stopped and provides an API
to check if a tranceiver is stopped.

peerconnection.go

@@ -379,15 +379,15 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 	for _, t := range pc.rtpTransceivers {
 		// https://www.w3.org/TR/webrtc/#dfn-update-the-negotiation-needed-flag
 		// Step 5.1
-		// if t.stopping && !t.stopped {
+		// if t.stopping && !t.IsStopped() {
 		// 	return true
 		// }
 		m := getByMid(t.Mid(), localDesc)
 		// Step 5.2
-		if !t.stopped && m == nil {
+		if !t.IsStopped() && m == nil {
 			return true
 		}
-		if !t.stopped && m != nil {
+		if !t.IsStopped() && m != nil {
 			// Step 5.3.1
 			if t.Direction() == RTPTransceiverDirectionSendrecv || t.Direction() == RTPTransceiverDirectionSendonly {
 				descMsid, okMsid := m.Attribute(sdp.AttrKeyMsid)
@@ -416,7 +416,7 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 			}
 		}
 		// Step 5.4
-		if t.stopped && t.Mid() != "" {
+		if t.IsStopped() && t.Mid() != "" {
 			if getByMid(t.Mid(), localDesc) != nil || getByMid(t.Mid(), remoteDesc) != nil {
 				return true
 			}
@@ -1765,7 +1765,7 @@ func (pc *PeerConnection) AddTrack(track TrackLocal) (*RTPSender, error) {
 		// transceiver can be reused only if it's currentDirection never be sendrecv or sendonly.
 		// But that will cause sdp inflate. So we only check currentDirection's current value,
 		// that's worked for all browsers.
-		if !t.stopped && t.kind == track.Kind() && t.Sender() == nil &&
+		if !t.IsStopped() && t.kind == track.Kind() && t.Sender() == nil &&
 			!(currentDirection == RTPTransceiverDirectionSendrecv || currentDirection == RTPTransceiverDirectionSendonly) {
 			sender, err := pc.api.NewRTPSender(track, pc.dtlsTransport)
 			if err == nil {
@@ -2025,9 +2025,7 @@ func (pc *PeerConnection) Close() error {
 	// https://www.w3.org/TR/webrtc/#dom-rtcpeerconnection-close (step #4)
 	pc.mu.Lock()
 	for _, t := range pc.rtpTransceivers {
-		if !t.stopped {
-			closeErrs = append(closeErrs, t.Stop())
-		}
+		closeErrs = append(closeErrs, t.Stop())
 	}
 	pc.mu.Unlock()
 

rtptransceiver.go

@@ -21,8 +21,8 @@ type RTPTransceiver struct {
 
 	codecs []RTPCodecParameters // User provided codecs via SetCodecPreferences
 
-	stopped bool
-	kind    RTPCodecType
+	isStopped *atomicBool
+	kind      RTPCodecType
 
 	api *API
 	mu  sync.RWMutex
@@ -35,7 +35,7 @@ func newRTPTransceiver(
 	kind RTPCodecType,
 	api *API,
 ) *RTPTransceiver {
-	t := &RTPTransceiver{kind: kind, api: api}
+	t := &RTPTransceiver{kind: kind, api: api, isStopped: &atomicBool{}}
 	t.setReceiver(receiver)
 	t.setSender(sender)
 	t.setDirection(direction)
@@ -150,6 +150,10 @@ func (t *RTPTransceiver) Direction() RTPTransceiverDirection {
 
 // Stop irreversibly stops the RTPTransceiver
 func (t *RTPTransceiver) Stop() error {
+	if t.isStopped.swap(true) {
+		return nil
+	}
+
 	if sender := t.Sender(); sender != nil {
 		if err := sender.Stop(); err != nil {
 			return err
@@ -166,6 +170,11 @@ func (t *RTPTransceiver) Stop() error {
 	return nil
 }
 
+// IsStopped returns true if RTPTransceiver is stopped, else returns false
+func (t *RTPTransceiver) IsStopped() bool {
+	return t.isStopped.get()
+}
+
 func (t *RTPTransceiver) setReceiver(r *RTPReceiver) {
 	if r != nil {
 		r.setRTPTransceiver(t)