Queue negotiation if not in stable state.

If we send two offers back to back, the second answer will arive in
stable state, which will confuse us.  Instead, queue the second offer.

rtpconn/rtpconn.go

@@ -129,6 +129,12 @@ func (down *rtpDownTrack) SetCname(cname string) {
 	down.cname.Store(cname)
 }
 
+const (
+	negotiationUnneeded = iota
+	negotiationNeeded
+	negotiationRestartIce
+)
+
 type rtpDownConnection struct {
 	id                string
 	pc                *webrtc.PeerConnection
@@ -136,6 +142,7 @@ type rtpDownConnection struct {
 	tracks            []*rtpDownTrack
 	maxREMBBitrate    *bitrate
 	iceCandidates     []*webrtc.ICECandidateInit
+	negotiationNeeded int
 }
 
 func newDownConn(c group.Client, id string, remote conn.Up) (*rtpDownConnection, error) {

rtpconn/webclient.go

@@ -423,6 +423,18 @@ func addDownTrack(c *webClient, conn *rtpDownConnection, remoteTrack conn.UpTrac
 }
 
 func negotiate(c *webClient, down *rtpDownConnection, renegotiate, restartIce bool) error {
+	if down.pc.SignalingState() == webrtc.SignalingStateHaveLocalOffer {
+		// avoid sending multiple offers back-to-back
+		if restartIce {
+			down.negotiationNeeded = negotiationRestartIce
+		} else if down.negotiationNeeded == negotiationUnneeded {
+			down.negotiationNeeded = negotiationNeeded
+		}
+		return nil
+	}
+
+	down.negotiationNeeded = negotiationUnneeded
+
 	options := webrtc.OfferOptions{ICERestart: restartIce}
 	offer, err := down.pc.CreateOffer(&options)
 	if err != nil {
@@ -808,6 +820,9 @@ func clientLoop(c *webClient, ws *websocket.Conn) error {
 				if down != nil {
 					err = negotiate(c, down, false, false)
 					if err != nil {
+						log.Printf(
+							"Negotiation failed: %v",
+							err)
 						delDownConn(c, down.id)
 						c.error(group.UserError(
 							"Negotiation failed",
@@ -1154,13 +1169,26 @@ func handleClientMessage(c *webClient, m clientMessage) error {
 			}
 			return failDownConnection(c, m.Id, message)
 		}
+		down := getDownConn(c, m.Id)
+		if down.negotiationNeeded > negotiationUnneeded {
+			err := negotiate(
+				c, down, true,
+				down.negotiationNeeded == negotiationRestartIce,
+			)
+			if err != nil {
+				return failDownConnection(
+					c, m.Id, "negotiation failed",
+				)
+			}
+		}
 	case "renegotiate":
 		down := getDownConn(c, m.Id)
 		if down != nil {
 			err := negotiate(c, down, true, true)
 			if err != nil {
-				return failDownConnection(c, m.Id,
+				return failDownConnection(
-					"renegotiation failed")
+					c, m.Id, "renegotiation failed",
+				)
 			}
 		} else {
 			log.Printf("Trying to renegotiate unknown connection")