Fix datarace for WebRTCPeer.closed

The race condition occurs because concurrent goroutines are intermixing
reads and writes of `WebRTCPeer.closed`.

Spotted when integrating Snowflake inside OONI in
https://github.com/ooni/probe-cli/pull/373.

client/lib/lib_test.go

@@ -33,7 +33,7 @@ type FakeDialer struct {
 
 func (w FakeDialer) Catch() (*WebRTCPeer, error) {
 	fmt.Println("Caught a dummy snowflake.")
-	return &WebRTCPeer{}, nil
+	return &WebRTCPeer{closed: make(chan struct{})}, nil
 }
 
 func (w FakeDialer) GetMax() int {
@@ -97,7 +97,7 @@ func TestSnowflakeClient(t *testing.T) {
 			So(err, ShouldNotBeNil)
 			So(p.Count(), ShouldEqual, c)
 
-			// But popping and closing allows it to continue.
+			// But popping allows it to continue.
 			s := p.Pop()
 			s.Close()
 			So(s, ShouldNotBeNil)
@@ -127,7 +127,7 @@ func TestSnowflakeClient(t *testing.T) {
 			cnt := 5
 			p, _ := NewPeers(FakeDialer{max: cnt})
 			for i := 0; i < cnt; i++ {
-				p.activePeers.PushBack(&WebRTCPeer{})
+				p.activePeers.PushBack(&WebRTCPeer{closed: make(chan struct{})})
 			}
 			So(p.Count(), ShouldEqual, cnt)
 			p.End()

client/lib/peers.go

@@ -83,7 +83,7 @@ func (p *Peers) Pop() *WebRTCPeer {
 		if !ok {
 			return nil
 		}
-		if snowflake.closed {
+		if snowflake.Closed() {
 			continue
 		}
 		// Set to use the same rate-limited traffic logger to keep consistency.
@@ -110,7 +110,7 @@ func (p *Peers) purgeClosedPeers() {
 		next := e.Next()
 		conn := e.Value.(*WebRTCPeer)
 		// Purge those marked for deletion.
-		if conn.closed {
+		if conn.Closed() {
 			p.activePeers.Remove(e)
 		}
 		e = next

client/lib/webrtc.go

@@ -28,7 +28,7 @@ type WebRTCPeer struct {
 	lastReceive time.Time
 
 	open   chan struct{} // Channel to notify when datachannel opens
-	closed bool
+	closed chan struct{}
 
 	once sync.Once // Synchronization for PeerConnection destruction
 
@@ -46,6 +46,7 @@ func NewWebRTCPeer(config *webrtc.Configuration,
 		}
 		connection.id = "snowflake-" + hex.EncodeToString(buf[:])
 	}
+	connection.closed = make(chan struct{})
 
 	// Override with something that's not NullLogger to have real logging.
 	connection.BytesLogger = &BytesNullLogger{}
@@ -78,9 +79,19 @@ func (c *WebRTCPeer) Write(b []byte) (int, error) {
 	return len(b), nil
 }
 
+//Returns a boolean indicated whether the peer is closed
+func (c *WebRTCPeer) Closed() bool {
+	select {
+	case <-c.closed:
+		return true
+	default:
+	}
+	return false
+}
+
 func (c *WebRTCPeer) Close() error {
 	c.once.Do(func() {
-		c.closed = true
+		close(c.closed)
 		c.cleanup()
 		log.Printf("WebRTC: Closing")
 	})
@@ -95,9 +106,6 @@ func (c *WebRTCPeer) checkForStaleness() {
 	c.lastReceive = time.Now()
 	c.mu.Unlock()
 	for {
-		if c.closed {
-			return
-		}
 		c.mu.Lock()
 		lastReceive := c.lastReceive
 		c.mu.Unlock()
@@ -107,7 +115,11 @@ func (c *WebRTCPeer) checkForStaleness() {
 			c.Close()
 			return
 		}
-		<-time.After(time.Second)
+		select {
+		case <-c.closed:
+			return
+		case <-time.After(time.Second):
+		}
 	}
 }