Provide ping handler that responds when client blocked

The websocket only handles responding to pings when reading messages.  If the client is blocked handling incoming messages and does not read additional messages for some time, websocker keep-alive pings are not responded to.  This may lead to the server dropping the websocket connection thinking that the client is dead.  This PR provides a ping handler that answers pings even when the client if not reading messages.

Addresses issue #245

transport/websocketpeer.go

@@ -52,6 +52,7 @@ type WebsocketConnection interface {
 	WriteMessage(messageType int, data []byte) error
 	ReadMessage() (messageType int, p []byte, err error)
 	SetPongHandler(h func(appData string) error)
+	SetPingHandler(h func(appData string) error)
 	Subprotocol() string
 }
 
@@ -255,6 +256,15 @@ func (w *websocketPeer) sendHandler() {
 	defer close(w.writerDone)
 	defer w.cancelSender()
 
+	pongs := make(chan string, 1) // capacity must be >= 1
+	w.conn.SetPingHandler(func(m string) error {
+		select {
+		case pongs <- m:
+		default:
+		}
+		return nil
+	})
+
 sendLoop:
 	for {
 		select {
@@ -271,6 +281,11 @@ sendLoop:
 				}
 				return
 			}
+		case m := <-pongs:
+			err := w.conn.WriteMessage(websocket.PongMessage, []byte(m))
+			if err != nil {
+				w.log.Print(err)
+			}
 		case <-w.ctxSender.Done():
 			return
 		}
@@ -281,6 +296,15 @@ func (w *websocketPeer) sendHandlerKeepAlive(keepAlive time.Duration) {
 	defer close(w.writerDone)
 	defer w.cancelSender()
 
+	pongs := make(chan string, 1) // capacity must be >= 1
+	w.conn.SetPingHandler(func(m string) error {
+		select {
+		case pongs <- m:
+		default:
+		}
+		return nil
+	})
+
 	var pendingPongs int32
 	w.conn.SetPongHandler(func(msg string) error {
 		// Any response resets counter.
@@ -322,6 +346,11 @@ recvLoop:
 				return
 			}
 			atomic.AddInt32(&pendingPongs, 1)
+		case m := <-pongs:
+			err := w.conn.WriteMessage(websocket.PongMessage, []byte(m))
+			if err != nil {
+				w.log.Print(err)
+			}
 		case <-senderDone:
 			return
 		}