Don't close writer channel at client shutdown.

Closing the channel could cause a panic if a c.write was pending.
Instead, ensure we send a closeMessage to the writer, and cause the
writer to exit when it receives a closeMessage.

webclient.go

@@ -676,6 +676,7 @@ func startClient(conn *websocket.Conn) (err error) {
 	defer func() {
 		if isWSNormalError(err) {
 			err = nil
+			c.close(nil)
 		} else {
 			m, e := errorToWSCloseMessage(err)
 			if m != "" {
@@ -685,13 +686,8 @@ func startClient(conn *websocket.Conn) (err error) {
 					Value: m,
 				})
 			}
-			select {
-			case c.writeCh <- closeMessage{e}:
-			case <-c.writerDone:
-			}
+			c.close(e)
 		}
-		close(c.writeCh)
-		c.writeCh = nil
 	}()
 
 	c.writerDone = make(chan struct{})
@@ -1153,10 +1149,13 @@ func clientWriter(conn *websocket.Conn, ch <-chan interface{}, done chan<- struc
 				return
 			}
 		case closeMessage:
-			err := conn.WriteMessage(websocket.CloseMessage, m.data)
-			if err != nil {
-				return
+			if m.data != nil {
+				conn.WriteMessage(
+					websocket.CloseMessage,
+					m.data,
+				)
 			}
+			return
 		default:
 			log.Printf("clientWiter: unexpected message %T", m)
 			return
@@ -1185,6 +1184,15 @@ func (c *webClient) write(m clientMessage) error {
 	}
 }
 
+func (c *webClient) close(data []byte) error {
+	select {
+	case c.writeCh <- closeMessage{data}:
+		return nil
+	case <-c.writerDone:
+		return ErrWriterDead
+	}
+}
+
 func (c *webClient) error(err error) error {
 	switch e := err.(type) {
 	case userError: