Stop using WaitGroup to detect finished message processing.

This causes flaky races if "Wait" and "Add" are being used from different
goroutines.

client.go

@@ -106,7 +106,7 @@ type Client struct {
 	mu sync.Mutex
 
 	closeChan         chan bool
-	messagesDone      sync.WaitGroup
+	messagesDone      chan bool
 	messageChan       chan *bytes.Buffer
 	messageProcessing uint32
 
@@ -125,14 +125,16 @@ func NewClient(conn *websocket.Conn, remoteAddress string, agent string) (*Clien
 	if agent == "" {
 		agent = "unknown user agent"
 	}
+
 	client := &Client{
 		conn:   conn,
 		addr:   remoteAddress,
 		agent:  agent,
 		logRTT: true,
 
-		closeChan:   make(chan bool, 1),
-		messageChan: make(chan *bytes.Buffer, 16),
+		closeChan:    make(chan bool, 1),
+		messageChan:  make(chan *bytes.Buffer, 16),
+		messagesDone: make(chan bool, 1),
 
 		OnLookupCountry:   func(client *Client) string { return unknownCountry },
 		OnClosed:          func(client *Client) {},
@@ -207,7 +209,7 @@ func (c *Client) Close() {
 
 func (c *Client) doClose() {
 	c.closeChan <- true
-	c.messagesDone.Wait()
+	<-c.messagesDone
 
 	c.OnClosed(c)
 	c.SetSession(nil)
@@ -334,25 +336,24 @@ func (c *Client) ReadPump() {
 			break
 		}
 
-		c.messagesDone.Add(1)
 		c.messageChan <- decodeBuffer
 	}
 }
 
 func (c *Client) processMessages() {
+	atomic.StoreUint32(&c.messageProcessing, 1)
 	for {
 		buffer := <-c.messageChan
 		if buffer == nil {
 			break
 		}
 
-		atomic.StoreUint32(&c.messageProcessing, 1)
 		c.OnMessageReceived(c, buffer.Bytes())
-		atomic.StoreUint32(&c.messageProcessing, 0)
-		c.messagesDone.Done()
 		bufferPool.Put(buffer)
 	}
+	atomic.StoreUint32(&c.messageProcessing, 0)
 
+	c.messagesDone <- true
 	if atomic.LoadUint32(&c.closed) == 2 {
 		c.doClose()
 	}