Fix write lock deadline handling

conn.go

@@ -90,6 +90,10 @@ var ErrCloseSent = errors.New("websocket: close sent")
 // read limit set for the connection.
 var ErrReadLimit = errors.New("websocket: read limit exceeded")
 
+// ErrWriteDeadlineExceeded is returned when a writing to a connection has exceeded
+// the specified deadline
+var ErrWriteDeadlineExceeded = errors.New("websocket: writing deadline exceeded")
+
 // netError satisfies the net Error interface.
 type netError struct {
 	msg       string
@@ -428,7 +432,36 @@ func (c *Conn) read(n int) ([]byte, error) {
 }
 
 func (c *Conn) write(frameType int, deadline time.Time, buf0, buf1 []byte) error {
-	<-c.mu
+	// if user has provided no deadline
+	if deadline.IsZero() {
+		// just wait for the mutex.
+		<-c.mu
+	} else {
+		// see how far the deadline is from now
+		sleepTime := deadline.Sub(time.Now())
+
+		// is the deadline pointing to the future, or the past ?
+		if sleepTime < 0 {
+			// operation timed out already.
+			return ErrWriteDeadlineExceeded
+		}
+
+		select {
+		case <-c.mu:
+			// good, we've got the writing lock.
+
+		// the default case would be taken if the above lock could not be aquired right away.
+		default:
+			// writing lock is currently taken.
+			select {
+			case <-c.mu:
+				// great ! we got the writing lock.
+			case <-time.After(sleepTime):
+				// we were not able to aquire the writing lock.
+				return ErrWriteDeadlineExceeded
+			}
+		}
+	}
 	defer func() { c.mu <- true }()
 
 	c.writeErrMu.Lock()