Improve Close handshake behaviour

- For JS we ensure we indicate which size initiated the close first from our POV
- For normal Go, concurrent closes block until the first one succeeds instead of returning early

conn.go

@@ -851,6 +851,13 @@ func (c *Conn) realWriteFrame(ctx context.Context, h header, p []byte) (n int, e
 // complete.
 func (c *Conn) Close(code StatusCode, reason string) error {
 	err := c.exportedClose(code, reason, true)
+	var ec errClosing
+	if errors.As(err, &ec) {
+		<-c.closed
+		// We wait until the connection closes.
+		// We use writeClose and not exportedClose to avoid a second failed to marshal close frame error.
+		err = c.writeClose(nil, ec.ce, true)
+	}
 	if err != nil {
 		return fmt.Errorf("failed to close websocket connection: %w", err)
 	}
@@ -878,15 +885,31 @@ func (c *Conn) exportedClose(code StatusCode, reason string, handshake bool) err
 	return c.writeClose(p, fmt.Errorf("sent close: %w", ce), handshake)
 }
 
+type errClosing struct {
+	ce error
+}
+
+func (e errClosing) Error() string {
+	return "already closing connection"
+}
+
 func (c *Conn) writeClose(p []byte, ce error, handshake bool) error {
-	select {
-	case <-c.closed:
-		return fmt.Errorf("tried to close with %v but connection already closed: %w", ce, c.closeErr)
-	default:
+	if c.isClosed() {
+		return fmt.Errorf("tried to close with %q but connection already closed: %w", ce, c.closeErr)
 	}
 
 	if !c.closing.CAS(0, 1) {
-		return fmt.Errorf("another goroutine is closing")
+		// Normally, we would want to wait until the connection is closed,
+		// at least for when a user calls into Close, so we handle that case in
+		// the exported Close function.
+		//
+		// But for internal library usage, we always want to return early, e.g.
+		// if we are performing a close handshake and the peer sends their close frame,
+		// we do not want to block here waiting for c.closed to close because it won't,
+		// at least not until we return since the gorouine that will close it is this one.
+		return errClosing{
+			ce: ce,
+		}
 	}
 
 	// No matter what happens next, close error should be set.

conn_common.go

@@ -234,3 +234,12 @@ func (v *atomicInt64) Increment(delta int64) int64 {
 func (v *atomicInt64) CAS(old, new int64) (swapped bool) {
 	return atomic.CompareAndSwapInt64(&v.v, old, new)
 }
+
+func (c *Conn) isClosed() bool {
+	select {
+	case <-c.closed:
+		return true
+	default:
+		return false
+	}
+}

conn_test.go

@@ -602,7 +602,11 @@ func TestConn(t *testing.T) {
 		{
 			name: "largeControlFrame",
 			server: func(ctx context.Context, c *websocket.Conn) error {
-				_, err := c.WriteFrame(ctx, true, websocket.OpClose, []byte(strings.Repeat("x", 4096)))
+				err := c.WriteHeader(ctx, websocket.Header{
+					Fin:           true,
+					OpCode:        websocket.OpClose,
+					PayloadLength: 4096,
+				})
 				if err != nil {
 					return err
 				}

websocket_js.go

@@ -23,7 +23,7 @@ type Conn struct {
 	// read limit for a message in bytes.
 	msgReadLimit *atomicInt64
 
-	closeMu       sync.Mutex
+	closingMu     sync.Mutex
 	isReadClosed  *atomicInt64
 	closeOnce     sync.Once
 	closed        chan struct{}
@@ -43,6 +43,9 @@ func (c *Conn) close(err error, wasClean bool) {
 	c.closeOnce.Do(func() {
 		runtime.SetFinalizer(c, nil)
 
+		if !wasClean {
+			err = fmt.Errorf("unclean connection close: %w", err)
+		}
 		c.setCloseErr(err)
 		c.closeWasClean = wasClean
 		close(c.closed)
@@ -59,14 +62,11 @@ func (c *Conn) init() {
 	c.isReadClosed = &atomicInt64{}
 
 	c.releaseOnClose = c.ws.OnClose(func(e wsjs.CloseEvent) {
-		var err error = CloseError{
+		err := CloseError{
 			Code:   StatusCode(e.Code),
 			Reason: e.Reason,
 		}
-		if !e.WasClean {
-			err = fmt.Errorf("connection close was not clean: %w", err)
-		}
-		c.close(err, e.WasClean)
+		c.close(fmt.Errorf("received close: %w", err), e.WasClean)
 
 		c.releaseOnClose()
 		c.releaseOnMessage()
@@ -182,15 +182,6 @@ func (c *Conn) write(ctx context.Context, typ MessageType, p []byte) error {
 	}
 }
 
-func (c *Conn) isClosed() bool {
-	select {
-	case <-c.closed:
-		return true
-	default:
-		return false
-	}
-}
-
 // Close closes the websocket with the given code and reason.
 // It will wait until the peer responds with a close frame
 // or the connection is closed.
@@ -204,13 +195,19 @@ func (c *Conn) Close(code StatusCode, reason string) error {
 }
 
 func (c *Conn) exportedClose(code StatusCode, reason string) error {
-	c.closeMu.Lock()
-	defer c.closeMu.Unlock()
+	c.closingMu.Lock()
+	defer c.closingMu.Unlock()
+
+	ce := fmt.Errorf("sent close: %w", CloseError{
+		Code:   code,
+		Reason: reason,
+	})
 
 	if c.isClosed() {
-		return fmt.Errorf("already closed: %w", c.closeErr)
+		return fmt.Errorf("tried to close with %q but connection already closed: %w", ce, c.closeErr)
 	}
 
+	c.setCloseErr(ce)
 	err := c.ws.Close(int(code), reason)
 	if err != nil {
 		return err