fix: websocket server upgrade in golang1.20

Author: wwqgtxx

transport/vmess/websocket.go

@@ -555,7 +555,7 @@ func StreamUpgradedWebsocketConn(w http.ResponseWriter, r *http.Request) (net.Co
 		w.Header().Set("Sec-Websocket-Accept", getSecAccept(r.Header.Get("Sec-WebSocket-Key")))
 	}
 	w.WriteHeader(http.StatusSwitchingProtocols)
-	if flusher, isFlusher := w.(interface{ FlushError() error }); isFlusher {
+	if flusher, isFlusher := w.(interface{ FlushError() error }); isFlusher && writeHeaderShouldFlush {
 		err = flusher.FlushError()
 		if err != nil {
 			return nil, fmt.Errorf("flush response: %w", err)

transport/vmess/websocket_go120.go

@@ -0,0 +1,7 @@
+//go:build !go1.21
+
+package vmess
+
+// Golang1.20's net.http Flush will mistakenly call w.WriteHeader(StatusOK) internally after w.WriteHeader(http.StatusSwitchingProtocols)
+// https://github.com/golang/go/issues/59564
+const writeHeaderShouldFlush = false

transport/vmess/websocket_go121.go

@@ -0,0 +1,5 @@
+//go:build go1.21
+
+package vmess
+
+const writeHeaderShouldFlush = true