VLESS Multiplex (mux.cool): Apply MutexWriter to avoid concurrent writes to the underlayer

https://github.com/XTLS/Xray-core/pull/7337#issuecomment-6692618802
https://github.com/XTLS/Xray-core/pull/7345#issuecomment-6714114884

Fixes https://github.com/XTLS/Xray-core/issues/5845 and https://github.com/XTLS/Xray-core/issues/7336

---------

Co-authored-by: StarProxima <34741787+StarProxima@users.noreply.github.com>

common/mux/client.go

@@ -188,6 +188,7 @@ var (
 
 // NewClientWorker creates a new mux.Client.
 func NewClientWorker(stream transport.Link, s ClientStrategy) (*ClientWorker, error) {
+	stream.Writer = &MutexWriter{Writer: stream.Writer}
 	c := &ClientWorker{
 		sessionManager: NewSessionManager(),
 		link:           stream,

common/mux/server.go

@@ -95,7 +95,7 @@ type ServerWorker struct {
 func NewServerWorker(ctx context.Context, d routing.Dispatcher, link *transport.Link) (*ServerWorker, error) {
 	worker := &ServerWorker{
 		dispatcher:     d,
-		link:           link,
+		link:           &transport.Link{Reader: link.Reader, Writer: &MutexWriter{Writer: link.Writer}},
 		sessionManager: NewSessionManager(),
 		done:           done.New(),
 		timer:          time.NewTicker(60 * time.Second),

common/mux/writer.go

@@ -1,6 +1,8 @@
 package mux
 
 import (
+	"sync"
+
 	"github.com/xtls/xray-core/common"
 	"github.com/xtls/xray-core/common/buf"
 	"github.com/xtls/xray-core/common/net"
@@ -9,6 +11,17 @@ import (
 	"github.com/xtls/xray-core/common/session"
 )
 
+type MutexWriter struct {
+	sync.Mutex
+	buf.Writer
+}
+
+func (w *MutexWriter) WriteMultiBuffer(mb buf.MultiBuffer) error {
+	w.Lock()
+	defer w.Unlock()
+	return w.Writer.WriteMultiBuffer(mb)
+}
+
 type Writer struct {
 	dest         net.Destination
 	writer       buf.Writer