WireGuard proxy: Fix potential startup races (#6852)

https://github.com/XTLS/Xray-core/pull/6852#issuecomment-5871816550

Fixes https://github.com/XTLS/Xray-core/issues/6850

proxy/wireguard/bind.go

@@ -52,9 +52,12 @@ func (b *bind) Open(port uint16) (fns []conn.ReceiveFunc, actualPort uint16, err
 						case <-ch:
 						default:
 							errors.LogErrorInner(context.Background(), err, "unexpected closed")
-							if b.downFunc != nil {
+							b.mu.Lock()
+							downFunc := b.downFunc
+							b.mu.Unlock()
+							if downFunc != nil {
 								go func() {
-									common.Must(b.downFunc())
+									common.Must(downFunc())
 								}()
 							}
 						}
@@ -76,6 +79,13 @@ func (b *bind) Open(port uint16) (fns []conn.ReceiveFunc, actualPort uint16, err
 	}, uint16(c.LocalAddr().(*net.UDPAddr).Port), nil
 }
 
+// setDownFunc sets downFunc after the device is created, since the device may already be using the bind.
+func (b *bind) setDownFunc(f func() error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.downFunc = f
+}
+
 func (b *bind) Close() error {
 	b.mu.Lock()
 	defer b.mu.Unlock()

proxy/wireguard/client.go

@@ -288,7 +288,13 @@ func (h *Handler) init(ctx context.Context) error {
 		}
 		return pktConn, nil
 	}
-	bind := &bind{}
+	// device.NewDevice may use the bind right away (Up -> BindUpdate -> Open),
+	// so everything it reads must be set before creating the device.
+	bind := &bind{
+		resolveFunc: resolveFunc,
+		listenFunc:  listenFunc,
+		reserved:    h.conf.Reserved,
+	}
 	logger := &device.Logger{
 		Verbosef: func(format string, args ...any) {
 			log.Record(&log.GeneralMessage{
@@ -304,10 +310,7 @@ func (h *Handler) init(ctx context.Context) error {
 		},
 	}
 	dev := device.NewDevice(h.tun, bind, logger)
-	bind.resolveFunc = resolveFunc
-	bind.listenFunc = listenFunc
-	bind.downFunc = dev.Down
-	bind.reserved = h.conf.Reserved
+	bind.setDownFunc(dev.Down)
 	var cfg strings.Builder
 	cfg.WriteString("private_key=" + h.conf.SecretKey + "\n")
 	for _, peer := range h.conf.Peers {

proxy/wireguard/server.go

@@ -113,7 +113,7 @@ func NewServer(ctx context.Context, conf *DeviceConfig) (*Server, error) {
 		users.Store(user.Account.(*MemoryAccount).Pub, user)
 	}
 
-	return &Server{
+	s := &Server{
 		conf:          conf,
 		ctx:           core.ToBackgroundDetachedContext(ctx),
 		policyManager: p,
@@ -131,7 +131,10 @@ func NewServer(ctx context.Context, conf *DeviceConfig) (*Server, error) {
 
 		pub:   pub,
 		users: users,
-	}, nil
+	}
+	// Install the stack's protocol handlers before the device can deliver packets to it (Start -> dev.Up).
+	CreateForwarder(stack, s.HandleConnection)
+	return s, nil
 }
 
 func (s *Server) AddUser(ctx context.Context, user *protocol.MemoryUser) error {
@@ -320,7 +323,6 @@ func (s *Server) Start() error {
 		return err
 	}
 	s.dev = dev
-	CreateForwarder(s.stack, s.HandleConnection)
 	return nil
 }