[client] Fix/pkg loss (#3338)

The Relayed connection setup is optimistic. It does not have any confirmation of an established end-to-end connection. Peers start sending WireGuard handshake packets immediately after the successful offer-answer handshake.
Meanwhile, for successful P2P connection negotiation, we change the WireGuard endpoint address, but this change does not trigger new handshake initiation. Because the peer switched from Relayed connection to P2P, the packets from the Relay server are dropped and must wait for the next WireGuard handshake via P2P.

To avoid this scenario, the relayed WireGuard proxy no longer drops the packets. Instead, it rewrites the source address to the new P2P endpoint and continues forwarding the packets.

We still have one corner case: if the Relayed server negotiation chooses a server that has not been used before. In this case, one side of the peer connection will be slower to reach the Relay server, and the Relay server will drop the handshake packet.

If everything goes well we should see exactly 5 seconds improvements between the WireGuard configuration time and the handshake time.

client/iface/wgproxy/udp/proxy.go

@@ -1,3 +1,5 @@
+//go:build linux && !android
+
 package udp
 
 import (
@@ -21,16 +23,18 @@ type WGUDPProxy struct {
 	localWGListenPort int
 	mtu               uint16
 
-	remoteConn net.Conn
-	localConn  net.Conn
-	ctx        context.Context
-	cancel     context.CancelFunc
-	closeMu    sync.Mutex
-	closed     bool
+	remoteConn   net.Conn
+	localConn    net.Conn
+	srcFakerConn *SrcFaker
+	sendPkg      func(data []byte) (int, error)
+	ctx          context.Context
+	cancel       context.CancelFunc
+	closeMu      sync.Mutex
+	closed       bool
 
-	pausedMu  sync.Mutex
-	paused    bool
-	isStarted bool
+	paused     bool
+	pausedCond *sync.Cond
+	isStarted  bool
 
 	closeListener *listener.CloseListener
 }
@@ -41,6 +45,7 @@ func NewWGUDPProxy(wgPort int, mtu uint16) *WGUDPProxy {
 	p := &WGUDPProxy{
 		localWGListenPort: wgPort,
 		mtu:               mtu,
+		pausedCond:        sync.NewCond(&sync.Mutex{}),
 		closeListener:     listener.NewCloseListener(),
 	}
 	return p
@@ -61,6 +66,7 @@ func (p *WGUDPProxy) AddTurnConn(ctx context.Context, endpoint *net.UDPAddr, rem
 
 	p.ctx, p.cancel = context.WithCancel(ctx)
 	p.localConn = localConn
+	p.sendPkg = p.localConn.Write
 	p.remoteConn = remoteConn
 
 	return err
@@ -84,15 +90,24 @@ func (p *WGUDPProxy) Work() {
 		return
 	}
 
-	p.pausedMu.Lock()
+	p.pausedCond.L.Lock()
 	p.paused = false
-	p.pausedMu.Unlock()
+	p.sendPkg = p.localConn.Write
+
+	if p.srcFakerConn != nil {
+		if err := p.srcFakerConn.Close(); err != nil {
+			log.Errorf("failed to close src faker conn: %s", err)
+		}
+		p.srcFakerConn = nil
+	}
 
 	if !p.isStarted {
 		p.isStarted = true
 		go p.proxyToRemote(p.ctx)
 		go p.proxyToLocal(p.ctx)
 	}
+	p.pausedCond.Signal()
+	p.pausedCond.L.Unlock()
 }
 
 // Pause pauses the proxy from receiving data from the remote peer
@@ -101,9 +116,35 @@ func (p *WGUDPProxy) Pause() {
 		return
 	}
 
-	p.pausedMu.Lock()
+	p.pausedCond.L.Lock()
 	p.paused = true
-	p.pausedMu.Unlock()
+	p.pausedCond.L.Unlock()
+}
+
+// RedirectAs start to use the fake sourced raw socket as package sender
+func (p *WGUDPProxy) RedirectAs(endpoint *net.UDPAddr) {
+	p.pausedCond.L.Lock()
+	defer func() {
+		p.pausedCond.Signal()
+		p.pausedCond.L.Unlock()
+	}()
+
+	p.paused = false
+	if p.srcFakerConn != nil {
+		if err := p.srcFakerConn.Close(); err != nil {
+			log.Errorf("failed to close src faker conn: %s", err)
+		}
+		p.srcFakerConn = nil
+	}
+	srcFakerConn, err := NewSrcFaker(p.localWGListenPort, endpoint)
+	if err != nil {
+		log.Errorf("failed to create src faker conn: %s", err)
+		// fallback to continue without redirecting
+		p.paused = true
+		return
+	}
+	p.srcFakerConn = srcFakerConn
+	p.sendPkg = p.srcFakerConn.SendPkg
 }
 
 // CloseConn close the localConn
@@ -115,6 +156,8 @@ func (p *WGUDPProxy) CloseConn() error {
 }
 
 func (p *WGUDPProxy) close() error {
+	var result *multierror.Error
+
 	p.closeMu.Lock()
 	defer p.closeMu.Unlock()
 
@@ -128,7 +171,11 @@ func (p *WGUDPProxy) close() error {
 
 	p.cancel()
 
-	var result *multierror.Error
+	p.pausedCond.L.Lock()
+	p.paused = false
+	p.pausedCond.Signal()
+	p.pausedCond.L.Unlock()
+
 	if err := p.remoteConn.Close(); err != nil && !errors.Is(err, net.ErrClosed) {
 		result = multierror.Append(result, fmt.Errorf("remote conn: %s", err))
 	}
@@ -136,6 +183,13 @@ func (p *WGUDPProxy) close() error {
 	if err := p.localConn.Close(); err != nil {
 		result = multierror.Append(result, fmt.Errorf("local conn: %s", err))
 	}
+
+	if p.srcFakerConn != nil {
+		if err := p.srcFakerConn.Close(); err != nil {
+			result = multierror.Append(result, fmt.Errorf("src faker raw conn: %s", err))
+		}
+	}
+
 	return cerrors.FormatErrorOrNil(result)
 }
 
@@ -194,14 +248,12 @@ func (p *WGUDPProxy) proxyToLocal(ctx context.Context) {
 			return
 		}
 
-		p.pausedMu.Lock()
-		if p.paused {
-			p.pausedMu.Unlock()
-			continue
+		p.pausedCond.L.Lock()
+		for p.paused {
+			p.pausedCond.Wait()
 		}
-
-		_, err = p.localConn.Write(buf[:n])
-		p.pausedMu.Unlock()
+		_, err = p.sendPkg(buf[:n])
+		p.pausedCond.L.Unlock()
 
 		if err != nil {
 			if ctx.Err() != nil {