[client] Deliver relay-borne packets when the relay comes up after ICE

When the relay connection becomes ready while ICE is already the active
path, the proxy was only stored for later: neither Work() nor RedirectAs()
ran, so nothing consumed the relayed connection. The remote peer's paths
can come up in the opposite order, in which case it is already sending
WireGuard traffic over the relay — those packets were never delivered. A
responder that had configured a nil endpoint then waited for a handshake
that could not arrive, until the 5s delayed-update fallback fired; on an
Android e2e run this stretched connected-to-first-ping to ~12s.

Redirect the standby proxy to the active ICE endpoint. RedirectAs now also
starts the proxy when it has not been started yet, with the attribution
set before the reader runs: starting through Work() first would hand
packets to WireGuard labelled with the relayed fake address, and WireGuard
would roam to it and send its replies there. The udp proxy starts its
writer worker together with the reader — Work() skips the isStarted block
on a later switch to relay, so the writer must already exist by then.

RedirectAs also gained the same nil-remoteConn guard Work() has: now that
it can start workers, calling it before AddTurnConn must stay a no-op.

client/iface/wgproxy/bind/proxy.go

@@ -114,6 +114,10 @@ func (p *ProxyBind) Pause() {
 }
 
 func (p *ProxyBind) RedirectAs(endpoint *net.UDPAddr) {
+	if p.remoteConn == nil {
+		return
+	}
+
 	ep, err := addrToEndpoint(endpoint)
 	if err != nil {
 		log.Errorf("failed to start package redirection: %v", err)
@@ -125,6 +129,12 @@ func (p *ProxyBind) RedirectAs(endpoint *net.UDPAddr) {
 
 	p.wgCurrentUsed = ep
 
+	// start here (not only in Work) so the first packet already carries the redirected address
+	if !p.isStarted {
+		p.isStarted = true
+		go p.proxyToLocal(p.ctx)
+	}
+
 	p.pausedCond.Signal()
 	p.pausedCond.L.Unlock()
 }

client/iface/wgproxy/ebpf/wrapper.go

@@ -188,6 +188,10 @@ func (p *ProxyWrapper) Pause() {
 }
 
 func (p *ProxyWrapper) RedirectAs(endpoint *net.UDPAddr) {
+	if p.remoteConn == nil {
+		return
+	}
+
 	if endpoint == nil || endpoint.IP == nil {
 		log.Errorf("failed to start package redirection, endpoint is nil")
 		return
@@ -215,6 +219,12 @@ func (p *ProxyWrapper) RedirectAs(endpoint *net.UDPAddr) {
 	p.headerCurrentUsed = header
 	p.rawConn = p.selectRawConn(header)
 
+	// start here (not only in Work) so the first packet already carries the rewritten headers
+	if !p.isStarted {
+		p.isStarted = true
+		go p.proxyToLocal(p.ctx)
+	}
+
 	p.pausedCond.Signal()
 	p.pausedCond.L.Unlock()
 }

client/iface/wgproxy/proxy.go

@@ -12,9 +12,10 @@ type Proxy interface {
 	Work()                      // Work start or resume the proxy
 	Pause()                     // Pause to forward the packages from remote connection to WireGuard. The opposite way still works.
 
-	//RedirectAs resume the forwarding the packages from relayed connection to WireGuard interface if it was paused
-	//and rewrite the src address to the endpoint address.
-	//With this logic can avoid the package loss from relayed connections.
+	//RedirectAs forwards the packages from the relayed connection to the WireGuard interface
+	//with the src address rewritten to the endpoint address, starting the proxy if needed and
+	//resuming it if it was paused. Never delivers a packet with the relayed fake address —
+	//WireGuard would roam to it.
 	RedirectAs(endpoint *net.UDPAddr)
 	CloseConn() error
 	SetDisconnectListener(disconnected func())

client/iface/wgproxy/udp/proxy.go

@@ -123,6 +123,10 @@ func (p *WGUDPProxy) Pause() {
 
 // RedirectAs start to use the fake sourced raw socket as package sender
 func (p *WGUDPProxy) RedirectAs(endpoint *net.UDPAddr) {
+	if p.remoteConn == nil {
+		return
+	}
+
 	p.pausedCond.L.Lock()
 	defer func() {
 		p.pausedCond.Signal()
@@ -145,6 +149,13 @@ func (p *WGUDPProxy) RedirectAs(endpoint *net.UDPAddr) {
 	}
 	p.srcFakerConn = srcFakerConn
 	p.sendPkg = p.srcFakerConn.SendPkg
+
+	// start here (not only in Work) so the first packet already carries the faked source
+	if !p.isStarted {
+		p.isStarted = true
+		go p.proxyToRemote(p.ctx)
+		go p.proxyToLocal(p.ctx)
+	}
 }
 
 // InjectPacket writes b to the remote peer over the underlying transport.