[client] Fix UDP proxy to notify listener when remote conn closed (#4199)

* Fix UDP proxy to notify listener when remote conn closed

* Fix sender tests to use t.Errorf for timeout assertions

* Fix potential nil pointer

client/iface/wgproxy/listener/listener.go

@@ -1,7 +1,10 @@
 package listener
 
+import "sync"
+
 type CloseListener struct {
 	listener func()
+	mu       sync.Mutex
 }
 
 func NewCloseListener() *CloseListener {
@@ -9,11 +12,21 @@ func NewCloseListener() *CloseListener {
 }
 
 func (c *CloseListener) SetCloseListener(listener func()) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
 	c.listener = listener
 }
 
 func (c *CloseListener) Notify() {
-	if c.listener != nil {
-		c.listener()
+	c.mu.Lock()
+
+	if c.listener == nil {
+		c.mu.Unlock()
+		return
 	}
+	listener := c.listener
+	c.mu.Unlock()
+
+	listener()
 }

client/iface/wgproxy/udp/proxy.go

@@ -183,6 +183,11 @@ func (p *WGUDPProxy) proxyToLocal(ctx context.Context) {
 	for {
 		n, err := p.remoteConnRead(ctx, buf)
 		if err != nil {
+			if ctx.Err() != nil {
+				return
+			}
+
+			p.closeListener.Notify()
 			return
 		}