fix(xdns): address code review findings

Author: nnemirovsky

transport/internet/finalmask/xdns/client.go

@@ -66,8 +66,10 @@ type xdnsConnClient struct {
 	readQueue  chan *packet
 	writeQueue chan *packet
 
-	closed atomic.Bool
-	mutex  sync.Mutex
+	closed    atomic.Bool
+	closeOnce sync.Once
+	closeErr  error
+	mutex     sync.Mutex
 }
 
 func NewConnClient(c *Config, raw net.PacketConn) (net.PacketConn, error) {
@@ -304,21 +306,23 @@ func (c *xdnsConnClient) WriteTo(p []byte, addr net.Addr) (n int, err error) {
 }
 
 func (c *xdnsConnClient) Close() error {
-	c.closed.Store(true)
-	for _, rc := range c.resolverConns {
-		rc.conn.Close()
-	}
-	err := c.PacketConn.Close()
-	if len(c.resolverConns) > 0 {
-		c.recvWg.Wait()
-		close(c.pollChan)
-		close(c.readQueue)
-		c.mutex.Lock()
-		close(c.writeQueue)
-		c.mutex.Unlock()
-	}
-	c.sendWg.Wait()
-	return err
+	c.closeOnce.Do(func() {
+		c.closed.Store(true)
+		for _, rc := range c.resolverConns {
+			rc.conn.Close()
+		}
+		c.closeErr = c.PacketConn.Close()
+		if len(c.resolverConns) > 0 {
+			c.recvWg.Wait()
+			close(c.pollChan)
+			close(c.readQueue)
+			c.mutex.Lock()
+			close(c.writeQueue)
+			c.mutex.Unlock()
+		}
+		c.sendWg.Wait()
+	})
+	return c.closeErr
 }
 
 func encode(p []byte, clientID []byte, domain Name) ([]byte, error) {