Fix: use the fastest whether the result is successful

dns/client.go

@@ -20,7 +20,22 @@ func (c *client) Exchange(m *D.Msg) (msg *D.Msg, err error) {
 func (c *client) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Msg, err error) {
 	c.Client.Dialer = dialer.Dialer()
 
-	// Please note that miekg/dns ExchangeContext doesn't respond to context cancel.
-	msg, _, err = c.Client.ExchangeContext(ctx, m, c.Address)
-	return
+	// miekg/dns ExchangeContext doesn't respond to context cancel.
+	// this is a workaround
+	type result struct {
+		msg *D.Msg
+		err error
+	}
+	ch := make(chan result, 1)
+	go func() {
+		msg, _, err := c.Client.ExchangeContext(ctx, m, c.Address)
+		ch <- result{msg, err}
+	}()
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case ret := <-ch:
+		return ret.msg, ret.err
+	}
 }