t.Fatal(err)
	}
	if _, _, err := c.WriteMsgUDP(wb, nil, c.LocalAddr().(*UDPAddr)); err != nil {
		condFatalf(t, c.LocalAddr().Network(), "%v", err)
	}
	if _, _, _, _, err := c.ReadMsgUDP(rb, nil); err != nil {
		condFatalf(t, c.LocalAddr().Network(), "%v", err)
	}

	if f, err := c.File(); err != nil {
		condFatalf(t, "file+net", "%v", err)
	} else {
		f.Close()
	}

	defer func() {

			return false
		}
		if (ip.To4() != nil || ip == nil) && cip.To16() != nil && cip.To4() == nil { // a pair of IPv4 server and IPv6 client
			return false
		}
	}

	return true
}

func condFatalf(t *testing.T, network string, format string, args ...any) {
	t.Helper()
	// A few APIs like File and Read/WriteMsg{UDP,IP} are not
	// fully implemented yet on Plan 9 and Windows.
	switch runtime.GOOS {

	fetch := func(n, retries int) string {
		condFatalf := func(format string, arg ...any) {
			if retries <= 0 {
				t.Fatalf(format, arg...)
			}
			t.Logf("retrying shortly after expected error: "+format, arg...)
			time.Sleep(time.Second / time.Duration(retries))
		}
		for retries >= 0 {
			retries--
			res, err := c.Get(ts.URL)
			if err != nil {
				condFatalf("error in req #%d, GET: %v", n, err)
				continue