}

func TestCreateListenerPreventUpgrades(t *testing.T) {
	l, port, err := CreateListener("tcp", "127.0.0.1:0", net.ListenConfig{})
	if err != nil {
		t.Fatalf("failed to create listener: %v", err)
	}
	defer l.Close()

	l2, _, err := CreateListener("tcp", fmt.Sprintf("127.0.0.1:%d", port), net.ListenConfig{Control: permitPortReuse})
	if err == nil {
		l2.Close()
		t.Fatalf("creating second listener without port sharing should fail")

	var ctrlCtxFn func(ctx context.Context, network, address string, c syscall.RawConn) error
	if sl.ListenConfig.Control != nil {
		ctrlCtxFn = func(ctx context.Context, network, address string, c syscall.RawConn) error {
			return sl.ListenConfig.Control(network, address, c)
		}
	}
	fd, err := internetSocket(ctx, sl.network, laddr, nil, syscall.SOCK_STREAM, proto, "listen", ctrlCtxFn)

		return nil, UnknownNetworkError(sl.network)
	}
	var ctrlCtxFn func(ctx context.Context, network, address string, c syscall.RawConn) error
	if sl.ListenConfig.Control != nil {
		ctrlCtxFn = func(ctx context.Context, network, address string, c syscall.RawConn) error {
			return sl.ListenConfig.Control(network, address, c)
		}
	}
	fd, err := internetSocket(ctx, network, laddr, nil, syscall.SOCK_RAW, proto, "listen", ctrlCtxFn)

The new type [KeepAliveConfig] permits fine-tuning the keep-alive
options for TCP connections, via a new [TCPConn.SetKeepAliveConfig]

	var ctrlCtxFn func(ctx context.Context, network, address string, c syscall.RawConn) error
	if sl.ListenConfig.Control != nil {
		ctrlCtxFn = func(ctx context.Context, network, address string, c syscall.RawConn) error {
			return sl.ListenConfig.Control(network, address, c)
		}
	}
	fd, err := unixSocket(ctx, sl.network, laddr, nil, "listen", ctrlCtxFn)
	if err != nil {
		return nil, err

	if err != nil {
		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
	}
	return c, nil
}

// ListenConfig contains options for listening to an address.
type ListenConfig struct {
	// If Control is not nil, it is called after creating the network
	// connection but before binding it to the operating system.
	//

	var ctrlCtxFn func(ctx context.Context, network, address string, c syscall.RawConn) error
	if sl.ListenConfig.Control != nil {
		ctrlCtxFn = func(ctx context.Context, network, address string, c syscall.RawConn) error {
			return sl.ListenConfig.Control(network, address, c)
		}
	}
	fd, err := internetSocket(ctx, sl.network, laddr, nil, syscall.SOCK_DGRAM, 0, "listen", ctrlCtxFn)

		if err := os.RemoveAll(d); err != nil {
			t.Error(err)
		}
	})
	return filepath.Join(d, "sock")
}

func newLocalListener(t testing.TB, network string, lcOpt ...*ListenConfig) Listener {
	var lc *ListenConfig
	switch len(lcOpt) {
	case 0:
		lc = new(ListenConfig)
	case 1:
		lc = lcOpt[0]
	default:
		t.Helper()
		t.Fatal("too many ListenConfigs passed to newLocalListener: want 0 or 1")
	}

	}

	handler := func(ls *localServer, ln Listener) {
		for {
			c, err := ln.Accept()
			if err != nil {
				return
			}
			c.Close()
		}
	}
	ln := newLocalListener(t, "tcp", &ListenConfig{
		KeepAlive: -1, // prevent calling hook from accepting
	})
	ls := (&streamListener{Listener: ln}).newLocalServer()
	defer ls.teardown()
	if err := ls.buildup(handler); err != nil {
		t.Fatal(err)
	}

// It is possible to have a disconnected client in this tiny window of time.
func CheckPortAvailability(host, port string, opts TCPOptions) (err error) {
	lc := &net.ListenConfig{}

	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
	defer cancel()

	l, err := lc.Listen(ctx, "tcp", net.JoinHostPort(host, port))
	if err != nil {
		return err
	}