go func() {
		client.Write(data)
		client.Close()
		close(done)
	}()

	return dst, server, data, hook, func() { <-done }
}

// mustContainData ensures that the specified file contains exactly the
// specified data.
func mustContainData(t *testing.T, f *File, data []byte) {
	t.Helper()

	got := make([]byte, len(data))
	if _, err := io.ReadFull(f, got); err != nil {
		t.Fatal(err)
	}