func (w *failOnceWriter) Write(b []byte) (int, error) {
	if !*w {
		return 0, io.ErrShortWrite
	}
	*w = true
	return len(b), nil
}

func TestWriterErrors(t *testing.T) {
	t.Run("HeaderOnly", func(t *testing.T) {
		tw := NewWriter(new(bytes.Buffer))
		hdr := &Header{Name: "dir/", Typeflag: TypeDir}
		if err := tw.WriteHeader(hdr); err != nil {
			t.Fatalf("WriteHeader() = %v, want nil", err)
		}