}
	})

	t.Run("Persistence", func(t *testing.T) {
		tw := NewWriter(new(failOnceWriter))
		if err := tw.WriteHeader(&Header{}); err != io.ErrShortWrite {
			t.Fatalf("WriteHeader() = %v, want %v", err, io.ErrShortWrite)
		}
		if err := tw.WriteHeader(&Header{Name: "small.txt"}); err == nil {
			t.Errorf("WriteHeader() = got %v, want non-nil error", err)
		}
		if _, err := tw.Write(nil); err == nil {

// Some fields may not be populated.
//
// For forward compatibility, users that retrieve a Header from Reader.Next,
// mutate it in some ways, and then pass it back to Writer.WriteHeader
// should do so by creating a new Header and copying the fields
// that they are interested in preserving.
type Header struct {
	// Typeflag is the type of header entry.

		ModTime:    time.Now().Round(time.Second),
		PAXRecords: map[string]string{"uid": "2097152"},
		Format:     FormatPAX,
		Typeflag:   TypeReg,
	}
	if err := tw.WriteHeader(hdr); err != nil {
		t.Fatalf("tw.WriteHeader: %v", err)
	}
	if _, err := tw.Write(data); err != nil {
		t.Fatalf("tw.Write: %v", err)
	}
	if err := tw.Close(); err != nil {
		t.Fatalf("tw.Close: %v", err)
	}

		}
		fw.rawCount = &countWriter{w: fw.comp}
		fw.header = h
		ow = fw
	}
	w.dir = append(w.dir, h)
	if err := writeHeader(w.cw, h); err != nil {
		return nil, err
	}
	// If we're creating a directory, fw is nil.
	w.last = fw
	return ow, nil
}

func writeHeader(w io.Writer, h *header) error {
	const maxUint16 = 1<<16 - 1
	if len(h.Name) > maxUint16 {
		return errLongName
	}