func alignSparseEntries(src []sparseEntry, size int64) []sparseEntry {
	dst := src[:0]
	for _, s := range src {
		pos, end := s.Offset, s.endOffset()
		pos += blockPadding(+pos) // Round-up to nearest blockSize
		if end != size {
			end -= blockPadding(-end) // Round-down to nearest blockSize
		}
		if pos < end {
			dst = append(dst, sparseEntry{Offset: pos, Length: end - pos})
		}
	}
	return dst
}

	var b bytes.Buffer
	tw := NewWriter(&b)
	hdr := &Header{
		Name:       "file.txt",
		Uid:        1 << 21, // Too big for 8 octal digits
		Size:       int64(len(data)),
		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)
	}

	reader := NewReader(&buf)
	hdr, err = reader.Next()
	if err != nil {
		t.Fatal(err)
	}
	if !reflect.DeepEqual(hdr.Xattrs, xattrs) {
		t.Fatalf("xattrs did not survive round trip: got %+v, want %+v",
			hdr.Xattrs, xattrs)
	}
}

func TestPaxHeadersSorted(t *testing.T) {
	fileinfo, err := os.Stat("testdata/small.txt")
	if err != nil {
		t.Fatal(err)
	}