type dirReader struct {
	err error
}

func (r *dirReader) Read([]byte) (int, error) {
	return 0, r.err
}

func (r *dirReader) Close() error {
	return nil
}

type checksumReader struct {
	rc    io.ReadCloser
	hash  hash.Hash32
	nread uint64 // number of bytes read so far
	f     *File
	desr  io.Reader // if non-nil, where to read the data descriptor
	err   error     // sticky error
}

	MD5, err := hex.DecodeString(md5Hex)
	if err != nil {
		return nil, BadDigest{ // TODO(aead): Return an error that indicates that an invalid ETag has been specified
			ExpectedMD5:   md5Hex,
			CalculatedMD5: "",
		}
	}
	SHA256, err := hex.DecodeString(sha256Hex)
	if err != nil {
		return nil, SHA256Mismatch{ // TODO(aead): Return an error that indicates that an invalid Content-SHA256 has been specified

//   - Exactly 0 bytes are read and EOF is hit.
//   - Exactly 1 block of zeros is read and EOF is hit.
//   - At least 2 blocks of zeros are read.
func (tr *Reader) readHeader() (*Header, *block, error) {
	// Two blocks of zero bytes marks the end of the archive.
	if _, err := io.ReadFull(tr.r, tr.blk[:]); err != nil {
		return nil, nil, err // EOF is okay here; exactly 0 bytes read
	}