return nil, errors.New("hash: already read from hash reader")
		}
		if len(r.checksum) != 0 && len(MD5) != 0 && !etag.Equal(r.checksum, MD5) {
			return nil, BadDigest{
				ExpectedMD5:   r.checksum.String(),
				CalculatedMD5: md5Hex,
			}
		}
		if len(r.contentSHA256) != 0 && len(SHA256) != 0 && !bytes.Equal(r.contentSHA256, SHA256) {
			return nil, SHA256Mismatch{
				ExpectedSHA256:   hex.EncodeToString(r.contentSHA256),

// Reader compares the etag with the computed
// MD5 sum once the r returns io.EOF.
func NewReader(ctx context.Context, r io.Reader, etag ETag, forceMD5 []byte) *Reader {
	if er, ok := r.(*Reader); ok {
		if er.readN == 0 && Equal(etag, er.checksum) {
			return er
		}
	}
	if len(forceMD5) != 0 {
		return &Reader{
			src:      r,
			md5:      NewUUIDHash(forceMD5),
			checksum: etag,
		}
	}
	return &Reader{