func NewExactReadCloser(rc io.ReadCloser, totalBytes int64) io.ReadCloser {
	return &exactReadCloser{rc: rc, totalBytes: totalBytes}
}
func NewExactReadCloser(rc io.ReadCloser, totalBytes int64) io.ReadCloser {
	return &exactReadCloser{rc: rc, totalBytes: totalBytes}
}
// NewExactReadCloser returns a ReadCloser that returns errors if the underlying
// reader does not read back exactly the requested number of bytes.