var nn int
		nn, err = r.Read(b[n:])
		n += nn
	}
	if len(b) == n && err == io.EOF {
		err = nil
	}
	return n, err
}

// readSpecialFile is like io.ReadAll except it returns
// ErrFieldTooLong if more than maxSpecialFileSize is read.
func readSpecialFile(r io.Reader) ([]byte, error) {
	buf, err := io.ReadAll(io.LimitReader(r, maxSpecialFileSize+1))
	if len(buf) > maxSpecialFileSize {
		return nil, ErrFieldTooLong