func mustReadFull(r io.Reader, b []byte) (int, error) {
	n, err := tryReadFull(r, b)
	if err == io.EOF {
		err = io.ErrUnexpectedEOF
	}
	return n, err
}

// tryReadFull is like io.ReadFull except it returns
// io.EOF when it is hit before len(b) bytes are read.
func tryReadFull(r io.Reader, b []byte) (n int, err error) {
	for len(b) > n && err == nil {
		var nn int

		if c != 0 {
			return i, errWriteHole
		}
	}
	return len(b), nil
}

// ensureEOF checks whether r is at EOF, reporting ErrWriteTooLong if not so.
func ensureEOF(r io.Reader) error {
	n, err := tryReadFull(r, []byte{0})
	switch {
	case n > 0:
		return ErrWriteTooLong
	case err == io.EOF:
		return nil
	default:
		return err
	}