func (zeroWriter) Write(b []byte) (int, error) {
	for i, c := range b {
		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: