code: func (w *Writer) addPadding(size int) error {
  	p := w.pos % uint64(size)
  	if p == 0 {
  		return nil
  	}
  	p = uint64(size) - p
  	return errors.Wrap(w.write(make([]byte, p)), "add padding")
  }
docs: // addPadding adds zero byte padding until the file size is a multiple size.