func (tw *Writer) Flush() error {
	if tw.err != nil {
		return tw.err
	}
	if nb := tw.curr.logicalRemaining(); nb > 0 {
		return fmt.Errorf("archive/tar: missed writing %d bytes", nb)
	}
	if _, tw.err = tw.w.Write(zeroBlock[:tw.pad]); tw.err != nil {
		return tw.err
	}
	tw.pad = 0
	return nil
}

// WriteHeader writes hdr and prepares to accept the file's contents.