// then readFrom uses Seek to skip past holes defined in Header.SparseHoles,
// assuming that skipped regions are all NULs.
// This always reads the last byte to ensure r is the right size.
//
// TODO(dsnet): Re-export this when adding sparse file support.
// See https://golang.org/issue/22735
func (tw *Writer) readFrom(r io.Reader) (int64, error) {
	if tw.err != nil {
		return 0, tw.err
	}
	n, err := tw.curr.ReadFrom(r)