}

type zeroReader struct{}

func (zeroReader) Read(b []byte) (int, error) {
	clear(b)
	return len(b), nil
}

// mustReadFull is like io.ReadFull except it returns
// io.ErrUnexpectedEOF when io.EOF is hit before len(b) bytes are read.
func mustReadFull(r io.Reader, b []byte) (int, error) {
	n, err := tryReadFull(r, b)
	if err == io.EOF {
		err = io.ErrUnexpectedEOF
	}
	return n, err
}

		}
	}

	// If the last fragment is a hole, then seek to 1-byte before EOF, and
	// read a single byte to ensure the file is the right size.
	if readLastByte && err == nil {
		_, err = mustReadFull(rs, []byte{0})
		sw.pos++
	}

	n = sw.pos - pos0
	switch {
	case err == io.EOF:
		return n, io.ErrUnexpectedEOF
	case err == ErrWriteTooLong: