Writes file footer (finishes the file).

def WriteFileFooter(self):
    """Writes file footer (finishes the file)."""

    if self.cur_file_size != self.cur_info.size:
      raise IOError("Incorrect file size: st_size=%d, but written %d bytes." %
                    (self.cur_info.size, self.cur_file_size))

    # TODO(user):pytype: BLOCKSIZE/NUL constants are not visible to type
    # checker.
    # pytype: disable=module-attr
    blocks, remainder = divmod(self.cur_file_size, tarfile.BLOCKSIZE)
    if remainder > 0:
      self._tar_fd.fileobj.write(tarfile.NUL * (tarfile.BLOCKSIZE - remainder))
      blocks += 1
    self._tar_fd.offset += blocks * tarfile.BLOCKSIZE
    # pytype: enable=module-attr

    self._ResetState()

    return self._stream.GetValueAndReset()