def flush(self):
        """Closes the underlying zip file, ensuring all records are written,
        then re-opens the file for further modifications."""
        if self.mode != 'r':
            with self.mutex:
                self.zf.close()
                # N.B., re-open with mode 'a' regardless of initial mode so we don't wipe
                # what's been written
                self.zf = zipfile.ZipFile(self.path, mode='a',
                                          compression=self.compression,
                                          allowZip64=self.allowZip64)
Closes the underlying zip file, ensuring all records are written,
        then re-opens the file for further modifications.