def _lock(self):
        """Lock the entire multistore."""
        self._thread_lock.acquire()
        try:
            self._file.open_and_lock()
        except __HOLE__ as e:
            if e.errno == errno.ENOSYS:
                logger.warn('File system does not support locking the '
                            'credentials file.')
            elif e.errno == errno.ENOLCK:
                logger.warn('File system is out of resources for writing the '
                            'credentials file (is your disk full?).')
            elif e.errno == errno.EDEADLK:
                logger.warn('Lock contention on multistore file, opening '
                            'in read-only mode.')
            elif e.errno == errno.EACCES:
                logger.warn('Cannot access credentials file.')
            else:
                raise
        if not self._file.is_locked():
            self._read_only = True
            if self._warn_on_readonly:
                logger.warn('The credentials file (%s) is not writable. '
                            'Opening in read-only mode. Any refreshed '
                            'credentials will only be '
                            'valid for this run.', self._file.filename())

        if os.path.getsize(self._file.filename()) == 0:
            logger.debug('Initializing empty multistore file')
            # The multistore is empty so write out an empty file.
            self._data = {}
            self._write()
        elif not self._read_only or self._data is None:
            # Only refresh the data if we are read/write or we haven't
            # cached the data yet. If we are readonly, we assume is isn't
            # changing out from under us and that we only have to read it
            # once. This prevents us from whacking any new access keys that
            # we have cached in memory but were unable to write out.
            self._refresh_data_cache()
dataset/ETHPy150Open google/oauth2client/oauth2client/contrib/multistore_file.py/_MultiStore._lock