code: function readBatchSetIdsFromLogFile(log, batchesPerDownloadSession, callback) {
    logfilesummary(log, function processSummary(err, summary) {
      if (!summary.changesComplete) {
        callback(new error.BackupError('IncompleteChangesInLogFile',
          'WARNING: Changes did not finish spooling'));
        return;
      }
      if (Object.keys(summary.batches).length === 0) {
        return callback(null, []);
      }
  
      // batch IDs are the property names of summary.batches
      var batchSetIds = getPropertyNames(summary.batches, batchesPerDownloadSession);
      callback(null, batchSetIds);
    });
  }
docs: Return a set of uncompleted download batch IDs from the log file.
  
  @param {string} log - log file path
  @param {number} batchesPerDownloadSession - maximum IDs to return
  @param {function} callback - sign (err, batchSetIds array)