'Return the timestamp of the latest or earliest entry in the oplog.'

def _get_oplog_timestamp(self, newest_entry):

    sort_order = (pymongo.DESCENDING if newest_entry else pymongo.ASCENDING)
    curr = self.oplog.find({'op': {'$ne': 'n'}}).sort('$natural', sort_order).limit((-1))
    try:
        ts = next(curr)['ts']
    except StopIteration:
        LOG.debug('OplogThread:    oplog    is    empty.')
        return None
    LOG.debug(('OplogThread:    %s    oplog    entry    has    timestamp    %s.' % (('Newest' if newest_entry else 'Oldest'), ts)))
    return ts
