Assumes that the caller has the _index_lock !

def _locked_refresh_doc_ids(self):
        """Assumes that the caller has the _index_lock !
        """
        d = {}
        for s in self._shards:
            for k in s.doc_index.keys():
                if k in d:
                    raise KeyError('doc "{i}" found in multiple repos'.format(i=k))
                d[k] = s
        self._doc2shard_map = d