Create revoc registry if need be for input revocation registry identifier;
        open and cache tails file reader.

        :param rr_id: revocation registry identifier
        :param rr_size: if new revocation registry necessary, its size (default as per _create_rev_reg())

async def _sync_revoc(self, rr_id: str, rr_size: int = None) -> None:
        """
        Create revoc registry if need be for input revocation registry identifier;
        open and cache tails file reader.

        :param rr_id: revocation registry identifier
        :param rr_size: if new revocation registry necessary, its size (default as per _create_rev_reg())
        """

        LOGGER.debug('Issuer._sync_revoc >>> rr_id: %s, rr_size: %s', rr_id, rr_size)

        (cd_id, tag) = rev_reg_id2cred_def_id__tag(rr_id)

        try:
            await self.get_cred_def(cd_id)
        except AbsentCredDef:
            LOGGER.debug(
                'Issuer._sync_revoc: <!< tails tree %s may be for another ledger; no cred def found on %s',
                self._dir_tails,
                cd_id)
            raise AbsentCredDef('Tails tree {} may be for another ledger; no cred def found on {}'.format(
                self._dir_tails,
                cd_id))

        with REVO_CACHE.lock:
            revo_cache_entry = REVO_CACHE.get(rr_id, None)
            tails = None if revo_cache_entry is None else revo_cache_entry.tails
            if tails is None:  #  it's a new revocation registry, or not yet set in cache
                try:
                    tails = await Tails(self._dir_tails, cd_id, tag).open()
                except AbsentTails:
                    await self._create_rev_reg(rr_id, rr_size)   # it's a new revocation registry
                    tails = await Tails(self._dir_tails, cd_id, tag).open()  # symlink should exist now

                if revo_cache_entry is None:
                    REVO_CACHE[rr_id] = RevoCacheEntry(None, tails)
                else:
                    REVO_CACHE[rr_id].tails = tails

        LOGGER.debug('Issuer._sync_revoc <<<')