path: root/synapse/storage/databases/main/roommember.py

# Copyright 2014-2016 OpenMarket Ltd
# Copyright 2018 New Vector Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
from itertools import chain
from typing import (
    TYPE_CHECKING,
    AbstractSet,
    Collection,
    Dict,
    FrozenSet,
    Iterable,
    List,
    Mapping,
    Optional,
    Sequence,
    Set,
    Tuple,
    Union,
)

import attr

from synapse.api.constants import EventTypes, Membership
from synapse.metrics import LaterGauge
from synapse.metrics.background_process_metrics import wrap_as_background_process
from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
from synapse.storage.database import (
    DatabasePool,
    LoggingDatabaseConnection,
    LoggingTransaction,
)
from synapse.storage.databases.main.cache import CacheInvalidationWorkerStore
from synapse.storage.databases.main.events_worker import EventsWorkerStore
from synapse.storage.engines import Sqlite3Engine
from synapse.storage.roommember import (
    GetRoomsForUserWithStreamOrdering,
    MemberSummary,
    ProfileInfo,
    RoomsForUser,
)
from synapse.types import (
    JsonDict,
    PersistedEventPosition,
    StateMap,
    StrCollection,
    get_domain_from_id,
)
from synapse.util.async_helpers import Linearizer
from synapse.util.caches import intern_string
from synapse.util.caches.descriptors import _CacheContext, cached, cachedList
from synapse.util.iterutils import batch_iter
from synapse.util.metrics import Measure

if TYPE_CHECKING:
    from synapse.server import HomeServer
    from synapse.state import _StateCacheEntry

logger = logging.getLogger(__name__)


_MEMBERSHIP_PROFILE_UPDATE_NAME = "room_membership_profile_update"
_CURRENT_STATE_MEMBERSHIP_UPDATE_NAME = "current_state_events_membership"


@attr.s(frozen=True, slots=True, auto_attribs=True)
class EventIdMembership:
    """Returned by `get_membership_from_event_ids`"""

    user_id: str
    membership: str


class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
    def __init__(
        self,
        database: DatabasePool,
        db_conn: LoggingDatabaseConnection,
        hs: "HomeServer",
    ):
        super().__init__(database, db_conn, hs)

        # Used by `_get_joined_hosts` to ensure only one thing mutates the cache
        # at a time. Keyed by room_id.
        self._joined_host_linearizer = Linearizer("_JoinedHostsCache")

        self._server_notices_mxid = hs.config.servernotices.server_notices_mxid

        if (
            self.hs.config.worker.run_background_tasks
            and self.hs.config.metrics.metrics_flags.known_servers
        ):
            self._known_servers_count = 1
            self.hs.get_clock().looping_call(
                self._count_known_servers,
                60 * 1000,
            )
            self.hs.get_clock().call_later(
                1,
                self._count_known_servers,
            )
            LaterGauge(
                "synapse_federation_known_servers",
                "",
                [],
                lambda: self._known_servers_count,
            )

    @wrap_as_background_process("_count_known_servers")
    async def _count_known_servers(self) -> int:
        """
        Count the servers that this server knows about.

        The statistic is stored on the class for the
        `synapse_federation_known_servers` LaterGauge to collect.
        """

        def _transact(txn: LoggingTransaction) -> int:
            if isinstance(self.database_engine, Sqlite3Engine):
                query = """
                    SELECT COUNT(DISTINCT substr(out.user_id, pos+1))
                    FROM (
                        SELECT rm.user_id as user_id, instr(rm.user_id, ':')
                            AS pos FROM room_memberships as rm
                        INNER JOIN current_state_events as c ON rm.event_id = c.event_id
                        WHERE c.type = 'm.room.member'
                    ) as out
                """
            else:
                query = """
                    SELECT COUNT(DISTINCT split_part(state_key, ':', 2))
                    FROM current_state_events
                    WHERE type = 'm.room.member' AND membership = 'join';
                """
            txn.execute(query)
            return list(txn)[0][0]

        count = await self.db_pool.runInteraction("get_known_servers", _transact)

        # We always know about ourselves, even if we have nothing in
        # room_memberships (for example, the server is new).
        self._known_servers_count = max([count, 1])
        return self._known_servers_count

    @cached(max_entries=100000, iterable=True)
    async def get_users_in_room(self, room_id: str) -> Sequence[str]:
        """Returns a list of users in the room.

        Will return inaccurate results for rooms with partial state, since the state for
        the forward extremities of those rooms will exclude most members. We may also
        calculate room state incorrectly for such rooms and believe that a member is or
        is not in the room when the opposite is true.

        Note: If you only care about users in the room local to the homeserver, use
        `get_local_users_in_room(...)` instead which will be more performant.
        """
        return await self.db_pool.simple_select_onecol(
            table="current_state_events",
            keyvalues={
                "type": EventTypes.Member,
                "room_id": room_id,
                "membership": Membership.JOIN,
            },
            retcol="state_key",
            desc="get_users_in_room",
        )

    def get_users_in_room_txn(self, txn: LoggingTransaction, room_id: str) -> List[str]:
        """Returns a list of users in the room."""

        return self.db_pool.simple_select_onecol_txn(
            txn,
            table="current_state_events",
            keyvalues={
                "type": EventTypes.Member,
                "room_id": room_id,
                "membership": Membership.JOIN,
            },
            retcol="state_key",
        )

    @cached()
    def get_user_in_room_with_profile(self, room_id: str, user_id: str) -> ProfileInfo:
        raise NotImplementedError()

    @cachedList(
        cached_method_name="get_user_in_room_with_profile", list_name="user_ids"
    )
    async def get_subset_users_in_room_with_profiles(
        self, room_id: str, user_ids: Collection[str]
    ) -> Dict[str, ProfileInfo]:
        """Get a mapping from user ID to profile information for a list of users
        in a given room.

        The profile information comes directly from this room's `m.room.member`
        events, and so may be specific to this room rather than part of a user's
        global profile. To avoid privacy leaks, the profile data should only be
        revealed to users who are already in this room.

        Args:
            room_id: The ID of the room to retrieve the users of.
            user_ids: a list of users in the room to run the query for

        Returns:
                A mapping from user ID to ProfileInfo.
        """

        def _get_subset_users_in_room_with_profiles(
            txn: LoggingTransaction,
        ) -> Dict[str, ProfileInfo]:
            clause, ids = make_in_list_sql_clause(
                self.database_engine, "c.state_key", user_ids
            )

            sql = """
                SELECT state_key, display_name, avatar_url FROM room_memberships as m
                INNER JOIN current_state_events as c
                ON m.event_id = c.event_id
                AND m.room_id = c.room_id
                AND m.user_id = c.state_key
                WHERE c.type = 'm.room.member' AND c.room_id = ? AND m.membership = ? AND %s
            """ % (
                clause,
            )
            txn.execute(sql, (room_id, Membership.JOIN, *ids))

            return {r[0]: ProfileInfo(display_name=r[1], avatar_url=r[2]) for r in txn}

        return await self.db_pool.runInteraction(
            "get_subset_users_in_room_with_profiles",
            _get_subset_users_in_room_with_profiles,
        )

    @cached(max_entries=100000, iterable=True)
    async def get_users_in_room_with_profiles(
        self, room_id: str
    ) -> Mapping[str, ProfileInfo]:
        """Get a mapping from user ID to profile information for all users in a given room.

        The profile information comes directly from this room's `m.room.member`
        events, and so may be specific to this room rather than part of a user's
        global profile. To avoid privacy leaks, the profile data should only be
        revealed to users who are already in this room.

        Args:
            room_id: The ID of the room to retrieve the users of.

        Returns:
            A mapping from user ID to ProfileInfo.

        Preconditions:
          - There is full state available for the room (it is not partial-stated).
        """

        def _get_users_in_room_with_profiles(
            txn: LoggingTransaction,
        ) -> Dict[str, ProfileInfo]:
            sql = """
                SELECT state_key, display_name, avatar_url FROM room_memberships as m
                INNER JOIN current_state_events as c
                ON m.event_id = c.event_id
                AND m.room_id = c.room_id
                AND m.user_id = c.state_key
                WHERE c.type = 'm.room.member' AND c.room_id = ? AND m.membership = ?
            """
            txn.execute(sql, (room_id, Membership.JOIN))

            return {r[0]: ProfileInfo(display_name=r[1], avatar_url=r[2]) for r in txn}

        return await self.db_pool.runInteraction(
            "get_users_in_room_with_profiles",
            _get_users_in_room_with_profiles,
        )

    @cached(max_entries=100000)
    async def get_room_summary(self, room_id: str) -> Mapping[str, MemberSummary]:
        """Get the details of a room roughly suitable for use by the room
        summary extension to /sync. Useful when lazy loading room members.
        Args:
            room_id: The room ID to query
        Returns:
            dict of membership states, pointing to a MemberSummary named tuple.
        """

        def _get_room_summary_txn(
            txn: LoggingTransaction,
        ) -> Dict[str, MemberSummary]:
            # first get counts.
            # We do this all in one transaction to keep the cache small.
            # FIXME: get rid of this when we have room_stats

            # Note, rejected events will have a null membership field, so
            # we we manually filter them out.
            sql = """
                SELECT count(*), membership FROM current_state_events
                WHERE type = 'm.room.member' AND room_id = ?
                    AND membership IS NOT NULL
                GROUP BY membership
            """

            txn.execute(sql, (room_id,))
            res: Dict[str, MemberSummary] = {}
            for count, membership in txn:
                res.setdefault(membership, MemberSummary([], count))

            # we order by membership and then fairly arbitrarily by event_id so
            # heroes are consistent
            # Note, rejected events will have a null membership field, so
            # we we manually filter them out.
            sql = """
                SELECT state_key, membership, event_id
                FROM current_state_events
                WHERE type = 'm.room.member' AND room_id = ?
                    AND membership IS NOT NULL
                ORDER BY
                    CASE membership WHEN ? THEN 1 WHEN ? THEN 2 ELSE 3 END ASC,
                    event_id ASC
                LIMIT ?
            """

            # 6 is 5 (number of heroes) plus 1, in case one of them is the calling user.
            txn.execute(sql, (room_id, Membership.JOIN, Membership.INVITE, 6))
            for user_id, membership, event_id in txn:
                summary = res[membership]
                # we will always have a summary for this membership type at this
                # point given the summary currently contains the counts.
                members = summary.members
                members.append((user_id, event_id))

            return res

        return await self.db_pool.runInteraction(
            "get_room_summary", _get_room_summary_txn
        )

    @cached()
    async def get_number_joined_users_in_room(self, room_id: str) -> int:
        return await self.db_pool.simple_select_one_onecol(
            table="current_state_events",
            keyvalues={"room_id": room_id, "membership": Membership.JOIN},
            retcol="COUNT(*)",
            desc="get_number_joined_users_in_room",
        )

    @cached()
    async def get_invited_rooms_for_local_user(
        self, user_id: str
    ) -> Sequence[RoomsForUser]:
        """Get all the rooms the *local* user is invited to.

        Args:
            user_id: The user ID.

        Returns:
            A list of RoomsForUser.
        """

        return await self.get_rooms_for_local_user_where_membership_is(
            user_id, [Membership.INVITE]
        )

    async def get_invite_for_local_user_in_room(
        self, user_id: str, room_id: str
    ) -> Optional[RoomsForUser]:
        """Gets the invite for the given *local* user and room.

        Args:
            user_id: The user ID to find the invite of.
            room_id: The room to user was invited to.

        Returns:
            Either a RoomsForUser or None if no invite was found.
        """
        invites = await self.get_invited_rooms_for_local_user(user_id)
        for invite in invites:
            if invite.room_id == room_id:
                return invite
        return None

    async def get_rooms_for_local_user_where_membership_is(
        self,
        user_id: str,
        membership_list: Collection[str],
        excluded_rooms: StrCollection = (),
    ) -> List[RoomsForUser]:
        """Get all the rooms for this *local* user where the membership for this user
        matches one in the membership list.

        Filters out forgotten rooms.

        Args:
            user_id: The user ID.
            membership_list: A list of synapse.api.constants.Membership
                values which the user must be in.
            excluded_rooms: A list of rooms to ignore.

        Returns:
            The RoomsForUser that the user matches the membership types.
        """
        if not membership_list:
            return []

        rooms = await self.db_pool.runInteraction(
            "get_rooms_for_local_user_where_membership_is",
            self._get_rooms_for_local_user_where_membership_is_txn,
            user_id,
            membership_list,
        )

        # Now we filter out forgotten and excluded rooms
        rooms_to_exclude: AbstractSet[str] = set()

        # Users can't forget joined/invited rooms, so we skip the check for such look ups.
        if any(m not in (Membership.JOIN, Membership.INVITE) for m in membership_list):
            rooms_to_exclude = await self.get_forgotten_rooms_for_user(user_id)

        if excluded_rooms is not None:
            # Take a copy to avoid mutating the in-cache set
            rooms_to_exclude = set(rooms_to_exclude)
            rooms_to_exclude.update(excluded_rooms)

        return [room for room in rooms if room.room_id not in rooms_to_exclude]

    def _get_rooms_for_local_user_where_membership_is_txn(
        self,
        txn: LoggingTransaction,
        user_id: str,
        membership_list: List[str],
    ) -> List[RoomsForUser]:
        """Get all the rooms for this *local* user where the membership for this user
        matches one in the membership list.

        Args:
            user_id: The user ID.
            membership_list: A list of synapse.api.constants.Membership
                    values which the user must be in.

        Returns:
            The RoomsForUser that the user matches the membership types.
        """
        # Paranoia check.
        if not self.hs.is_mine_id(user_id):
            raise Exception(
                "Cannot call 'get_rooms_for_local_user_where_membership_is' on non-local user %r"
                % (user_id,),
            )

        clause, args = make_in_list_sql_clause(
            self.database_engine, "c.membership", membership_list
        )

        sql = """
            SELECT room_id, e.sender, c.membership, event_id, e.stream_ordering, r.room_version
            FROM local_current_membership AS c
            INNER JOIN events AS e USING (room_id, event_id)
            INNER JOIN rooms AS r USING (room_id)
            WHERE
                user_id = ?
                AND %s
        """ % (
            clause,
        )

        txn.execute(sql, (user_id, *args))
        results = [RoomsForUser(*r) for r in txn]

        return results

    @cached(iterable=True)
    async def get_local_users_in_room(self, room_id: str) -> Sequence[str]:
        """
        Retrieves a list of the current roommembers who are local to the server.
        """
        return await self.db_pool.simple_select_onecol(
            table="local_current_membership",
            keyvalues={"room_id": room_id, "membership": Membership.JOIN},
            retcol="user_id",
            desc="get_local_users_in_room",
        )

    async def check_local_user_in_room(self, user_id: str, room_id: str) -> bool:
        """
        Check whether a given local user is currently joined to the given room.

        Returns:
            A boolean indicating whether the user is currently joined to the room

        Raises:
            Exeption when called with a non-local user to this homeserver
        """
        if not self.hs.is_mine_id(user_id):
            raise Exception(
                "Cannot call 'check_local_user_in_room' on "
                "non-local user %s" % (user_id,),
            )

        (
            membership,
            member_event_id,
        ) = await self.get_local_current_membership_for_user_in_room(
            user_id=user_id,
            room_id=room_id,
        )

        return membership == Membership.JOIN

    async def is_server_notice_room(self, room_id: str) -> bool:
        """
        Determines whether the given room is a 'Server Notices' room, used for
        sending server notices to a user.

        This is determined by seeing whether the server notices user is present
        in the room.
        """
        if self._server_notices_mxid is None:
            return False
        is_server_notices_room = await self.check_local_user_in_room(
            user_id=self._server_notices_mxid, room_id=room_id
        )
        return is_server_notices_room

    async def get_local_current_membership_for_user_in_room(
        self, user_id: str, room_id: str
    ) -> Tuple[Optional[str], Optional[str]]:
        """Retrieve the current local membership state and event ID for a user in a room.

        Args:
            user_id: The ID of the user.
            room_id: The ID of the room.

        Returns:
            A tuple of (membership_type, event_id). Both will be None if a
                room_id/user_id pair is not found.
        """
        # Paranoia check.
        if not self.hs.is_mine_id(user_id):
            raise Exception(
                "Cannot call 'get_local_current_membership_for_user_in_room' on "
                "non-local user %s" % (user_id,),
            )

        results_dict = await self.db_pool.simple_select_one(
            "local_current_membership",
            {"room_id": room_id, "user_id": user_id},
            ("membership", "event_id"),
            allow_none=True,
            desc="get_local_current_membership_for_user_in_room",
        )
        if not results_dict:
            return None, None

        return results_dict.get("membership"), results_dict.get("event_id")

    @cached(max_entries=500000, iterable=True)
    async def get_rooms_for_user_with_stream_ordering(
        self, user_id: str
    ) -> FrozenSet[GetRoomsForUserWithStreamOrdering]:
        """Returns a set of room_ids the user is currently joined to.

        If a remote user only returns rooms this server is currently
        participating in.

        Args:
            user_id

        Returns:
            Returns the rooms the user is in currently, along with the stream
            ordering of the most recent join for that user and room, along with
            the room version of the room.
        """
        return await self.db_pool.runInteraction(
            "get_rooms_for_user_with_stream_ordering",
            self._get_rooms_for_user_with_stream_ordering_txn,
            user_id,
        )

    def _get_rooms_for_user_with_stream_ordering_txn(
        self, txn: LoggingTransaction, user_id: str
    ) -> FrozenSet[GetRoomsForUserWithStreamOrdering]:
        # We use `current_state_events` here and not `local_current_membership`
        # as a) this gets called with remote users and b) this only gets called
        # for rooms the server is participating in.
        sql = """
            SELECT room_id, e.instance_name, e.stream_ordering
            FROM current_state_events AS c
            INNER JOIN events AS e USING (room_id, event_id)
            WHERE
                c.type = 'm.room.member'
                AND c.state_key = ?
                AND c.membership = ?
        """

        txn.execute(sql, (user_id, Membership.JOIN))
        return frozenset(
            GetRoomsForUserWithStreamOrdering(
                room_id, PersistedEventPosition(instance, stream_id)
            )
            for room_id, instance, stream_id in txn
        )

    async def get_users_server_still_shares_room_with(
        self, user_ids: Collection[str]
    ) -> Set[str]:
        """Given a list of users return the set that the server still share a
        room with.
        """

        if not user_ids:
            return set()

        return await self.db_pool.runInteraction(
            "get_users_server_still_shares_room_with",
            self.get_users_server_still_shares_room_with_txn,
            user_ids,
        )

    def get_users_server_still_shares_room_with_txn(
        self,
        txn: LoggingTransaction,
        user_ids: Collection[str],
    ) -> Set[str]:
        if not user_ids:
            return set()

        sql = """
            SELECT state_key FROM current_state_events
            WHERE
                type = 'm.room.member'
                AND membership = 'join'
                AND %s
            GROUP BY state_key
        """

        clause, args = make_in_list_sql_clause(
            self.database_engine, "state_key", user_ids
        )

        txn.execute(sql % (clause,), args)

        return {row[0] for row in txn}

    @cached(max_entries=500000, iterable=True)
    async def get_rooms_for_user(self, user_id: str) -> FrozenSet[str]:
        """Returns a set of room_ids the user is currently joined to.

        If a remote user only returns rooms this server is currently
        participating in.
        """
        rooms = self.get_rooms_for_user_with_stream_ordering.cache.get_immediate(
            (user_id,),
            None,
            update_metrics=False,
        )
        if rooms:
            return frozenset(r.room_id for r in rooms)

        room_ids = await self.db_pool.simple_select_onecol(
            table="current_state_events",
            keyvalues={
                "type": EventTypes.Member,
                "membership": Membership.JOIN,
                "state_key": user_id,
            },
            retcol="room_id",
            desc="get_rooms_for_user",
        )

        return frozenset(room_ids)

    @cachedList(
        cached_method_name="get_rooms_for_user",
        list_name="user_ids",
    )
    async def _get_rooms_for_users(
        self, user_ids: Collection[str]
    ) -> Dict[str, FrozenSet[str]]:
        """A batched version of `get_rooms_for_user`.

        Returns:
            Map from user_id to set of rooms that is currently in.
        """

        rows = await self.db_pool.simple_select_many_batch(
            table="current_state_events",
            column="state_key",
            iterable=user_ids,
            retcols=(
                "state_key",
                "room_id",
            ),
            keyvalues={
                "type": EventTypes.Member,
                "membership": Membership.JOIN,
            },
            desc="get_rooms_for_users",
        )

        user_rooms: Dict[str, Set[str]] = {user_id: set() for user_id in user_ids}

        for row in rows:
            user_rooms[row["state_key"]].add(row["room_id"])

        return {key: frozenset(rooms) for key, rooms in user_rooms.items()}

    async def get_rooms_for_users(
        self, user_ids: Collection[str]
    ) -> Dict[str, FrozenSet[str]]:
        """A batched wrapper around `_get_rooms_for_users`, to prevent locking
        other calls to `get_rooms_for_user` for large user lists.
        """
        all_user_rooms: Dict[str, FrozenSet[str]] = {}

        # 250 users is pretty arbitrary but the data can be quite large if users
        # are in many rooms.
        for batch_user_ids in batch_iter(user_ids, 250):
            all_user_rooms.update(await self._get_rooms_for_users(batch_user_ids))

        return all_user_rooms

    @cached(max_entries=10000)
    async def does_pair_of_users_share_a_room(
        self, user_id: str, other_user_id: str
    ) -> bool:
        raise NotImplementedError()

    @cachedList(
        cached_method_name="does_pair_of_users_share_a_room", list_name="other_user_ids"
    )
    async def _do_users_share_a_room(
        self, user_id: str, other_user_ids: Collection[str]
    ) -> Mapping[str, Optional[bool]]:
        """Return mapping from user ID to whether they share a room with the
        given user.

        Note: `None` and `False` are equivalent and mean they don't share a
        room.
        """

        def do_users_share_a_room_txn(
            txn: LoggingTransaction, user_ids: Collection[str]
        ) -> Dict[str, bool]:
            clause, args = make_in_list_sql_clause(
                self.database_engine, "state_key", user_ids
            )

            # This query works by fetching both the list of rooms for the target
            # user and the set of other users, and then checking if there is any
            # overlap.
            sql = f"""
                SELECT DISTINCT b.state_key
                FROM (
                    SELECT room_id FROM current_state_events
                    WHERE type = 'm.room.member' AND membership = 'join' AND state_key = ?
                ) AS a
                INNER JOIN (
                    SELECT room_id, state_key FROM current_state_events
                    WHERE type = 'm.room.member' AND membership = 'join' AND {clause}
                ) AS b using (room_id)
            """

            txn.execute(sql, (user_id, *args))
            return {u: True for u, in txn}

        to_return = {}
        for batch_user_ids in batch_iter(other_user_ids, 1000):
            res = await self.db_pool.runInteraction(
                "do_users_share_a_room", do_users_share_a_room_txn, batch_user_ids
            )
            to_return.update(res)

        return to_return

    async def do_users_share_a_room(
        self, user_id: str, other_user_ids: Collection[str]
    ) -> Set[str]:
        """Return the set of users who share a room with the first users"""

        user_dict = await self._do_users_share_a_room(user_id, other_user_ids)

        return {u for u, share_room in user_dict.items() if share_room}

    async def get_users_who_share_room_with_user(self, user_id: str) -> Set[str]:
        """Returns the set of users who share a room with `user_id`"""
        room_ids = await self.get_rooms_for_user(user_id)

        user_who_share_room: Set[str] = set()
        for room_id in room_ids:
            user_ids = await self.get_users_in_room(room_id)
            user_who_share_room.update(user_ids)

        return user_who_share_room

    @cached(cache_context=True, iterable=True)
    async def get_mutual_rooms_between_users(
        self, user_ids: FrozenSet[str], cache_context: _CacheContext
    ) -> FrozenSet[str]:
        """
        Returns the set of rooms that all users in `user_ids` share.

        Args:
            user_ids: A frozen set of all users to investigate and return
              overlapping joined rooms for.
            cache_context
        """
        shared_room_ids: Optional[FrozenSet[str]] = None
        for user_id in user_ids:
            room_ids = await self.get_rooms_for_user(
                user_id, on_invalidate=cache_context.invalidate
            )
            if shared_room_ids is not None:
                shared_room_ids &= room_ids
            else:
                shared_room_ids = room_ids

        return shared_room_ids or frozenset()

    async def get_joined_user_ids_from_state(
        self, room_id: str, state: StateMap[str]
    ) -> Set[str]:
        """
        For a given set of state IDs, get a set of user IDs in the room.

        This method checks the local event cache, before calling
        `_get_user_ids_from_membership_event_ids` for any uncached events.
        """

        with Measure(self._clock, "get_joined_user_ids_from_state"):
            users_in_room = set()
            member_event_ids = [
                e_id for key, e_id in state.items() if key[0] == EventTypes.Member
            ]

            # We check if we have any of the member event ids in the event cache
            # before we ask the DB

            # We don't update the event cache hit ratio as it completely throws off
            # the hit ratio counts. After all, we don't populate the cache if we
            # miss it here
            event_map = self._get_events_from_local_cache(
                member_event_ids, update_metrics=False
            )

            missing_member_event_ids = []
            for event_id in member_event_ids:
                ev_entry = event_map.get(event_id)
                if ev_entry and not ev_entry.event.rejected_reason:
                    if ev_entry.event.membership == Membership.JOIN:
                        users_in_room.add(ev_entry.event.state_key)
                else:
                    missing_member_event_ids.append(event_id)

            if missing_member_event_ids:
                event_to_memberships = (
                    await self._get_user_ids_from_membership_event_ids(
                        missing_member_event_ids
                    )
                )
                users_in_room.update(
                    user_id for user_id in event_to_memberships.values() if user_id
                )

            return users_in_room

    @cached(
        max_entries=10000,
        # This name matches the old function that has been replaced - the cache name
        # is kept here to maintain backwards compatibility.
        name="_get_joined_profile_from_event_id",
    )
    def _get_user_id_from_membership_event_id(
        self, event_id: str
    ) -> Optional[Tuple[str, ProfileInfo]]:
        raise NotImplementedError()

    @cachedList(
        cached_method_name="_get_user_id_from_membership_event_id",
        list_name="event_ids",
    )
    async def _get_user_ids_from_membership_event_ids(
        self, event_ids: Iterable[str]
    ) -> Dict[str, Optional[str]]:
        """For given set of member event_ids check if they point to a join
        event.

        Args:
            event_ids: The member event IDs to lookup

        Returns:
            Map from event ID to `user_id`, or None if event is not a join.
        """

        rows = await self.db_pool.simple_select_many_batch(
            table="room_memberships",
            column="event_id",
            iterable=event_ids,
            retcols=("user_id", "event_id"),
            keyvalues={"membership": Membership.JOIN},
            batch_size=1000,
            desc="_get_user_ids_from_membership_event_ids",
        )

        return {row["event_id"]: row["user_id"] for row in rows}

    @cached(max_entries=10000)
    async def is_host_joined(self, room_id: str, host: str) -> bool:
        return await self._check_host_room_membership(room_id, host, Membership.JOIN)

    @cached(max_entries=10000)
    async def is_host_invited(self, room_id: str, host: str) -> bool:
        return await self._check_host_room_membership(room_id, host, Membership.INVITE)

    async def _check_host_room_membership(
        self, room_id: str, host: str, membership: str
    ) -> bool:
        if "%" in host or "_" in host:
            raise Exception("Invalid host name")

        sql = """
            SELECT state_key FROM current_state_events
            WHERE membership = ?
                AND type = 'm.room.member'
                AND room_id = ?
                AND state_key LIKE ?
            LIMIT 1
        """

        # We do need to be careful to ensure that host doesn't have any wild cards
        # in it, but we checked above for known ones and we'll check below that
        # the returned user actually has the correct domain.
        like_clause = "%:" + host

        rows = await self.db_pool.execute(
            "is_host_joined", None, sql, membership, room_id, like_clause
        )

        if not rows:
            return False

        user_id = rows[0][0]
        if get_domain_from_id(user_id) != host:
            # This can only happen if the host name has something funky in it
            raise Exception("Invalid host name")

        return True

    @cached(iterable=True, max_entries=10000)
    async def get_current_hosts_in_room(self, room_id: str) -> AbstractSet[str]:
        """Get current hosts in room based on current state."""

        # First we check if we already have `get_users_in_room` in the cache, as
        # we can just calculate result from that
        users = self.get_users_in_room.cache.get_immediate(
            (room_id,), None, update_metrics=False
        )
        if users is not None:
            return {get_domain_from_id(u) for u in users}

        if isinstance(self.database_engine, Sqlite3Engine):
            # If we're using SQLite then let's just always use
            # `get_users_in_room` rather than funky SQL.
            users = await self.get_users_in_room(room_id)
            return {get_domain_from_id(u) for u in users}

        # For PostgreSQL we can use a regex to pull out the domains from the
        # joined users in `current_state_events` via regex.

        def get_current_hosts_in_room_txn(txn: LoggingTransaction) -> Set[str]:
            sql = """
                SELECT DISTINCT substring(state_key FROM '@[^:]*:(.*)$')
                FROM current_state_events
                WHERE
                    type = 'm.room.member'
                    AND membership = 'join'
                    AND room_id = ?
            """
            txn.execute(sql, (room_id,))
            return {d for d, in txn}

        return await self.db_pool.runInteraction(
            "get_current_hosts_in_room", get_current_hosts_in_room_txn
        )

    @cached(iterable=True, max_entries=10000)
    async def get_current_hosts_in_room_ordered(self, room_id: str) -> List[str]:
        """
        Get current hosts in room based on current state.

        The heuristic of sorting by servers who have been in the room the
        longest is good because they're most likely to have anything we ask
        about.

        For SQLite the returned list is not ordered, as SQLite doesn't support
        the appropriate SQL.

        Uses `m.room.member`s in the room state at the current forward
        extremities to determine which hosts are in the room.

        Will return inaccurate results for rooms with partial state, since the
        state for the forward extremities of those rooms will exclude most
        members. We may also calculate room state incorrectly for such rooms and
        believe that a host is or is not in the room when the opposite is true.

        Returns:
            Returns a list of servers sorted by longest in the room first. (aka.
            sorted by join with the lowest depth first).
        """

        if isinstance(self.database_engine, Sqlite3Engine):
            # If we're using SQLite then let's just always use
            # `get_users_in_room` rather than funky SQL.

            domains = await self.get_current_hosts_in_room(room_id)
            return list(domains)

        # For PostgreSQL we can use a regex to pull out the domains from the
        # joined users in `current_state_events` via regex.

        def get_current_hosts_in_room_ordered_txn(txn: LoggingTransaction) -> List[str]:
            # Returns a list of servers currently joined in the room sorted by
            # longest in the room first (aka. with the lowest depth). The
            # heuristic of sorting by servers who have been in the room the
            # longest is good because they're most likely to have anything we
            # ask about.
            sql = """
                SELECT
                    /* Match the domain part of the MXID */
                    substring(c.state_key FROM '@[^:]*:(.*)$') as server_domain
                FROM current_state_events c
                /* Get the depth of the event from the events table */
                INNER JOIN events AS e USING (event_id)
                WHERE
                    /* Find any join state events in the room */
                    c.type = 'm.room.member'
                    AND c.membership = 'join'
                    AND c.room_id = ?
                /* Group all state events from the same domain into their own buckets (groups) */
                GROUP BY server_domain
                /* Sorted by lowest depth first */
                ORDER BY min(e.depth) ASC;
            """
            txn.execute(sql, (room_id,))
            # `server_domain` will be `NULL` for malformed MXIDs with no colons.
            return [d for d, in txn if d is not None]

        return await self.db_pool.runInteraction(
            "get_current_hosts_in_room_ordered", get_current_hosts_in_room_ordered_txn
        )

    async def get_joined_hosts(
        self, room_id: str, state: StateMap[str], state_entry: "_StateCacheEntry"
    ) -> FrozenSet[str]:
        state_group: Union[object, int] = state_entry.state_group
        if not state_group:
            # If state_group is None it means it has yet to be assigned a
            # state group, i.e. we need to make sure that calls with a state_group
            # of None don't hit previous cached calls with a None state_group.
            # To do this we set the state_group to a new object as object() != object()
            state_group = object()

        assert state_group is not None
        with Measure(self._clock, "get_joined_hosts"):
            return await self._get_joined_hosts(
                room_id, state_group, state, state_entry=state_entry
            )

    @cached(num_args=2, max_entries=10000, iterable=True)
    async def _get_joined_hosts(
        self,
        room_id: str,
        state_group: Union[object, int],
        state: StateMap[str],
        state_entry: "_StateCacheEntry",
    ) -> FrozenSet[str]:
        # We don't use `state_group`, it's there so that we can cache based on
        # it. However, its important that its never None, since two
        # current_state's with a state_group of None are likely to be different.
        #
        # The `state_group` must match the `state_entry.state_group` (if not None).
        assert state_group is not None
        assert state_entry.state_group is None or state_entry.state_group == state_group

        # We use a secondary cache of previous work to allow us to build up the
        # joined hosts for the given state group based on previous state groups.
        #
        # We cache one object per room containing the results of the last state
        # group we got joined hosts for. The idea is that generally
        # `get_joined_hosts` is called with the "current" state group for the
        # room, and so consecutive calls will be for consecutive state groups
        # which point to the previous state group.
        cache = await self._get_joined_hosts_cache(room_id)

        # If the state group in the cache matches, we already have the data we need.
        if state_entry.state_group == cache.state_group:
            return frozenset(cache.hosts_to_joined_users)

        # Since we'll mutate the cache we need to lock.
        async with self._joined_host_linearizer.queue(room_id):
            if state_entry.state_group == cache.state_group:
                # Same state group, so nothing to do. We've already checked for
                # this above, but the cache may have changed while waiting on
                # the lock.
                pass
            elif state_entry.prev_group == cache.state_group:
                # The cached work is for the previous state group, so we work out
                # the delta.
                assert state_entry.delta_ids is not None
                for (typ, state_key), event_id in state_entry.delta_ids.items():
                    if typ != EventTypes.Member:
                        continue

                    host = intern_string(get_domain_from_id(state_key))
                    user_id = state_key
                    known_joins = cache.hosts_to_joined_users.setdefault(host, set())

                    event = await self.get_event(event_id)
                    if event.membership == Membership.JOIN:
                        known_joins.add(user_id)
                    else:
                        known_joins.discard(user_id)

                        if not known_joins:
                            cache.hosts_to_joined_users.pop(host, None)
            else:
                # The cache doesn't match the state group or prev state group,
                # so we calculate the result from first principles.
                #
                # We need to fetch all hosts joined to the room according to `state` by
                # inspecting all join memberships in `state`. However, if the `state` is
                # relatively recent then many of its events are likely to be held in
                # the current state of the room, which is easily available and likely
                # cached.
                #
                # We therefore compute the set of `state` events not in the
                # current state and only fetch those.
                current_memberships = (
                    await self._get_approximate_current_memberships_in_room(room_id)
                )
                unknown_state_events = {}
                joined_users_in_current_state = []

                for (type, state_key), event_id in state.items():
                    if event_id not in current_memberships:
                        unknown_state_events[type, state_key] = event_id
                    elif current_memberships[event_id] == Membership.JOIN:
                        joined_users_in_current_state.append(state_key)

                joined_user_ids = await self.get_joined_user_ids_from_state(
                    room_id, unknown_state_events
                )

                cache.hosts_to_joined_users = {}
                for user_id in chain(joined_user_ids, joined_users_in_current_state):
                    host = intern_string(get_domain_from_id(user_id))
                    cache.hosts_to_joined_users.setdefault(host, set()).add(user_id)

            if state_entry.state_group:
                cache.state_group = state_entry.state_group
            else:
                cache.state_group = object()

        return frozenset(cache.hosts_to_joined_users)

    async def _get_approximate_current_memberships_in_room(
        self, room_id: str
    ) -> Mapping[str, Optional[str]]:
        """Build a map from event id to membership, for all events in the current state.

        The event ids of non-memberships events (e.g. `m.room.power_levels`) are present
        in the result, mapped to values of `None`.

        The result is approximate for partially-joined rooms. It is fully accurate
        for fully-joined rooms.
        """

        rows = await self.db_pool.simple_select_list(
            "current_state_events",
            keyvalues={"room_id": room_id},
            retcols=("event_id", "membership"),
            desc="has_completed_background_updates",
        )
        return {row["event_id"]: row["membership"] for row in rows}

    @cached(max_entries=10000)
    def _get_joined_hosts_cache(self, room_id: str) -> "_JoinedHostsCache":
        return _JoinedHostsCache()

    @cached(num_args=2)
    async def did_forget(self, user_id: str, room_id: str) -> bool:
        """Returns whether user_id has elected to discard history for room_id.

        Returns False if they have since re-joined."""

        def f(txn: LoggingTransaction) -> int:
            sql = (
                "SELECT"
                "  COUNT(*)"
                " FROM"
                "  room_memberships"
                " WHERE"
                "  user_id = ?"
                " AND"
                "  room_id = ?"
                " AND"
                "  forgotten = 0"
            )
            txn.execute(sql, (user_id, room_id))
            rows = txn.fetchall()
            return rows[0][0]

        count = await self.db_pool.runInteraction("did_forget_membership", f)
        return count == 0

    @cached()
    async def get_forgotten_rooms_for_user(self, user_id: str) -> AbstractSet[str]:
        """Gets all rooms the user has forgotten.

        Args:
            user_id: The user ID to query the rooms of.

        Returns:
            The forgotten rooms.
        """

        def _get_forgotten_rooms_for_user_txn(txn: LoggingTransaction) -> Set[str]:
            # This is a slightly convoluted query that first looks up all rooms
            # that the user has forgotten in the past, then rechecks that list
            # to see if any have subsequently been updated. This is done so that
            # we can use a partial index on `forgotten = 1` on the assumption
            # that few users will actually forget many rooms.
            #
            # Note that a room is considered "forgotten" if *all* membership
            # events for that user and room have the forgotten field set (as
            # when a user forgets a room we update all rows for that user and
            # room, not just the current one).
            sql = """
                SELECT room_id, (
                    SELECT count(*) FROM room_memberships
                    WHERE room_id = m.room_id AND user_id = m.user_id AND forgotten = 0
                ) AS count
                FROM room_memberships AS m
                WHERE user_id = ? AND forgotten = 1
                GROUP BY room_id, user_id;
            """
            txn.execute(sql, (user_id,))
            return {row[0] for row in txn if row[1] == 0}

        return await self.db_pool.runInteraction(
            "get_forgotten_rooms_for_user", _get_forgotten_rooms_for_user_txn
        )

    async def is_locally_forgotten_room(self, room_id: str) -> bool:
        """Returns whether all local users have forgotten this room_id.

        Args:
            room_id: The room ID to query.

        Returns:
            Whether the room is forgotten.
        """

        sql = """
            SELECT count(*) > 0 FROM local_current_membership
            INNER JOIN room_memberships USING (room_id, event_id)
            WHERE
                room_id = ?
                AND forgotten = 0;
        """

        rows = await self.db_pool.execute("is_forgotten_room", None, sql, room_id)

        # `count(*)` returns always an integer
        # If any rows still exist it means someone has not forgotten this room yet
        return not rows[0][0]

    async def get_rooms_user_has_been_in(self, user_id: str) -> Set[str]:
        """Get all rooms that the user has ever been in.

        Args:
            user_id: The user ID to get the rooms of.

        Returns:
            Set of room IDs.
        """

        room_ids = await self.db_pool.simple_select_onecol(
            table="room_memberships",
            keyvalues={"membership": Membership.JOIN, "user_id": user_id},
            retcol="room_id",
            desc="get_rooms_user_has_been_in",
        )

        return set(room_ids)

    @cached(max_entries=5000)
    async def _get_membership_from_event_id(
        self, member_event_id: str
    ) -> Optional[EventIdMembership]:
        raise NotImplementedError()

    @cachedList(
        cached_method_name="_get_membership_from_event_id", list_name="member_event_ids"
    )
    async def get_membership_from_event_ids(
        self, member_event_ids: Iterable[str]
    ) -> Dict[str, Optional[EventIdMembership]]:
        """Get user_id and membership of a set of event IDs.

        Returns:
            Mapping from event ID to `EventIdMembership` if the event is a
            membership event, otherwise the value is None.
        """

        rows = await self.db_pool.simple_select_many_batch(
            table="room_memberships",
            column="event_id",
            iterable=member_event_ids,
            retcols=("user_id", "membership", "event_id"),
            keyvalues={},
            batch_size=500,
            desc="get_membership_from_event_ids",
        )

        return {
            row["event_id"]: EventIdMembership(
                membership=row["membership"], user_id=row["user_id"]
            )
            for row in rows
        }

    async def is_local_host_in_room_ignoring_users(
        self, room_id: str, ignore_users: Collection[str]
    ) -> bool:
        """Check if there are any local users, excluding those in the given
        list, in the room.
        """

        clause, args = make_in_list_sql_clause(
            self.database_engine, "user_id", ignore_users
        )

        sql = """
            SELECT 1 FROM local_current_membership
            WHERE
                room_id = ? AND membership = ?
                AND NOT (%s)
                LIMIT 1
        """ % (
            clause,
        )

        def _is_local_host_in_room_ignoring_users_txn(
            txn: LoggingTransaction,
        ) -> bool:
            txn.execute(sql, (room_id, Membership.JOIN, *args))

            return bool(txn.fetchone())

        return await self.db_pool.runInteraction(
            "is_local_host_in_room_ignoring_users",
            _is_local_host_in_room_ignoring_users_txn,
        )

    async def forget(self, user_id: str, room_id: str) -> None:
        """Indicate that user_id wishes to discard history for room_id."""

        def f(txn: LoggingTransaction) -> None:
            self.db_pool.simple_update_txn(
                txn,
                table="room_memberships",
                keyvalues={"user_id": user_id, "room_id": room_id},
                updatevalues={"forgotten": 1},
            )

            self._invalidate_cache_and_stream(txn, self.did_forget, (user_id, room_id))
            self._invalidate_cache_and_stream(
                txn, self.get_forgotten_rooms_for_user, (user_id,)
            )

        await self.db_pool.runInteraction("forget_membership", f)

    async def get_room_forgetter_stream_pos(self) -> int:
        """Get the stream position of the background process to forget rooms when left
        by users.
        """
        return await self.db_pool.simple_select_one_onecol(
            table="room_forgetter_stream_pos",
            keyvalues={},
            retcol="stream_id",
            desc="room_forgetter_stream_pos",
        )

    async def update_room_forgetter_stream_pos(self, stream_id: int) -> None:
        """Update the stream position of the background process to forget rooms when
        left by users.

        Must only be used by the worker running the background process.
        """
        assert self.hs.config.worker.run_background_tasks

        await self.db_pool.simple_update_one(
            table="room_forgetter_stream_pos",
            keyvalues={},
            updatevalues={"stream_id": stream_id},
            desc="room_forgetter_stream_pos",
        )


class RoomMemberBackgroundUpdateStore(SQLBaseStore):
    def __init__(
        self,
        database: DatabasePool,
        db_conn: LoggingDatabaseConnection,
        hs: "HomeServer",
    ):
        super().__init__(database, db_conn, hs)
        self.db_pool.updates.register_background_update_handler(
            _MEMBERSHIP_PROFILE_UPDATE_NAME, self._background_add_membership_profile
        )
        self.db_pool.updates.register_background_update_handler(
            _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME,
            self._background_current_state_membership,
        )
        self.db_pool.updates.register_background_index_update(
            "room_membership_forgotten_idx",
            index_name="room_memberships_user_room_forgotten",
            table="room_memberships",
            columns=["user_id", "room_id"],
            where_clause="forgotten = 1",
        )
        self.db_pool.updates.register_background_index_update(
            "room_membership_user_room_index",
            index_name="room_membership_user_room_idx",
            table="room_memberships",
            columns=["user_id", "room_id"],
        )

    async def _background_add_membership_profile(
        self, progress: JsonDict, batch_size: int
    ) -> int:
        target_min_stream_id = progress.get(
            "target_min_stream_id_inclusive", self._min_stream_order_on_start  # type: ignore[attr-defined]
        )
        max_stream_id = progress.get(
            "max_stream_id_exclusive", self._stream_order_on_start + 1  # type: ignore[attr-defined]
        )

        def add_membership_profile_txn(txn: LoggingTransaction) -> int:
            sql = """
                SELECT stream_ordering, event_id, events.room_id, event_json.json
                FROM events
                INNER JOIN event_json USING (event_id)
                WHERE ? <= stream_ordering AND stream_ordering < ?
                AND type = 'm.room.member'
                ORDER BY stream_ordering DESC
                LIMIT ?
            """

            txn.execute(sql, (target_min_stream_id, max_stream_id, batch_size))

            rows = self.db_pool.cursor_to_dict(txn)
            if not rows:
                return 0

            min_stream_id = rows[-1]["stream_ordering"]

            to_update = []
            for row in rows:
                event_id = row["event_id"]
                room_id = row["room_id"]
                try:
                    event_json = db_to_json(row["json"])
                    content = event_json["content"]
                except Exception:
                    continue

                display_name = content.get("displayname", None)
                avatar_url = content.get("avatar_url", None)

                if display_name or avatar_url:
                    to_update.append((display_name, avatar_url, event_id, room_id))

            to_update_sql = """
                UPDATE room_memberships SET display_name = ?, avatar_url = ?
                WHERE event_id = ? AND room_id = ?
            """
            txn.execute_batch(to_update_sql, to_update)

            progress = {
                "target_min_stream_id_inclusive": target_min_stream_id,
                "max_stream_id_exclusive": min_stream_id,
            }

            self.db_pool.updates._background_update_progress_txn(
                txn, _MEMBERSHIP_PROFILE_UPDATE_NAME, progress
            )

            return len(rows)

        result = await self.db_pool.runInteraction(
            _MEMBERSHIP_PROFILE_UPDATE_NAME, add_membership_profile_txn
        )

        if not result:
            await self.db_pool.updates._end_background_update(
                _MEMBERSHIP_PROFILE_UPDATE_NAME
            )

        return result

    async def _background_current_state_membership(
        self, progress: JsonDict, batch_size: int
    ) -> int:
        """Update the new membership column on current_state_events.

        This works by iterating over all rooms in alphebetical order.
        """

        def _background_current_state_membership_txn(
            txn: LoggingTransaction, last_processed_room: str
        ) -> Tuple[int, bool]:
            processed = 0
            while processed < batch_size:
                txn.execute(
                    """
                        SELECT MIN(room_id) FROM current_state_events WHERE room_id > ?
                    """,
                    (last_processed_room,),
                )
                row = txn.fetchone()
                if not row or not row[0]:
                    return processed, True

                (next_room,) = row

                sql = """
                    UPDATE current_state_events
                    SET membership = (
                        SELECT membership FROM room_memberships
                        WHERE event_id = current_state_events.event_id
                    )
                    WHERE room_id = ?
                """
                txn.execute(sql, (next_room,))
                processed += txn.rowcount

                last_processed_room = next_room

            self.db_pool.updates._background_update_progress_txn(
                txn,
                _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME,
                {"last_processed_room": last_processed_room},
            )

            return processed, False

        # If we haven't got a last processed room then just use the empty
        # string, which will compare before all room IDs correctly.
        last_processed_room = progress.get("last_processed_room", "")

        row_count, finished = await self.db_pool.runInteraction(
            "_background_current_state_membership_update",
            _background_current_state_membership_txn,
            last_processed_room,
        )

        if finished:
            await self.db_pool.updates._end_background_update(
                _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME
            )

        return row_count


class RoomMemberStore(
    RoomMemberWorkerStore,
    RoomMemberBackgroundUpdateStore,
    CacheInvalidationWorkerStore,
):
    def __init__(
        self,
        database: DatabasePool,
        db_conn: LoggingDatabaseConnection,
        hs: "HomeServer",
    ):
        super().__init__(database, db_conn, hs)


def extract_heroes_from_room_summary(
    details: Mapping[str, MemberSummary], me: str
) -> List[str]:
    """Determine the users that represent a room, from the perspective of the `me` user.

    The rules which say which users we select are specified in the "Room Summary"
    section of
    https://spec.matrix.org/v1.4/client-server-api/#get_matrixclientv3sync

    Returns a list (possibly empty) of heroes' mxids.
    """
    empty_ms = MemberSummary([], 0)

    joined_user_ids = [
        r[0] for r in details.get(Membership.JOIN, empty_ms).members if r[0] != me
    ]
    invited_user_ids = [
        r[0] for r in details.get(Membership.INVITE, empty_ms).members if r[0] != me
    ]
    gone_user_ids = [
        r[0] for r in details.get(Membership.LEAVE, empty_ms).members if r[0] != me
    ] + [r[0] for r in details.get(Membership.BAN, empty_ms).members if r[0] != me]

    # FIXME: order by stream ordering rather than as returned by SQL
    if joined_user_ids or invited_user_ids:
        return sorted(joined_user_ids + invited_user_ids)[0:5]
    else:
        return sorted(gone_user_ids)[0:5]


@attr.s(slots=True, auto_attribs=True)
class _JoinedHostsCache:
    """The cached data used by the `_get_joined_hosts_cache`."""

    # Dict of host to the set of their users in the room at the state group.
    hosts_to_joined_users: Dict[str, Set[str]] = attr.Factory(dict)

    # The state group `hosts_to_joined_users` is derived from. Will be an object
    # if the instance is newly created or if the state is not based on a state
    # group. (An object is used as a sentinel value to ensure that it never is
    # equal to anything else).
    state_group: Union[object, int] = attr.Factory(object)

    def __len__(self) -> int:
        return sum(len(v) for v in self.hosts_to_joined_users.values())