# 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 typing import (
    TYPE_CHECKING,
    Collection,
    Dict,
    FrozenSet,
    Iterable,
    List,
    Optional,
    Set,
    Tuple,
    Union,
)

import attr

from synapse.api.constants import EventTypes, Membership
from synapse.events import EventBase
from synapse.events.snapshot import EventContext
from synapse.metrics import LaterGauge
from synapse.metrics.background_process_metrics import (
    run_as_background_process,
    wrap_as_background_process,
)
from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
from synapse.storage.database import DatabasePool
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 PersistedEventPosition, StateMap, 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.metrics import Measure

if TYPE_CHECKING:
    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"


class RoomMemberWorkerStore(EventsWorkerStore):
    def __init__(self, database: DatabasePool, db_conn, hs):
        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")

        # Is the current_state_events.membership up to date? Or is the
        # background update still running?
        self._current_state_events_membership_up_to_date = False

        txn = db_conn.cursor(
            txn_name="_check_safe_current_state_events_membership_updated"
        )
        self._check_safe_current_state_events_membership_updated_txn(txn)
        txn.close()

        if (
            self.hs.config.run_background_tasks
            and self.hs.config.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):
        """
        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):
            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

    def _check_safe_current_state_events_membership_updated_txn(self, txn):
        """Checks if it is safe to assume the new current_state_events
        membership column is up to date
        """

        pending_update = self.db_pool.simple_select_one_txn(
            txn,
            table="background_updates",
            keyvalues={"update_name": _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME},
            retcols=["update_name"],
            allow_none=True,
        )

        self._current_state_events_membership_up_to_date = not pending_update

        # If the update is still running, reschedule to run.
        if pending_update:
            self._clock.call_later(
                15.0,
                run_as_background_process,
                "_check_safe_current_state_events_membership_updated",
                self.db_pool.runInteraction,
                "_check_safe_current_state_events_membership_updated",
                self._check_safe_current_state_events_membership_updated_txn,
            )

    @cached(max_entries=100000, iterable=True)
    async def get_users_in_room(self, room_id: str) -> List[str]:
        return await self.db_pool.runInteraction(
            "get_users_in_room", self.get_users_in_room_txn, room_id
        )

    def get_users_in_room_txn(self, txn, room_id: str) -> List[str]:
        # If we can assume current_state_events.membership is up to date
        # then we can avoid a join, which is a Very Good Thing given how
        # frequently this function gets called.
        if self._current_state_events_membership_up_to_date:
            sql = """
                SELECT state_key FROM current_state_events
                WHERE type = 'm.room.member' AND room_id = ? AND membership = ?
            """
        else:
            sql = """
                SELECT state_key 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] for r in txn]

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

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

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

        def _get_users_in_room_with_profiles(txn) -> 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) -> Dict[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):
            # 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

            # If we can assume current_state_events.membership is up to date
            # then we can avoid a join, which is a Very Good Thing given how
            # frequently this function gets called.
            if self._current_state_events_membership_up_to_date:
                # 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
                """
            else:
                sql = """
                    SELECT count(*), m.membership 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 = ?
                    GROUP BY m.membership
                """

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

            # we order by membership and then fairly arbitrarily by event_id so
            # heroes are consistent
            if self._current_state_events_membership_up_to_date:
                # 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 ?
                """
            else:
                sql = """
                    SELECT c.state_key, m.membership, c.event_id
                    FROM room_memberships as m
                    INNER JOIN current_state_events as c USING (room_id, event_id)
                    WHERE c.type = 'm.room.member' AND c.room_id = ?
                    ORDER BY
                        CASE m.membership WHEN ? THEN 1 WHEN ? THEN 2 ELSE 3 END ASC,
                        c.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_invited_rooms_for_local_user(self, user_id: str) -> 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]
    ) -> 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.

        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 rooms
        forgotten_rooms = await self.get_forgotten_rooms_for_user(user_id)
        return [room for room in rooms if room.room_id not in forgotten_rooms]

    def _get_rooms_for_local_user_where_membership_is_txn(
        self, txn, user_id: str, membership_list: List[str]
    ) -> List[RoomsForUser]:
        # 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
            FROM local_current_membership AS c
            INNER JOIN events AS e USING (room_id, event_id)
            WHERE
                user_id = ?
                AND %s
        """ % (
            clause,
        )

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

        return results

    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.
        """
        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, 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.
        if self._current_state_events_membership_up_to_date:
            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 state_key = ?
                    AND c.membership = ?
            """
        else:
            sql = """
                SELECT room_id, e.instance_name, e.stream_ordering
                FROM current_state_events AS c
                INNER JOIN room_memberships AS m USING (room_id, event_id)
                INNER JOIN events AS e USING (room_id, event_id)
                WHERE
                    c.type = 'm.room.member'
                    AND state_key = ?
                    AND m.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()

        def _get_users_server_still_shares_room_with_txn(txn):
            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}

        return await self.db_pool.runInteraction(
            "get_users_server_still_shares_room_with",
            _get_users_server_still_shares_room_with_txn,
        )

    async def get_rooms_for_user(self, user_id: str, on_invalidate=None):
        """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 = await self.get_rooms_for_user_with_stream_ordering(
            user_id, on_invalidate=on_invalidate
        )
        return frozenset(r.room_id for r in rooms)

    @cached(max_entries=500000, cache_context=True, iterable=True)
    async def get_users_who_share_room_with_user(
        self, user_id: str, cache_context: _CacheContext
    ) -> Set[str]:
        """Returns the set of users who share a room with `user_id`"""
        room_ids = await self.get_rooms_for_user(
            user_id, on_invalidate=cache_context.invalidate
        )

        user_who_share_room = set()
        for room_id in room_ids:
            user_ids = await self.get_users_in_room(
                room_id, on_invalidate=cache_context.invalidate
            )
            user_who_share_room.update(user_ids)

        return user_who_share_room

    async def get_joined_users_from_context(
        self, event: EventBase, context: EventContext
    ):
        state_group = context.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()

        current_state_ids = await context.get_current_state_ids()
        return await self._get_joined_users_from_context(
            event.room_id, state_group, current_state_ids, event=event, context=context
        )

    async def get_joined_users_from_state(self, room_id, state_entry):
        state_group = 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()

        with Measure(self._clock, "get_joined_users_from_state"):
            return await self._get_joined_users_from_context(
                room_id, state_group, state_entry.state, context=state_entry
            )

    @cached(num_args=2, cache_context=True, iterable=True, max_entries=100000)
    async def _get_joined_users_from_context(
        self,
        room_id,
        state_group,
        current_state_ids,
        cache_context,
        event=None,
        context=None,
    ):
        # We don't use `state_group`, it's there so that we can cache based
        # on it. However, it's important that it's never None, since two current_states
        # with a state_group of None are likely to be different.
        assert state_group is not None

        users_in_room = {}
        member_event_ids = [
            e_id
            for key, e_id in current_state_ids.items()
            if key[0] == EventTypes.Member
        ]

        if context is not None:
            # If we have a context with a delta from a previous state group,
            # check if we also have the result from the previous group in cache.
            # If we do then we can reuse that result and simply update it with
            # any membership changes in `delta_ids`
            if context.prev_group and context.delta_ids:
                prev_res = self._get_joined_users_from_context.cache.get_immediate(
                    (room_id, context.prev_group), None
                )
                if prev_res and isinstance(prev_res, dict):
                    users_in_room = dict(prev_res)
                    member_event_ids = [
                        e_id
                        for key, e_id in context.delta_ids.items()
                        if key[0] == EventTypes.Member
                    ]
                    for etype, state_key in context.delta_ids:
                        if etype == EventTypes.Member:
                            users_in_room.pop(state_key, None)

        # 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_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[ev_entry.event.state_key] = ProfileInfo(
                        display_name=ev_entry.event.content.get("displayname", None),
                        avatar_url=ev_entry.event.content.get("avatar_url", None),
                    )
            else:
                missing_member_event_ids.append(event_id)

        if missing_member_event_ids:
            event_to_memberships = await self._get_joined_profiles_from_event_ids(
                missing_member_event_ids
            )
            users_in_room.update(row for row in event_to_memberships.values() if row)

        if event is not None and event.type == EventTypes.Member:
            if event.membership == Membership.JOIN:
                if event.event_id in member_event_ids:
                    users_in_room[event.state_key] = ProfileInfo(
                        display_name=event.content.get("displayname", None),
                        avatar_url=event.content.get("avatar_url", None),
                    )

        return users_in_room

    @cached(max_entries=10000)
    def _get_joined_profile_from_event_id(self, event_id):
        raise NotImplementedError()

    @cachedList(
        cached_method_name="_get_joined_profile_from_event_id",
        list_name="event_ids",
    )
    async def _get_joined_profiles_from_event_ids(self, event_ids: Iterable[str]):
        """For given set of member event_ids check if they point to a join
        event and if so return the associated user and profile info.

        Args:
            event_ids: The member event IDs to lookup

        Returns:
            dict[str, Tuple[str, ProfileInfo]|None]: Map from event ID
            to `user_id` and ProfileInfo (or None if not join event).
        """

        rows = await self.db_pool.simple_select_many_batch(
            table="room_memberships",
            column="event_id",
            iterable=event_ids,
            retcols=("user_id", "display_name", "avatar_url", "event_id"),
            keyvalues={"membership": Membership.JOIN},
            batch_size=500,
            desc="_get_membership_from_event_ids",
        )

        return {
            row["event_id"]: (
                row["user_id"],
                ProfileInfo(
                    avatar_url=row["avatar_url"], display_name=row["display_name"]
                ),
            )
            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 AS c
            INNER JOIN room_memberships AS m USING (event_id)
            WHERE m.membership = ?
                AND type = 'm.room.member'
                AND c.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

    async def get_joined_hosts(self, room_id: str, state_entry):
        state_group = 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()

        with Measure(self._clock, "get_joined_hosts"):
            return await self._get_joined_hosts(
                room_id, state_group, state_entry.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: int,
        current_state_ids: StateMap[str],
        state_entry: "_StateCacheEntry",
    ) -> FrozenSet[str]:
        # We don't use `state_group`, its 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.
        with (await 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.
                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.
                joined_users = await self.get_joined_users_from_state(
                    room_id, state_entry
                )

                cache.hosts_to_joined_users = {}
                for user_id in joined_users:
                    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)

    @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):
            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) -> Set[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):
            # 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 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)

    async def get_membership_from_event_ids(
        self, member_event_ids: Iterable[str]
    ) -> List[dict]:
        """Get user_id and membership of a set of event IDs."""

        return 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",
        )

    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):
            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,
        )


class RoomMemberBackgroundUpdateStore(SQLBaseStore):
    def __init__(self, database: DatabasePool, db_conn, hs):
        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",
        )

    async def _background_add_membership_profile(self, progress, batch_size):
        target_min_stream_id = progress.get(
            "target_min_stream_id_inclusive", self._min_stream_order_on_start
        )
        max_stream_id = progress.get(
            "max_stream_id_exclusive", self._stream_order_on_start + 1
        )

        def add_membership_profile_txn(txn):
            sql = """
                SELECT stream_ordering, event_id, events.room_id, event_json.json
                FROM events
                INNER JOIN event_json USING (event_id)
                INNER JOIN room_memberships 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, batch_size):
        """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, last_processed_room):
            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):
    def __init__(self, database: DatabasePool, db_conn, hs):
        super().__init__(database, db_conn, hs)

    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):
            sql = (
                "UPDATE"
                "  room_memberships"
                " SET"
                "  forgotten = 1"
                " WHERE"
                "  user_id = ?"
                " AND"
                "  room_id = ?"
            )
            txn.execute(sql, (user_id, room_id))

            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)


@attr.s(slots=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 = attr.ib(type=Dict[str, Set[str]], 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 = attr.ib(type=Union[object, int], factory=object)

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