forked-synapse/synapse/handlers/room_member.py

# Copyright 2016-2020 The Matrix.org Foundation C.I.C.
#
# 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 abc
import logging
import random
from http import HTTPStatus
from typing import TYPE_CHECKING, Iterable, List, Optional, Tuple

from synapse import types
from synapse.api.constants import AccountDataTypes, EventTypes, Membership
from synapse.api.errors import (
    AuthError,
    Codes,
    LimitExceededError,
    ShadowBanError,
    SynapseError,
)
from synapse.api.ratelimiting import Ratelimiter
from synapse.events import EventBase
from synapse.events.snapshot import EventContext
from synapse.types import JsonDict, Requester, RoomAlias, RoomID, StateMap, UserID
from synapse.util.async_helpers import Linearizer
from synapse.util.distributor import user_left_room

from ._base import BaseHandler

if TYPE_CHECKING:
    from synapse.server import HomeServer


logger = logging.getLogger(__name__)


class RoomMemberHandler(metaclass=abc.ABCMeta):
    # TODO(paul): This handler currently contains a messy conflation of
    #   low-level API that works on UserID objects and so on, and REST-level
    #   API that takes ID strings and returns pagination chunks. These concerns
    #   ought to be separated out a lot better.

    def __init__(self, hs: "HomeServer"):
        self.hs = hs
        self.store = hs.get_datastore()
        self.auth = hs.get_auth()
        self.state_handler = hs.get_state_handler()
        self.config = hs.config

        self.federation_handler = hs.get_federation_handler()
        self.directory_handler = hs.get_directory_handler()
        self.identity_handler = hs.get_identity_handler()
        self.registration_handler = hs.get_registration_handler()
        self.profile_handler = hs.get_profile_handler()
        self.event_creation_handler = hs.get_event_creation_handler()
        self.account_data_handler = hs.get_account_data_handler()
        self.event_auth_handler = hs.get_event_auth_handler()

        self.member_linearizer = Linearizer(name="member")

        self.clock = hs.get_clock()
        self.spam_checker = hs.get_spam_checker()
        self.third_party_event_rules = hs.get_third_party_event_rules()
        self._server_notices_mxid = self.config.server_notices_mxid
        self._enable_lookup = hs.config.enable_3pid_lookup
        self.allow_per_room_profiles = self.config.allow_per_room_profiles

        self._join_rate_limiter_local = Ratelimiter(
            store=self.store,
            clock=self.clock,
            rate_hz=hs.config.ratelimiting.rc_joins_local.per_second,
            burst_count=hs.config.ratelimiting.rc_joins_local.burst_count,
        )
        self._join_rate_limiter_remote = Ratelimiter(
            store=self.store,
            clock=self.clock,
            rate_hz=hs.config.ratelimiting.rc_joins_remote.per_second,
            burst_count=hs.config.ratelimiting.rc_joins_remote.burst_count,
        )

        self._invites_per_room_limiter = Ratelimiter(
            store=self.store,
            clock=self.clock,
            rate_hz=hs.config.ratelimiting.rc_invites_per_room.per_second,
            burst_count=hs.config.ratelimiting.rc_invites_per_room.burst_count,
        )
        self._invites_per_user_limiter = Ratelimiter(
            store=self.store,
            clock=self.clock,
            rate_hz=hs.config.ratelimiting.rc_invites_per_user.per_second,
            burst_count=hs.config.ratelimiting.rc_invites_per_user.burst_count,
        )

        # This is only used to get at ratelimit function, and
        # maybe_kick_guest_users. It's fine there are multiple of these as
        # it doesn't store state.
        self.base_handler = BaseHandler(hs)

    @abc.abstractmethod
    async def _remote_join(
        self,
        requester: Requester,
        remote_room_hosts: List[str],
        room_id: str,
        user: UserID,
        content: dict,
    ) -> Tuple[str, int]:
        """Try and join a room that this server is not in

        Args:
            requester
            remote_room_hosts: List of servers that can be used to join via.
            room_id: Room that we are trying to join
            user: User who is trying to join
            content: A dict that should be used as the content of the join event.
        """
        raise NotImplementedError()

    @abc.abstractmethod
    async def remote_reject_invite(
        self,
        invite_event_id: str,
        txn_id: Optional[str],
        requester: Requester,
        content: JsonDict,
    ) -> Tuple[str, int]:
        """
        Rejects an out-of-band invite we have received from a remote server

        Args:
            invite_event_id: ID of the invite to be rejected
            txn_id: optional transaction ID supplied by the client
            requester: user making the rejection request, according to the access token
            content: additional content to include in the rejection event.
               Normally an empty dict.

        Returns:
            event id, stream_id of the leave event
        """
        raise NotImplementedError()

    @abc.abstractmethod
    async def _user_left_room(self, target: UserID, room_id: str) -> None:
        """Notifies distributor on master process that the user has left the
        room.

        Args:
            target
            room_id
        """
        raise NotImplementedError()

    @abc.abstractmethod
    async def forget(self, user: UserID, room_id: str) -> None:
        raise NotImplementedError()

    async def ratelimit_invite(
        self,
        requester: Optional[Requester],
        room_id: Optional[str],
        invitee_user_id: str,
    ):
        """Ratelimit invites by room and by target user.

        If room ID is missing then we just rate limit by target user.
        """
        if room_id:
            await self._invites_per_room_limiter.ratelimit(requester, room_id)

        await self._invites_per_user_limiter.ratelimit(requester, invitee_user_id)

    async def _local_membership_update(
        self,
        requester: Requester,
        target: UserID,
        room_id: str,
        membership: str,
        prev_event_ids: List[str],
        txn_id: Optional[str] = None,
        ratelimit: bool = True,
        content: Optional[dict] = None,
        require_consent: bool = True,
    ) -> Tuple[str, int]:
        user_id = target.to_string()

        if content is None:
            content = {}

        content["membership"] = membership
        if requester.is_guest:
            content["kind"] = "guest"

        # Check if we already have an event with a matching transaction ID. (We
        # do this check just before we persist an event as well, but may as well
        # do it up front for efficiency.)
        if txn_id and requester.access_token_id:
            existing_event_id = await self.store.get_event_id_from_transaction_id(
                room_id,
                requester.user.to_string(),
                requester.access_token_id,
                txn_id,
            )
            if existing_event_id:
                event_pos = await self.store.get_position_for_event(existing_event_id)
                return existing_event_id, event_pos.stream

        event, context = await self.event_creation_handler.create_event(
            requester,
            {
                "type": EventTypes.Member,
                "content": content,
                "room_id": room_id,
                "sender": requester.user.to_string(),
                "state_key": user_id,
                # For backwards compatibility:
                "membership": membership,
            },
            txn_id=txn_id,
            prev_event_ids=prev_event_ids,
            require_consent=require_consent,
        )

        prev_state_ids = await context.get_prev_state_ids()

        prev_member_event_id = prev_state_ids.get((EventTypes.Member, user_id), None)

        if event.membership == Membership.JOIN:
            newly_joined = True
            user_is_invited = False
            if prev_member_event_id:
                prev_member_event = await self.store.get_event(prev_member_event_id)
                newly_joined = prev_member_event.membership != Membership.JOIN
                user_is_invited = prev_member_event.membership == Membership.INVITE

            # If the member is not already in the room and is not accepting an invite,
            # check if they should be allowed access via membership in a space.
            if (
                newly_joined
                and not user_is_invited
                and not await self.event_auth_handler.can_join_without_invite(
                    prev_state_ids, event.room_version, user_id
                )
            ):
                raise AuthError(
                    403,
                    "You do not belong to any of the required spaces to join this room.",
                )

            # Only rate-limit if the user actually joined the room, otherwise we'll end
            # up blocking profile updates.
            if newly_joined and ratelimit:
                time_now_s = self.clock.time()
                (
                    allowed,
                    time_allowed,
                ) = await self._join_rate_limiter_local.can_do_action(requester)

                if not allowed:
                    raise LimitExceededError(
                        retry_after_ms=int(1000 * (time_allowed - time_now_s))
                    )

        result_event = await self.event_creation_handler.handle_new_client_event(
            requester,
            event,
            context,
            extra_users=[target],
            ratelimit=ratelimit,
        )

        if event.membership == Membership.LEAVE:
            if prev_member_event_id:
                prev_member_event = await self.store.get_event(prev_member_event_id)
                if prev_member_event.membership == Membership.JOIN:
                    await self._user_left_room(target, room_id)

        # we know it was persisted, so should have a stream ordering
        assert result_event.internal_metadata.stream_ordering
        return result_event.event_id, result_event.internal_metadata.stream_ordering

    async def copy_room_tags_and_direct_to_room(
        self, old_room_id, new_room_id, user_id
    ) -> None:
        """Copies the tags and direct room state from one room to another.

        Args:
            old_room_id: The room ID of the old room.
            new_room_id: The room ID of the new room.
            user_id: The user's ID.
        """
        # Retrieve user account data for predecessor room
        user_account_data, _ = await self.store.get_account_data_for_user(user_id)

        # Copy direct message state if applicable
        direct_rooms = user_account_data.get(AccountDataTypes.DIRECT, {})

        # Check which key this room is under
        if isinstance(direct_rooms, dict):
            for key, room_id_list in direct_rooms.items():
                if old_room_id in room_id_list and new_room_id not in room_id_list:
                    # Add new room_id to this key
                    direct_rooms[key].append(new_room_id)

                    # Save back to user's m.direct account data
                    await self.account_data_handler.add_account_data_for_user(
                        user_id, AccountDataTypes.DIRECT, direct_rooms
                    )
                    break

        # Copy room tags if applicable
        room_tags = await self.store.get_tags_for_room(user_id, old_room_id)

        # Copy each room tag to the new room
        for tag, tag_content in room_tags.items():
            await self.account_data_handler.add_tag_to_room(
                user_id, new_room_id, tag, tag_content
            )

    async def update_membership(
        self,
        requester: Requester,
        target: UserID,
        room_id: str,
        action: str,
        txn_id: Optional[str] = None,
        remote_room_hosts: Optional[List[str]] = None,
        third_party_signed: Optional[dict] = None,
        ratelimit: bool = True,
        content: Optional[dict] = None,
        require_consent: bool = True,
    ) -> Tuple[str, int]:
        """Update a user's membership in a room.

        Params:
            requester: The user who is performing the update.
            target: The user whose membership is being updated.
            room_id: The room ID whose membership is being updated.
            action: The membership change, see synapse.api.constants.Membership.
            txn_id: The transaction ID, if given.
            remote_room_hosts: Remote servers to send the update to.
            third_party_signed: Information from a 3PID invite.
            ratelimit: Whether to rate limit the request.
            content: The content of the created event.
            require_consent: Whether consent is required.

        Returns:
            A tuple of the new event ID and stream ID.

        Raises:
            ShadowBanError if a shadow-banned requester attempts to send an invite.
        """
        if action == Membership.INVITE and requester.shadow_banned:
            # We randomly sleep a bit just to annoy the requester.
            await self.clock.sleep(random.randint(1, 10))
            raise ShadowBanError()

        key = (room_id,)

        with (await self.member_linearizer.queue(key)):
            result = await self.update_membership_locked(
                requester,
                target,
                room_id,
                action,
                txn_id=txn_id,
                remote_room_hosts=remote_room_hosts,
                third_party_signed=third_party_signed,
                ratelimit=ratelimit,
                content=content,
                require_consent=require_consent,
            )

        return result

    async def update_membership_locked(
        self,
        requester: Requester,
        target: UserID,
        room_id: str,
        action: str,
        txn_id: Optional[str] = None,
        remote_room_hosts: Optional[List[str]] = None,
        third_party_signed: Optional[dict] = None,
        ratelimit: bool = True,
        content: Optional[dict] = None,
        require_consent: bool = True,
    ) -> Tuple[str, int]:
        """Helper for update_membership.

        Assumes that the membership linearizer is already held for the room.
        """
        content_specified = bool(content)
        if content is None:
            content = {}
        else:
            # We do a copy here as we potentially change some keys
            # later on.
            content = dict(content)

        # allow the server notices mxid to set room-level profile
        is_requester_server_notices_user = (
            self._server_notices_mxid is not None
            and requester.user.to_string() == self._server_notices_mxid
        )

        if (
            not self.allow_per_room_profiles and not is_requester_server_notices_user
        ) or requester.shadow_banned:
            # Strip profile data, knowing that new profile data will be added to the
            # event's content in event_creation_handler.create_event() using the target's
            # global profile.
            content.pop("displayname", None)
            content.pop("avatar_url", None)

        effective_membership_state = action
        if action in ["kick", "unban"]:
            effective_membership_state = "leave"

        # if this is a join with a 3pid signature, we may need to turn a 3pid
        # invite into a normal invite before we can handle the join.
        if third_party_signed is not None:
            await self.federation_handler.exchange_third_party_invite(
                third_party_signed["sender"],
                target.to_string(),
                room_id,
                third_party_signed,
            )

        if not remote_room_hosts:
            remote_room_hosts = []

        if effective_membership_state not in ("leave", "ban"):
            is_blocked = await self.store.is_room_blocked(room_id)
            if is_blocked:
                raise SynapseError(403, "This room has been blocked on this server")

        if effective_membership_state == Membership.INVITE:
            target_id = target.to_string()
            if ratelimit:
                await self.ratelimit_invite(requester, room_id, target_id)

            # block any attempts to invite the server notices mxid
            if target_id == self._server_notices_mxid:
                raise SynapseError(HTTPStatus.FORBIDDEN, "Cannot invite this user")

            block_invite = False

            if (
                self._server_notices_mxid is not None
                and requester.user.to_string() == self._server_notices_mxid
            ):
                # allow the server notices mxid to send invites
                is_requester_admin = True

            else:
                is_requester_admin = await self.auth.is_server_admin(requester.user)

            if not is_requester_admin:
                if self.config.block_non_admin_invites:
                    logger.info(
                        "Blocking invite: user is not admin and non-admin "
                        "invites disabled"
                    )
                    block_invite = True

                if not await self.spam_checker.user_may_invite(
                    requester.user.to_string(), target_id, room_id
                ):
                    logger.info("Blocking invite due to spam checker")
                    block_invite = True

            if block_invite:
                raise SynapseError(403, "Invites have been disabled on this server")

        latest_event_ids = await self.store.get_prev_events_for_room(room_id)

        current_state_ids = await self.state_handler.get_current_state_ids(
            room_id, latest_event_ids=latest_event_ids
        )

        # TODO: Refactor into dictionary of explicitly allowed transitions
        # between old and new state, with specific error messages for some
        # transitions and generic otherwise
        old_state_id = current_state_ids.get((EventTypes.Member, target.to_string()))
        if old_state_id:
            old_state = await self.store.get_event(old_state_id, allow_none=True)
            old_membership = old_state.content.get("membership") if old_state else None
            if action == "unban" and old_membership != "ban":
                raise SynapseError(
                    403,
                    "Cannot unban user who was not banned"
                    " (membership=%s)" % old_membership,
                    errcode=Codes.BAD_STATE,
                )
            if old_membership == "ban" and action != "unban":
                raise SynapseError(
                    403,
                    "Cannot %s user who was banned" % (action,),
                    errcode=Codes.BAD_STATE,
                )

            if old_state:
                same_content = content == old_state.content
                same_membership = old_membership == effective_membership_state
                same_sender = requester.user.to_string() == old_state.sender
                if same_sender and same_membership and same_content:
                    # duplicate event.
                    # we know it was persisted, so must have a stream ordering.
                    assert old_state.internal_metadata.stream_ordering
                    return (
                        old_state.event_id,
                        old_state.internal_metadata.stream_ordering,
                    )

            if old_membership in ["ban", "leave"] and action == "kick":
                raise AuthError(403, "The target user is not in the room")

            # we don't allow people to reject invites to the server notice
            # room, but they can leave it once they are joined.
            if (
                old_membership == Membership.INVITE
                and effective_membership_state == Membership.LEAVE
            ):
                is_blocked = await self._is_server_notice_room(room_id)
                if is_blocked:
                    raise SynapseError(
                        HTTPStatus.FORBIDDEN,
                        "You cannot reject this invite",
                        errcode=Codes.CANNOT_LEAVE_SERVER_NOTICE_ROOM,
                    )
        else:
            if action == "kick":
                raise AuthError(403, "The target user is not in the room")

        is_host_in_room = await self._is_host_in_room(current_state_ids)

        if effective_membership_state == Membership.JOIN:
            if requester.is_guest:
                guest_can_join = await self._can_guest_join(current_state_ids)
                if not guest_can_join:
                    # This should be an auth check, but guests are a local concept,
                    # so don't really fit into the general auth process.
                    raise AuthError(403, "Guest access not allowed")

            if not is_host_in_room:
                if ratelimit:
                    time_now_s = self.clock.time()
                    (
                        allowed,
                        time_allowed,
                    ) = await self._join_rate_limiter_remote.can_do_action(
                        requester,
                    )

                    if not allowed:
                        raise LimitExceededError(
                            retry_after_ms=int(1000 * (time_allowed - time_now_s))
                        )

                inviter = await self._get_inviter(target.to_string(), room_id)
                if inviter and not self.hs.is_mine(inviter):
                    remote_room_hosts.append(inviter.domain)

                content["membership"] = Membership.JOIN

                profile = self.profile_handler
                if not content_specified:
                    content["displayname"] = await profile.get_displayname(target)
                    content["avatar_url"] = await profile.get_avatar_url(target)

                if requester.is_guest:
                    content["kind"] = "guest"

                remote_join_response = await self._remote_join(
                    requester, remote_room_hosts, room_id, target, content
                )

                return remote_join_response

        elif effective_membership_state == Membership.LEAVE:
            if not is_host_in_room:
                # perhaps we've been invited
                (
                    current_membership_type,
                    current_membership_event_id,
                ) = await self.store.get_local_current_membership_for_user_in_room(
                    target.to_string(), room_id
                )
                if (
                    current_membership_type != Membership.INVITE
                    or not current_membership_event_id
                ):
                    logger.info(
                        "%s sent a leave request to %s, but that is not an active room "
                        "on this server, and there is no pending invite",
                        target,
                        room_id,
                    )

                    raise SynapseError(404, "Not a known room")

                invite = await self.store.get_event(current_membership_event_id)
                logger.info(
                    "%s rejects invite to %s from %s", target, room_id, invite.sender
                )

                if not self.hs.is_mine_id(invite.sender):
                    # send the rejection to the inviter's HS (with fallback to
                    # local event)
                    return await self.remote_reject_invite(
                        invite.event_id,
                        txn_id,
                        requester,
                        content,
                    )

                # the inviter was on our server, but has now left. Carry on
                # with the normal rejection codepath, which will also send the
                # rejection out to any other servers we believe are still in the room.

                # thanks to overzealous cleaning up of event_forward_extremities in
                # `delete_old_current_state_events`, it's possible to end up with no
                # forward extremities here. If that happens, let's just hang the
                # rejection off the invite event.
                #
                # see: https://github.com/matrix-org/synapse/issues/7139
                if len(latest_event_ids) == 0:
                    latest_event_ids = [invite.event_id]

        return await self._local_membership_update(
            requester=requester,
            target=target,
            room_id=room_id,
            membership=effective_membership_state,
            txn_id=txn_id,
            ratelimit=ratelimit,
            prev_event_ids=latest_event_ids,
            content=content,
            require_consent=require_consent,
        )

    async def transfer_room_state_on_room_upgrade(
        self, old_room_id: str, room_id: str
    ) -> None:
        """Upon our server becoming aware of an upgraded room, either by upgrading a room
        ourselves or joining one, we can transfer over information from the previous room.

        Copies user state (tags/push rules) for every local user that was in the old room, as
        well as migrating the room directory state.

        Args:
            old_room_id: The ID of the old room
            room_id: The ID of the new room
        """
        logger.info("Transferring room state from %s to %s", old_room_id, room_id)

        # Find all local users that were in the old room and copy over each user's state
        users = await self.store.get_users_in_room(old_room_id)
        await self.copy_user_state_on_room_upgrade(old_room_id, room_id, users)

        # Add new room to the room directory if the old room was there
        # Remove old room from the room directory
        old_room = await self.store.get_room(old_room_id)
        if old_room and old_room["is_public"]:
            await self.store.set_room_is_public(old_room_id, False)
            await self.store.set_room_is_public(room_id, True)

        # Transfer alias mappings in the room directory
        await self.store.update_aliases_for_room(old_room_id, room_id)

        # Check if any groups we own contain the predecessor room
        local_group_ids = await self.store.get_local_groups_for_room(old_room_id)
        for group_id in local_group_ids:
            # Add new the new room to those groups
            await self.store.add_room_to_group(group_id, room_id, old_room["is_public"])

            # Remove the old room from those groups
            await self.store.remove_room_from_group(group_id, old_room_id)

    async def copy_user_state_on_room_upgrade(
        self, old_room_id: str, new_room_id: str, user_ids: Iterable[str]
    ) -> None:
        """Copy user-specific information when they join a new room when that new room is the
        result of a room upgrade

        Args:
            old_room_id: The ID of upgraded room
            new_room_id: The ID of the new room
            user_ids: User IDs to copy state for
        """

        logger.debug(
            "Copying over room tags and push rules from %s to %s for users %s",
            old_room_id,
            new_room_id,
            user_ids,
        )

        for user_id in user_ids:
            try:
                # It is an upgraded room. Copy over old tags
                await self.copy_room_tags_and_direct_to_room(
                    old_room_id, new_room_id, user_id
                )
                # Copy over push rules
                await self.store.copy_push_rules_from_room_to_room_for_user(
                    old_room_id, new_room_id, user_id
                )
            except Exception:
                logger.exception(
                    "Error copying tags and/or push rules from rooms %s to %s for user %s. "
                    "Skipping...",
                    old_room_id,
                    new_room_id,
                    user_id,
                )
                continue

    async def send_membership_event(
        self,
        requester: Optional[Requester],
        event: EventBase,
        context: EventContext,
        ratelimit: bool = True,
    ):
        """
        Change the membership status of a user in a room.

        Args:
            requester: The local user who requested the membership
                event. If None, certain checks, like whether this homeserver can
                act as the sender, will be skipped.
            event: The membership event.
            context: The context of the event.
            ratelimit: Whether to rate limit this request.
        Raises:
            SynapseError if there was a problem changing the membership.
        """
        target_user = UserID.from_string(event.state_key)
        room_id = event.room_id

        if requester is not None:
            sender = UserID.from_string(event.sender)
            assert (
                sender == requester.user
            ), "Sender (%s) must be same as requester (%s)" % (sender, requester.user)
            assert self.hs.is_mine(sender), "Sender must be our own: %s" % (sender,)
        else:
            requester = types.create_requester(target_user)

        prev_state_ids = await context.get_prev_state_ids()
        if event.membership == Membership.JOIN:
            if requester.is_guest:
                guest_can_join = await self._can_guest_join(prev_state_ids)
                if not guest_can_join:
                    # This should be an auth check, but guests are a local concept,
                    # so don't really fit into the general auth process.
                    raise AuthError(403, "Guest access not allowed")

        if event.membership not in (Membership.LEAVE, Membership.BAN):
            is_blocked = await self.store.is_room_blocked(room_id)
            if is_blocked:
                raise SynapseError(403, "This room has been blocked on this server")

        event = await self.event_creation_handler.handle_new_client_event(
            requester, event, context, extra_users=[target_user], ratelimit=ratelimit
        )

        prev_member_event_id = prev_state_ids.get(
            (EventTypes.Member, event.state_key), None
        )

        if event.membership == Membership.LEAVE:
            if prev_member_event_id:
                prev_member_event = await self.store.get_event(prev_member_event_id)
                if prev_member_event.membership == Membership.JOIN:
                    await self._user_left_room(target_user, room_id)

    async def _can_guest_join(self, current_state_ids: StateMap[str]) -> bool:
        """
        Returns whether a guest can join a room based on its current state.
        """
        guest_access_id = current_state_ids.get((EventTypes.GuestAccess, ""), None)
        if not guest_access_id:
            return False

        guest_access = await self.store.get_event(guest_access_id)

        return bool(
            guest_access
            and guest_access.content
            and "guest_access" in guest_access.content
            and guest_access.content["guest_access"] == "can_join"
        )

    async def lookup_room_alias(
        self, room_alias: RoomAlias
    ) -> Tuple[RoomID, List[str]]:
        """
        Get the room ID associated with a room alias.

        Args:
            room_alias: The alias to look up.
        Returns:
            A tuple of:
                The room ID as a RoomID object.
                Hosts likely to be participating in the room ([str]).
        Raises:
            SynapseError if room alias could not be found.
        """
        directory_handler = self.directory_handler
        mapping = await directory_handler.get_association(room_alias)

        if not mapping:
            raise SynapseError(404, "No such room alias")

        room_id = mapping["room_id"]
        servers = mapping["servers"]

        # put the server which owns the alias at the front of the server list.
        if room_alias.domain in servers:
            servers.remove(room_alias.domain)
        servers.insert(0, room_alias.domain)

        return RoomID.from_string(room_id), servers

    async def _get_inviter(self, user_id: str, room_id: str) -> Optional[UserID]:
        invite = await self.store.get_invite_for_local_user_in_room(
            user_id=user_id, room_id=room_id
        )
        if invite:
            return UserID.from_string(invite.sender)
        return None

    async def do_3pid_invite(
        self,
        room_id: str,
        inviter: UserID,
        medium: str,
        address: str,
        id_server: str,
        requester: Requester,
        txn_id: Optional[str],
        id_access_token: Optional[str] = None,
    ) -> int:
        """Invite a 3PID to a room.

        Args:
            room_id: The room to invite the 3PID to.
            inviter: The user sending the invite.
            medium: The 3PID's medium.
            address: The 3PID's address.
            id_server: The identity server to use.
            requester: The user making the request.
            txn_id: The transaction ID this is part of, or None if this is not
                part of a transaction.
            id_access_token: The optional identity server access token.

        Returns:
             The new stream ID.

        Raises:
            ShadowBanError if the requester has been shadow-banned.
        """
        if self.config.block_non_admin_invites:
            is_requester_admin = await self.auth.is_server_admin(requester.user)
            if not is_requester_admin:
                raise SynapseError(
                    403, "Invites have been disabled on this server", Codes.FORBIDDEN
                )

        if requester.shadow_banned:
            # We randomly sleep a bit just to annoy the requester.
            await self.clock.sleep(random.randint(1, 10))
            raise ShadowBanError()

        # We need to rate limit *before* we send out any 3PID invites, so we
        # can't just rely on the standard ratelimiting of events.
        await self.base_handler.ratelimit(requester)

        can_invite = await self.third_party_event_rules.check_threepid_can_be_invited(
            medium, address, room_id
        )
        if not can_invite:
            raise SynapseError(
                403,
                "This third-party identifier can not be invited in this room",
                Codes.FORBIDDEN,
            )

        if not self._enable_lookup:
            raise SynapseError(
                403, "Looking up third-party identifiers is denied from this server"
            )

        invitee = await self.identity_handler.lookup_3pid(
            id_server, medium, address, id_access_token
        )

        if invitee:
            # Note that update_membership with an action of "invite" can raise
            # a ShadowBanError, but this was done above already.
            _, stream_id = await self.update_membership(
                requester, UserID.from_string(invitee), room_id, "invite", txn_id=txn_id
            )
        else:
            stream_id = await self._make_and_store_3pid_invite(
                requester,
                id_server,
                medium,
                address,
                room_id,
                inviter,
                txn_id=txn_id,
                id_access_token=id_access_token,
            )

        return stream_id

    async def _make_and_store_3pid_invite(
        self,
        requester: Requester,
        id_server: str,
        medium: str,
        address: str,
        room_id: str,
        user: UserID,
        txn_id: Optional[str],
        id_access_token: Optional[str] = None,
    ) -> int:
        room_state = await self.state_handler.get_current_state(room_id)

        inviter_display_name = ""
        inviter_avatar_url = ""
        member_event = room_state.get((EventTypes.Member, user.to_string()))
        if member_event:
            inviter_display_name = member_event.content.get("displayname", "")
            inviter_avatar_url = member_event.content.get("avatar_url", "")

        # if user has no display name, default to their MXID
        if not inviter_display_name:
            inviter_display_name = user.to_string()

        canonical_room_alias = ""
        canonical_alias_event = room_state.get((EventTypes.CanonicalAlias, ""))
        if canonical_alias_event:
            canonical_room_alias = canonical_alias_event.content.get("alias", "")

        room_name = ""
        room_name_event = room_state.get((EventTypes.Name, ""))
        if room_name_event:
            room_name = room_name_event.content.get("name", "")

        room_join_rules = ""
        join_rules_event = room_state.get((EventTypes.JoinRules, ""))
        if join_rules_event:
            room_join_rules = join_rules_event.content.get("join_rule", "")

        room_avatar_url = ""
        room_avatar_event = room_state.get((EventTypes.RoomAvatar, ""))
        if room_avatar_event:
            room_avatar_url = room_avatar_event.content.get("url", "")

        (
            token,
            public_keys,
            fallback_public_key,
            display_name,
        ) = await self.identity_handler.ask_id_server_for_third_party_invite(
            requester=requester,
            id_server=id_server,
            medium=medium,
            address=address,
            room_id=room_id,
            inviter_user_id=user.to_string(),
            room_alias=canonical_room_alias,
            room_avatar_url=room_avatar_url,
            room_join_rules=room_join_rules,
            room_name=room_name,
            inviter_display_name=inviter_display_name,
            inviter_avatar_url=inviter_avatar_url,
            id_access_token=id_access_token,
        )

        (
            event,
            stream_id,
        ) = await self.event_creation_handler.create_and_send_nonmember_event(
            requester,
            {
                "type": EventTypes.ThirdPartyInvite,
                "content": {
                    "display_name": display_name,
                    "public_keys": public_keys,
                    # For backwards compatibility:
                    "key_validity_url": fallback_public_key["key_validity_url"],
                    "public_key": fallback_public_key["public_key"],
                },
                "room_id": room_id,
                "sender": user.to_string(),
                "state_key": token,
            },
            ratelimit=False,
            txn_id=txn_id,
        )
        return stream_id

    async def _is_host_in_room(self, current_state_ids: StateMap[str]) -> bool:
        # Have we just created the room, and is this about to be the very
        # first member event?
        create_event_id = current_state_ids.get(("m.room.create", ""))
        if len(current_state_ids) == 1 and create_event_id:
            # We can only get here if we're in the process of creating the room
            return True

        for etype, state_key in current_state_ids:
            if etype != EventTypes.Member or not self.hs.is_mine_id(state_key):
                continue

            event_id = current_state_ids[(etype, state_key)]
            event = await self.store.get_event(event_id, allow_none=True)
            if not event:
                continue

            if event.membership == Membership.JOIN:
                return True

        return False

    async def _is_server_notice_room(self, room_id: str) -> bool:
        if self._server_notices_mxid is None:
            return False
        user_ids = await self.store.get_users_in_room(room_id)
        return self._server_notices_mxid in user_ids


class RoomMemberMasterHandler(RoomMemberHandler):
    def __init__(self, hs: "HomeServer"):
        super().__init__(hs)

        self.distributor = hs.get_distributor()
        self.distributor.declare("user_left_room")
        self._server_name = hs.hostname

    async def _is_remote_room_too_complex(
        self, room_id: str, remote_room_hosts: List[str]
    ) -> Optional[bool]:
        """
        Check if complexity of a remote room is too great.

        Args:
            room_id
            remote_room_hosts

        Returns: bool of whether the complexity is too great, or None
            if unable to be fetched
        """
        max_complexity = self.hs.config.limit_remote_rooms.complexity
        complexity = await self.federation_handler.get_room_complexity(
            remote_room_hosts, room_id
        )

        if complexity:
            return complexity["v1"] > max_complexity
        return None

    async def _is_local_room_too_complex(self, room_id: str) -> bool:
        """
        Check if the complexity of a local room is too great.

        Args:
            room_id: The room ID to check for complexity.
        """
        max_complexity = self.hs.config.limit_remote_rooms.complexity
        complexity = await self.store.get_room_complexity(room_id)

        return complexity["v1"] > max_complexity

    async def _remote_join(
        self,
        requester: Requester,
        remote_room_hosts: List[str],
        room_id: str,
        user: UserID,
        content: dict,
    ) -> Tuple[str, int]:
        """Implements RoomMemberHandler._remote_join"""
        # filter ourselves out of remote_room_hosts: do_invite_join ignores it
        # and if it is the only entry we'd like to return a 404 rather than a
        # 500.
        remote_room_hosts = [
            host for host in remote_room_hosts if host != self.hs.hostname
        ]

        if len(remote_room_hosts) == 0:
            raise SynapseError(404, "No known servers")

        check_complexity = self.hs.config.limit_remote_rooms.enabled
        if check_complexity and self.hs.config.limit_remote_rooms.admins_can_join:
            check_complexity = not await self.auth.is_server_admin(user)

        if check_complexity:
            # Fetch the room complexity
            too_complex = await self._is_remote_room_too_complex(
                room_id, remote_room_hosts
            )
            if too_complex is True:
                raise SynapseError(
                    code=400,
                    msg=self.hs.config.limit_remote_rooms.complexity_error,
                    errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
                )

        # We don't do an auth check if we are doing an invite
        # join dance for now, since we're kinda implicitly checking
        # that we are allowed to join when we decide whether or not we
        # need to do the invite/join dance.
        event_id, stream_id = await self.federation_handler.do_invite_join(
            remote_room_hosts, room_id, user.to_string(), content
        )

        # Check the room we just joined wasn't too large, if we didn't fetch the
        # complexity of it before.
        if check_complexity:
            if too_complex is False:
                # We checked, and we're under the limit.
                return event_id, stream_id

            # Check again, but with the local state events
            too_complex = await self._is_local_room_too_complex(room_id)

            if too_complex is False:
                # We're under the limit.
                return event_id, stream_id

            # The room is too large. Leave.
            requester = types.create_requester(
                user, authenticated_entity=self._server_name
            )
            await self.update_membership(
                requester=requester, target=user, room_id=room_id, action="leave"
            )
            raise SynapseError(
                code=400,
                msg=self.hs.config.limit_remote_rooms.complexity_error,
                errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
            )

        return event_id, stream_id

    async def remote_reject_invite(
        self,
        invite_event_id: str,
        txn_id: Optional[str],
        requester: Requester,
        content: JsonDict,
    ) -> Tuple[str, int]:
        """
        Rejects an out-of-band invite received from a remote user

        Implements RoomMemberHandler.remote_reject_invite
        """
        invite_event = await self.store.get_event(invite_event_id)
        room_id = invite_event.room_id
        target_user = invite_event.state_key

        # first of all, try doing a rejection via the inviting server
        fed_handler = self.federation_handler
        try:
            inviter_id = UserID.from_string(invite_event.sender)
            event, stream_id = await fed_handler.do_remotely_reject_invite(
                [inviter_id.domain], room_id, target_user, content=content
            )
            return event.event_id, stream_id
        except Exception as e:
            # if we were unable to reject the invite, we will generate our own
            # leave event.
            #
            # The 'except' clause is very broad, but we need to
            # capture everything from DNS failures upwards
            #
            logger.warning("Failed to reject invite: %s", e)

            return await self._generate_local_out_of_band_leave(
                invite_event, txn_id, requester, content
            )

    async def _generate_local_out_of_band_leave(
        self,
        previous_membership_event: EventBase,
        txn_id: Optional[str],
        requester: Requester,
        content: JsonDict,
    ) -> Tuple[str, int]:
        """Generate a local leave event for a room

        This can be called after we e.g fail to reject an invite via a remote server.
        It generates an out-of-band membership event locally.

        Args:
            previous_membership_event: the previous membership event for this user
            txn_id: optional transaction ID supplied by the client
            requester: user making the request, according to the access token
            content: additional content to include in the leave event.
               Normally an empty dict.

        Returns:
            A tuple containing (event_id, stream_id of the leave event)
        """
        room_id = previous_membership_event.room_id
        target_user = previous_membership_event.state_key

        content["membership"] = Membership.LEAVE

        event_dict = {
            "type": EventTypes.Member,
            "room_id": room_id,
            "sender": target_user,
            "content": content,
            "state_key": target_user,
        }

        # the auth events for the new event are the same as that of the previous event, plus
        # the event itself.
        #
        # the prev_events consist solely of the previous membership event.
        prev_event_ids = [previous_membership_event.event_id]
        auth_event_ids = previous_membership_event.auth_event_ids() + prev_event_ids

        event, context = await self.event_creation_handler.create_event(
            requester,
            event_dict,
            txn_id=txn_id,
            prev_event_ids=prev_event_ids,
            auth_event_ids=auth_event_ids,
        )
        event.internal_metadata.outlier = True
        event.internal_metadata.out_of_band_membership = True

        result_event = await self.event_creation_handler.handle_new_client_event(
            requester,
            event,
            context,
            extra_users=[UserID.from_string(target_user)],
        )
        # we know it was persisted, so must have a stream ordering
        assert result_event.internal_metadata.stream_ordering

        return result_event.event_id, result_event.internal_metadata.stream_ordering

    async def _user_left_room(self, target: UserID, room_id: str) -> None:
        """Implements RoomMemberHandler._user_left_room"""
        user_left_room(self.distributor, target, room_id)

    async def forget(self, user: UserID, room_id: str) -> None:
        user_id = user.to_string()

        member = await self.state_handler.get_current_state(
            room_id=room_id, event_type=EventTypes.Member, state_key=user_id
        )
        membership = member.membership if member else None

        if membership is not None and membership not in [
            Membership.LEAVE,
            Membership.BAN,
        ]:
            raise SynapseError(400, "User %s in room %s" % (user_id, room_id))

        if membership:
            await self.store.forget(user_id, room_id)