synapse/synapse/handlers/message.py

# Copyright 2014-2016 OpenMarket Ltd
# Copyright 2017-2018 New Vector Ltd
# Copyright 2019-2020 The Matrix.org Foundation C.I.C.
# Copyrignt 2020 Sorunome
#
# 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
import random
from http import HTTPStatus
from typing import TYPE_CHECKING, Any, Dict, List, Mapping, Optional, Tuple

from canonicaljson import encode_canonical_json

from twisted.internet.interfaces import IDelayedCall

from synapse import event_auth
from synapse.api.constants import (
    EventContentFields,
    EventTypes,
    GuestAccess,
    HistoryVisibility,
    Membership,
    RelationTypes,
    UserTypes,
)
from synapse.api.errors import (
    AuthError,
    Codes,
    ConsentNotGivenError,
    NotFoundError,
    PartialStateConflictError,
    ShadowBanError,
    SynapseError,
    UnstableSpecAuthError,
    UnsupportedRoomVersionError,
)
from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
from synapse.api.urls import ConsentURIBuilder
from synapse.event_auth import validate_event_for_room_version
from synapse.events import EventBase, relation_from_event
from synapse.events.builder import EventBuilder
from synapse.events.snapshot import EventContext, UnpersistedEventContextBase
from synapse.events.utils import SerializeEventConfig, maybe_upsert_event_field
from synapse.events.validator import EventValidator
from synapse.handlers.directory import DirectoryHandler
from synapse.handlers.worker_lock import NEW_EVENT_DURING_PURGE_LOCK_NAME
from synapse.logging import opentracing
from synapse.logging.context import make_deferred_yieldable, run_in_background
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.replication.http.send_event import ReplicationSendEventRestServlet
from synapse.replication.http.send_events import ReplicationSendEventsRestServlet
from synapse.storage.databases.main.events_worker import EventRedactBehaviour
from synapse.types import (
    PersistedEventPosition,
    Requester,
    RoomAlias,
    StateMap,
    StreamToken,
    UserID,
    create_requester,
)
from synapse.types.state import StateFilter
from synapse.util import json_decoder, json_encoder, log_failure, unwrapFirstError
from synapse.util.async_helpers import Linearizer, gather_results
from synapse.util.caches.expiringcache import ExpiringCache
from synapse.util.metrics import measure_func
from synapse.visibility import get_effective_room_visibility_from_state

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)


class MessageHandler:
    """Contains some read only APIs to get state about a room"""

    def __init__(self, hs: "HomeServer"):
        self.auth = hs.get_auth()
        self.clock = hs.get_clock()
        self.state = hs.get_state_handler()
        self.store = hs.get_datastores().main
        self._storage_controllers = hs.get_storage_controllers()
        self._state_storage_controller = self._storage_controllers.state
        self._event_serializer = hs.get_event_client_serializer()
        self._ephemeral_events_enabled = hs.config.server.enable_ephemeral_messages

        # The scheduled call to self._expire_event. None if no call is currently
        # scheduled.
        self._scheduled_expiry: Optional[IDelayedCall] = None

        if not hs.config.worker.worker_app:
            run_as_background_process(
                "_schedule_next_expiry", self._schedule_next_expiry
            )

    async def get_room_data(
        self,
        requester: Requester,
        room_id: str,
        event_type: str,
        state_key: str,
    ) -> Optional[EventBase]:
        """Get data from a room.

        Args:
            requester: The user who did the request.
            room_id
            event_type
            state_key
        Returns:
            The path data content.
        Raises:
            SynapseError or AuthError if the user is not in the room
        """
        (
            membership,
            membership_event_id,
        ) = await self.auth.check_user_in_room_or_world_readable(
            room_id, requester, allow_departed_users=True
        )

        if membership == Membership.JOIN:
            data = await self._storage_controllers.state.get_current_state_event(
                room_id, event_type, state_key
            )
        elif membership == Membership.LEAVE:
            key = (event_type, state_key)
            # If the membership is not JOIN, then the event ID should exist.
            assert (
                membership_event_id is not None
            ), "check_user_in_room_or_world_readable returned invalid data"
            room_state = await self._state_storage_controller.get_state_for_events(
                [membership_event_id], StateFilter.from_types([key])
            )
            data = room_state[membership_event_id].get(key)
        else:
            # check_user_in_room_or_world_readable, if it doesn't raise an AuthError, should
            # only ever return a Membership.JOIN/LEAVE object
            #
            # Safeguard in case it returned something else
            logger.error(
                "Attempted to retrieve data from a room for a user that has never been in it. "
                "This should not have happened."
            )
            raise UnstableSpecAuthError(
                403,
                "User not in room",
                errcode=Codes.NOT_JOINED,
            )

        return data

    async def get_state_events(
        self,
        requester: Requester,
        room_id: str,
        state_filter: Optional[StateFilter] = None,
        at_token: Optional[StreamToken] = None,
    ) -> List[dict]:
        """Retrieve all state events for a given room. If the user is
        joined to the room then return the current state. If the user has
        left the room return the state events from when they left. If an explicit
        'at' parameter is passed, return the state events as of that event, if
        visible.

        Args:
            requester: The user requesting state events.
            room_id: The room ID to get all state events from.
            state_filter: The state filter used to fetch state from the database.
            at_token: the stream token of the at which we are requesting
                the stats. If the user is not allowed to view the state as of that
                stream token, we raise a 403 SynapseError. If None, returns the current
                state based on the current_state_events table.
        Returns:
            A list of dicts representing state events. [{}, {}, {}]
        Raises:
            NotFoundError (404) if the at token does not yield an event

            AuthError (403) if the user doesn't have permission to view
            members of this room.
        """
        state_filter = state_filter or StateFilter.all()
        user_id = requester.user.to_string()

        if at_token:
            last_event_id = (
                await self.store.get_last_event_in_room_before_stream_ordering(
                    room_id,
                    end_token=at_token.room_key,
                )
            )

            if not last_event_id:
                raise NotFoundError("Can't find event for token %s" % (at_token,))

            if not await self._user_can_see_state_at_event(
                user_id, room_id, last_event_id
            ):
                raise AuthError(
                    403,
                    "User %s not allowed to view events in room %s at token %s"
                    % (user_id, room_id, at_token),
                )

            room_state_events = (
                await self._state_storage_controller.get_state_for_events(
                    [last_event_id], state_filter=state_filter
                )
            )
            room_state: Mapping[Any, EventBase] = room_state_events[last_event_id]
        else:
            (
                membership,
                membership_event_id,
            ) = await self.auth.check_user_in_room_or_world_readable(
                room_id, requester, allow_departed_users=True
            )

            if membership == Membership.JOIN:
                state_ids = await self._state_storage_controller.get_current_state_ids(
                    room_id, state_filter=state_filter
                )
                room_state = await self.store.get_events(state_ids.values())
            elif membership == Membership.LEAVE:
                # If the membership is not JOIN, then the event ID should exist.
                assert (
                    membership_event_id is not None
                ), "check_user_in_room_or_world_readable returned invalid data"
                room_state_events = (
                    await self._state_storage_controller.get_state_for_events(
                        [membership_event_id], state_filter=state_filter
                    )
                )
                room_state = room_state_events[membership_event_id]

        events = self._event_serializer.serialize_events(
            room_state.values(),
            self.clock.time_msec(),
            config=SerializeEventConfig(requester=requester),
        )
        return events

    async def _user_can_see_state_at_event(
        self, user_id: str, room_id: str, event_id: str
    ) -> bool:
        # check whether the user was in the room, and the history visibility,
        # at that time.
        state_map = await self._state_storage_controller.get_state_for_event(
            event_id,
            StateFilter.from_types(
                [
                    (EventTypes.Member, user_id),
                    (EventTypes.RoomHistoryVisibility, ""),
                ]
            ),
        )

        membership = None
        membership_event = state_map.get((EventTypes.Member, user_id))
        if membership_event:
            membership = membership_event.membership

        # if the user was a member of the room at the time of the event,
        # they can see it.
        if membership == Membership.JOIN:
            return True

        # otherwise, it depends on the history visibility.
        visibility = get_effective_room_visibility_from_state(state_map)

        if visibility == HistoryVisibility.JOINED:
            # we weren't a member at the time of the event, so we can't see this event.
            return False

        # otherwise *invited* is good enough
        if membership == Membership.INVITE:
            return True

        if visibility == HistoryVisibility.INVITED:
            # we weren't invited, so we can't see this event.
            return False

        if visibility == HistoryVisibility.WORLD_READABLE:
            return True

        # So it's SHARED, and the user was not a member at the time. The user cannot
        # see history, unless they have *subsequently* joined the room.
        #
        # XXX: if the user has subsequently joined and then left again,
        # ideally we would share history up to the point they left. But
        # we don't know when they left. We just treat it as though they
        # never joined, and restrict access.

        (
            current_membership,
            _,
        ) = await self.store.get_local_current_membership_for_user_in_room(
            user_id, event_id
        )
        return current_membership == Membership.JOIN

    async def get_joined_members(self, requester: Requester, room_id: str) -> dict:
        """Get all the joined members in the room and their profile information.

        If the user has left the room return the state events from when they left.

        Args:
            requester: The user requesting state events.
            room_id: The room ID to get all state events from.
        Returns:
            A dict of user_id to profile info
        """
        if not requester.app_service:
            # We check AS auth after fetching the room membership, as it
            # requires us to pull out all joined members anyway.
            membership, _ = await self.auth.check_user_in_room_or_world_readable(
                room_id, requester, allow_departed_users=True
            )
            if membership != Membership.JOIN:
                raise SynapseError(
                    code=403,
                    errcode=Codes.FORBIDDEN,
                    msg="Getting joined members while not being a current member of the room is forbidden.",
                )

        users_with_profile = (
            await self._state_storage_controller.get_users_in_room_with_profiles(
                room_id
            )
        )

        # If this is an AS, double check that they are allowed to see the members.
        # This can either be because the AS user is in the room or because there
        # is a user in the room that the AS is "interested in"
        if (
            requester.app_service
            and requester.user.to_string() not in users_with_profile
        ):
            for uid in users_with_profile:
                if requester.app_service.is_interested_in_user(uid):
                    break
            else:
                # Loop fell through, AS has no interested users in room
                raise UnstableSpecAuthError(
                    403,
                    "Appservice not in room",
                    errcode=Codes.NOT_JOINED,
                )

        return {
            user_id: {
                "avatar_url": profile.avatar_url,
                "display_name": profile.display_name,
            }
            for user_id, profile in users_with_profile.items()
        }

    def maybe_schedule_expiry(self, event: EventBase) -> None:
        """Schedule the expiry of an event if there's not already one scheduled,
        or if the one running is for an event that will expire after the provided
        timestamp.

        This function needs to invalidate the event cache, which is only possible on
        the master process, and therefore needs to be run on there.

        Args:
            event: The event to schedule the expiry of.
        """

        expiry_ts = event.content.get(EventContentFields.SELF_DESTRUCT_AFTER)
        if type(expiry_ts) is not int or event.is_state():  # noqa: E721
            return

        # _schedule_expiry_for_event won't actually schedule anything if there's already
        # a task scheduled for a timestamp that's sooner than the provided one.
        self._schedule_expiry_for_event(event.event_id, expiry_ts)

    async def _schedule_next_expiry(self) -> None:
        """Retrieve the ID and the expiry timestamp of the next event to be expired,
        and schedule an expiry task for it.

        If there's no event left to expire, set _expiry_scheduled to None so that a
        future call to save_expiry_ts can schedule a new expiry task.
        """
        # Try to get the expiry timestamp of the next event to expire.
        res = await self.store.get_next_event_to_expire()
        if res:
            event_id, expiry_ts = res
            self._schedule_expiry_for_event(event_id, expiry_ts)

    def _schedule_expiry_for_event(self, event_id: str, expiry_ts: int) -> None:
        """Schedule an expiry task for the provided event if there's not already one
        scheduled at a timestamp that's sooner than the provided one.

        Args:
            event_id: The ID of the event to expire.
            expiry_ts: The timestamp at which to expire the event.
        """
        if self._scheduled_expiry:
            # If the provided timestamp refers to a time before the scheduled time of the
            # next expiry task, cancel that task and reschedule it for this timestamp.
            next_scheduled_expiry_ts = self._scheduled_expiry.getTime() * 1000
            if expiry_ts < next_scheduled_expiry_ts:
                self._scheduled_expiry.cancel()
            else:
                return

        # Figure out how many seconds we need to wait before expiring the event.
        now_ms = self.clock.time_msec()
        delay = (expiry_ts - now_ms) / 1000

        # callLater doesn't support negative delays, so trim the delay to 0 if we're
        # in that case.
        if delay < 0:
            delay = 0

        logger.info("Scheduling expiry for event %s in %.3fs", event_id, delay)

        self._scheduled_expiry = self.clock.call_later(
            delay,
            run_as_background_process,
            "_expire_event",
            self._expire_event,
            event_id,
        )

    async def _expire_event(self, event_id: str) -> None:
        """Retrieve and expire an event that needs to be expired from the database.

        If the event doesn't exist in the database, log it and delete the expiry date
        from the database (so that we don't try to expire it again).
        """
        assert self._ephemeral_events_enabled

        self._scheduled_expiry = None

        logger.info("Expiring event %s", event_id)

        try:
            # Expire the event if we know about it. This function also deletes the expiry
            # date from the database in the same database transaction.
            await self.store.expire_event(event_id)
        except Exception as e:
            logger.error("Could not expire event %s: %r", event_id, e)

        # Schedule the expiry of the next event to expire.
        await self._schedule_next_expiry()


# The duration (in ms) after which rooms should be removed
# `_rooms_to_exclude_from_dummy_event_insertion` (with the effect that we will try
# to generate a dummy event for them once more)
#
_DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY = 7 * 24 * 60 * 60 * 1000


class EventCreationHandler:
    def __init__(self, hs: "HomeServer"):
        self.hs = hs
        self.auth_blocking = hs.get_auth_blocking()
        self._event_auth_handler = hs.get_event_auth_handler()
        self.store = hs.get_datastores().main
        self._storage_controllers = hs.get_storage_controllers()
        self.state = hs.get_state_handler()
        self.clock = hs.get_clock()
        self.validator = EventValidator()
        self.profile_handler = hs.get_profile_handler()
        self.event_builder_factory = hs.get_event_builder_factory()
        self.server_name = hs.hostname
        self.notifier = hs.get_notifier()
        self.config = hs.config
        self.require_membership_for_aliases = (
            hs.config.server.require_membership_for_aliases
        )
        self._events_shard_config = self.config.worker.events_shard_config
        self._instance_name = hs.get_instance_name()
        self._notifier = hs.get_notifier()
        self._worker_lock_handler = hs.get_worker_locks_handler()

        self.room_prejoin_state_types = self.hs.config.api.room_prejoin_state

        self.membership_types_to_include_profile_data_in = {
            Membership.JOIN,
            Membership.KNOCK,
        }
        if self.hs.config.server.include_profile_data_on_invite:
            self.membership_types_to_include_profile_data_in.add(Membership.INVITE)

        self.send_event = ReplicationSendEventRestServlet.make_client(hs)
        self.send_events = ReplicationSendEventsRestServlet.make_client(hs)

        self.request_ratelimiter = hs.get_request_ratelimiter()

        # We limit concurrent event creation for a room to 1. This prevents state resolution
        # from occurring when sending bursts of events to a local room
        self.limiter = Linearizer(max_count=1, name="room_event_creation_limit")

        self._bulk_push_rule_evaluator = hs.get_bulk_push_rule_evaluator()

        self._spam_checker_module_callbacks = hs.get_module_api_callbacks().spam_checker
        self._third_party_event_rules = (
            self.hs.get_module_api_callbacks().third_party_event_rules
        )

        self._block_events_without_consent_error = (
            self.config.consent.block_events_without_consent_error
        )

        # we need to construct a ConsentURIBuilder here, as it checks that the necessary
        # config options, but *only* if we have a configuration for which we are
        # going to need it.
        if self._block_events_without_consent_error:
            self._consent_uri_builder = ConsentURIBuilder(self.config)

        # Rooms which should be excluded from dummy insertion. (For instance,
        # those without local users who can send events into the room).
        #
        # map from room id to time-of-last-attempt.
        #
        self._rooms_to_exclude_from_dummy_event_insertion: Dict[str, int] = {}
        # The number of forward extremeities before a dummy event is sent.
        self._dummy_events_threshold = hs.config.server.dummy_events_threshold

        if (
            self.config.worker.run_background_tasks
            and self.config.server.cleanup_extremities_with_dummy_events
        ):
            self.clock.looping_call(
                lambda: run_as_background_process(
                    "send_dummy_events_to_fill_extremities",
                    self._send_dummy_events_to_fill_extremities,
                ),
                5 * 60 * 1000,
            )

        self._message_handler = hs.get_message_handler()

        self._ephemeral_events_enabled = hs.config.server.enable_ephemeral_messages

        self._external_cache = hs.get_external_cache()

        # Stores the state groups we've recently added to the joined hosts
        # external cache. Note that the timeout must be significantly less than
        # the TTL on the external cache.
        self._external_cache_joined_hosts_updates: Optional[ExpiringCache] = None
        if self._external_cache.is_enabled():
            self._external_cache_joined_hosts_updates = ExpiringCache(
                "_external_cache_joined_hosts_updates",
                self.clock,
                expiry_ms=30 * 60 * 1000,
            )

    async def create_event(
        self,
        requester: Requester,
        event_dict: dict,
        txn_id: Optional[str] = None,
        allow_no_prev_events: bool = False,
        prev_event_ids: Optional[List[str]] = None,
        auth_event_ids: Optional[List[str]] = None,
        state_event_ids: Optional[List[str]] = None,
        require_consent: bool = True,
        outlier: bool = False,
        depth: Optional[int] = None,
        state_map: Optional[StateMap[str]] = None,
        for_batch: bool = False,
        current_state_group: Optional[int] = None,
    ) -> Tuple[EventBase, UnpersistedEventContextBase]:
        """
        Given a dict from a client, create a new event. If bool for_batch is true, will
        create an event using the prev_event_ids, and will create an event context for
        the event using the parameters state_map and current_state_group, thus these parameters
        must be provided in this case if for_batch is True. The subsequently created event
        and context are suitable for being batched up and bulk persisted to the database
        with other similarly created events.

        Creates an FrozenEvent object, filling out auth_events, prev_events,
        etc.

        Adds display names to Join membership events.

        Args:
            requester
            event_dict: An entire event
            txn_id
            allow_no_prev_events: Whether to allow this event to be created an empty
                list of prev_events. Normally this is prohibited just because most
                events should have a prev_event and we should only use this in special
                cases (previously useful for MSC2716).
            prev_event_ids:
                the forward extremities to use as the prev_events for the
                new event.

                If None, they will be requested from the database.

            auth_event_ids:
                The event ids to use as the auth_events for the new event.
                Should normally be left as None, which will cause them to be calculated
                based on the room state at the prev_events.

                If non-None, prev_event_ids must also be provided.

            state_event_ids:
                The full state at a given event. This was previously used particularly
                by the MSC2716 /batch_send endpoint. This should normally be left as
                None, which will cause the auth_event_ids to be calculated based on the
                room state at the prev_events.

            require_consent: Whether to check if the requester has
                consented to the privacy policy.

            outlier: Indicates whether the event is an `outlier`, i.e. if
                it's from an arbitrary point and floating in the DAG as
                opposed to being inline with the current DAG.

            depth: Override the depth used to order the event in the DAG.
                Should normally be set to None, which will cause the depth to be calculated
                based on the prev_events.

            state_map: A state map of previously created events, used only when creating events
                for batch persisting

            for_batch: whether the event is being created for batch persisting to the db

            current_state_group: the current state group, used only for creating events for
                batch persisting

        Raises:
            ResourceLimitError if server is blocked to some resource being
            exceeded

        Returns:
            Tuple of created event, Context
        """
        await self.auth_blocking.check_auth_blocking(requester=requester)

        if event_dict["type"] == EventTypes.Create and event_dict["state_key"] == "":
            room_version_id = event_dict["content"]["room_version"]
            maybe_room_version_obj = KNOWN_ROOM_VERSIONS.get(room_version_id)
            if not maybe_room_version_obj:
                # this can happen if support is withdrawn for a room version
                raise UnsupportedRoomVersionError(room_version_id)
            room_version_obj = maybe_room_version_obj
        else:
            try:
                room_version_obj = await self.store.get_room_version(
                    event_dict["room_id"]
                )
            except NotFoundError:
                raise AuthError(403, "Unknown room")

        builder = self.event_builder_factory.for_room_version(
            room_version_obj, event_dict
        )

        self.validator.validate_builder(builder)

        if builder.type == EventTypes.Member:
            membership = builder.content.get("membership", None)
            target = UserID.from_string(builder.state_key)

            if membership in self.membership_types_to_include_profile_data_in:
                # If event doesn't include a display name, add one.
                profile = self.profile_handler
                content = builder.content

                try:
                    if "displayname" not in content:
                        displayname = await profile.get_displayname(target)
                        if displayname is not None:
                            content["displayname"] = displayname
                    if "avatar_url" not in content:
                        avatar_url = await profile.get_avatar_url(target)
                        if avatar_url is not None:
                            content["avatar_url"] = avatar_url
                except Exception as e:
                    logger.info(
                        "Failed to get profile information for %r: %s", target, e
                    )

        is_exempt = await self._is_exempt_from_privacy_policy(builder, requester)
        if require_consent and not is_exempt:
            await self.assert_accepted_privacy_policy(requester)

        # Save the access token ID, the device ID and the transaction ID in the event
        # internal metadata. This is useful to determine if we should echo the
        # transaction_id in events.
        # See `synapse.events.utils.EventClientSerializer.serialize_event`
        if requester.access_token_id is not None:
            builder.internal_metadata.token_id = requester.access_token_id

        if requester.device_id is not None:
            builder.internal_metadata.device_id = requester.device_id

        if txn_id is not None:
            builder.internal_metadata.txn_id = txn_id

        builder.internal_metadata.outlier = outlier

        event, unpersisted_context = await self.create_new_client_event(
            builder=builder,
            requester=requester,
            allow_no_prev_events=allow_no_prev_events,
            prev_event_ids=prev_event_ids,
            auth_event_ids=auth_event_ids,
            state_event_ids=state_event_ids,
            depth=depth,
            state_map=state_map,
            for_batch=for_batch,
            current_state_group=current_state_group,
        )

        # In an ideal world we wouldn't need the second part of this condition. However,
        # this behaviour isn't spec'd yet, meaning we should be able to deactivate this
        # behaviour. Another reason is that this code is also evaluated each time a new
        # m.room.aliases event is created, which includes hitting a /directory route.
        # Therefore not including this condition here would render the similar one in
        # synapse.handlers.directory pointless.
        if builder.type == EventTypes.Aliases and self.require_membership_for_aliases:
            # Ideally we'd do the membership check in event_auth.check(), which
            # describes a spec'd algorithm for authenticating events received over
            # federation as well as those created locally. As of room v3, aliases events
            # can be created by users that are not in the room, therefore we have to
            # tolerate them in event_auth.check().
            if for_batch:
                assert state_map is not None
                prev_event_id = state_map.get((EventTypes.Member, event.sender))
            else:
                prev_state_ids = await unpersisted_context.get_prev_state_ids(
                    StateFilter.from_types([(EventTypes.Member, event.sender)])
                )
                prev_event_id = prev_state_ids.get((EventTypes.Member, event.sender))
            prev_event = (
                await self.store.get_event(prev_event_id, allow_none=True)
                if prev_event_id
                else None
            )
            if not prev_event or prev_event.membership != Membership.JOIN:
                logger.warning(
                    (
                        "Attempt to send `m.room.aliases` in room %s by user %s but"
                        " membership is %s"
                    ),
                    event.room_id,
                    event.sender,
                    prev_event.membership if prev_event else None,
                )

                raise AuthError(
                    403, "You must be in the room to create an alias for it"
                )

        self.validator.validate_new(event, self.config)
        return event, unpersisted_context

    async def _is_exempt_from_privacy_policy(
        self, builder: EventBuilder, requester: Requester
    ) -> bool:
        """ "Determine if an event to be sent is exempt from having to consent
        to the privacy policy

        Args:
            builder: event being created
            requester: user requesting this event

        Returns:
            true if the event can be sent without the user consenting
        """
        # the only thing the user can do is join the server notices room.
        if builder.type == EventTypes.Member:
            membership = builder.content.get("membership", None)
            if membership == Membership.JOIN:
                return await self.store.is_server_notice_room(builder.room_id)
            elif membership == Membership.LEAVE:
                # the user is always allowed to leave (but not kick people)
                return builder.state_key == requester.user.to_string()
        return False

    async def assert_accepted_privacy_policy(self, requester: Requester) -> None:
        """Check if a user has accepted the privacy policy

        Called when the given user is about to do something that requires
        privacy consent. We see if the user is exempt and otherwise check that
        they have given consent. If they have not, a ConsentNotGiven error is
        raised.

        Args:
            requester: The user making the request

        Returns:
            Returns normally if the user has consented or is exempt

        Raises:
            ConsentNotGivenError: if the user has not given consent yet
        """
        if self._block_events_without_consent_error is None:
            return

        # exempt AS users from needing consent
        if requester.app_service is not None:
            return

        user_id = requester.authenticated_entity
        if not user_id.startswith("@"):
            # The authenticated entity might not be a user, e.g. if it's the
            # server puppetting the user.
            return

        user = UserID.from_string(user_id)

        # exempt the system notices user
        if (
            self.config.servernotices.server_notices_mxid is not None
            and user_id == self.config.servernotices.server_notices_mxid
        ):
            return

        u = await self.store.get_user_by_id(user_id)
        assert u is not None
        if u.user_type in (UserTypes.SUPPORT, UserTypes.BOT):
            # support and bot users are not required to consent
            return
        if u.appservice_id is not None:
            # users registered by an appservice are exempt
            return
        if u.consent_version == self.config.consent.user_consent_version:
            return

        consent_uri = self._consent_uri_builder.build_user_consent_uri(user.localpart)
        msg = self._block_events_without_consent_error % {"consent_uri": consent_uri}
        raise ConsentNotGivenError(msg=msg, consent_uri=consent_uri)

    async def deduplicate_state_event(
        self, event: EventBase, context: EventContext
    ) -> Optional[EventBase]:
        """
        Checks whether event is in the latest resolved state in context.

        Args:
            event: The event to check for duplication.
            context: The event context.

        Returns:
            The previous version of the event is returned, if it is found in the
            event context. Otherwise, None is returned.
        """
        if event.internal_metadata.is_outlier():
            # This can happen due to out of band memberships
            return None

        prev_state_ids = await context.get_prev_state_ids(
            StateFilter.from_types([(event.type, event.state_key)])
        )
        prev_event_id = prev_state_ids.get((event.type, event.state_key))
        if not prev_event_id:
            return None
        prev_event = await self.store.get_event(prev_event_id, allow_none=True)
        if not prev_event:
            return None

        if prev_event and event.user_id == prev_event.user_id:
            prev_content = encode_canonical_json(prev_event.content)
            next_content = encode_canonical_json(event.content)
            if prev_content == next_content:
                return prev_event
        return None

    async def get_event_id_from_transaction(
        self,
        requester: Requester,
        txn_id: str,
        room_id: str,
    ) -> Optional[str]:
        """For the given transaction ID and room ID, check if there is a matching event ID.

        Args:
            requester: The requester making the request in the context of which we want
                to fetch the event.
            txn_id: The transaction ID.
            room_id: The room ID.

        Returns:
            An event ID if one could be found, None otherwise.
        """
        existing_event_id = None

        # According to the spec, transactions are scoped to a user's device ID.
        if requester.device_id:
            existing_event_id = (
                await self.store.get_event_id_from_transaction_id_and_device_id(
                    room_id,
                    requester.user.to_string(),
                    requester.device_id,
                    txn_id,
                )
            )
            if existing_event_id:
                return existing_event_id

        return existing_event_id

    async def get_event_from_transaction(
        self,
        requester: Requester,
        txn_id: str,
        room_id: str,
    ) -> Optional[EventBase]:
        """For the given transaction ID and room ID, check if there is a matching event.
        If so, fetch it and return it.

        Args:
            requester: The requester making the request in the context of which we want
                to fetch the event.
            txn_id: The transaction ID.
            room_id: The room ID.

        Returns:
            An event if one could be found, None otherwise.
        """
        existing_event_id = await self.get_event_id_from_transaction(
            requester, txn_id, room_id
        )
        if existing_event_id:
            return await self.store.get_event(existing_event_id)
        return None

    async def create_and_send_nonmember_event(
        self,
        requester: Requester,
        event_dict: dict,
        allow_no_prev_events: bool = False,
        prev_event_ids: Optional[List[str]] = None,
        state_event_ids: Optional[List[str]] = None,
        ratelimit: bool = True,
        txn_id: Optional[str] = None,
        ignore_shadow_ban: bool = False,
        outlier: bool = False,
        depth: Optional[int] = None,
    ) -> Tuple[EventBase, int]:
        """
        Creates an event, then sends it.

        See self.create_event and self.handle_new_client_event.

        Args:
            requester: The requester sending the event.
            event_dict: An entire event.
            allow_no_prev_events: Whether to allow this event to be created an empty
                list of prev_events. Normally this is prohibited just because most
                events should have a prev_event and we should only use this in special
                cases (previously useful for MSC2716).
            prev_event_ids:
                The event IDs to use as the prev events.
                Should normally be left as None to automatically request them
                from the database.
            state_event_ids:
                The full state at a given event. This was previously used particularly
                by the MSC2716 /batch_send endpoint. This should normally be left as
                None, which will cause the auth_event_ids to be calculated based on the
                room state at the prev_events.
            ratelimit: Whether to rate limit this send.
            txn_id: The transaction ID.
            ignore_shadow_ban: True if shadow-banned users should be allowed to
                send this event.
            outlier: Indicates whether the event is an `outlier`, i.e. if
                it's from an arbitrary point and floating in the DAG as
                opposed to being inline with the current DAG.
            depth: Override the depth used to order the event in the DAG.
                Should normally be set to None, which will cause the depth to be calculated
                based on the prev_events.

        Returns:
            The event, and its stream ordering (if deduplication happened,
            the previous, duplicate event).

        Raises:
            ShadowBanError if the requester has been shadow-banned.
        """

        if event_dict["type"] == EventTypes.Member:
            raise SynapseError(
                500, "Tried to send member event through non-member codepath"
            )

        if not ignore_shadow_ban and requester.shadow_banned:
            # We randomly sleep a bit just to annoy the requester.
            await self.clock.sleep(random.randint(1, 10))
            raise ShadowBanError()

        if ratelimit:
            await self.request_ratelimiter.ratelimit(requester, update=False)

        # We limit the number of concurrent event sends in a room so that we
        # don't fork the DAG too much. If we don't limit then we can end up in
        # a situation where event persistence can't keep up, causing
        # extremities to pile up, which in turn leads to state resolution
        # taking longer.
        room_id = event_dict["room_id"]
        async with self.limiter.queue(room_id):
            if txn_id:
                event = await self.get_event_from_transaction(
                    requester, txn_id, room_id
                )
                if event:
                    # we know it was persisted, so must have a stream ordering
                    assert event.internal_metadata.stream_ordering
                    return (
                        event,
                        event.internal_metadata.stream_ordering,
                    )

        async with self._worker_lock_handler.acquire_read_write_lock(
            NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
        ):
            return await self._create_and_send_nonmember_event_locked(
                requester=requester,
                event_dict=event_dict,
                allow_no_prev_events=allow_no_prev_events,
                prev_event_ids=prev_event_ids,
                state_event_ids=state_event_ids,
                ratelimit=ratelimit,
                txn_id=txn_id,
                ignore_shadow_ban=ignore_shadow_ban,
                outlier=outlier,
                depth=depth,
            )

    async def _create_and_send_nonmember_event_locked(
        self,
        requester: Requester,
        event_dict: dict,
        allow_no_prev_events: bool = False,
        prev_event_ids: Optional[List[str]] = None,
        state_event_ids: Optional[List[str]] = None,
        ratelimit: bool = True,
        txn_id: Optional[str] = None,
        ignore_shadow_ban: bool = False,
        outlier: bool = False,
        depth: Optional[int] = None,
    ) -> Tuple[EventBase, int]:
        room_id = event_dict["room_id"]

        # If we don't have any prev event IDs specified then we need to
        # check that the host is in the room (as otherwise populating the
        # prev events will fail), at which point we may as well check the
        # local user is in the room.
        if not prev_event_ids:
            user_id = requester.user.to_string()
            is_user_in_room = await self.store.check_local_user_in_room(
                user_id, room_id
            )
            if not is_user_in_room:
                raise AuthError(403, f"User {user_id} not in room {room_id}")

        # Try several times, it could fail with PartialStateConflictError
        # in handle_new_client_event, cf comment in except block.
        max_retries = 5
        for i in range(max_retries):
            try:
                event, unpersisted_context = await self.create_event(
                    requester,
                    event_dict,
                    txn_id=txn_id,
                    allow_no_prev_events=allow_no_prev_events,
                    prev_event_ids=prev_event_ids,
                    state_event_ids=state_event_ids,
                    outlier=outlier,
                    depth=depth,
                )
                context = await unpersisted_context.persist(event)

                assert self.hs.is_mine_id(event.sender), "User must be our own: %s" % (
                    event.sender,
                )

                spam_check_result = (
                    await self._spam_checker_module_callbacks.check_event_for_spam(
                        event
                    )
                )
                if spam_check_result != self._spam_checker_module_callbacks.NOT_SPAM:
                    if isinstance(spam_check_result, tuple):
                        try:
                            [code, dict] = spam_check_result
                            raise SynapseError(
                                403,
                                "This message had been rejected as probable spam",
                                code,
                                dict,
                            )
                        except ValueError:
                            logger.error(
                                "Spam-check module returned invalid error value. Expecting [code, dict], got %s",
                                spam_check_result,
                            )

                            raise SynapseError(
                                403,
                                "This message has been rejected as probable spam",
                                Codes.FORBIDDEN,
                            )

                    # Backwards compatibility: if the return value is not an error code, it
                    # means the module returned an error message to be included in the
                    # SynapseError (which is now deprecated).
                    raise SynapseError(
                        403,
                        spam_check_result,
                        Codes.FORBIDDEN,
                    )

                ev = await self.handle_new_client_event(
                    requester=requester,
                    events_and_context=[(event, context)],
                    ratelimit=ratelimit,
                    ignore_shadow_ban=ignore_shadow_ban,
                )

                break
            except PartialStateConflictError as e:
                # Persisting couldn't happen because the room got un-partial stated
                # in the meantime and context needs to be recomputed, so let's do so.
                if i == max_retries - 1:
                    raise e
                pass

        # we know it was persisted, so must have a stream ordering
        assert ev.internal_metadata.stream_ordering
        return ev, ev.internal_metadata.stream_ordering

    @measure_func("create_new_client_event")
    async def create_new_client_event(
        self,
        builder: EventBuilder,
        requester: Optional[Requester] = None,
        allow_no_prev_events: bool = False,
        prev_event_ids: Optional[List[str]] = None,
        auth_event_ids: Optional[List[str]] = None,
        state_event_ids: Optional[List[str]] = None,
        depth: Optional[int] = None,
        state_map: Optional[StateMap[str]] = None,
        for_batch: bool = False,
        current_state_group: Optional[int] = None,
    ) -> Tuple[EventBase, UnpersistedEventContextBase]:
        """Create a new event for a local client. If bool for_batch is true, will
        create an event using the prev_event_ids, and will create an event context for
        the event using the parameters state_map and current_state_group, thus these parameters
        must be provided in this case if for_batch is True. The subsequently created event
        and context are suitable for being batched up and bulk persisted to the database
        with other similarly created events. Note that this returns an UnpersistedEventContext,
        which must be converted to an EventContext before it can be sent to the DB.

        Args:
            builder:
            requester:
            allow_no_prev_events: Whether to allow this event to be created an empty
                list of prev_events. Normally this is prohibited just because most
                events should have a prev_event and we should only use this in special
                cases (previously useful for MSC2716).
            prev_event_ids:
                the forward extremities to use as the prev_events for the
                new event.

                If None, they will be requested from the database.

            auth_event_ids:
                The event ids to use as the auth_events for the new event.
                Should normally be left as None, which will cause them to be calculated
                based on the room state at the prev_events.

            state_event_ids:
                The full state at a given event. This was previously used particularly
                by the MSC2716 /batch_send endpoint. This should normally be left as
                None, which will cause the auth_event_ids to be calculated based on the
                room state at the prev_events.

            depth: Override the depth used to order the event in the DAG.
                Should normally be set to None, which will cause the depth to be calculated
                based on the prev_events.

            state_map: A state map of previously created events, used only when creating events
                for batch persisting

            for_batch: whether the event is being created for batch persisting to the db

            current_state_group: the current state group, used only for creating events for
                batch persisting

        Returns:
            Tuple of created event, UnpersistedEventContext
        """
        # Strip down the state_event_ids to only what we need to auth the event.
        # For example, we don't need extra m.room.member that don't match event.sender
        if state_event_ids is not None:
            # Do a quick check to make sure that prev_event_ids is present to
            # make the type-checking around `builder.build` happy.
            # prev_event_ids could be an empty array though.
            assert prev_event_ids is not None

            temp_event = await builder.build(
                prev_event_ids=prev_event_ids,
                auth_event_ids=state_event_ids,
                depth=depth,
            )
            state_events = await self.store.get_events_as_list(state_event_ids)
            # Create a StateMap[str]
            current_state_ids = {
                (e.type, e.state_key): e.event_id for e in state_events
            }
            # Actually strip down and only use the necessary auth events
            auth_event_ids = self._event_auth_handler.compute_auth_events(
                event=temp_event,
                current_state_ids=current_state_ids,
                for_verification=False,
            )

        if prev_event_ids is not None:
            assert (
                len(prev_event_ids) <= 10
            ), "Attempting to create an event with %i prev_events" % (
                len(prev_event_ids),
            )
        else:
            prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)

        # Do a quick sanity check here, rather than waiting until we've created the
        # event and then try to auth it (which fails with a somewhat confusing "No
        # create event in auth events")
        if allow_no_prev_events:
            # We allow events with no `prev_events` but it better have some `auth_events`
            assert (
                builder.type == EventTypes.Create
                # Allow an event to have empty list of prev_event_ids
                # only if it has auth_event_ids.
                or auth_event_ids
            ), "Attempting to create a non-m.room.create event with no prev_events or auth_event_ids"
        else:
            # we now ought to have some prev_events (unless it's a create event).
            assert (
                builder.type == EventTypes.Create or prev_event_ids
            ), "Attempting to create a non-m.room.create event with no prev_events"

        if for_batch:
            assert prev_event_ids is not None
            assert state_map is not None
            auth_ids = self._event_auth_handler.compute_auth_events(builder, state_map)
            event = await builder.build(
                prev_event_ids=prev_event_ids, auth_event_ids=auth_ids, depth=depth
            )

            context: UnpersistedEventContextBase = (
                await self.state.calculate_context_info(
                    event,
                    state_ids_before_event=state_map,
                    partial_state=False,
                    state_group_before_event=current_state_group,
                )
            )

        else:
            event = await builder.build(
                prev_event_ids=prev_event_ids,
                auth_event_ids=auth_event_ids,
                depth=depth,
            )

            # Pass on the outlier property from the builder to the event
            # after it is created
            if builder.internal_metadata.outlier:
                event.internal_metadata.outlier = True
                context = EventContext.for_outlier(self._storage_controllers)
            else:
                context = await self.state.calculate_context_info(event)

        if requester:
            context.app_service = requester.app_service

        res, new_content = await self._third_party_event_rules.check_event_allowed(
            event, context
        )
        if res is False:
            logger.info(
                "Event %s forbidden by third-party rules",
                event,
            )
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )
        elif new_content is not None:
            # the third-party rules want to replace the event. We'll need to build a new
            # event.
            event, context = await self._rebuild_event_after_third_party_rules(
                new_content, event
            )

        self.validator.validate_new(event, self.config)
        await self._validate_event_relation(event)
        logger.debug("Created event %s", event.event_id)

        return event, context

    async def _validate_event_relation(self, event: EventBase) -> None:
        """
        Ensure the relation data on a new event is not bogus.

        Args:
            event: The event being created.

        Raises:
            SynapseError if the event is invalid.
        """

        relation = relation_from_event(event)
        if not relation:
            return

        parent_event = await self.store.get_event(relation.parent_id, allow_none=True)
        if parent_event:
            # And in the same room.
            if parent_event.room_id != event.room_id:
                raise SynapseError(400, "Relations must be in the same room")

        else:
            # There must be some reason that the client knows the event exists,
            # see if there are existing relations. If so, assume everything is fine.
            if not await self.store.event_is_target_of_relation(relation.parent_id):
                # Otherwise, the client can't know about the parent event!
                raise SynapseError(400, "Can't send relation to unknown event")

        # If this event is an annotation then we check that that the sender
        # can't annotate the same way twice (e.g. stops users from liking an
        # event multiple times).
        if relation.rel_type == RelationTypes.ANNOTATION:
            aggregation_key = relation.aggregation_key

            if aggregation_key is None:
                raise SynapseError(400, "Missing aggregation key")

            if len(aggregation_key) > 500:
                raise SynapseError(400, "Aggregation key is too long")

            already_exists = await self.store.has_user_annotated_event(
                relation.parent_id, event.type, aggregation_key, event.sender
            )
            if already_exists:
                raise SynapseError(
                    400,
                    "Can't send same reaction twice",
                    errcode=Codes.DUPLICATE_ANNOTATION,
                )

        # Don't attempt to start a thread if the parent event is a relation.
        elif relation.rel_type == RelationTypes.THREAD:
            if await self.store.event_includes_relation(relation.parent_id):
                raise SynapseError(
                    400, "Cannot start threads from an event with a relation"
                )

    @measure_func("handle_new_client_event")
    async def handle_new_client_event(
        self,
        requester: Requester,
        events_and_context: List[Tuple[EventBase, EventContext]],
        ratelimit: bool = True,
        extra_users: Optional[List[UserID]] = None,
        ignore_shadow_ban: bool = False,
    ) -> EventBase:
        """Processes new events. Please note that if batch persisting events, an error in
        handling any one of these events will result in all of the events being dropped.

        This includes deduplicating, checking auth, persisting,
        notifying users, sending to remote servers, etc.

        If called from a worker will hit out to the master process for final
        processing.

        Args:
            requester
            events_and_context: A list of one or more tuples of event, context to be persisted
            ratelimit
            extra_users: Any extra users to notify about event

            ignore_shadow_ban: True if shadow-banned users should be allowed to
                send this event.

        Return:
            If the event was deduplicated, the previous, duplicate, event. Otherwise,
            `event`.

        Raises:
            ShadowBanError if the requester has been shadow-banned.
            PartialStateConflictError if attempting to persist a partial state event in
                a room that has been un-partial stated.
        """
        extra_users = extra_users or []

        for event, context in events_and_context:
            # we don't apply shadow-banning to membership events here. Invites are blocked
            # higher up the stack, and we allow shadow-banned users to send join and leave
            # events as normal.
            if (
                event.type != EventTypes.Member
                and not ignore_shadow_ban
                and requester.shadow_banned
            ):
                # We randomly sleep a bit just to annoy the requester.
                await self.clock.sleep(random.randint(1, 10))
                raise ShadowBanError()

            if event.is_state():
                prev_event = await self.deduplicate_state_event(event, context)
                if prev_event is not None:
                    logger.info(
                        "Not bothering to persist state event %s duplicated by %s",
                        event.event_id,
                        prev_event.event_id,
                    )
                    return prev_event

            if event.internal_metadata.is_out_of_band_membership():
                # the only sort of out-of-band-membership events we expect to see here are
                # invite rejections and rescinded knocks that we have generated ourselves.
                assert event.type == EventTypes.Member
                assert event.content["membership"] == Membership.LEAVE
            else:
                try:
                    validate_event_for_room_version(event)
                    # If we are persisting a batch of events the event(s) needed to auth the
                    # current event may be part of the batch and will not be in the DB yet
                    event_id_to_event = {e.event_id: e for e, _ in events_and_context}
                    batched_auth_events = {}
                    for event_id in event.auth_event_ids():
                        auth_event = event_id_to_event.get(event_id)
                        if auth_event:
                            batched_auth_events[event_id] = auth_event
                    await self._event_auth_handler.check_auth_rules_from_context(
                        event, batched_auth_events
                    )
                except AuthError as err:
                    logger.warning("Denying new event %r because %s", event, err)
                    raise err

            # Ensure that we can round trip before trying to persist in db
            try:
                dump = json_encoder.encode(event.content)
                json_decoder.decode(dump)
            except Exception:
                logger.exception("Failed to encode content: %r", event.content)
                raise

        # We now persist the event (and update the cache in parallel, since we
        # don't want to block on it).
        #
        # Note: mypy gets confused if we inline dl and check with twisted#11770.
        # Some kind of bug in mypy's deduction?
        deferreds = (
            run_in_background(
                self._persist_events,
                requester=requester,
                events_and_context=events_and_context,
                ratelimit=ratelimit,
                extra_users=extra_users,
            ),
            run_in_background(
                self.cache_joined_hosts_for_events, events_and_context
            ).addErrback(log_failure, "cache_joined_hosts_for_event failed"),
        )
        result, _ = await make_deferred_yieldable(
            gather_results(deferreds, consumeErrors=True)
        ).addErrback(unwrapFirstError)

        return result

    async def _persist_events(
        self,
        requester: Requester,
        events_and_context: List[Tuple[EventBase, EventContext]],
        ratelimit: bool = True,
        extra_users: Optional[List[UserID]] = None,
    ) -> EventBase:
        """Actually persists new events. Should only be called by
        `handle_new_client_event`, and see its docstring for documentation of
        the arguments. Please note that if batch persisting events, an error in
        handling any one of these events will result in all of the events being dropped.

        PartialStateConflictError: if attempting to persist a partial state event in
            a room that has been un-partial stated.
        """

        await self._bulk_push_rule_evaluator.action_for_events_by_user(
            events_and_context
        )

        try:
            # If we're a worker we need to hit out to the master.
            first_event, _ = events_and_context[0]
            writer_instance = self._events_shard_config.get_instance(
                first_event.room_id
            )
            if writer_instance != self._instance_name:
                try:
                    result = await self.send_events(
                        instance_name=writer_instance,
                        events_and_context=events_and_context,
                        store=self.store,
                        requester=requester,
                        ratelimit=ratelimit,
                        extra_users=extra_users,
                    )
                except SynapseError as e:
                    if e.code == HTTPStatus.CONFLICT:
                        raise PartialStateConflictError()
                    raise
                stream_id = result["stream_id"]
                event_id = result["event_id"]

                # If we batch persisted events we return the last persisted event, otherwise
                # we return the one event that was persisted
                event, _ = events_and_context[-1]

                if event_id != event.event_id:
                    # If we get a different event back then it means that its
                    # been de-duplicated, so we replace the given event with the
                    # one already persisted.
                    event = await self.store.get_event(event_id)
                else:
                    # If we newly persisted the event then we need to update its
                    # stream_ordering entry manually (as it was persisted on
                    # another worker).
                    event.internal_metadata.stream_ordering = stream_id
                return event

            event = await self.persist_and_notify_client_events(
                requester,
                events_and_context,
                ratelimit=ratelimit,
                extra_users=extra_users,
            )

            return event
        except Exception:
            for event, _ in events_and_context:
                # Ensure that we actually remove the entries in the push actions
                # staging area, if we calculated them.
                await self.store.remove_push_actions_from_staging(event.event_id)
            raise

    async def cache_joined_hosts_for_events(
        self, events_and_context: List[Tuple[EventBase, EventContext]]
    ) -> None:
        """Precalculate the joined hosts at each of the given events, when using Redis, so that
        external federation senders don't have to recalculate it themselves.
        """

        if not self._external_cache.is_enabled():
            return

        # If external cache is enabled we should always have this.
        assert self._external_cache_joined_hosts_updates is not None

        for event, event_context in events_and_context:
            if event_context.partial_state:
                # To populate the cache for a partial-state event, we either have to
                # block until full state, which the code below does, or change the
                # meaning of cache values to be the list of hosts to which we plan to
                # send events and calculate that instead.
                #
                # The federation senders don't use the external cache when sending
                # events in partial-state rooms anyway, so let's not bother populating
                # the cache.
                continue

            # We actually store two mappings, event ID -> prev state group,
            # state group -> joined hosts, which is much more space efficient
            # than event ID -> joined hosts.
            #
            # Note: We have to cache event ID -> prev state group, as we don't
            # store that in the DB.
            #
            # Note: We set the state group -> joined hosts cache if it hasn't been
            # set for a while, so that the expiry time is reset.

            state_entry = await self.state.resolve_state_groups_for_events(
                event.room_id, event_ids=event.prev_event_ids()
            )

            if state_entry.state_group:
                await self._external_cache.set(
                    "event_to_prev_state_group",
                    event.event_id,
                    state_entry.state_group,
                    expiry_ms=60 * 60 * 1000,
                )

                if state_entry.state_group in self._external_cache_joined_hosts_updates:
                    return

                with opentracing.start_active_span("get_joined_hosts"):
                    joined_hosts = (
                        await self._storage_controllers.state.get_joined_hosts(
                            event.room_id, state_entry
                        )
                    )

                # Note that the expiry times must be larger than the expiry time in
                # _external_cache_joined_hosts_updates.
                await self._external_cache.set(
                    "get_joined_hosts",
                    str(state_entry.state_group),
                    list(joined_hosts),
                    expiry_ms=60 * 60 * 1000,
                )

                self._external_cache_joined_hosts_updates[
                    state_entry.state_group
                ] = None

    async def _validate_canonical_alias(
        self,
        directory_handler: DirectoryHandler,
        room_alias_str: str,
        expected_room_id: str,
    ) -> None:
        """
        Ensure that the given room alias points to the expected room ID.

        Args:
            directory_handler: The directory handler object.
            room_alias_str: The room alias to check.
            expected_room_id: The room ID that the alias should point to.
        """
        room_alias = RoomAlias.from_string(room_alias_str)
        try:
            mapping = await directory_handler.get_association(room_alias)
        except SynapseError as e:
            # Turn M_NOT_FOUND errors into M_BAD_ALIAS errors.
            if e.errcode == Codes.NOT_FOUND:
                raise SynapseError(
                    400,
                    "Room alias %s does not point to the room" % (room_alias_str,),
                    Codes.BAD_ALIAS,
                )
            raise

        if mapping["room_id"] != expected_room_id:
            raise SynapseError(
                400,
                "Room alias %s does not point to the room" % (room_alias_str,),
                Codes.BAD_ALIAS,
            )

    async def persist_and_notify_client_events(
        self,
        requester: Requester,
        events_and_context: List[Tuple[EventBase, EventContext]],
        ratelimit: bool = True,
        extra_users: Optional[List[UserID]] = None,
    ) -> EventBase:
        """Called when we have fully built the events, have already
        calculated the push actions for the events, and checked auth.

        This should only be run on the instance in charge of persisting events.

        Please note that if batch persisting events, an error in
        handling any one of these events will result in all of the events being dropped.

        Returns:
            The persisted event, if one event is passed in, or the last event in the
            list in the case of batch persisting. If only one event was persisted, the
            returned event may be different than the given event if it was de-duplicated
            (e.g. because we had already persisted an event with the same transaction ID.)

        Raises:
            PartialStateConflictError: if attempting to persist a partial state event in
                a room that has been un-partial stated.
        """
        extra_users = extra_users or []

        for event, context in events_and_context:
            assert self._events_shard_config.should_handle(
                self._instance_name, event.room_id
            )

            if ratelimit:
                # We check if this is a room admin redacting an event so that we
                # can apply different ratelimiting. We do this by simply checking
                # it's not a self-redaction (to avoid having to look up whether the
                # user is actually admin or not).
                is_admin_redaction = False
                if event.type == EventTypes.Redaction:
                    assert event.redacts is not None

                    original_event = await self.store.get_event(
                        event.redacts,
                        redact_behaviour=EventRedactBehaviour.as_is,
                        get_prev_content=False,
                        allow_rejected=False,
                        allow_none=True,
                    )

                    is_admin_redaction = bool(
                        original_event and event.sender != original_event.sender
                    )

                await self.request_ratelimiter.ratelimit(
                    requester, is_admin_redaction=is_admin_redaction
                )

            # run checks/actions on event based on type
            if event.type == EventTypes.Member and event.membership == Membership.JOIN:
                (
                    current_membership,
                    _,
                ) = await self.store.get_local_current_membership_for_user_in_room(
                    event.state_key, event.room_id
                )
                if current_membership != Membership.JOIN:
                    self._notifier.notify_user_joined_room(
                        event.event_id, event.room_id
                    )

            await self._maybe_kick_guest_users(event, context)

            if event.type == EventTypes.CanonicalAlias:
                # Validate a newly added alias or newly added alt_aliases.

                original_alias = None
                original_alt_aliases: object = []

                original_event_id = event.unsigned.get("replaces_state")
                if original_event_id:
                    original_alias_event = await self.store.get_event(original_event_id)

                    if original_alias_event:
                        original_alias = original_alias_event.content.get("alias", None)
                        original_alt_aliases = original_alias_event.content.get(
                            "alt_aliases", []
                        )

                # Check the alias is currently valid (if it has changed).
                room_alias_str = event.content.get("alias", None)
                directory_handler = self.hs.get_directory_handler()
                if room_alias_str and room_alias_str != original_alias:
                    await self._validate_canonical_alias(
                        directory_handler, room_alias_str, event.room_id
                    )

                # Check that alt_aliases is the proper form.
                alt_aliases = event.content.get("alt_aliases", [])
                if not isinstance(alt_aliases, (list, tuple)):
                    raise SynapseError(
                        400,
                        "The alt_aliases property must be a list.",
                        Codes.INVALID_PARAM,
                    )

                # If the old version of alt_aliases is of an unknown form,
                # completely replace it.
                if not isinstance(original_alt_aliases, (list, tuple)):
                    # TODO: check that the original_alt_aliases' entries are all strings
                    original_alt_aliases = []

                # Check that each alias is currently valid.
                new_alt_aliases = set(alt_aliases) - set(original_alt_aliases)
                if new_alt_aliases:
                    for alias_str in new_alt_aliases:
                        await self._validate_canonical_alias(
                            directory_handler, alias_str, event.room_id
                        )

            federation_handler = self.hs.get_federation_handler()

            if event.type == EventTypes.Member:
                if event.content["membership"] == Membership.INVITE:
                    maybe_upsert_event_field(
                        event,
                        event.unsigned,
                        "invite_room_state",
                        await self.store.get_stripped_room_state_from_event_context(
                            context,
                            self.room_prejoin_state_types,
                            membership_user_id=event.sender,
                        ),
                    )

                    invitee = UserID.from_string(event.state_key)
                    if not self.hs.is_mine(invitee):
                        # TODO: Can we add signature from remote server in a nicer
                        # way? If we have been invited by a remote server, we need
                        # to get them to sign the event.

                        returned_invite = await federation_handler.send_invite(
                            invitee.domain, event
                        )
                        event.unsigned.pop("room_state", None)

                        # TODO: Make sure the signatures actually are correct.
                        event.signatures.update(returned_invite.signatures)

                if event.content["membership"] == Membership.KNOCK:
                    maybe_upsert_event_field(
                        event,
                        event.unsigned,
                        "knock_room_state",
                        await self.store.get_stripped_room_state_from_event_context(
                            context,
                            self.room_prejoin_state_types,
                        ),
                    )

            if event.type == EventTypes.Redaction:
                assert event.redacts is not None

                original_event = await self.store.get_event(
                    event.redacts,
                    redact_behaviour=EventRedactBehaviour.as_is,
                    get_prev_content=False,
                    allow_rejected=False,
                    allow_none=True,
                )

                room_version = await self.store.get_room_version_id(event.room_id)
                room_version_obj = KNOWN_ROOM_VERSIONS[room_version]

                # we can make some additional checks now if we have the original event.
                if original_event:
                    if original_event.type == EventTypes.Create:
                        raise AuthError(403, "Redacting create events is not permitted")

                    if original_event.room_id != event.room_id:
                        raise SynapseError(
                            400, "Cannot redact event from a different room"
                        )

                    if original_event.type == EventTypes.ServerACL:
                        raise AuthError(
                            403, "Redacting server ACL events is not permitted"
                        )

                event_types = event_auth.auth_types_for_event(event.room_version, event)
                prev_state_ids = await context.get_prev_state_ids(
                    StateFilter.from_types(event_types)
                )

                auth_events_ids = self._event_auth_handler.compute_auth_events(
                    event, prev_state_ids, for_verification=True
                )
                auth_events_map = await self.store.get_events(auth_events_ids)
                auth_events = {
                    (e.type, e.state_key): e for e in auth_events_map.values()
                }

                if event_auth.check_redaction(
                    room_version_obj, event, auth_events=auth_events
                ):
                    # this user doesn't have 'redact' rights, so we need to do some more
                    # checks on the original event. Let's start by checking the original
                    # event exists.
                    if not original_event:
                        raise NotFoundError(
                            "Could not find event %s" % (event.redacts,)
                        )

                    if event.user_id != original_event.user_id:
                        raise AuthError(
                            403, "You don't have permission to redact events"
                        )

                    # all the checks are done.
                    event.internal_metadata.recheck_redaction = False

            if event.type == EventTypes.Create:
                prev_state_ids = await context.get_prev_state_ids()
                if prev_state_ids:
                    raise AuthError(403, "Changing the room create event is forbidden")

        assert self._storage_controllers.persistence is not None
        (
            persisted_events,
            max_stream_token,
        ) = await self._storage_controllers.persistence.persist_events(
            events_and_context,
        )

        events_and_pos = []
        for event in persisted_events:
            if self._ephemeral_events_enabled:
                # If there's an expiry timestamp on the event, schedule its expiry.
                self._message_handler.maybe_schedule_expiry(event)

            stream_ordering = event.internal_metadata.stream_ordering
            assert stream_ordering is not None
            pos = PersistedEventPosition(self._instance_name, stream_ordering)
            events_and_pos.append((event, pos))

            if event.type == EventTypes.Message:
                # We don't want to block sending messages on any presence code. This
                # matters as sometimes presence code can take a while.
                run_as_background_process(
                    "bump_presence_active_time",
                    self._bump_active_time,
                    requester.user,
                    requester.device_id,
                )

        async def _notify() -> None:
            try:
                await self.notifier.on_new_room_events(
                    events_and_pos, max_stream_token, extra_users=extra_users
                )
            except Exception:
                logger.exception("Error notifying about new room events")

        run_in_background(_notify)

        return persisted_events[-1]

    async def _maybe_kick_guest_users(
        self, event: EventBase, context: EventContext
    ) -> None:
        if event.type != EventTypes.GuestAccess:
            return

        guest_access = event.content.get(EventContentFields.GUEST_ACCESS)
        if guest_access == GuestAccess.CAN_JOIN:
            return

        current_state_ids = await context.get_current_state_ids()

        # since this is a client-generated event, it cannot be an outlier and we must
        # therefore have the state ids.
        assert current_state_ids is not None
        current_state_dict = await self.store.get_events(
            list(current_state_ids.values())
        )
        current_state = list(current_state_dict.values())
        logger.info("maybe_kick_guest_users %r", current_state)
        await self.hs.get_room_member_handler().kick_guest_users(current_state)

    async def _bump_active_time(self, user: UserID, device_id: Optional[str]) -> None:
        try:
            presence = self.hs.get_presence_handler()
            await presence.bump_presence_active_time(user, device_id)
        except Exception:
            logger.exception("Error bumping presence active time")

    async def _send_dummy_events_to_fill_extremities(self) -> None:
        """Background task to send dummy events into rooms that have a large
        number of extremities
        """
        self._expire_rooms_to_exclude_from_dummy_event_insertion()
        room_ids = await self.store.get_rooms_with_many_extremities(
            min_count=self._dummy_events_threshold,
            limit=5,
            room_id_filter=self._rooms_to_exclude_from_dummy_event_insertion.keys(),
        )

        for room_id in room_ids:
            async with self._worker_lock_handler.acquire_read_write_lock(
                NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
            ):
                dummy_event_sent = await self._send_dummy_event_for_room(room_id)

            if not dummy_event_sent:
                # Did not find a valid user in the room, so remove from future attempts
                # Exclusion is time limited, so the room will be rechecked in the future
                # dependent on _DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY
                logger.info(
                    "Failed to send dummy event into room %s. Will exclude it from "
                    "future attempts until cache expires" % (room_id,)
                )
                now = self.clock.time_msec()
                self._rooms_to_exclude_from_dummy_event_insertion[room_id] = now

    async def _send_dummy_event_for_room(self, room_id: str) -> bool:
        """Attempt to send a dummy event for the given room.

        Args:
            room_id: room to try to send an event from

        Returns:
            True if a dummy event was successfully sent. False if no user was able
            to send an event.
        """

        # For each room we need to find a joined member we can use to send
        # the dummy event with.
        members = await self.store.get_local_users_in_room(room_id)
        for user_id in members:
            requester = create_requester(user_id, authenticated_entity=self.server_name)
            try:
                # Try several times, it could fail with PartialStateConflictError
                # in handle_new_client_event, cf comment in except block.
                max_retries = 5
                for i in range(max_retries):
                    try:
                        event, unpersisted_context = await self.create_event(
                            requester,
                            {
                                "type": EventTypes.Dummy,
                                "content": {},
                                "room_id": room_id,
                                "sender": user_id,
                            },
                        )
                        context = await unpersisted_context.persist(event)

                        event.internal_metadata.proactively_send = False

                        # Since this is a dummy-event it is OK if it is sent by a
                        # shadow-banned user.
                        await self.handle_new_client_event(
                            requester,
                            events_and_context=[(event, context)],
                            ratelimit=False,
                            ignore_shadow_ban=True,
                        )

                        break
                    except PartialStateConflictError as e:
                        # Persisting couldn't happen because the room got un-partial stated
                        # in the meantime and context needs to be recomputed, so let's do so.
                        if i == max_retries - 1:
                            raise e
                        pass
                return True
            except AuthError:
                logger.info(
                    "Failed to send dummy event into room %s for user %s due to "
                    "lack of power. Will try another user" % (room_id, user_id)
                )
        return False

    def _expire_rooms_to_exclude_from_dummy_event_insertion(self) -> None:
        expire_before = self.clock.time_msec() - _DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY
        to_expire = set()
        for room_id, time in self._rooms_to_exclude_from_dummy_event_insertion.items():
            if time < expire_before:
                to_expire.add(room_id)
        for room_id in to_expire:
            logger.debug(
                "Expiring room id %s from dummy event insertion exclusion cache",
                room_id,
            )
            del self._rooms_to_exclude_from_dummy_event_insertion[room_id]

    async def _rebuild_event_after_third_party_rules(
        self, third_party_result: dict, original_event: EventBase
    ) -> Tuple[EventBase, UnpersistedEventContextBase]:
        # the third_party_event_rules want to replace the event.
        # we do some basic checks, and then return the replacement event.

        # Construct a new EventBuilder and validate it, which helps with the
        # rest of these checks.
        try:
            builder = self.event_builder_factory.for_room_version(
                original_event.room_version, third_party_result
            )
            self.validator.validate_builder(builder)
        except SynapseError as e:
            raise Exception(
                "Third party rules module created an invalid event: " + e.msg,
            )

        immutable_fields = [
            # changing the room is going to break things: we've already checked that the
            # room exists, and are holding a concurrency limiter token for that room.
            # Also, we might need to use a different room version.
            "room_id",
            # changing the type or state key might work, but we'd need to check that the
            # calling functions aren't making assumptions about them.
            "type",
            "state_key",
        ]

        for k in immutable_fields:
            if getattr(builder, k, None) != original_event.get(k):
                raise Exception(
                    "Third party rules module created an invalid event: "
                    "cannot change field " + k
                )

        # check that the new sender belongs to this HS
        if not self.hs.is_mine_id(builder.sender):
            raise Exception(
                "Third party rules module created an invalid event: "
                "invalid sender " + builder.sender
            )

        # copy over the original internal metadata
        for k, v in original_event.internal_metadata.get_dict().items():
            setattr(builder.internal_metadata, k, v)

        # modules can send new state events, so we re-calculate the auth events just in
        # case.
        prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)

        event = await builder.build(
            prev_event_ids=prev_event_ids,
            auth_event_ids=None,
        )

        # we rebuild the event context, to be on the safe side. If nothing else,
        # delta_ids might need an update.
        context = await self.state.calculate_context_info(event)

        return event, context