synapse-product/synapse/handlers/federation_event.py

# Copyright 2021 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 itertools
import logging
from http import HTTPStatus
from typing import (
    TYPE_CHECKING,
    Collection,
    Container,
    Dict,
    Iterable,
    List,
    Optional,
    Sequence,
    Set,
    Tuple,
)

from prometheus_client import Counter

from synapse.api.constants import (
    EventContentFields,
    EventTypes,
    GuestAccess,
    Membership,
    RejectedReason,
    RoomEncryptionAlgorithms,
)
from synapse.api.errors import (
    AuthError,
    Codes,
    FederationError,
    HttpResponseException,
    RequestSendFailed,
    SynapseError,
)
from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion, RoomVersions
from synapse.event_auth import (
    auth_types_for_event,
    check_auth_rules_for_event,
    validate_event_for_room_version,
)
from synapse.events import EventBase
from synapse.events.snapshot import EventContext
from synapse.federation.federation_client import InvalidResponseError
from synapse.logging.context import nested_logging_context, run_in_background
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.replication.http.devices import ReplicationUserDevicesResyncRestServlet
from synapse.replication.http.federation import (
    ReplicationFederationSendEventsRestServlet,
)
from synapse.state import StateResolutionStore
from synapse.storage.databases.main.events_worker import EventRedactBehaviour
from synapse.types import (
    PersistedEventPosition,
    RoomStreamToken,
    StateMap,
    UserID,
    get_domain_from_id,
)
from synapse.util.async_helpers import Linearizer, concurrently_execute
from synapse.util.iterutils import batch_iter
from synapse.util.retryutils import NotRetryingDestination
from synapse.util.stringutils import shortstr

if TYPE_CHECKING:
    from synapse.server import HomeServer


logger = logging.getLogger(__name__)

soft_failed_event_counter = Counter(
    "synapse_federation_soft_failed_events_total",
    "Events received over federation that we marked as soft_failed",
)


class FederationEventHandler:
    """Handles events that originated from federation.

    Responsible for handing incoming events and passing them on to the rest
    of the homeserver (including auth and state conflict resolutions)
    """

    def __init__(self, hs: "HomeServer"):
        self._store = hs.get_datastores().main
        self._storage = hs.get_storage()
        self._state_store = self._storage.state

        self._state_handler = hs.get_state_handler()
        self._event_creation_handler = hs.get_event_creation_handler()
        self._event_auth_handler = hs.get_event_auth_handler()
        self._message_handler = hs.get_message_handler()
        self._bulk_push_rule_evaluator = hs.get_bulk_push_rule_evaluator()
        self._state_resolution_handler = hs.get_state_resolution_handler()
        # avoid a circular dependency by deferring execution here
        self._get_room_member_handler = hs.get_room_member_handler

        self._federation_client = hs.get_federation_client()
        self._third_party_event_rules = hs.get_third_party_event_rules()
        self._notifier = hs.get_notifier()

        self._is_mine_id = hs.is_mine_id
        self._server_name = hs.hostname
        self._instance_name = hs.get_instance_name()

        self._config = hs.config
        self._ephemeral_messages_enabled = hs.config.server.enable_ephemeral_messages

        self._send_events = ReplicationFederationSendEventsRestServlet.make_client(hs)
        if hs.config.worker.worker_app:
            self._user_device_resync = (
                ReplicationUserDevicesResyncRestServlet.make_client(hs)
            )
        else:
            self._device_list_updater = hs.get_device_handler().device_list_updater

        # When joining a room we need to queue any events for that room up.
        # For each room, a list of (pdu, origin) tuples.
        # TODO: replace this with something more elegant, probably based around the
        # federation event staging area.
        self.room_queues: Dict[str, List[Tuple[EventBase, str]]] = {}

        self._room_pdu_linearizer = Linearizer("fed_room_pdu")

    async def on_receive_pdu(self, origin: str, pdu: EventBase) -> None:
        """Process a PDU received via a federation /send/ transaction

        Args:
            origin: server which initiated the /send/ transaction. Will
                be used to fetch missing events or state.
            pdu: received PDU
        """

        # We should never see any outliers here.
        assert not pdu.internal_metadata.outlier

        room_id = pdu.room_id
        event_id = pdu.event_id

        # We reprocess pdus when we have seen them only as outliers
        existing = await self._store.get_event(
            event_id, allow_none=True, allow_rejected=True
        )

        # FIXME: Currently we fetch an event again when we already have it
        # if it has been marked as an outlier.
        if existing:
            if not existing.internal_metadata.is_outlier():
                logger.info(
                    "Ignoring received event %s which we have already seen", event_id
                )
                return
            if pdu.internal_metadata.is_outlier():
                logger.info(
                    "Ignoring received outlier %s which we already have as an outlier",
                    event_id,
                )
                return
            logger.info("De-outliering event %s", event_id)

        # do some initial sanity-checking of the event. In particular, make
        # sure it doesn't have hundreds of prev_events or auth_events, which
        # could cause a huge state resolution or cascade of event fetches.
        try:
            self._sanity_check_event(pdu)
        except SynapseError as err:
            logger.warning("Received event failed sanity checks")
            raise FederationError("ERROR", err.code, err.msg, affected=pdu.event_id)

        # If we are currently in the process of joining this room, then we
        # queue up events for later processing.
        if room_id in self.room_queues:
            logger.info(
                "Queuing PDU from %s for now: join in progress",
                origin,
            )
            self.room_queues[room_id].append((pdu, origin))
            return

        # If we're not in the room just ditch the event entirely. This is
        # probably an old server that has come back and thinks we're still in
        # the room (or we've been rejoined to the room by a state reset).
        #
        # Note that if we were never in the room then we would have already
        # dropped the event, since we wouldn't know the room version.
        is_in_room = await self._event_auth_handler.check_host_in_room(
            room_id, self._server_name
        )
        if not is_in_room:
            logger.info(
                "Ignoring PDU from %s as we're not in the room",
                origin,
            )
            return None

        # Try to fetch any missing prev events to fill in gaps in the graph
        prevs = set(pdu.prev_event_ids())
        seen = await self._store.have_events_in_timeline(prevs)
        missing_prevs = prevs - seen

        if missing_prevs:
            # We only backfill backwards to the min depth.
            min_depth = await self._store.get_min_depth(pdu.room_id)
            logger.debug("min_depth: %d", min_depth)

            if min_depth is not None and pdu.depth > min_depth:
                # If we're missing stuff, ensure we only fetch stuff one
                # at a time.
                logger.info(
                    "Acquiring room lock to fetch %d missing prev_events: %s",
                    len(missing_prevs),
                    shortstr(missing_prevs),
                )
                async with self._room_pdu_linearizer.queue(pdu.room_id):
                    logger.info(
                        "Acquired room lock to fetch %d missing prev_events",
                        len(missing_prevs),
                    )

                    try:
                        await self._get_missing_events_for_pdu(
                            origin, pdu, prevs, min_depth
                        )
                    except Exception as e:
                        raise Exception(
                            "Error fetching missing prev_events for %s: %s"
                            % (event_id, e)
                        ) from e

                # Update the set of things we've seen after trying to
                # fetch the missing stuff
                seen = await self._store.have_events_in_timeline(prevs)
                missing_prevs = prevs - seen

                if not missing_prevs:
                    logger.info("Found all missing prev_events")

            if missing_prevs:
                # since this event was pushed to us, it is possible for it to
                # become the only forward-extremity in the room, and we would then
                # trust its state to be the state for the whole room. This is very
                # bad. Further, if the event was pushed to us, there is no excuse
                # for us not to have all the prev_events. (XXX: apart from
                # min_depth?)
                #
                # We therefore reject any such events.
                logger.warning(
                    "Rejecting: failed to fetch %d prev events: %s",
                    len(missing_prevs),
                    shortstr(missing_prevs),
                )
                raise FederationError(
                    "ERROR",
                    403,
                    (
                        "Your server isn't divulging details about prev_events "
                        "referenced in this event."
                    ),
                    affected=pdu.event_id,
                )

        await self._process_received_pdu(origin, pdu, state=None)

    async def on_send_membership_event(
        self, origin: str, event: EventBase
    ) -> Tuple[EventBase, EventContext]:
        """
        We have received a join/leave/knock event for a room via send_join/leave/knock.

        Verify that event and send it into the room on the remote homeserver's behalf.

        This is quite similar to on_receive_pdu, with the following principal
        differences:
          * only membership events are permitted (and only events with
            sender==state_key -- ie, no kicks or bans)
          * *We* send out the event on behalf of the remote server.
          * We enforce the membership restrictions of restricted rooms.
          * Rejected events result in an exception rather than being stored.

        There are also other differences, however it is not clear if these are by
        design or omission. In particular, we do not attempt to backfill any missing
        prev_events.

        Args:
            origin: The homeserver of the remote (joining/invited/knocking) user.
            event: The member event that has been signed by the remote homeserver.

        Returns:
            The event and context of the event after inserting it into the room graph.

        Raises:
            SynapseError if the event is not accepted into the room
        """
        logger.debug(
            "on_send_membership_event: Got event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        if get_domain_from_id(event.sender) != origin:
            logger.info(
                "Got send_membership request for user %r from different origin %s",
                event.sender,
                origin,
            )
            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)

        if event.sender != event.state_key:
            raise SynapseError(400, "state_key and sender must match", Codes.BAD_JSON)

        assert not event.internal_metadata.outlier

        # Send this event on behalf of the other server.
        #
        # The remote server isn't a full participant in the room at this point, so
        # may not have an up-to-date list of the other homeservers participating in
        # the room, so we send it on their behalf.
        event.internal_metadata.send_on_behalf_of = origin

        context = await self._state_handler.compute_event_context(event)
        context = await self._check_event_auth(origin, event, context)
        if context.rejected:
            raise SynapseError(
                403, f"{event.membership} event was rejected", Codes.FORBIDDEN
            )

        # for joins, we need to check the restrictions of restricted rooms
        if event.membership == Membership.JOIN:
            await self.check_join_restrictions(context, event)

        # for knock events, we run the third-party event rules. It's not entirely clear
        # why we don't do this for other sorts of membership events.
        if event.membership == Membership.KNOCK:
            event_allowed, _ = await self._third_party_event_rules.check_event_allowed(
                event, context
            )
            if not event_allowed:
                logger.info("Sending of knock %s forbidden by third-party rules", event)
                raise SynapseError(
                    403, "This event is not allowed in this context", Codes.FORBIDDEN
                )

        # all looks good, we can persist the event.

        # First, precalculate the joined hosts so that the federation sender doesn't
        # need to.
        await self._event_creation_handler.cache_joined_hosts_for_event(event, context)

        await self._check_for_soft_fail(event, None, origin=origin)
        await self._run_push_actions_and_persist_event(event, context)
        return event, context

    async def check_join_restrictions(
        self, context: EventContext, event: EventBase
    ) -> None:
        """Check that restrictions in restricted join rules are matched

        Called when we receive a join event via send_join.

        Raises an auth error if the restrictions are not matched.
        """
        prev_state_ids = await context.get_prev_state_ids()

        # Check if the user is already in the room or invited to the room.
        user_id = event.state_key
        prev_member_event_id = prev_state_ids.get((EventTypes.Member, user_id), None)
        prev_member_event = None
        if prev_member_event_id:
            prev_member_event = await self._store.get_event(prev_member_event_id)

        # Check if the member should be allowed access via membership in a space.
        await self._event_auth_handler.check_restricted_join_rules(
            prev_state_ids,
            event.room_version,
            user_id,
            prev_member_event,
        )

    async def process_remote_join(
        self,
        origin: str,
        room_id: str,
        auth_events: List[EventBase],
        state: List[EventBase],
        event: EventBase,
        room_version: RoomVersion,
        partial_state: bool,
    ) -> int:
        """Persists the events returned by a send_join

        Checks the auth chain is valid (and passes auth checks) for the
        state and event. Then persists all of the events.
        Notifies about the persisted events where appropriate.

        Args:
            origin: Where the events came from
            room_id:
            auth_events
            state
            event
            room_version: The room version we expect this room to have, and
                will raise if it doesn't match the version in the create event.
            partial_state: True if the state omits non-critical membership events

        Returns:
            The stream ID after which all events have been persisted.

        Raises:
            SynapseError if the response is in some way invalid.
        """
        create_event = None
        for e in state:
            if (e.type, e.state_key) == (EventTypes.Create, ""):
                create_event = e
                break

        if create_event is None:
            # If the state doesn't have a create event then the room is
            # invalid, and it would fail auth checks anyway.
            raise SynapseError(400, "No create event in state")

        room_version_id = create_event.content.get(
            "room_version", RoomVersions.V1.identifier
        )

        if room_version.identifier != room_version_id:
            raise SynapseError(400, "Room version mismatch")

        # persist the auth chain and state events.
        #
        # any invalid events here will be marked as rejected, and we'll carry on.
        #
        # any events whose auth events are missing (ie, not in the send_join response,
        # and not already in our db) will just be ignored. This is correct behaviour,
        # because the reason that auth_events are missing might be due to us being
        # unable to validate their signatures. The fact that we can't validate their
        # signatures right now doesn't mean that we will *never* be able to, so it
        # is premature to reject them.
        #
        await self._auth_and_persist_outliers(
            room_id, itertools.chain(auth_events, state)
        )

        # and now persist the join event itself.
        logger.info(
            "Peristing join-via-remote %s (partial_state: %s)", event, partial_state
        )
        with nested_logging_context(suffix=event.event_id):
            context = await self._state_handler.compute_event_context(
                event,
                old_state=state,
                partial_state=partial_state,
            )

            context = await self._check_event_auth(origin, event, context)
            if context.rejected:
                raise SynapseError(400, "Join event was rejected")

            # the remote server is responsible for sending our join event to the rest
            # of the federation. Indeed, attempting to do so will result in problems
            # when we try to look up the state before the join (to get the server list)
            # and discover that we do not have it.
            event.internal_metadata.proactively_send = False

            return await self.persist_events_and_notify(room_id, [(event, context)])

    async def update_state_for_partial_state_event(
        self, destination: str, event: EventBase
    ) -> None:
        """Recalculate the state at an event as part of a de-partial-stating process

        Args:
            destination: server to request full state from
            event: partial-state event to be de-partial-stated
        """
        logger.info("Updating state for %s", event.event_id)
        with nested_logging_context(suffix=event.event_id):
            # if we have all the event's prev_events, then we can work out the
            # state based on their states. Otherwise, we request it from the destination
            # server.
            #
            # This is the same operation as we do when we receive a regular event
            # over federation.
            state = await self._resolve_state_at_missing_prevs(destination, event)

            # build a new state group for it if need be
            context = await self._state_handler.compute_event_context(
                event,
                old_state=state,
            )
            if context.partial_state:
                # this can happen if some or all of the event's prev_events still have
                # partial state - ie, an event has an earlier stream_ordering than one
                # or more of its prev_events, so we de-partial-state it before its
                # prev_events.
                #
                # TODO(faster_joins): we probably need to be more intelligent, and
                #    exclude partial-state prev_events from consideration
                logger.warning(
                    "%s still has partial state: can't de-partial-state it yet",
                    event.event_id,
                )
                return
            await self._store.update_state_for_partial_state_event(event, context)
            self._state_store.notify_event_un_partial_stated(event.event_id)

    async def backfill(
        self, dest: str, room_id: str, limit: int, extremities: Collection[str]
    ) -> None:
        """Trigger a backfill request to `dest` for the given `room_id`

        This will attempt to get more events from the remote. If the other side
        has no new events to offer, this will return an empty list.

        As the events are received, we check their signatures, and also do some
        sanity-checking on them. If any of the backfilled events are invalid,
        this method throws a SynapseError.

        We might also raise an InvalidResponseError if the response from the remote
        server is just bogus.

        TODO: make this more useful to distinguish failures of the remote
        server from invalid events (there is probably no point in trying to
        re-fetch invalid events from every other HS in the room.)
        """
        if dest == self._server_name:
            raise SynapseError(400, "Can't backfill from self.")

        events = await self._federation_client.backfill(
            dest, room_id, limit=limit, extremities=extremities
        )

        if not events:
            return

        # if there are any events in the wrong room, the remote server is buggy and
        # should not be trusted.
        for ev in events:
            if ev.room_id != room_id:
                raise InvalidResponseError(
                    f"Remote server {dest} returned event {ev.event_id} which is in "
                    f"room {ev.room_id}, when we were backfilling in {room_id}"
                )

        await self._process_pulled_events(
            dest,
            events,
            backfilled=True,
        )

    async def _get_missing_events_for_pdu(
        self, origin: str, pdu: EventBase, prevs: Set[str], min_depth: int
    ) -> None:
        """
        Args:
            origin: Origin of the pdu. Will be called to get the missing events
            pdu: received pdu
            prevs: List of event ids which we are missing
            min_depth: Minimum depth of events to return.
        """

        room_id = pdu.room_id
        event_id = pdu.event_id

        seen = await self._store.have_events_in_timeline(prevs)

        if not prevs - seen:
            return

        latest_list = await self._store.get_latest_event_ids_in_room(room_id)

        # We add the prev events that we have seen to the latest
        # list to ensure the remote server doesn't give them to us
        latest = set(latest_list)
        latest |= seen

        logger.info(
            "Requesting missing events between %s and %s",
            shortstr(latest),
            event_id,
        )

        # XXX: we set timeout to 10s to help workaround
        # https://github.com/matrix-org/synapse/issues/1733.
        # The reason is to avoid holding the linearizer lock
        # whilst processing inbound /send transactions, causing
        # FDs to stack up and block other inbound transactions
        # which empirically can currently take up to 30 minutes.
        #
        # N.B. this explicitly disables retry attempts.
        #
        # N.B. this also increases our chances of falling back to
        # fetching fresh state for the room if the missing event
        # can't be found, which slightly reduces our security.
        # it may also increase our DAG extremity count for the room,
        # causing additional state resolution?  See #1760.
        # However, fetching state doesn't hold the linearizer lock
        # apparently.
        #
        # see https://github.com/matrix-org/synapse/pull/1744
        #
        # ----
        #
        # Update richvdh 2018/09/18: There are a number of problems with timing this
        # request out aggressively on the client side:
        #
        # - it plays badly with the server-side rate-limiter, which starts tarpitting you
        #   if you send too many requests at once, so you end up with the server carefully
        #   working through the backlog of your requests, which you have already timed
        #   out.
        #
        # - for this request in particular, we now (as of
        #   https://github.com/matrix-org/synapse/pull/3456) reject any PDUs where the
        #   server can't produce a plausible-looking set of prev_events - so we becone
        #   much more likely to reject the event.
        #
        # - contrary to what it says above, we do *not* fall back to fetching fresh state
        #   for the room if get_missing_events times out. Rather, we give up processing
        #   the PDU whose prevs we are missing, which then makes it much more likely that
        #   we'll end up back here for the *next* PDU in the list, which exacerbates the
        #   problem.
        #
        # - the aggressive 10s timeout was introduced to deal with incoming federation
        #   requests taking 8 hours to process. It's not entirely clear why that was going
        #   on; certainly there were other issues causing traffic storms which are now
        #   resolved, and I think in any case we may be more sensible about our locking
        #   now. We're *certainly* more sensible about our logging.
        #
        # All that said: Let's try increasing the timeout to 60s and see what happens.

        try:
            missing_events = await self._federation_client.get_missing_events(
                origin,
                room_id,
                earliest_events_ids=list(latest),
                latest_events=[pdu],
                limit=10,
                min_depth=min_depth,
                timeout=60000,
            )
        except (RequestSendFailed, HttpResponseException, NotRetryingDestination) as e:
            # We failed to get the missing events, but since we need to handle
            # the case of `get_missing_events` not returning the necessary
            # events anyway, it is safe to simply log the error and continue.
            logger.warning("Failed to get prev_events: %s", e)
            return

        logger.info("Got %d prev_events", len(missing_events))
        await self._process_pulled_events(origin, missing_events, backfilled=False)

    async def _process_pulled_events(
        self, origin: str, events: Iterable[EventBase], backfilled: bool
    ) -> None:
        """Process a batch of events we have pulled from a remote server

        Pulls in any events required to auth the events, persists the received events,
        and notifies clients, if appropriate.

        Assumes the events have already had their signatures and hashes checked.

        Params:
            origin: The server we received these events from
            events: The received events.
            backfilled: True if this is part of a historical batch of events (inhibits
                notification to clients, and validation of device keys.)
        """
        logger.debug(
            "processing pulled backfilled=%s events=%s",
            backfilled,
            [
                "event_id=%s,depth=%d,body=%s,prevs=%s\n"
                % (
                    event.event_id,
                    event.depth,
                    event.content.get("body", event.type),
                    event.prev_event_ids(),
                )
                for event in events
            ],
        )

        # We want to sort these by depth so we process them and
        # tell clients about them in order.
        sorted_events = sorted(events, key=lambda x: x.depth)
        for ev in sorted_events:
            with nested_logging_context(ev.event_id):
                await self._process_pulled_event(origin, ev, backfilled=backfilled)

    async def _process_pulled_event(
        self, origin: str, event: EventBase, backfilled: bool
    ) -> None:
        """Process a single event that we have pulled from a remote server

        Pulls in any events required to auth the event, persists the received event,
        and notifies clients, if appropriate.

        Assumes the event has already had its signatures and hashes checked.

        This is somewhat equivalent to on_receive_pdu, but applies somewhat different
        logic in the case that we are missing prev_events (in particular, it just
        requests the state at that point, rather than triggering a get_missing_events) -
        so is appropriate when we have pulled the event from a remote server, rather
        than having it pushed to us.

        Params:
            origin: The server we received this event from
            events: The received event
            backfilled: True if this is part of a historical batch of events (inhibits
                notification to clients, and validation of device keys.)
        """
        logger.info("Processing pulled event %s", event)

        # these should not be outliers.
        assert (
            not event.internal_metadata.is_outlier()
        ), "pulled event unexpectedly flagged as outlier"

        event_id = event.event_id

        existing = await self._store.get_event(
            event_id, allow_none=True, allow_rejected=True
        )
        if existing:
            if not existing.internal_metadata.is_outlier():
                logger.info(
                    "Ignoring received event %s which we have already seen",
                    event_id,
                )
                return
            logger.info("De-outliering event %s", event_id)

        try:
            self._sanity_check_event(event)
        except SynapseError as err:
            logger.warning("Event %s failed sanity check: %s", event_id, err)
            return

        try:
            state = await self._resolve_state_at_missing_prevs(origin, event)
            # TODO(faster_joins): make sure that _resolve_state_at_missing_prevs does
            #   not return partial state
            await self._process_received_pdu(
                origin, event, state=state, backfilled=backfilled
            )
        except FederationError as e:
            if e.code == 403:
                logger.warning("Pulled event %s failed history check.", event_id)
            else:
                raise

    async def _resolve_state_at_missing_prevs(
        self, dest: str, event: EventBase
    ) -> Optional[Iterable[EventBase]]:
        """Calculate the state at an event with missing prev_events.

        This is used when we have pulled a batch of events from a remote server, and
        still don't have all the prev_events.

        If we already have all the prev_events for `event`, this method does nothing.

        Otherwise, the missing prevs become new backwards extremities, and we fall back
        to asking the remote server for the state after each missing `prev_event`,
        and resolving across them.

        That's ok provided we then resolve the state against other bits of the DAG
        before using it - in other words, that the received event `event` is not going
        to become the only forwards_extremity in the room (which will ensure that you
        can't just take over a room by sending an event, withholding its prev_events,
        and declaring yourself to be an admin in the subsequent state request).

        In other words: we should only call this method if `event` has been *pulled*
        as part of a batch of missing prev events, or similar.

        Params:
            dest: the remote server to ask for state at the missing prevs. Typically,
                this will be the server we got `event` from.
            event: an event to check for missing prevs.

        Returns:
            if we already had all the prev events, `None`. Otherwise, returns a list of
            the events in the state at `event`.
        """
        room_id = event.room_id
        event_id = event.event_id

        prevs = set(event.prev_event_ids())
        seen = await self._store.have_events_in_timeline(prevs)
        missing_prevs = prevs - seen

        if not missing_prevs:
            return None

        logger.info(
            "Event %s is missing prev_events %s: calculating state for a "
            "backwards extremity",
            event_id,
            shortstr(missing_prevs),
        )
        # Calculate the state after each of the previous events, and
        # resolve them to find the correct state at the current event.
        event_map = {event_id: event}
        try:
            # Get the state of the events we know about
            ours = await self._state_store.get_state_groups_ids(room_id, seen)

            # state_maps is a list of mappings from (type, state_key) to event_id
            state_maps: List[StateMap[str]] = list(ours.values())

            # we don't need this any more, let's delete it.
            del ours

            # Ask the remote server for the states we don't
            # know about
            for p in missing_prevs:
                logger.info("Requesting state after missing prev_event %s", p)

                with nested_logging_context(p):
                    # note that if any of the missing prevs share missing state or
                    # auth events, the requests to fetch those events are deduped
                    # by the get_pdu_cache in federation_client.
                    remote_state = await self._get_state_after_missing_prev_event(
                        dest, room_id, p
                    )

                    remote_state_map = {
                        (x.type, x.state_key): x.event_id for x in remote_state
                    }
                    state_maps.append(remote_state_map)

                    for x in remote_state:
                        event_map[x.event_id] = x

            room_version = await self._store.get_room_version_id(room_id)
            state_map = await self._state_resolution_handler.resolve_events_with_store(
                room_id,
                room_version,
                state_maps,
                event_map,
                state_res_store=StateResolutionStore(self._store),
            )

            # We need to give _process_received_pdu the actual state events
            # rather than event ids, so generate that now.

            # First though we need to fetch all the events that are in
            # state_map, so we can build up the state below.
            evs = await self._store.get_events(
                list(state_map.values()),
                get_prev_content=False,
                redact_behaviour=EventRedactBehaviour.as_is,
            )
            event_map.update(evs)

            state = [event_map[e] for e in state_map.values()]
        except Exception:
            logger.warning(
                "Error attempting to resolve state at missing prev_events",
                exc_info=True,
            )
            raise FederationError(
                "ERROR",
                403,
                "We can't get valid state history.",
                affected=event_id,
            )
        return state

    async def _get_state_after_missing_prev_event(
        self,
        destination: str,
        room_id: str,
        event_id: str,
    ) -> List[EventBase]:
        """Requests all of the room state at a given event from a remote homeserver.

        Args:
            destination: The remote homeserver to query for the state.
            room_id: The id of the room we're interested in.
            event_id: The id of the event we want the state at.

        Returns:
            A list of events in the state, including the event itself
        """
        (
            state_event_ids,
            auth_event_ids,
        ) = await self._federation_client.get_room_state_ids(
            destination, room_id, event_id=event_id
        )

        logger.debug(
            "state_ids returned %i state events, %i auth events",
            len(state_event_ids),
            len(auth_event_ids),
        )

        # start by just trying to fetch the events from the store
        desired_events = set(state_event_ids)
        desired_events.add(event_id)
        logger.debug("Fetching %i events from cache/store", len(desired_events))
        fetched_events = await self._store.get_events(
            desired_events, allow_rejected=True
        )

        missing_desired_events = desired_events - fetched_events.keys()
        logger.debug(
            "We are missing %i events (got %i)",
            len(missing_desired_events),
            len(fetched_events),
        )

        # We probably won't need most of the auth events, so let's just check which
        # we have for now, rather than thrashing the event cache with them all
        # unnecessarily.

        # TODO: we probably won't actually need all of the auth events, since we
        #   already have a bunch of the state events. It would be nice if the
        #   federation api gave us a way of finding out which we actually need.

        missing_auth_events = set(auth_event_ids) - fetched_events.keys()
        missing_auth_events.difference_update(
            await self._store.have_seen_events(room_id, missing_auth_events)
        )
        logger.debug("We are also missing %i auth events", len(missing_auth_events))

        missing_events = missing_desired_events | missing_auth_events

        # Making an individual request for each of 1000s of events has a lot of
        # overhead. On the other hand, we don't really want to fetch all of the events
        # if we already have most of them.
        #
        # As an arbitrary heuristic, if we are missing more than 10% of the events, then
        # we fetch the whole state.
        #
        # TODO: might it be better to have an API which lets us do an aggregate event
        #   request
        if (len(missing_events) * 10) >= len(auth_event_ids) + len(state_event_ids):
            logger.debug("Requesting complete state from remote")
            await self._get_state_and_persist(destination, room_id, event_id)
        else:
            logger.debug("Fetching %i events from remote", len(missing_events))
            await self._get_events_and_persist(
                destination=destination, room_id=room_id, event_ids=missing_events
            )

        # we need to make sure we re-load from the database to get the rejected
        # state correct.
        fetched_events.update(
            await self._store.get_events(missing_desired_events, allow_rejected=True)
        )

        # check for events which were in the wrong room.
        #
        # this can happen if a remote server claims that the state or
        # auth_events at an event in room A are actually events in room B

        bad_events = [
            (event_id, event.room_id)
            for event_id, event in fetched_events.items()
            if event.room_id != room_id
        ]

        for bad_event_id, bad_room_id in bad_events:
            # This is a bogus situation, but since we may only discover it a long time
            # after it happened, we try our best to carry on, by just omitting the
            # bad events from the returned state set.
            logger.warning(
                "Remote server %s claims event %s in room %s is an auth/state "
                "event in room %s",
                destination,
                bad_event_id,
                bad_room_id,
                room_id,
            )

            del fetched_events[bad_event_id]

        # if we couldn't get the prev event in question, that's a problem.
        remote_event = fetched_events.get(event_id)
        if not remote_event:
            raise Exception("Unable to get missing prev_event %s" % (event_id,))

        # missing state at that event is a warning, not a blocker
        # XXX: this doesn't sound right? it means that we'll end up with incomplete
        #   state.
        failed_to_fetch = desired_events - fetched_events.keys()
        if failed_to_fetch:
            logger.warning(
                "Failed to fetch missing state events for %s %s",
                event_id,
                failed_to_fetch,
            )

        remote_state = [
            fetched_events[e_id] for e_id in state_event_ids if e_id in fetched_events
        ]

        if remote_event.is_state() and remote_event.rejected_reason is None:
            remote_state.append(remote_event)

        return remote_state

    async def _get_state_and_persist(
        self, destination: str, room_id: str, event_id: str
    ) -> None:
        """Get the complete room state at a given event, and persist any new events
        as outliers"""
        room_version = await self._store.get_room_version(room_id)
        auth_events, state_events = await self._federation_client.get_room_state(
            destination, room_id, event_id=event_id, room_version=room_version
        )
        logger.info("/state returned %i events", len(auth_events) + len(state_events))

        await self._auth_and_persist_outliers(
            room_id, itertools.chain(auth_events, state_events)
        )

        # we also need the event itself.
        if not await self._store.have_seen_event(room_id, event_id):
            await self._get_events_and_persist(
                destination=destination, room_id=room_id, event_ids=(event_id,)
            )

    async def _process_received_pdu(
        self,
        origin: str,
        event: EventBase,
        state: Optional[Iterable[EventBase]],
        backfilled: bool = False,
    ) -> None:
        """Called when we have a new non-outlier event.

        This is called when we have a new event to add to the room DAG. This can be
        due to:
           * events received directly via a /send request
           * events retrieved via get_missing_events after a /send request
           * events backfilled after a client request.

        It's not currently used for events received from incoming send_{join,knock,leave}
        requests (which go via on_send_membership_event), nor for joins created by a
        remote join dance (which go via process_remote_join).

        We need to do auth checks and put it through the StateHandler.

        Args:
            origin: server sending the event

            event: event to be persisted

            state: Normally None, but if we are handling a gap in the graph
                (ie, we are missing one or more prev_events), the resolved state at the
                event

            backfilled: True if this is part of a historical batch of events (inhibits
                notification to clients, and validation of device keys.)
        """
        logger.debug("Processing event: %s", event)
        assert not event.internal_metadata.outlier

        try:
            context = await self._state_handler.compute_event_context(
                event, old_state=state
            )
            context = await self._check_event_auth(
                origin,
                event,
                context,
            )
        except AuthError as e:
            # FIXME richvdh 2021/10/07 I don't think this is reachable. Let's log it
            #   for now
            logger.exception("Unexpected AuthError from _check_event_auth")
            raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)

        if not backfilled and not context.rejected:
            # For new (non-backfilled and non-outlier) events we check if the event
            # passes auth based on the current state. If it doesn't then we
            # "soft-fail" the event.
            await self._check_for_soft_fail(event, state, origin=origin)

        await self._run_push_actions_and_persist_event(event, context, backfilled)

        await self._handle_marker_event(origin, event)

        if backfilled or context.rejected:
            return

        await self._maybe_kick_guest_users(event)

        # For encrypted messages we check that we know about the sending device,
        # if we don't then we mark the device cache for that user as stale.
        if event.type == EventTypes.Encrypted:
            device_id = event.content.get("device_id")
            sender_key = event.content.get("sender_key")

            cached_devices = await self._store.get_cached_devices_for_user(event.sender)

            resync = False  # Whether we should resync device lists.

            device = None
            if device_id is not None:
                device = cached_devices.get(device_id)
                if device is None:
                    logger.info(
                        "Received event from remote device not in our cache: %s %s",
                        event.sender,
                        device_id,
                    )
                    resync = True

            # We also check if the `sender_key` matches what we expect.
            if sender_key is not None:
                # Figure out what sender key we're expecting. If we know the
                # device and recognize the algorithm then we can work out the
                # exact key to expect. Otherwise check it matches any key we
                # have for that device.

                current_keys: Container[str] = []

                if device:
                    keys = device.get("keys", {}).get("keys", {})

                    if (
                        event.content.get("algorithm")
                        == RoomEncryptionAlgorithms.MEGOLM_V1_AES_SHA2
                    ):
                        # For this algorithm we expect a curve25519 key.
                        key_name = "curve25519:%s" % (device_id,)
                        current_keys = [keys.get(key_name)]
                    else:
                        # We don't know understand the algorithm, so we just
                        # check it matches a key for the device.
                        current_keys = keys.values()
                elif device_id:
                    # We don't have any keys for the device ID.
                    pass
                else:
                    # The event didn't include a device ID, so we just look for
                    # keys across all devices.
                    current_keys = [
                        key
                        for device in cached_devices.values()
                        for key in device.get("keys", {}).get("keys", {}).values()
                    ]

                # We now check that the sender key matches (one of) the expected
                # keys.
                if sender_key not in current_keys:
                    logger.info(
                        "Received event from remote device with unexpected sender key: %s %s: %s",
                        event.sender,
                        device_id or "<no device_id>",
                        sender_key,
                    )
                    resync = True

            if resync:
                run_as_background_process(
                    "resync_device_due_to_pdu",
                    self._resync_device,
                    event.sender,
                )

    async def _resync_device(self, sender: str) -> None:
        """We have detected that the device list for the given user may be out
        of sync, so we try and resync them.
        """

        try:
            await self._store.mark_remote_user_device_cache_as_stale(sender)

            # Immediately attempt a resync in the background
            if self._config.worker.worker_app:
                await self._user_device_resync(user_id=sender)
            else:
                await self._device_list_updater.user_device_resync(sender)
        except Exception:
            logger.exception("Failed to resync device for %s", sender)

    async def _handle_marker_event(self, origin: str, marker_event: EventBase) -> None:
        """Handles backfilling the insertion event when we receive a marker
        event that points to one.

        Args:
            origin: Origin of the event. Will be called to get the insertion event
            marker_event: The event to process
        """

        if marker_event.type != EventTypes.MSC2716_MARKER:
            # Not a marker event
            return

        if marker_event.rejected_reason is not None:
            # Rejected event
            return

        # Skip processing a marker event if the room version doesn't
        # support it or the event is not from the room creator.
        room_version = await self._store.get_room_version(marker_event.room_id)
        create_event = await self._store.get_create_event_for_room(marker_event.room_id)
        room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
        if not room_version.msc2716_historical and (
            not self._config.experimental.msc2716_enabled
            or marker_event.sender != room_creator
        ):
            return

        logger.debug("_handle_marker_event: received %s", marker_event)

        insertion_event_id = marker_event.content.get(
            EventContentFields.MSC2716_MARKER_INSERTION
        )

        if insertion_event_id is None:
            # Nothing to retrieve then (invalid marker)
            return

        logger.debug(
            "_handle_marker_event: backfilling insertion event %s", insertion_event_id
        )

        await self._get_events_and_persist(
            origin,
            marker_event.room_id,
            [insertion_event_id],
        )

        insertion_event = await self._store.get_event(
            insertion_event_id, allow_none=True
        )
        if insertion_event is None:
            logger.warning(
                "_handle_marker_event: server %s didn't return insertion event %s for marker %s",
                origin,
                insertion_event_id,
                marker_event.event_id,
            )
            return

        logger.debug(
            "_handle_marker_event: succesfully backfilled insertion event %s from marker event %s",
            insertion_event,
            marker_event,
        )

        await self._store.insert_insertion_extremity(
            insertion_event_id, marker_event.room_id
        )

        logger.debug(
            "_handle_marker_event: insertion extremity added for %s from marker event %s",
            insertion_event,
            marker_event,
        )

    async def _get_events_and_persist(
        self, destination: str, room_id: str, event_ids: Collection[str]
    ) -> None:
        """Fetch the given events from a server, and persist them as outliers.

        This function *does not* recursively get missing auth events of the
        newly fetched events. Callers must include in the `event_ids` argument
        any missing events from the auth chain.

        Logs a warning if we can't find the given event.
        """

        room_version = await self._store.get_room_version(room_id)

        events: List[EventBase] = []

        async def get_event(event_id: str) -> None:
            with nested_logging_context(event_id):
                try:
                    event = await self._federation_client.get_pdu(
                        [destination],
                        event_id,
                        room_version,
                    )
                    if event is None:
                        logger.warning(
                            "Server %s didn't return event %s",
                            destination,
                            event_id,
                        )
                        return
                    events.append(event)

                except Exception as e:
                    logger.warning(
                        "Error fetching missing state/auth event %s: %s %s",
                        event_id,
                        type(e),
                        e,
                    )

        await concurrently_execute(get_event, event_ids, 5)
        logger.info("Fetched %i events of %i requested", len(events), len(event_ids))
        await self._auth_and_persist_outliers(room_id, events)

    async def _auth_and_persist_outliers(
        self, room_id: str, events: Iterable[EventBase]
    ) -> None:
        """Persist a batch of outlier events fetched from remote servers.

        We first sort the events to make sure that we process each event's auth_events
        before the event itself.

        We then mark the events as outliers, persist them to the database, and, where
        appropriate (eg, an invite), awake the notifier.

        Params:
            room_id: the room that the events are meant to be in (though this has
               not yet been checked)
            events: the events that have been fetched
        """
        event_map = {event.event_id: event for event in events}

        # filter out any events we have already seen. This might happen because
        # the events were eagerly pushed to us (eg, during a room join), or because
        # another thread has raced against us since we decided to request the event.
        #
        # This is just an optimisation, so it doesn't need to be watertight - the event
        # persister does another round of deduplication.
        seen_remotes = await self._store.have_seen_events(room_id, event_map.keys())
        for s in seen_remotes:
            event_map.pop(s, None)

        # XXX: it might be possible to kick this process off in parallel with fetching
        # the events.
        while event_map:
            # build a list of events whose auth events are not in the queue.
            roots = tuple(
                ev
                for ev in event_map.values()
                if not any(aid in event_map for aid in ev.auth_event_ids())
            )

            if not roots:
                # if *none* of the remaining events are ready, that means
                # we have a loop. This either means a bug in our logic, or that
                # somebody has managed to create a loop (which requires finding a
                # hash collision in room v2 and later).
                logger.warning(
                    "Loop found in auth events while fetching missing state/auth "
                    "events: %s",
                    shortstr(event_map.keys()),
                )
                return

            logger.info(
                "Persisting %i of %i remaining outliers: %s",
                len(roots),
                len(event_map),
                shortstr(e.event_id for e in roots),
            )

            await self._auth_and_persist_outliers_inner(room_id, roots)

            for ev in roots:
                del event_map[ev.event_id]

    async def _auth_and_persist_outliers_inner(
        self, room_id: str, fetched_events: Collection[EventBase]
    ) -> None:
        """Helper for _auth_and_persist_outliers

        Persists a batch of events where we have (theoretically) already persisted all
        of their auth events.

        Marks the events as outliers, auths them, persists them to the database, and,
        where appropriate (eg, an invite), awakes the notifier.

        Params:
            origin: where the events came from
            room_id: the room that the events are meant to be in (though this has
               not yet been checked)
            fetched_events: the events to persist
        """
        # get all the auth events for all the events in this batch. By now, they should
        # have been persisted.
        auth_events = {
            aid for event in fetched_events for aid in event.auth_event_ids()
        }
        persisted_events = await self._store.get_events(
            auth_events,
            allow_rejected=True,
        )

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

        def prep(event: EventBase) -> Optional[Tuple[EventBase, EventContext]]:
            with nested_logging_context(suffix=event.event_id):
                auth = []
                for auth_event_id in event.auth_event_ids():
                    ae = persisted_events.get(auth_event_id)
                    if not ae:
                        # the fact we can't find the auth event doesn't mean it doesn't
                        # exist, which means it is premature to reject `event`. Instead we
                        # just ignore it for now.
                        logger.warning(
                            "Dropping event %s, which relies on auth_event %s, which could not be found",
                            event,
                            auth_event_id,
                        )
                        return None
                    auth.append(ae)

                # we're not bothering about room state, so flag the event as an outlier.
                event.internal_metadata.outlier = True

                context = EventContext.for_outlier(self._storage)
                try:
                    validate_event_for_room_version(room_version_obj, event)
                    check_auth_rules_for_event(room_version_obj, event, auth)
                except AuthError as e:
                    logger.warning("Rejecting %r because %s", event, e)
                    context.rejected = RejectedReason.AUTH_ERROR

            return event, context

        events_to_persist = (x for x in (prep(event) for event in fetched_events) if x)
        await self.persist_events_and_notify(
            room_id,
            tuple(events_to_persist),
            # Mark these events backfilled as they're historic events that will
            # eventually be backfilled. For example, missing events we fetch
            # during backfill should be marked as backfilled as well.
            backfilled=True,
        )

    async def _check_event_auth(
        self,
        origin: str,
        event: EventBase,
        context: EventContext,
    ) -> EventContext:
        """
        Checks whether an event should be rejected (for failing auth checks).

        Args:
            origin: The host the event originates from.
            event: The event itself.
            context:
                The event context.

        Returns:
            The updated context object.

        Raises:
            AuthError if we were unable to find copies of the event's auth events.
               (Most other failures just cause us to set `context.rejected`.)
        """
        # This method should only be used for non-outliers
        assert not event.internal_metadata.outlier

        # first of all, check that the event itself is valid.
        room_version = await self._store.get_room_version_id(event.room_id)
        room_version_obj = KNOWN_ROOM_VERSIONS[room_version]

        try:
            validate_event_for_room_version(room_version_obj, event)
        except AuthError as e:
            logger.warning("While validating received event %r: %s", event, e)
            # TODO: use a different rejected reason here?
            context.rejected = RejectedReason.AUTH_ERROR
            return context

        # next, check that we have all of the event's auth events.
        #
        # Note that this can raise AuthError, which we want to propagate to the
        # caller rather than swallow with `context.rejected` (since we cannot be
        # certain that there is a permanent problem with the event).
        claimed_auth_events = await self._load_or_fetch_auth_events_for_event(
            origin, event
        )

        # ... and check that the event passes auth at those auth events.
        try:
            check_auth_rules_for_event(room_version_obj, event, claimed_auth_events)
        except AuthError as e:
            logger.warning(
                "While checking auth of %r against auth_events: %s", event, e
            )
            context.rejected = RejectedReason.AUTH_ERROR
            return context

        # now check auth against what we think the auth events *should* be.
        prev_state_ids = await context.get_prev_state_ids()
        auth_events_ids = self._event_auth_handler.compute_auth_events(
            event, prev_state_ids, for_verification=True
        )
        auth_events_x = await self._store.get_events(auth_events_ids)
        calculated_auth_event_map = {
            (e.type, e.state_key): e for e in auth_events_x.values()
        }

        try:
            updated_auth_events = await self._update_auth_events_for_auth(
                event,
                calculated_auth_event_map=calculated_auth_event_map,
            )
        except Exception:
            # We don't really mind if the above fails, so lets not fail
            # processing if it does. However, it really shouldn't fail so
            # let's still log as an exception since we'll still want to fix
            # any bugs.
            logger.exception(
                "Failed to double check auth events for %s with remote. "
                "Ignoring failure and continuing processing of event.",
                event.event_id,
            )
            updated_auth_events = None

        if updated_auth_events:
            context = await self._update_context_for_auth_events(
                event, context, updated_auth_events
            )
            auth_events_for_auth = updated_auth_events
        else:
            auth_events_for_auth = calculated_auth_event_map

        try:
            check_auth_rules_for_event(
                room_version_obj, event, auth_events_for_auth.values()
            )
        except AuthError as e:
            logger.warning("Failed auth resolution for %r because %s", event, e)
            context.rejected = RejectedReason.AUTH_ERROR

        return context

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

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

        current_state_map = await self._state_handler.get_current_state(event.room_id)
        current_state = list(current_state_map.values())
        await self._get_room_member_handler().kick_guest_users(current_state)

    async def _check_for_soft_fail(
        self,
        event: EventBase,
        state: Optional[Iterable[EventBase]],
        origin: str,
    ) -> None:
        """Checks if we should soft fail the event; if so, marks the event as
        such.

        Args:
            event
            state: The state at the event if we don't have all the event's prev events
            origin: The host the event originates from.
        """
        extrem_ids_list = await self._store.get_latest_event_ids_in_room(event.room_id)
        extrem_ids = set(extrem_ids_list)
        prev_event_ids = set(event.prev_event_ids())

        if extrem_ids == prev_event_ids:
            # If they're the same then the current state is the same as the
            # state at the event, so no point rechecking auth for soft fail.
            return

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

        # Calculate the "current state".
        if state is not None:
            # If we're explicitly given the state then we won't have all the
            # prev events, and so we have a gap in the graph. In this case
            # we want to be a little careful as we might have been down for
            # a while and have an incorrect view of the current state,
            # however we still want to do checks as gaps are easy to
            # maliciously manufacture.
            #
            # So we use a "current state" that is actually a state
            # resolution across the current forward extremities and the
            # given state at the event. This should correctly handle cases
            # like bans, especially with state res v2.

            state_sets_d = await self._state_store.get_state_groups(
                event.room_id, extrem_ids
            )
            state_sets: List[Iterable[EventBase]] = list(state_sets_d.values())
            state_sets.append(state)
            current_states = await self._state_handler.resolve_events(
                room_version, state_sets, event
            )
            current_state_ids: StateMap[str] = {
                k: e.event_id for k, e in current_states.items()
            }
        else:
            current_state_ids = await self._state_handler.get_current_state_ids(
                event.room_id, latest_event_ids=extrem_ids
            )

        logger.debug(
            "Doing soft-fail check for %s: state %s",
            event.event_id,
            current_state_ids,
        )

        # Now check if event pass auth against said current state
        auth_types = auth_types_for_event(room_version_obj, event)
        current_state_ids_list = [
            e for k, e in current_state_ids.items() if k in auth_types
        ]
        current_auth_events = await self._store.get_events_as_list(
            current_state_ids_list
        )

        try:
            check_auth_rules_for_event(room_version_obj, event, current_auth_events)
        except AuthError as e:
            logger.warning(
                "Soft-failing %r (from %s) because %s",
                event,
                e,
                origin,
                extra={
                    "room_id": event.room_id,
                    "mxid": event.sender,
                    "hs": origin,
                },
            )
            soft_failed_event_counter.inc()
            event.internal_metadata.soft_failed = True

    async def _update_auth_events_for_auth(
        self,
        event: EventBase,
        calculated_auth_event_map: StateMap[EventBase],
    ) -> Optional[StateMap[EventBase]]:
        """Helper for _check_event_auth. See there for docs.

        Checks whether a given event has the expected auth events. If it
        doesn't then we talk to the remote server to compare state to see if
        we can come to a consensus (e.g. if one server missed some valid
        state).

        This attempts to resolve any potential divergence of state between
        servers, but is not essential and so failures should not block further
        processing of the event.

        Args:
            event:

            calculated_auth_event_map:
                Our calculated auth_events based on the state of the room
                at the event's position in the DAG.

        Returns:
            updated auth event map, or None if no changes are needed.

        """
        assert not event.internal_metadata.outlier

        # check for events which are in the event's claimed auth_events, but not
        # in our calculated event map.
        event_auth_events = set(event.auth_event_ids())
        different_auth = event_auth_events.difference(
            e.event_id for e in calculated_auth_event_map.values()
        )

        if not different_auth:
            return None

        logger.info(
            "auth_events refers to events which are not in our calculated auth "
            "chain: %s",
            different_auth,
        )

        # XXX: currently this checks for redactions but I'm not convinced that is
        # necessary?
        different_events = await self._store.get_events_as_list(different_auth)

        # double-check they're all in the same room - we should already have checked
        # this but it doesn't hurt to check again.
        for d in different_events:
            assert (
                d.room_id == event.room_id
            ), f"Event {event.event_id} refers to auth_event {d.event_id} which is in a different room"

        # now we state-resolve between our own idea of the auth events, and the remote's
        # idea of them.

        local_state = calculated_auth_event_map.values()
        remote_auth_events = dict(calculated_auth_event_map)
        remote_auth_events.update({(d.type, d.state_key): d for d in different_events})
        remote_state = remote_auth_events.values()

        room_version = await self._store.get_room_version_id(event.room_id)
        new_state = await self._state_handler.resolve_events(
            room_version, (local_state, remote_state), event
        )
        different_state = {
            (d.type, d.state_key): d
            for d in new_state.values()
            if calculated_auth_event_map.get((d.type, d.state_key)) != d
        }
        if not different_state:
            logger.info("State res returned no new state")
            return None

        logger.info(
            "After state res: updating auth_events with new state %s",
            different_state.values(),
        )

        # take a copy of calculated_auth_event_map before we modify it.
        auth_events = dict(calculated_auth_event_map)
        auth_events.update(different_state)
        return auth_events

    async def _load_or_fetch_auth_events_for_event(
        self, destination: str, event: EventBase
    ) -> Collection[EventBase]:
        """Fetch this event's auth_events, from database or remote

        Loads any of the auth_events that we already have from the database/cache. If
        there are any that are missing, calls /event_auth to get the complete auth
        chain for the event (and then attempts to load the auth_events again).

        If any of the auth_events cannot be found, raises an AuthError. This can happen
        for a number of reasons; eg: the events don't exist, or we were unable to talk
        to `destination`, or we couldn't validate the signature on the event (which
        in turn has multiple potential causes).

        Args:
            destination: where to send the /event_auth request. Typically the server
               that sent us `event` in the first place.
            event: the event whose auth_events we want

        Returns:
            all of the events listed in `event.auth_events_ids`, after deduplication

        Raises:
            AuthError if we were unable to fetch the auth_events for any reason.
        """
        event_auth_event_ids = set(event.auth_event_ids())
        event_auth_events = await self._store.get_events(
            event_auth_event_ids, allow_rejected=True
        )
        missing_auth_event_ids = event_auth_event_ids.difference(
            event_auth_events.keys()
        )
        if not missing_auth_event_ids:
            return event_auth_events.values()

        logger.info(
            "Event %s refers to unknown auth events %s: fetching auth chain",
            event,
            missing_auth_event_ids,
        )
        try:
            await self._get_remote_auth_chain_for_event(
                destination, event.room_id, event.event_id
            )
        except Exception as e:
            logger.warning("Failed to get auth chain for %s: %s", event, e)
            # in this case, it's very likely we still won't have all the auth
            # events - but we pick that up below.

        # try to fetch the auth events we missed list time.
        extra_auth_events = await self._store.get_events(
            missing_auth_event_ids, allow_rejected=True
        )
        missing_auth_event_ids.difference_update(extra_auth_events.keys())
        event_auth_events.update(extra_auth_events)
        if not missing_auth_event_ids:
            return event_auth_events.values()

        # we still don't have all the auth events.
        logger.warning(
            "Missing auth events for %s: %s",
            event,
            shortstr(missing_auth_event_ids),
        )
        # the fact we can't find the auth event doesn't mean it doesn't
        # exist, which means it is premature to store `event` as rejected.
        # instead we raise an AuthError, which will make the caller ignore it.
        raise AuthError(code=HTTPStatus.FORBIDDEN, msg="Auth events could not be found")

    async def _get_remote_auth_chain_for_event(
        self, destination: str, room_id: str, event_id: str
    ) -> None:
        """If we are missing some of an event's auth events, attempt to request them

        Args:
            destination: where to fetch the auth tree from
            room_id: the room in which we are lacking auth events
            event_id: the event for which we are lacking auth events
        """
        try:
            remote_events = await self._federation_client.get_event_auth(
                destination, room_id, event_id
            )

        except RequestSendFailed as e1:
            # The other side isn't around or doesn't implement the
            # endpoint, so lets just bail out.
            logger.info("Failed to get event auth from remote: %s", e1)
            return

        logger.info("/event_auth returned %i events", len(remote_events))

        # `event` may be returned, but we should not yet process it.
        remote_auth_events = (e for e in remote_events if e.event_id != event_id)

        await self._auth_and_persist_outliers(room_id, remote_auth_events)

    async def _update_context_for_auth_events(
        self, event: EventBase, context: EventContext, auth_events: StateMap[EventBase]
    ) -> EventContext:
        """Update the state_ids in an event context after auth event resolution,
        storing the changes as a new state group.

        Args:
            event: The event we're handling the context for

            context: initial event context

            auth_events: Events to update in the event context.

        Returns:
            new event context
        """
        # exclude the state key of the new event from the current_state in the context.
        if event.is_state():
            event_key: Optional[Tuple[str, str]] = (event.type, event.state_key)
        else:
            event_key = None
        state_updates = {
            k: a.event_id for k, a in auth_events.items() if k != event_key
        }

        current_state_ids = await context.get_current_state_ids()
        current_state_ids = dict(current_state_ids)  # type: ignore

        current_state_ids.update(state_updates)

        prev_state_ids = await context.get_prev_state_ids()
        prev_state_ids = dict(prev_state_ids)

        prev_state_ids.update({k: a.event_id for k, a in auth_events.items()})

        # create a new state group as a delta from the existing one.
        prev_group = context.state_group
        state_group = await self._state_store.store_state_group(
            event.event_id,
            event.room_id,
            prev_group=prev_group,
            delta_ids=state_updates,
            current_state_ids=current_state_ids,
        )

        return EventContext.with_state(
            storage=self._storage,
            state_group=state_group,
            state_group_before_event=context.state_group_before_event,
            state_delta_due_to_event=state_updates,
            prev_group=prev_group,
            delta_ids=state_updates,
            partial_state=context.partial_state,
        )

    async def _run_push_actions_and_persist_event(
        self, event: EventBase, context: EventContext, backfilled: bool = False
    ) -> None:
        """Run the push actions for a received event, and persist it.

        Args:
            event: The event itself.
            context: The event context.
            backfilled: True if the event was backfilled.
        """
        # this method should not be called on outliers (those code paths call
        # persist_events_and_notify directly.)
        assert not event.internal_metadata.outlier

        if not backfilled and not context.rejected:
            min_depth = await self._store.get_min_depth(event.room_id)
            if min_depth is None or min_depth > event.depth:
                # XXX richvdh 2021/10/07: I don't really understand what this
                # condition is doing. I think it's trying not to send pushes
                # for events that predate our join - but that's not really what
                # min_depth means, and anyway ancient events are a more general
                # problem.
                #
                # for now I'm just going to log about it.
                logger.info(
                    "Skipping push actions for old event with depth %s < %s",
                    event.depth,
                    min_depth,
                )
            else:
                await self._bulk_push_rule_evaluator.action_for_event_by_user(
                    event, context
                )

        try:
            await self.persist_events_and_notify(
                event.room_id, [(event, context)], backfilled=backfilled
            )
        except Exception:
            run_in_background(
                self._store.remove_push_actions_from_staging, event.event_id
            )
            raise

    async def persist_events_and_notify(
        self,
        room_id: str,
        event_and_contexts: Sequence[Tuple[EventBase, EventContext]],
        backfilled: bool = False,
    ) -> int:
        """Persists events and tells the notifier/pushers about them, if
        necessary.

        Args:
            room_id: The room ID of events being persisted.
            event_and_contexts: Sequence of events with their associated
                context that should be persisted. All events must belong to
                the same room.
            backfilled: Whether these events are a result of
                backfilling or not

        Returns:
            The stream ID after which all events have been persisted.
        """
        if not event_and_contexts:
            return self._store.get_room_max_stream_ordering()

        instance = self._config.worker.events_shard_config.get_instance(room_id)
        if instance != self._instance_name:
            # Limit the number of events sent over replication. We choose 200
            # here as that is what we default to in `max_request_body_size(..)`
            for batch in batch_iter(event_and_contexts, 200):
                result = await self._send_events(
                    instance_name=instance,
                    store=self._store,
                    room_id=room_id,
                    event_and_contexts=batch,
                    backfilled=backfilled,
                )
            return result["max_stream_id"]
        else:
            assert self._storage.persistence

            # Note that this returns the events that were persisted, which may not be
            # the same as were passed in if some were deduplicated due to transaction IDs.
            events, max_stream_token = await self._storage.persistence.persist_events(
                event_and_contexts, backfilled=backfilled
            )

            if self._ephemeral_messages_enabled:
                for event in events:
                    # If there's an expiry timestamp on the event, schedule its expiry.
                    self._message_handler.maybe_schedule_expiry(event)

            if not backfilled:  # Never notify for backfilled events
                for event in events:
                    await self._notify_persisted_event(event, max_stream_token)

            return max_stream_token.stream

    async def _notify_persisted_event(
        self, event: EventBase, max_stream_token: RoomStreamToken
    ) -> None:
        """Checks to see if notifier/pushers should be notified about the
        event or not.

        Args:
            event:
            max_stream_token: The max_stream_id returned by persist_events
        """

        extra_users = []
        if event.type == EventTypes.Member:
            target_user_id = event.state_key

            # We notify for memberships if its an invite for one of our
            # users
            if event.internal_metadata.is_outlier():
                if event.membership != Membership.INVITE:
                    if not self._is_mine_id(target_user_id):
                        return

            target_user = UserID.from_string(target_user_id)
            extra_users.append(target_user)
        elif event.internal_metadata.is_outlier():
            return

        # the event has been persisted so it should have a stream ordering.
        assert event.internal_metadata.stream_ordering

        event_pos = PersistedEventPosition(
            self._instance_name, event.internal_metadata.stream_ordering
        )
        await self._notifier.on_new_room_event(
            event, event_pos, max_stream_token, extra_users=extra_users
        )

    def _sanity_check_event(self, ev: EventBase) -> None:
        """
        Do some early sanity checks of a received event

        In particular, checks it doesn't have an excessive number of
        prev_events or auth_events, which could cause a huge state resolution
        or cascade of event fetches.

        Args:
            ev: event to be checked

        Raises:
            SynapseError if the event does not pass muster
        """
        if len(ev.prev_event_ids()) > 20:
            logger.warning(
                "Rejecting event %s which has %i prev_events",
                ev.event_id,
                len(ev.prev_event_ids()),
            )
            raise SynapseError(HTTPStatus.BAD_REQUEST, "Too many prev_events")

        if len(ev.auth_event_ids()) > 10:
            logger.warning(
                "Rejecting event %s which has %i auth_events",
                ev.event_id,
                len(ev.auth_event_ids()),
            )
            raise SynapseError(HTTPStatus.BAD_REQUEST, "Too many auth_events")