Split `FederationHandler` in half (#10692)

The idea here is to take anything to do with incoming events and move it out to a separate handler, as a way of making FederationHandler smaller.

1 files changed, 1825 insertions, 0 deletions

diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
new file mode 100644
index 0000000000..9f055f00cf
--- /dev/null
+++ b/synapse/handlers/federation_event.py
@@ -0,0 +1,1825 @@
+# 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 logging
+from http import HTTPStatus
+from typing import (
+    TYPE_CHECKING,
+    Collection,
+    Container,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+)
+
+import attr
+from prometheus_client import Counter
+
+from twisted.internet import defer
+
+from synapse import event_auth
+from synapse.api.constants import (
+    EventContentFields,
+    EventTypes,
+    Membership,
+    RejectedReason,
+    RoomEncryptionAlgorithms,
+)
+from synapse.api.errors import (
+    AuthError,
+    Codes,
+    FederationError,
+    HttpResponseException,
+    RequestSendFailed,
+    SynapseError,
+)
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
+from synapse.event_auth import auth_types_for_event
+from synapse.events import EventBase
+from synapse.events.snapshot import EventContext
+from synapse.federation.federation_client import InvalidResponseError
+from synapse.handlers._base import BaseHandler
+from synapse.logging.context import (
+    make_deferred_yieldable,
+    nested_logging_context,
+    run_in_background,
+)
+from synapse.logging.utils import log_function
+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 (
+    MutableStateMap,
+    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",
+)
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class _NewEventInfo:
+    """Holds information about a received event, ready for passing to _auth_and_persist_events
+
+    Attributes:
+        event: the received event
+
+        claimed_auth_event_map: a map of (type, state_key) => event for the event's
+            claimed auth_events.
+
+            This can include events which have not yet been persisted, in the case that
+            we are backfilling a batch of events.
+
+            Note: May be incomplete: if we were unable to find all of the claimed auth
+            events. Also, treat the contents with caution: the events might also have
+            been rejected, might not yet have been authorized themselves, or they might
+            be in the wrong room.
+
+    """
+
+    event: EventBase
+    claimed_auth_event_map: StateMap[EventBase]
+
+
+class FederationEventHandler(BaseHandler):
+    """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"):
+        super().__init__(hs)
+
+        self.store = hs.get_datastore()
+        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.action_generator = hs.get_action_generator()
+        self._state_resolution_handler = hs.get_state_resolution_handler()
+
+        self.federation_client = hs.get_federation_client()
+        self.third_party_event_rules = hs.get_third_party_event_rules()
+
+        self.is_mine_id = hs.is_mine_id
+        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_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
+        """
+
+        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
+
+        # Check that the event passes auth based on the state at the event. This is
+        # done for events that are to be added to the timeline (non-outliers).
+        #
+        # Get missing pdus if necessary:
+        #  - Fetching any missing prev events to fill in gaps in the graph
+        #  - Fetching state if we have a hole in the graph
+        if not pdu.internal_metadata.is_outlier():
+            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.get_min_depth_for_context(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),
+                    )
+                    with (await 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)
+
+    @log_function
+    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.
+        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,
+        )
+
+    @log_function
+    async def backfill(
+        self, dest: str, room_id: str, limit: int, extremities: List[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.)
+        """
+
+        # 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()
+
+        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)
+            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
+        logger.debug("Fetching %i events from remote", len(missing_events))
+        await self._get_events_and_persist(
+            destination=destination, room_id=room_id, events=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 _process_received_pdu(
+        self,
+        origin: str,
+        event: EventBase,
+        state: Optional[Iterable[EventBase]],
+        backfilled: bool = False,
+    ) -> None:
+        """Called when we have a new pdu. 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)
+
+        try:
+            context = await self.state_handler.compute_event_context(
+                event, old_state=state
+            )
+            await self._auth_and_persist_event(
+                origin, event, context, state=state, backfilled=backfilled
+            )
+        except AuthError as e:
+            raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)
+
+        if backfilled:
+            return
+
+        # 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,
+                )
+
+        await self._handle_marker_event(origin, event)
+
+    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_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):
+        """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.
+        room_version = await self.store.get_room_version(marker_event.room_id)
+        if not room_version.msc2716_historical:
+            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, events: Iterable[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 `events` 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)
+
+        event_map: Dict[str, EventBase] = {}
+
+        async def get_event(event_id: str):
+            with nested_logging_context(event_id):
+                try:
+                    event = await self.federation_client.get_pdu(
+                        [destination],
+                        event_id,
+                        room_version,
+                        outlier=True,
+                    )
+                    if event is None:
+                        logger.warning(
+                            "Server %s didn't return event %s",
+                            destination,
+                            event_id,
+                        )
+                        return
+
+                    event_map[event.event_id] = 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, events, 5)
+
+        # Make a map of auth events for each event. We do this after fetching
+        # all the events as some of the events' auth events will be in the list
+        # of requested events.
+
+        auth_events = [
+            aid
+            for event in event_map.values()
+            for aid in event.auth_event_ids()
+            if aid not in event_map
+        ]
+        persisted_events = await self.store.get_events(
+            auth_events,
+            allow_rejected=True,
+        )
+
+        event_infos = []
+        for event in event_map.values():
+            auth = {}
+            for auth_event_id in event.auth_event_ids():
+                ae = persisted_events.get(auth_event_id) or event_map.get(auth_event_id)
+                if ae:
+                    auth[(ae.type, ae.state_key)] = ae
+                else:
+                    logger.info("Missing auth event %s", auth_event_id)
+
+            event_infos.append(_NewEventInfo(event, auth))
+
+        if event_infos:
+            await self._auth_and_persist_events(
+                destination,
+                room_id,
+                event_infos,
+            )
+
+    async def _auth_and_persist_events(
+        self,
+        origin: str,
+        room_id: str,
+        event_infos: Collection[_NewEventInfo],
+    ) -> None:
+        """Creates the appropriate contexts and persists events. The events
+        should not depend on one another, e.g. this should be used to persist
+        a bunch of outliers, but not a chunk of individual events that depend
+        on each other for state calculations.
+
+        Notifies about the events where appropriate.
+        """
+
+        if not event_infos:
+            return
+
+        async def prep(ev_info: _NewEventInfo):
+            event = ev_info.event
+            with nested_logging_context(suffix=event.event_id):
+                res = await self.state_handler.compute_event_context(event)
+                res = await self._check_event_auth(
+                    origin,
+                    event,
+                    res,
+                    claimed_auth_event_map=ev_info.claimed_auth_event_map,
+                )
+            return res
+
+        contexts = await make_deferred_yieldable(
+            defer.gatherResults(
+                [run_in_background(prep, ev_info) for ev_info in event_infos],
+                consumeErrors=True,
+            )
+        )
+
+        await self.persist_events_and_notify(
+            room_id,
+            [
+                (ev_info.event, context)
+                for ev_info, context in zip(event_infos, contexts)
+            ],
+        )
+
+    async def _auth_and_persist_event(
+        self,
+        origin: str,
+        event: EventBase,
+        context: EventContext,
+        state: Optional[Iterable[EventBase]] = None,
+        claimed_auth_event_map: Optional[StateMap[EventBase]] = None,
+        backfilled: bool = False,
+    ) -> None:
+        """
+        Process an event by performing auth checks and then persisting to the database.
+
+        Args:
+            origin: The host the event originates from.
+            event: The event itself.
+            context:
+                The event context.
+
+            state:
+                The state events used to check the event for soft-fail. If this is
+                not provided the current state events will be used.
+
+            claimed_auth_event_map:
+                A map of (type, state_key) => event for the event's claimed auth_events.
+                Possibly incomplete, and possibly including events that are not yet
+                persisted, or authed, or in the right room.
+
+                Only populated where we may not already have persisted these events -
+                for example, when populating outliers.
+
+            backfilled: True if the event was backfilled.
+        """
+        context = await self._check_event_auth(
+            origin,
+            event,
+            context,
+            state=state,
+            claimed_auth_event_map=claimed_auth_event_map,
+            backfilled=backfilled,
+        )
+
+        await self._run_push_actions_and_persist_event(event, context, backfilled)
+
+    async def _check_event_auth(
+        self,
+        origin: str,
+        event: EventBase,
+        context: EventContext,
+        state: Optional[Iterable[EventBase]] = None,
+        claimed_auth_event_map: Optional[StateMap[EventBase]] = None,
+        backfilled: bool = False,
+    ) -> 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.
+
+            state:
+                The state events used to check the event for soft-fail. If this is
+                not provided the current state events will be used.
+
+            claimed_auth_event_map:
+                A map of (type, state_key) => event for the event's claimed auth_events.
+                Possibly incomplete, and possibly including events that are not yet
+                persisted, or authed, or in the right room.
+
+                Only populated where we may not already have persisted these events -
+                for example, when populating outliers, or the state for a backwards
+                extremity.
+
+            backfilled: True if the event was backfilled.
+
+        Returns:
+            The updated context object.
+        """
+        room_version = await self.store.get_room_version_id(event.room_id)
+        room_version_obj = KNOWN_ROOM_VERSIONS[room_version]
+
+        if claimed_auth_event_map:
+            # if we have a copy of the auth events from the event, use that as the
+            # basis for auth.
+            auth_events = claimed_auth_event_map
+        else:
+            # otherwise, we calculate what the auth events *should* be, and use that
+            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)
+            auth_events = {(e.type, e.state_key): e for e in auth_events_x.values()}
+
+        try:
+            (
+                context,
+                auth_events_for_auth,
+            ) = await self._update_auth_events_and_context_for_auth(
+                origin, event, context, auth_events
+            )
+        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,
+            )
+            auth_events_for_auth = auth_events
+
+        try:
+            event_auth.check(room_version_obj, event, auth_events=auth_events_for_auth)
+        except AuthError as e:
+            logger.warning("Failed auth resolution for %r because %s", event, e)
+            context.rejected = RejectedReason.AUTH_ERROR
+
+        if not context.rejected:
+            await self._check_for_soft_fail(event, state, backfilled, origin=origin)
+
+        if event.type == EventTypes.GuestAccess and not context.rejected:
+            await self.maybe_kick_guest_users(event)
+
+        # If we are going to send this event over federation we precaclculate
+        # the joined hosts.
+        if event.internal_metadata.get_send_on_behalf_of():
+            await self.event_creation_handler.cache_joined_hosts_for_event(
+                event, context
+            )
+
+        return context
+
+    async def _check_for_soft_fail(
+        self,
+        event: EventBase,
+        state: Optional[Iterable[EventBase]],
+        backfilled: bool,
+        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
+            backfilled: Whether the event is from backfill
+            origin: The host the event originates from.
+        """
+        # 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.
+        if backfilled or event.internal_metadata.is_outlier():
+            return
+
+        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
+        ]
+
+        auth_events_map = await self.store.get_events(current_state_ids_list)
+        current_auth_events = {
+            (e.type, e.state_key): e for e in auth_events_map.values()
+        }
+
+        try:
+            event_auth.check(room_version_obj, event, auth_events=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_and_context_for_auth(
+        self,
+        origin: str,
+        event: EventBase,
+        context: EventContext,
+        input_auth_events: StateMap[EventBase],
+    ) -> Tuple[EventContext, 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:
+            origin:
+            event:
+            context:
+
+            input_auth_events:
+                Map from (event_type, state_key) to event
+
+                Normally, our calculated auth_events based on the state of the room
+                at the event's position in the DAG, though occasionally (eg if the
+                event is an outlier), may be the auth events claimed by the remote
+                server.
+
+        Returns:
+            updated context, updated auth event map
+        """
+        # take a copy of input_auth_events before we modify it.
+        auth_events: MutableStateMap[EventBase] = dict(input_auth_events)
+
+        event_auth_events = set(event.auth_event_ids())
+
+        # missing_auth is the set of the event's auth_events which we don't yet have
+        # in auth_events.
+        missing_auth = event_auth_events.difference(
+            e.event_id for e in auth_events.values()
+        )
+
+        # if we have missing events, we need to fetch those events from somewhere.
+        #
+        # we start by checking if they are in the store, and then try calling /event_auth/.
+        if missing_auth:
+            have_events = await self.store.have_seen_events(event.room_id, missing_auth)
+            logger.debug("Events %s are in the store", have_events)
+            missing_auth.difference_update(have_events)
+
+        if missing_auth:
+            # If we don't have all the auth events, we need to get them.
+            logger.info("auth_events contains unknown events: %s", missing_auth)
+            try:
+                try:
+                    remote_auth_chain = await self.federation_client.get_event_auth(
+                        origin, event.room_id, event.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 context, auth_events
+
+                seen_remotes = await self.store.have_seen_events(
+                    event.room_id, [e.event_id for e in remote_auth_chain]
+                )
+
+                for e in remote_auth_chain:
+                    if e.event_id in seen_remotes:
+                        continue
+
+                    if e.event_id == event.event_id:
+                        continue
+
+                    try:
+                        auth_ids = e.auth_event_ids()
+                        auth = {
+                            (e.type, e.state_key): e
+                            for e in remote_auth_chain
+                            if e.event_id in auth_ids or e.type == EventTypes.Create
+                        }
+                        e.internal_metadata.outlier = True
+
+                        logger.debug(
+                            "_check_event_auth %s missing_auth: %s",
+                            event.event_id,
+                            e.event_id,
+                        )
+                        missing_auth_event_context = (
+                            await self.state_handler.compute_event_context(e)
+                        )
+                        await self._auth_and_persist_event(
+                            origin,
+                            e,
+                            missing_auth_event_context,
+                            claimed_auth_event_map=auth,
+                        )
+
+                        if e.event_id in event_auth_events:
+                            auth_events[(e.type, e.state_key)] = e
+                    except AuthError:
+                        pass
+
+            except Exception:
+                logger.exception("Failed to get auth chain")
+
+        if event.internal_metadata.is_outlier():
+            # XXX: given that, for an outlier, we'll be working with the
+            # event's *claimed* auth events rather than those we calculated:
+            # (a) is there any point in this test, since different_auth below will
+            # obviously be empty
+            # (b) alternatively, why don't we do it earlier?
+            logger.info("Skipping auth_event fetch for outlier")
+            return context, auth_events
+
+        different_auth = event_auth_events.difference(
+            e.event_id for e in auth_events.values()
+        )
+
+        if not different_auth:
+            return context, auth_events
+
+        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)
+
+        for d in different_events:
+            if d.room_id != event.room_id:
+                logger.warning(
+                    "Event %s refers to auth_event %s which is in a different room",
+                    event.event_id,
+                    d.event_id,
+                )
+
+                # don't attempt to resolve the claimed auth events against our own
+                # in this case: just use our own auth events.
+                #
+                # XXX: should we reject the event in this case? It feels like we should,
+                # but then shouldn't we also do so if we've failed to fetch any of the
+                # auth events?
+                return context, auth_events
+
+        # now we state-resolve between our own idea of the auth events, and the remote's
+        # idea of them.
+
+        local_state = auth_events.values()
+        remote_auth_events = dict(auth_events)
+        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
+        )
+
+        logger.info(
+            "After state res: updating auth_events with new state %s",
+            {
+                (d.type, d.state_key): d.event_id
+                for d in new_state.values()
+                if auth_events.get((d.type, d.state_key)) != d
+            },
+        )
+
+        auth_events.update(new_state)
+
+        context = await self._update_context_for_auth_events(
+            event, context, auth_events
+        )
+
+        return context, 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(
+            state_group=state_group,
+            state_group_before_event=context.state_group_before_event,
+            current_state_ids=current_state_ids,
+            prev_state_ids=prev_state_ids,
+            prev_group=prev_group,
+            delta_ids=state_updates,
+        )
+
+    async def _run_push_actions_and_persist_event(
+        self, event: EventBase, context: EventContext, backfilled: bool = False
+    ):
+        """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.
+        """
+        try:
+            if (
+                not event.internal_metadata.is_outlier()
+                and not backfilled
+                and not context.rejected
+                and (await self.store.get_min_depth(event.room_id)) <= event.depth
+            ):
+                await self.action_generator.handle_push_actions_for_event(
+                    event, context
+                )
+
+            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_current_events_token()
+
+        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
+        )
+        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")
+
+    async def get_min_depth_for_context(self, context: str) -> int:
+        return await self.store.get_min_depth(context)