anonymousland-synapse/synapse/handlers/federation.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Contains handlers for federation events."""
from signedjson.key import decode_verify_key_bytes
from signedjson.sign import verify_signed_json
from unpaddedbase64 import decode_base64

from ._base import BaseHandler

from synapse.api.errors import (
    AuthError, FederationError, StoreError, CodeMessageException, SynapseError,
)
from synapse.api.constants import EventTypes, Membership, RejectedReason
from synapse.events.validator import EventValidator
from synapse.util import unwrapFirstError
from synapse.util.logcontext import PreserveLoggingContext, preserve_fn
from synapse.util.logutils import log_function
from synapse.util.async import run_on_reactor
from synapse.util.frozenutils import unfreeze
from synapse.crypto.event_signing import (
    compute_event_signature, add_hashes_and_signatures,
)
from synapse.types import UserID, get_domain_from_id

from synapse.events.utils import prune_event

from synapse.util.retryutils import NotRetryingDestination

from synapse.push.action_generator import ActionGenerator
from synapse.util.distributor import user_joined_room

from twisted.internet import defer

import itertools
import logging

logger = logging.getLogger(__name__)


class FederationHandler(BaseHandler):
    """Handles events that originated from federation.
        Responsible for:
        a) handling received Pdus before handing them on as Events to the rest
        of the home server (including auth and state conflict resoultion)
        b) converting events that were produced by local clients that may need
        to be sent to remote home servers.
        c) doing the necessary dances to invite remote users and join remote
        rooms.
    """

    def __init__(self, hs):
        super(FederationHandler, self).__init__(hs)

        self.hs = hs

        self.store = hs.get_datastore()
        self.replication_layer = hs.get_replication_layer()
        self.state_handler = hs.get_state_handler()
        self.server_name = hs.hostname
        self.keyring = hs.get_keyring()

        self.replication_layer.set_handler(self)

        # When joining a room we need to queue any events for that room up
        self.room_queues = {}

    def handle_new_event(self, event, destinations):
        """ Takes in an event from the client to server side, that has already
        been authed and handled by the state module, and sends it to any
        remote home servers that may be interested.

        Args:
            event: The event to send
            destinations: A list of destinations to send it to

        Returns:
            Deferred: Resolved when it has successfully been queued for
            processing.
        """

        return self.replication_layer.send_pdu(event, destinations)

    @log_function
    @defer.inlineCallbacks
    def on_receive_pdu(self, origin, pdu, state=None, auth_chain=None):
        """ Called by the ReplicationLayer when we have a new pdu. We need to
        do auth checks and put it through the StateHandler.
        """
        event = pdu

        logger.debug("Got event: %s", event.event_id)

        # If we are currently in the process of joining this room, then we
        # queue up events for later processing.
        if event.room_id in self.room_queues:
            self.room_queues[event.room_id].append((pdu, origin))
            return

        logger.debug("Processing event: %s", event.event_id)

        logger.debug("Event: %s", event)

        # FIXME (erikj): Awful hack to make the case where we are not currently
        # in the room work
        is_in_room = yield self.auth.check_host_in_room(
            event.room_id,
            self.server_name
        )
        if not is_in_room and not event.internal_metadata.is_outlier():
            logger.debug("Got event for room we're not in.")

            try:
                event_stream_id, max_stream_id = yield self._persist_auth_tree(
                    origin, auth_chain, state, event
                )
            except AuthError as e:
                raise FederationError(
                    "ERROR",
                    e.code,
                    e.msg,
                    affected=event.event_id,
                )

        else:
            event_ids = set()
            if state:
                event_ids |= {e.event_id for e in state}
            if auth_chain:
                event_ids |= {e.event_id for e in auth_chain}

            seen_ids = set(
                (yield self.store.have_events(event_ids)).keys()
            )

            if state and auth_chain is not None:
                # If we have any state or auth_chain given to us by the replication
                # layer, then we should handle them (if we haven't before.)

                event_infos = []

                for e in itertools.chain(auth_chain, state):
                    if e.event_id in seen_ids:
                        continue
                    e.internal_metadata.outlier = True
                    auth_ids = [e_id for e_id, _ in e.auth_events]
                    auth = {
                        (e.type, e.state_key): e for e in auth_chain
                        if e.event_id in auth_ids or e.type == EventTypes.Create
                    }
                    event_infos.append({
                        "event": e,
                        "auth_events": auth,
                    })
                    seen_ids.add(e.event_id)

                yield self._handle_new_events(origin, event_infos)

            try:
                context, event_stream_id, max_stream_id = yield self._handle_new_event(
                    origin,
                    event,
                    state=state,
                )
            except AuthError as e:
                raise FederationError(
                    "ERROR",
                    e.code,
                    e.msg,
                    affected=event.event_id,
                )

        # if we're receiving valid events from an origin,
        # it's probably a good idea to mark it as not in retry-state
        # for sending (although this is a bit of a leap)
        retry_timings = yield self.store.get_destination_retry_timings(origin)
        if retry_timings and retry_timings["retry_last_ts"]:
            self.store.set_destination_retry_timings(origin, 0, 0)

        room = yield self.store.get_room(event.room_id)

        if not room:
            try:
                yield self.store.store_room(
                    room_id=event.room_id,
                    room_creator_user_id="",
                    is_public=False,
                )
            except StoreError:
                logger.exception("Failed to store room.")

        extra_users = []
        if event.type == EventTypes.Member:
            target_user_id = event.state_key
            target_user = UserID.from_string(target_user_id)
            extra_users.append(target_user)

        with PreserveLoggingContext():
            self.notifier.on_new_room_event(
                event, event_stream_id, max_stream_id,
                extra_users=extra_users
            )

        if event.type == EventTypes.Member:
            if event.membership == Membership.JOIN:
                prev_state = context.current_state.get((event.type, event.state_key))
                if not prev_state or prev_state.membership != Membership.JOIN:
                    # Only fire user_joined_room if the user has acutally
                    # joined the room. Don't bother if the user is just
                    # changing their profile info.
                    user = UserID.from_string(event.state_key)
                    yield user_joined_room(self.distributor, user, event.room_id)

    @defer.inlineCallbacks
    def _filter_events_for_server(self, server_name, room_id, events):
        event_to_state = yield self.store.get_state_for_events(
            frozenset(e.event_id for e in events),
            types=(
                (EventTypes.RoomHistoryVisibility, ""),
                (EventTypes.Member, None),
            )
        )

        def redact_disallowed(event, state):
            if not state:
                return event

            history = state.get((EventTypes.RoomHistoryVisibility, ''), None)
            if history:
                visibility = history.content.get("history_visibility", "shared")
                if visibility in ["invited", "joined"]:
                    # We now loop through all state events looking for
                    # membership states for the requesting server to determine
                    # if the server is either in the room or has been invited
                    # into the room.
                    for ev in state.values():
                        if ev.type != EventTypes.Member:
                            continue
                        try:
                            domain = get_domain_from_id(ev.state_key)
                        except:
                            continue

                        if domain != server_name:
                            continue

                        memtype = ev.membership
                        if memtype == Membership.JOIN:
                            return event
                        elif memtype == Membership.INVITE:
                            if visibility == "invited":
                                return event
                    else:
                        return prune_event(event)

            return event

        defer.returnValue([
            redact_disallowed(e, event_to_state[e.event_id])
            for e in events
        ])

    @log_function
    @defer.inlineCallbacks
    def backfill(self, dest, room_id, limit, extremities=[]):
        """ Trigger a backfill request to `dest` for the given `room_id`

        This will attempt to get more events from the remote. This may return
        be successfull and still return no events if the other side has no new
        events to offer.
        """
        if dest == self.server_name:
            raise SynapseError(400, "Can't backfill from self.")

        if not extremities:
            extremities = yield self.store.get_oldest_events_in_room(room_id)

        events = yield self.replication_layer.backfill(
            dest,
            room_id,
            limit=limit,
            extremities=extremities,
        )

        # Don't bother processing events we already have.
        seen_events = yield self.store.have_events_in_timeline(
            set(e.event_id for e in events)
        )

        events = [e for e in events if e.event_id not in seen_events]

        if not events:
            defer.returnValue([])

        event_map = {e.event_id: e for e in events}

        event_ids = set(e.event_id for e in events)

        edges = [
            ev.event_id
            for ev in events
            if set(e_id for e_id, _ in ev.prev_events) - event_ids
        ]

        logger.info(
            "backfill: Got %d events with %d edges",
            len(events), len(edges),
        )

        # For each edge get the current state.

        auth_events = {}
        state_events = {}
        events_to_state = {}
        for e_id in edges:
            state, auth = yield self.replication_layer.get_state_for_room(
                destination=dest,
                room_id=room_id,
                event_id=e_id
            )
            auth_events.update({a.event_id: a for a in auth})
            auth_events.update({s.event_id: s for s in state})
            state_events.update({s.event_id: s for s in state})
            events_to_state[e_id] = state

        required_auth = set(
            a_id
            for event in events + state_events.values() + auth_events.values()
            for a_id, _ in event.auth_events
        )
        auth_events.update({
            e_id: event_map[e_id] for e_id in required_auth if e_id in event_map
        })
        missing_auth = required_auth - set(auth_events)
        failed_to_fetch = set()

        # Try and fetch any missing auth events from both DB and remote servers.
        # We repeatedly do this until we stop finding new auth events.
        while missing_auth - failed_to_fetch:
            logger.info("Missing auth for backfill: %r", missing_auth)
            ret_events = yield self.store.get_events(missing_auth - failed_to_fetch)
            auth_events.update(ret_events)

            required_auth.update(
                a_id for event in ret_events.values() for a_id, _ in event.auth_events
            )
            missing_auth = required_auth - set(auth_events)

            if missing_auth - failed_to_fetch:
                logger.info(
                    "Fetching missing auth for backfill: %r",
                    missing_auth - failed_to_fetch
                )

                results = yield defer.gatherResults(
                    [
                        self.replication_layer.get_pdu(
                            [dest],
                            event_id,
                            outlier=True,
                            timeout=10000,
                        )
                        for event_id in missing_auth - failed_to_fetch
                    ],
                    consumeErrors=True
                ).addErrback(unwrapFirstError)
                auth_events.update({a.event_id: a for a in results})
                required_auth.update(
                    a_id for event in results for a_id, _ in event.auth_events
                )
                missing_auth = required_auth - set(auth_events)

                failed_to_fetch = missing_auth - set(auth_events)

        seen_events = yield self.store.have_events(
            set(auth_events.keys()) | set(state_events.keys())
        )

        ev_infos = []
        for a in auth_events.values():
            if a.event_id in seen_events:
                continue
            a.internal_metadata.outlier = True
            ev_infos.append({
                "event": a,
                "auth_events": {
                    (auth_events[a_id].type, auth_events[a_id].state_key):
                    auth_events[a_id]
                    for a_id, _ in a.auth_events
                    if a_id in auth_events
                }
            })

        for e_id in events_to_state:
            ev_infos.append({
                "event": event_map[e_id],
                "state": events_to_state[e_id],
                "auth_events": {
                    (auth_events[a_id].type, auth_events[a_id].state_key):
                    auth_events[a_id]
                    for a_id, _ in event_map[e_id].auth_events
                    if a_id in auth_events
                }
            })

        yield self._handle_new_events(
            dest, ev_infos,
            backfilled=True,
        )

        events.sort(key=lambda e: e.depth)

        for event in events:
            if event in events_to_state:
                continue

            # We store these one at a time since each event depends on the
            # previous to work out the state.
            # TODO: We can probably do something more clever here.
            yield self._handle_new_event(
                dest, event, backfilled=True,
            )

        defer.returnValue(events)

    @defer.inlineCallbacks
    def maybe_backfill(self, room_id, current_depth):
        """Checks the database to see if we should backfill before paginating,
        and if so do.
        """
        extremities = yield self.store.get_oldest_events_with_depth_in_room(
            room_id
        )

        if not extremities:
            logger.debug("Not backfilling as no extremeties found.")
            return

        # Check if we reached a point where we should start backfilling.
        sorted_extremeties_tuple = sorted(
            extremities.items(),
            key=lambda e: -int(e[1])
        )
        max_depth = sorted_extremeties_tuple[0][1]

        if current_depth > max_depth:
            logger.debug(
                "Not backfilling as we don't need to. %d < %d",
                max_depth, current_depth,
            )
            return

        # Now we need to decide which hosts to hit first.

        # First we try hosts that are already in the room
        # TODO: HEURISTIC ALERT.

        curr_state = yield self.state_handler.get_current_state(room_id)

        def get_domains_from_state(state):
            joined_users = [
                (state_key, int(event.depth))
                for (e_type, state_key), event in state.items()
                if e_type == EventTypes.Member
                and event.membership == Membership.JOIN
            ]

            joined_domains = {}
            for u, d in joined_users:
                try:
                    dom = get_domain_from_id(u)
                    old_d = joined_domains.get(dom)
                    if old_d:
                        joined_domains[dom] = min(d, old_d)
                    else:
                        joined_domains[dom] = d
                except:
                    pass

            return sorted(joined_domains.items(), key=lambda d: d[1])

        curr_domains = get_domains_from_state(curr_state)

        likely_domains = [
            domain for domain, depth in curr_domains
            if domain != self.server_name
        ]

        @defer.inlineCallbacks
        def try_backfill(domains):
            # TODO: Should we try multiple of these at a time?
            for dom in domains:
                try:
                    yield self.backfill(
                        dom, room_id,
                        limit=100,
                        extremities=[e for e in extremities.keys()]
                    )
                    # If this succeeded then we probably already have the
                    # appropriate stuff.
                    # TODO: We can probably do something more intelligent here.
                    defer.returnValue(True)
                except SynapseError as e:
                    logger.info(
                        "Failed to backfill from %s because %s",
                        dom, e,
                    )
                    continue
                except CodeMessageException as e:
                    if 400 <= e.code < 500:
                        raise

                    logger.info(
                        "Failed to backfill from %s because %s",
                        dom, e,
                    )
                    continue
                except NotRetryingDestination as e:
                    logger.info(e.message)
                    continue
                except Exception as e:
                    logger.exception(
                        "Failed to backfill from %s because %s",
                        dom, e,
                    )
                    continue

            defer.returnValue(False)

        success = yield try_backfill(likely_domains)
        if success:
            defer.returnValue(True)

        # Huh, well *those* domains didn't work out. Lets try some domains
        # from the time.

        tried_domains = set(likely_domains)
        tried_domains.add(self.server_name)

        event_ids = list(extremities.keys())

        states = yield defer.gatherResults([
            self.state_handler.resolve_state_groups(room_id, [e])
            for e in event_ids
        ])
        states = dict(zip(event_ids, [s[1] for s in states]))

        for e_id, _ in sorted_extremeties_tuple:
            likely_domains = get_domains_from_state(states[e_id])

            success = yield try_backfill([
                dom for dom in likely_domains
                if dom not in tried_domains
            ])
            if success:
                defer.returnValue(True)

            tried_domains.update(likely_domains)

        defer.returnValue(False)

    @defer.inlineCallbacks
    def send_invite(self, target_host, event):
        """ Sends the invite to the remote server for signing.

        Invites must be signed by the invitee's server before distribution.
        """
        pdu = yield self.replication_layer.send_invite(
            destination=target_host,
            room_id=event.room_id,
            event_id=event.event_id,
            pdu=event
        )

        defer.returnValue(pdu)

    @defer.inlineCallbacks
    def on_event_auth(self, event_id):
        auth = yield self.store.get_auth_chain([event_id])

        for event in auth:
            event.signatures.update(
                compute_event_signature(
                    event,
                    self.hs.hostname,
                    self.hs.config.signing_key[0]
                )
            )

        defer.returnValue([e for e in auth])

    @log_function
    @defer.inlineCallbacks
    def do_invite_join(self, target_hosts, room_id, joinee, content):
        """ Attempts to join the `joinee` to the room `room_id` via the
        server `target_host`.

        This first triggers a /make_join/ request that returns a partial
        event that we can fill out and sign. This is then sent to the
        remote server via /send_join/ which responds with the state at that
        event and the auth_chains.

        We suspend processing of any received events from this room until we
        have finished processing the join.
        """
        logger.debug("Joining %s to %s", joinee, room_id)

        yield self.store.clean_room_for_join(room_id)

        origin, event = yield self._make_and_verify_event(
            target_hosts,
            room_id,
            joinee,
            "join",
            content,
        )

        self.room_queues[room_id] = []
        handled_events = set()

        try:
            event = self._sign_event(event)
            # Try the host we successfully got a response to /make_join/
            # request first.
            try:
                target_hosts.remove(origin)
                target_hosts.insert(0, origin)
            except ValueError:
                pass
            ret = yield self.replication_layer.send_join(target_hosts, event)

            origin = ret["origin"]
            state = ret["state"]
            auth_chain = ret["auth_chain"]
            auth_chain.sort(key=lambda e: e.depth)

            handled_events.update([s.event_id for s in state])
            handled_events.update([a.event_id for a in auth_chain])
            handled_events.add(event.event_id)

            logger.debug("do_invite_join auth_chain: %s", auth_chain)
            logger.debug("do_invite_join state: %s", state)

            logger.debug("do_invite_join event: %s", event)

            try:
                yield self.store.store_room(
                    room_id=room_id,
                    room_creator_user_id="",
                    is_public=False
                )
            except:
                # FIXME
                pass

            event_stream_id, max_stream_id = yield self._persist_auth_tree(
                origin, auth_chain, state, event
            )

            with PreserveLoggingContext():
                self.notifier.on_new_room_event(
                    event, event_stream_id, max_stream_id,
                    extra_users=[joinee]
                )

            logger.debug("Finished joining %s to %s", joinee, room_id)
        finally:
            room_queue = self.room_queues[room_id]
            del self.room_queues[room_id]

            for p, origin in room_queue:
                if p.event_id in handled_events:
                    continue

                try:
                    self.on_receive_pdu(origin, p)
                except:
                    logger.exception("Couldn't handle pdu")

        defer.returnValue(True)

    @defer.inlineCallbacks
    @log_function
    def on_make_join_request(self, room_id, user_id):
        """ We've received a /make_join/ request, so we create a partial
        join event for the room and return that. We do *not* persist or
        process it until the other server has signed it and sent it back.
        """
        event_content = {"membership": Membership.JOIN}

        builder = self.event_builder_factory.new({
            "type": EventTypes.Member,
            "content": event_content,
            "room_id": room_id,
            "sender": user_id,
            "state_key": user_id,
        })

        try:
            message_handler = self.hs.get_handlers().message_handler
            event, context = yield message_handler._create_new_client_event(
                builder=builder,
            )
        except AuthError as e:
            logger.warn("Failed to create join %r because %s", event, e)
            raise e

        # The remote hasn't signed it yet, obviously. We'll do the full checks
        # when we get the event back in `on_send_join_request`
        self.auth.check(event, auth_events=context.current_state, do_sig_check=False)

        defer.returnValue(event)

    @defer.inlineCallbacks
    @log_function
    def on_send_join_request(self, origin, pdu):
        """ We have received a join event for a room. Fully process it and
        respond with the current state and auth chains.
        """
        event = pdu

        logger.debug(
            "on_send_join_request: Got event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        event.internal_metadata.outlier = False

        context, event_stream_id, max_stream_id = yield self._handle_new_event(
            origin, event
        )

        logger.debug(
            "on_send_join_request: After _handle_new_event: %s, sigs: %s",
            event.event_id,
            event.signatures,
        )

        extra_users = []
        if event.type == EventTypes.Member:
            target_user_id = event.state_key
            target_user = UserID.from_string(target_user_id)
            extra_users.append(target_user)

        with PreserveLoggingContext():
            self.notifier.on_new_room_event(
                event, event_stream_id, max_stream_id, extra_users=extra_users
            )

        if event.type == EventTypes.Member:
            if event.content["membership"] == Membership.JOIN:
                user = UserID.from_string(event.state_key)
                yield user_joined_room(self.distributor, user, event.room_id)

        new_pdu = event

        destinations = set()

        for k, s in context.current_state.items():
            try:
                if k[0] == EventTypes.Member:
                    if s.content["membership"] == Membership.JOIN:
                        destinations.add(get_domain_from_id(s.state_key))
            except:
                logger.warn(
                    "Failed to get destination from event %s", s.event_id
                )

        destinations.discard(origin)

        logger.debug(
            "on_send_join_request: Sending event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        self.replication_layer.send_pdu(new_pdu, destinations)

        state_ids = [e.event_id for e in context.current_state.values()]
        auth_chain = yield self.store.get_auth_chain(set(
            [event.event_id] + state_ids
        ))

        defer.returnValue({
            "state": context.current_state.values(),
            "auth_chain": auth_chain,
        })

    @defer.inlineCallbacks
    def on_invite_request(self, origin, pdu):
        """ We've got an invite event. Process and persist it. Sign it.

        Respond with the now signed event.
        """
        event = pdu

        event.internal_metadata.outlier = True
        event.internal_metadata.invite_from_remote = True

        event.signatures.update(
            compute_event_signature(
                event,
                self.hs.hostname,
                self.hs.config.signing_key[0]
            )
        )

        context = yield self.state_handler.compute_event_context(event)

        event_stream_id, max_stream_id = yield self.store.persist_event(
            event,
            context=context,
        )

        target_user = UserID.from_string(event.state_key)
        with PreserveLoggingContext():
            self.notifier.on_new_room_event(
                event, event_stream_id, max_stream_id,
                extra_users=[target_user],
            )

        defer.returnValue(event)

    @defer.inlineCallbacks
    def do_remotely_reject_invite(self, target_hosts, room_id, user_id):
        try:
            origin, event = yield self._make_and_verify_event(
                target_hosts,
                room_id,
                user_id,
                "leave"
            )
            signed_event = self._sign_event(event)
        except SynapseError:
            raise
        except CodeMessageException as e:
            logger.warn("Failed to reject invite: %s", e)
            raise SynapseError(500, "Failed to reject invite")

        # Try the host we successfully got a response to /make_join/
        # request first.
        try:
            target_hosts.remove(origin)
            target_hosts.insert(0, origin)
        except ValueError:
            pass

        try:
            yield self.replication_layer.send_leave(
                target_hosts,
                signed_event
            )
        except SynapseError:
            raise
        except CodeMessageException as e:
            logger.warn("Failed to reject invite: %s", e)
            raise SynapseError(500, "Failed to reject invite")

        context = yield self.state_handler.compute_event_context(event)

        event_stream_id, max_stream_id = yield self.store.persist_event(
            event,
            context=context,
        )

        target_user = UserID.from_string(event.state_key)
        self.notifier.on_new_room_event(
            event, event_stream_id, max_stream_id,
            extra_users=[target_user],
        )

        defer.returnValue(event)

    @defer.inlineCallbacks
    def _make_and_verify_event(self, target_hosts, room_id, user_id, membership,
                               content={},):
        origin, pdu = yield self.replication_layer.make_membership_event(
            target_hosts,
            room_id,
            user_id,
            membership,
            content,
        )

        logger.debug("Got response to make_%s: %s", membership, pdu)

        event = pdu

        # We should assert some things.
        # FIXME: Do this in a nicer way
        assert(event.type == EventTypes.Member)
        assert(event.user_id == user_id)
        assert(event.state_key == user_id)
        assert(event.room_id == room_id)
        defer.returnValue((origin, event))

    def _sign_event(self, event):
        event.internal_metadata.outlier = False

        builder = self.event_builder_factory.new(
            unfreeze(event.get_pdu_json())
        )

        builder.event_id = self.event_builder_factory.create_event_id()
        builder.origin = self.hs.hostname

        if not hasattr(event, "signatures"):
            builder.signatures = {}

        add_hashes_and_signatures(
            builder,
            self.hs.hostname,
            self.hs.config.signing_key[0],
        )

        return builder.build()

    @defer.inlineCallbacks
    @log_function
    def on_make_leave_request(self, room_id, user_id):
        """ We've received a /make_leave/ request, so we create a partial
        join event for the room and return that. We do *not* persist or
        process it until the other server has signed it and sent it back.
        """
        builder = self.event_builder_factory.new({
            "type": EventTypes.Member,
            "content": {"membership": Membership.LEAVE},
            "room_id": room_id,
            "sender": user_id,
            "state_key": user_id,
        })

        message_handler = self.hs.get_handlers().message_handler
        event, context = yield message_handler._create_new_client_event(
            builder=builder,
        )

        try:
            # The remote hasn't signed it yet, obviously. We'll do the full checks
            # when we get the event back in `on_send_leave_request`
            self.auth.check(event, auth_events=context.current_state, do_sig_check=False)
        except AuthError as e:
            logger.warn("Failed to create new leave %r because %s", event, e)
            raise e

        defer.returnValue(event)

    @defer.inlineCallbacks
    @log_function
    def on_send_leave_request(self, origin, pdu):
        """ We have received a leave event for a room. Fully process it."""
        event = pdu

        logger.debug(
            "on_send_leave_request: Got event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        event.internal_metadata.outlier = False

        context, event_stream_id, max_stream_id = yield self._handle_new_event(
            origin, event
        )

        logger.debug(
            "on_send_leave_request: After _handle_new_event: %s, sigs: %s",
            event.event_id,
            event.signatures,
        )

        extra_users = []
        if event.type == EventTypes.Member:
            target_user_id = event.state_key
            target_user = UserID.from_string(target_user_id)
            extra_users.append(target_user)

        with PreserveLoggingContext():
            self.notifier.on_new_room_event(
                event, event_stream_id, max_stream_id, extra_users=extra_users
            )

        new_pdu = event

        destinations = set()

        for k, s in context.current_state.items():
            try:
                if k[0] == EventTypes.Member:
                    if s.content["membership"] == Membership.LEAVE:
                        destinations.add(get_domain_from_id(s.state_key))
            except:
                logger.warn(
                    "Failed to get destination from event %s", s.event_id
                )

        destinations.discard(origin)

        logger.debug(
            "on_send_leave_request: Sending event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        self.replication_layer.send_pdu(new_pdu, destinations)

        defer.returnValue(None)

    @defer.inlineCallbacks
    def get_state_for_pdu(self, room_id, event_id):
        yield run_on_reactor()

        state_groups = yield self.store.get_state_groups(
            room_id, [event_id]
        )

        if state_groups:
            _, state = state_groups.items().pop()
            results = {
                (e.type, e.state_key): e for e in state
            }

            event = yield self.store.get_event(event_id)
            if event and event.is_state():
                # Get previous state
                if "replaces_state" in event.unsigned:
                    prev_id = event.unsigned["replaces_state"]
                    if prev_id != event.event_id:
                        prev_event = yield self.store.get_event(prev_id)
                        results[(event.type, event.state_key)] = prev_event
                else:
                    del results[(event.type, event.state_key)]

            res = results.values()
            for event in res:
                # We sign these again because there was a bug where we
                # incorrectly signed things the first time round
                if self.hs.is_mine_id(event.event_id):
                    event.signatures.update(
                        compute_event_signature(
                            event,
                            self.hs.hostname,
                            self.hs.config.signing_key[0]
                        )
                    )

            defer.returnValue(res)
        else:
            defer.returnValue([])

    @defer.inlineCallbacks
    @log_function
    def on_backfill_request(self, origin, room_id, pdu_list, limit):
        in_room = yield self.auth.check_host_in_room(room_id, origin)
        if not in_room:
            raise AuthError(403, "Host not in room.")

        events = yield self.store.get_backfill_events(
            room_id,
            pdu_list,
            limit
        )

        events = yield self._filter_events_for_server(origin, room_id, events)

        defer.returnValue(events)

    @defer.inlineCallbacks
    @log_function
    def get_persisted_pdu(self, origin, event_id, do_auth=True):
        """ Get a PDU from the database with given origin and id.

        Returns:
            Deferred: Results in a `Pdu`.
        """
        event = yield self.store.get_event(
            event_id,
            allow_none=True,
            allow_rejected=True,
        )

        if event:
            if self.hs.is_mine_id(event.event_id):
                # FIXME: This is a temporary work around where we occasionally
                # return events slightly differently than when they were
                # originally signed
                event.signatures.update(
                    compute_event_signature(
                        event,
                        self.hs.hostname,
                        self.hs.config.signing_key[0]
                    )
                )

            if do_auth:
                in_room = yield self.auth.check_host_in_room(
                    event.room_id,
                    origin
                )
                if not in_room:
                    raise AuthError(403, "Host not in room.")

                events = yield self._filter_events_for_server(
                    origin, event.room_id, [event]
                )

                event = events[0]

            defer.returnValue(event)
        else:
            defer.returnValue(None)

    @log_function
    def get_min_depth_for_context(self, context):
        return self.store.get_min_depth(context)

    @defer.inlineCallbacks
    @log_function
    def _handle_new_event(self, origin, event, state=None, auth_events=None,
                          backfilled=False):
        context = yield self._prep_event(
            origin, event,
            state=state,
            auth_events=auth_events,
        )

        if not event.internal_metadata.is_outlier():
            action_generator = ActionGenerator(self.hs)
            yield action_generator.handle_push_actions_for_event(
                event, context
            )

        event_stream_id, max_stream_id = yield self.store.persist_event(
            event,
            context=context,
            backfilled=backfilled,
        )

        if not backfilled:
            # this intentionally does not yield: we don't care about the result
            # and don't need to wait for it.
            preserve_fn(self.hs.get_pusherpool().on_new_notifications)(
                event_stream_id, max_stream_id
            )

        defer.returnValue((context, event_stream_id, max_stream_id))

    @defer.inlineCallbacks
    def _handle_new_events(self, origin, event_infos, backfilled=False):
        """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.
        """
        contexts = yield defer.gatherResults(
            [
                self._prep_event(
                    origin,
                    ev_info["event"],
                    state=ev_info.get("state"),
                    auth_events=ev_info.get("auth_events"),
                )
                for ev_info in event_infos
            ]
        )

        yield self.store.persist_events(
            [
                (ev_info["event"], context)
                for ev_info, context in itertools.izip(event_infos, contexts)
            ],
            backfilled=backfilled,
        )

    @defer.inlineCallbacks
    def _persist_auth_tree(self, origin, auth_events, state, event):
        """Checks the auth chain is valid (and passes auth checks) for the
        state and event. Then persists the auth chain and state atomically.
        Persists the event seperately.

        Will attempt to fetch missing auth events.

        Args:
            origin (str): Where the events came from
            auth_events (list)
            state (list)
            event (Event)

        Returns:
            2-tuple of (event_stream_id, max_stream_id) from the persist_event
            call for `event`
        """
        events_to_context = {}
        for e in itertools.chain(auth_events, state):
            e.internal_metadata.outlier = True
            ctx = yield self.state_handler.compute_event_context(e)
            events_to_context[e.event_id] = ctx

        event_map = {
            e.event_id: e
            for e in itertools.chain(auth_events, state, [event])
        }

        create_event = None
        for e in auth_events:
            if (e.type, e.state_key) == (EventTypes.Create, ""):
                create_event = e
                break

        missing_auth_events = set()
        for e in itertools.chain(auth_events, state, [event]):
            for e_id, _ in e.auth_events:
                if e_id not in event_map:
                    missing_auth_events.add(e_id)

        for e_id in missing_auth_events:
            m_ev = yield self.replication_layer.get_pdu(
                [origin],
                e_id,
                outlier=True,
                timeout=10000,
            )
            if m_ev and m_ev.event_id == e_id:
                event_map[e_id] = m_ev
            else:
                logger.info("Failed to find auth event %r", e_id)

        for e in itertools.chain(auth_events, state, [event]):
            auth_for_e = {
                (event_map[e_id].type, event_map[e_id].state_key): event_map[e_id]
                for e_id, _ in e.auth_events
                if e_id in event_map
            }
            if create_event:
                auth_for_e[(EventTypes.Create, "")] = create_event

            try:
                self.auth.check(e, auth_events=auth_for_e)
            except SynapseError as err:
                # we may get SynapseErrors here as well as AuthErrors. For
                # instance, there are a couple of (ancient) events in some
                # rooms whose senders do not have the correct sigil; these
                # cause SynapseErrors in auth.check. We don't want to give up
                # the attempt to federate altogether in such cases.

                logger.warn(
                    "Rejecting %s because %s",
                    e.event_id, err.msg
                )

                if e == event:
                    raise
                events_to_context[e.event_id].rejected = RejectedReason.AUTH_ERROR

        yield self.store.persist_events(
            [
                (e, events_to_context[e.event_id])
                for e in itertools.chain(auth_events, state)
            ],
        )

        new_event_context = yield self.state_handler.compute_event_context(
            event, old_state=state
        )

        event_stream_id, max_stream_id = yield self.store.persist_event(
            event, new_event_context,
            current_state=state,
        )

        defer.returnValue((event_stream_id, max_stream_id))

    @defer.inlineCallbacks
    def _prep_event(self, origin, event, state=None, auth_events=None):

        context = yield self.state_handler.compute_event_context(
            event, old_state=state,
        )

        if not auth_events:
            auth_events = context.current_state

        # This is a hack to fix some old rooms where the initial join event
        # didn't reference the create event in its auth events.
        if event.type == EventTypes.Member and not event.auth_events:
            if len(event.prev_events) == 1 and event.depth < 5:
                c = yield self.store.get_event(
                    event.prev_events[0][0],
                    allow_none=True,
                )
                if c and c.type == EventTypes.Create:
                    auth_events[(c.type, c.state_key)] = c

        try:
            yield self.do_auth(
                origin, event, context, auth_events=auth_events
            )
        except AuthError as e:
            logger.warn(
                "Rejecting %s because %s",
                event.event_id, e.msg
            )

            context.rejected = RejectedReason.AUTH_ERROR

        if event.type == EventTypes.GuestAccess:
            full_context = yield self.store.get_current_state(room_id=event.room_id)
            yield self.maybe_kick_guest_users(event, full_context)

        defer.returnValue(context)

    @defer.inlineCallbacks
    def on_query_auth(self, origin, event_id, remote_auth_chain, rejects,
                      missing):
        # Just go through and process each event in `remote_auth_chain`. We
        # don't want to fall into the trap of `missing` being wrong.
        for e in remote_auth_chain:
            try:
                yield self._handle_new_event(origin, e)
            except AuthError:
                pass

        # Now get the current auth_chain for the event.
        local_auth_chain = yield self.store.get_auth_chain([event_id])

        # TODO: Check if we would now reject event_id. If so we need to tell
        # everyone.

        ret = yield self.construct_auth_difference(
            local_auth_chain, remote_auth_chain
        )

        for event in ret["auth_chain"]:
            event.signatures.update(
                compute_event_signature(
                    event,
                    self.hs.hostname,
                    self.hs.config.signing_key[0]
                )
            )

        logger.debug("on_query_auth returning: %s", ret)

        defer.returnValue(ret)

    @defer.inlineCallbacks
    def on_get_missing_events(self, origin, room_id, earliest_events,
                              latest_events, limit, min_depth):
        in_room = yield self.auth.check_host_in_room(
            room_id,
            origin
        )
        if not in_room:
            raise AuthError(403, "Host not in room.")

        limit = min(limit, 20)
        min_depth = max(min_depth, 0)

        missing_events = yield self.store.get_missing_events(
            room_id=room_id,
            earliest_events=earliest_events,
            latest_events=latest_events,
            limit=limit,
            min_depth=min_depth,
        )

        defer.returnValue(missing_events)

    @defer.inlineCallbacks
    @log_function
    def do_auth(self, origin, event, context, auth_events):
        # Check if we have all the auth events.
        current_state = set(e.event_id for e in auth_events.values())
        event_auth_events = set(e_id for e_id, _ in event.auth_events)

        if event_auth_events - current_state:
            have_events = yield self.store.have_events(
                event_auth_events - current_state
            )
        else:
            have_events = {}

        have_events.update({
            e.event_id: ""
            for e in auth_events.values()
        })

        seen_events = set(have_events.keys())

        missing_auth = event_auth_events - seen_events - current_state

        if missing_auth:
            logger.info("Missing auth: %s", missing_auth)
            # If we don't have all the auth events, we need to get them.
            try:
                remote_auth_chain = yield self.replication_layer.get_event_auth(
                    origin, event.room_id, event.event_id
                )

                seen_remotes = yield self.store.have_events(
                    [e.event_id for e in remote_auth_chain]
                )

                for e in remote_auth_chain:
                    if e.event_id in seen_remotes.keys():
                        continue

                    if e.event_id == event.event_id:
                        continue

                    try:
                        auth_ids = [e_id for e_id, _ in e.auth_events]
                        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(
                            "do_auth %s missing_auth: %s",
                            event.event_id, e.event_id
                        )
                        yield self._handle_new_event(
                            origin, e, auth_events=auth
                        )

                        if e.event_id in event_auth_events:
                            auth_events[(e.type, e.state_key)] = e
                    except AuthError:
                        pass

                have_events = yield self.store.have_events(
                    [e_id for e_id, _ in event.auth_events]
                )
                seen_events = set(have_events.keys())
            except:
                # FIXME:
                logger.exception("Failed to get auth chain")

        # FIXME: Assumes we have and stored all the state for all the
        # prev_events
        current_state = set(e.event_id for e in auth_events.values())
        different_auth = event_auth_events - current_state

        if different_auth and not event.internal_metadata.is_outlier():
            # Do auth conflict res.
            logger.info("Different auth: %s", different_auth)

            different_events = yield defer.gatherResults(
                [
                    self.store.get_event(
                        d,
                        allow_none=True,
                        allow_rejected=False,
                    )
                    for d in different_auth
                    if d in have_events and not have_events[d]
                ],
                consumeErrors=True
            ).addErrback(unwrapFirstError)

            if different_events:
                local_view = dict(auth_events)
                remote_view = dict(auth_events)
                remote_view.update({
                    (d.type, d.state_key): d for d in different_events if d
                })

                new_state, prev_state = self.state_handler.resolve_events(
                    [local_view.values(), remote_view.values()],
                    event
                )

                auth_events.update(new_state)

                current_state = set(e.event_id for e in auth_events.values())
                different_auth = event_auth_events - current_state

                context.current_state.update(auth_events)
                context.state_group = None

        if different_auth and not event.internal_metadata.is_outlier():
            logger.info("Different auth after resolution: %s", different_auth)

            # Only do auth resolution if we have something new to say.
            # We can't rove an auth failure.
            do_resolution = False

            provable = [
                RejectedReason.NOT_ANCESTOR, RejectedReason.NOT_ANCESTOR,
            ]

            for e_id in different_auth:
                if e_id in have_events:
                    if have_events[e_id] in provable:
                        do_resolution = True
                        break

            if do_resolution:
                # 1. Get what we think is the auth chain.
                auth_ids = self.auth.compute_auth_events(
                    event, context.current_state
                )
                local_auth_chain = yield self.store.get_auth_chain(auth_ids)

                try:
                    # 2. Get remote difference.
                    result = yield self.replication_layer.query_auth(
                        origin,
                        event.room_id,
                        event.event_id,
                        local_auth_chain,
                    )

                    seen_remotes = yield self.store.have_events(
                        [e.event_id for e in result["auth_chain"]]
                    )

                    # 3. Process any remote auth chain events we haven't seen.
                    for ev in result["auth_chain"]:
                        if ev.event_id in seen_remotes.keys():
                            continue

                        if ev.event_id == event.event_id:
                            continue

                        try:
                            auth_ids = [e_id for e_id, _ in ev.auth_events]
                            auth = {
                                (e.type, e.state_key): e
                                for e in result["auth_chain"]
                                if e.event_id in auth_ids
                                or event.type == EventTypes.Create
                            }
                            ev.internal_metadata.outlier = True

                            logger.debug(
                                "do_auth %s different_auth: %s",
                                event.event_id, e.event_id
                            )

                            yield self._handle_new_event(
                                origin, ev, auth_events=auth
                            )

                            if ev.event_id in event_auth_events:
                                auth_events[(ev.type, ev.state_key)] = ev
                        except AuthError:
                            pass

                except:
                    # FIXME:
                    logger.exception("Failed to query auth chain")

                # 4. Look at rejects and their proofs.
                # TODO.

                context.current_state.update(auth_events)
                context.state_group = None

        try:
            self.auth.check(event, auth_events=auth_events)
        except AuthError as e:
            logger.warn("Failed auth resolution for %r because %s", event, e)
            raise e

    @defer.inlineCallbacks
    def construct_auth_difference(self, local_auth, remote_auth):
        """ Given a local and remote auth chain, find the differences. This
        assumes that we have already processed all events in remote_auth

        Params:
            local_auth (list)
            remote_auth (list)

        Returns:
            dict
        """

        logger.debug("construct_auth_difference Start!")

        # TODO: Make sure we are OK with local_auth or remote_auth having more
        # auth events in them than strictly necessary.

        def sort_fun(ev):
            return ev.depth, ev.event_id

        logger.debug("construct_auth_difference after sort_fun!")

        # We find the differences by starting at the "bottom" of each list
        # and iterating up on both lists. The lists are ordered by depth and
        # then event_id, we iterate up both lists until we find the event ids
        # don't match. Then we look at depth/event_id to see which side is
        # missing that event, and iterate only up that list. Repeat.

        remote_list = list(remote_auth)
        remote_list.sort(key=sort_fun)

        local_list = list(local_auth)
        local_list.sort(key=sort_fun)

        local_iter = iter(local_list)
        remote_iter = iter(remote_list)

        logger.debug("construct_auth_difference before get_next!")

        def get_next(it, opt=None):
            try:
                return it.next()
            except:
                return opt

        current_local = get_next(local_iter)
        current_remote = get_next(remote_iter)

        logger.debug("construct_auth_difference before while")

        missing_remotes = []
        missing_locals = []
        while current_local or current_remote:
            if current_remote is None:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)
                continue

            if current_local is None:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

            if current_local.event_id == current_remote.event_id:
                current_local = get_next(local_iter)
                current_remote = get_next(remote_iter)
                continue

            if current_local.depth < current_remote.depth:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)
                continue

            if current_local.depth > current_remote.depth:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

            # They have the same depth, so we fall back to the event_id order
            if current_local.event_id < current_remote.event_id:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)

            if current_local.event_id > current_remote.event_id:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

        logger.debug("construct_auth_difference after while")

        # missing locals should be sent to the server
        # We should find why we are missing remotes, as they will have been
        # rejected.

        # Remove events from missing_remotes if they are referencing a missing
        # remote. We only care about the "root" rejected ones.
        missing_remote_ids = [e.event_id for e in missing_remotes]
        base_remote_rejected = list(missing_remotes)
        for e in missing_remotes:
            for e_id, _ in e.auth_events:
                if e_id in missing_remote_ids:
                    try:
                        base_remote_rejected.remove(e)
                    except ValueError:
                        pass

        reason_map = {}

        for e in base_remote_rejected:
            reason = yield self.store.get_rejection_reason(e.event_id)
            if reason is None:
                # TODO: e is not in the current state, so we should
                # construct some proof of that.
                continue

            reason_map[e.event_id] = reason

            if reason == RejectedReason.AUTH_ERROR:
                pass
            elif reason == RejectedReason.REPLACED:
                # TODO: Get proof
                pass
            elif reason == RejectedReason.NOT_ANCESTOR:
                # TODO: Get proof.
                pass

        logger.debug("construct_auth_difference returning")

        defer.returnValue({
            "auth_chain": local_auth,
            "rejects": {
                e.event_id: {
                    "reason": reason_map[e.event_id],
                    "proof": None,
                }
                for e in base_remote_rejected
            },
            "missing": [e.event_id for e in missing_locals],
        })

    @defer.inlineCallbacks
    @log_function
    def exchange_third_party_invite(
            self,
            sender_user_id,
            target_user_id,
            room_id,
            signed,
    ):
        third_party_invite = {
            "signed": signed,
        }

        event_dict = {
            "type": EventTypes.Member,
            "content": {
                "membership": Membership.INVITE,
                "third_party_invite": third_party_invite,
            },
            "room_id": room_id,
            "sender": sender_user_id,
            "state_key": target_user_id,
        }

        if (yield self.auth.check_host_in_room(room_id, self.hs.hostname)):
            builder = self.event_builder_factory.new(event_dict)
            EventValidator().validate_new(builder)
            message_handler = self.hs.get_handlers().message_handler
            event, context = yield message_handler._create_new_client_event(
                builder=builder
            )

            event, context = yield self.add_display_name_to_third_party_invite(
                event_dict, event, context
            )

            try:
                self.auth.check(event, context.current_state)
            except AuthError as e:
                logger.warn("Denying new third party invite %r because %s", event, e)
                raise e

            yield self._check_signature(event, auth_events=context.current_state)
            member_handler = self.hs.get_handlers().room_member_handler
            yield member_handler.send_membership_event(None, event, context)
        else:
            destinations = set(x.split(":", 1)[-1] for x in (sender_user_id, room_id))
            yield self.replication_layer.forward_third_party_invite(
                destinations,
                room_id,
                event_dict,
            )

    @defer.inlineCallbacks
    @log_function
    def on_exchange_third_party_invite_request(self, origin, room_id, event_dict):
        builder = self.event_builder_factory.new(event_dict)

        message_handler = self.hs.get_handlers().message_handler
        event, context = yield message_handler._create_new_client_event(
            builder=builder,
        )

        event, context = yield self.add_display_name_to_third_party_invite(
            event_dict, event, context
        )

        try:
            self.auth.check(event, auth_events=context.current_state)
        except AuthError as e:
            logger.warn("Denying third party invite %r because %s", event, e)
            raise e
        yield self._check_signature(event, auth_events=context.current_state)

        returned_invite = yield self.send_invite(origin, event)
        # TODO: Make sure the signatures actually are correct.
        event.signatures.update(returned_invite.signatures)
        member_handler = self.hs.get_handlers().room_member_handler
        yield member_handler.send_membership_event(None, event, context)

    @defer.inlineCallbacks
    def add_display_name_to_third_party_invite(self, event_dict, event, context):
        key = (
            EventTypes.ThirdPartyInvite,
            event.content["third_party_invite"]["signed"]["token"]
        )
        original_invite = context.current_state.get(key)
        if not original_invite:
            logger.info(
                "Could not find invite event for third_party_invite - "
                "discarding: %s" % (event_dict,)
            )
            return

        display_name = original_invite.content["display_name"]
        event_dict["content"]["third_party_invite"]["display_name"] = display_name
        builder = self.event_builder_factory.new(event_dict)
        EventValidator().validate_new(builder)
        message_handler = self.hs.get_handlers().message_handler
        event, context = yield message_handler._create_new_client_event(builder=builder)
        defer.returnValue((event, context))

    @defer.inlineCallbacks
    def _check_signature(self, event, auth_events):
        """
        Checks that the signature in the event is consistent with its invite.

        Args:
            event (Event): The m.room.member event to check
            auth_events (dict<(event type, state_key), event>):

        Raises:
            AuthError: if signature didn't match any keys, or key has been
                revoked,
            SynapseError: if a transient error meant a key couldn't be checked
                for revocation.
        """
        signed = event.content["third_party_invite"]["signed"]
        token = signed["token"]

        invite_event = auth_events.get(
            (EventTypes.ThirdPartyInvite, token,)
        )

        if not invite_event:
            raise AuthError(403, "Could not find invite")

        last_exception = None
        for public_key_object in self.hs.get_auth().get_public_keys(invite_event):
            try:
                for server, signature_block in signed["signatures"].items():
                    for key_name, encoded_signature in signature_block.items():
                        if not key_name.startswith("ed25519:"):
                            continue

                        public_key = public_key_object["public_key"]
                        verify_key = decode_verify_key_bytes(
                            key_name,
                            decode_base64(public_key)
                        )
                        verify_signed_json(signed, server, verify_key)
                        if "key_validity_url" in public_key_object:
                            yield self._check_key_revocation(
                                public_key,
                                public_key_object["key_validity_url"]
                            )
                        return
            except Exception as e:
                last_exception = e
        raise last_exception

    @defer.inlineCallbacks
    def _check_key_revocation(self, public_key, url):
        """
        Checks whether public_key has been revoked.

        Args:
            public_key (str): base-64 encoded public key.
            url (str): Key revocation URL.

        Raises:
            AuthError: if they key has been revoked.
            SynapseError: if a transient error meant a key couldn't be checked
                for revocation.
        """
        try:
            response = yield self.hs.get_simple_http_client().get_json(
                url,
                {"public_key": public_key}
            )
        except Exception:
            raise SynapseError(
                502,
                "Third party certificate could not be checked"
            )
        if "valid" not in response or not response["valid"]:
            raise AuthError(403, "Third party certificate was invalid")