forked-synapse/synapse/handlers/device.py

# Copyright 2016 OpenMarket Ltd
# Copyright 2019 New Vector Ltd
# Copyright 2019,2020 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
from typing import (
    TYPE_CHECKING,
    Any,
    Collection,
    Dict,
    Iterable,
    List,
    Mapping,
    Optional,
    Set,
    Tuple,
)

from synapse.api import errors
from synapse.api.constants import EventTypes
from synapse.api.errors import (
    Codes,
    FederationDeniedError,
    HttpResponseException,
    RequestSendFailed,
    SynapseError,
)
from synapse.logging.opentracing import log_kv, set_tag, trace
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.types import (
    JsonDict,
    StreamToken,
    UserID,
    get_domain_from_id,
    get_verify_key_from_cross_signing_key,
)
from synapse.util import stringutils
from synapse.util.async_helpers import Linearizer
from synapse.util.caches.expiringcache import ExpiringCache
from synapse.util.metrics import measure_func
from synapse.util.retryutils import NotRetryingDestination

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

MAX_DEVICE_DISPLAY_NAME_LEN = 100


class DeviceWorkerHandler:
    def __init__(self, hs: "HomeServer"):
        self.clock = hs.get_clock()
        self.hs = hs
        self.store = hs.get_datastore()
        self.notifier = hs.get_notifier()
        self.state = hs.get_state_handler()
        self.state_store = hs.get_storage().state
        self._auth_handler = hs.get_auth_handler()
        self.server_name = hs.hostname

    @trace
    async def get_devices_by_user(self, user_id: str) -> List[JsonDict]:
        """
        Retrieve the given user's devices

        Args:
            user_id: The user ID to query for devices.
        Returns:
            info on each device
        """

        set_tag("user_id", user_id)
        device_map = await self.store.get_devices_by_user(user_id)

        ips = await self.store.get_last_client_ip_by_device(user_id, device_id=None)

        devices = list(device_map.values())
        for device in devices:
            _update_device_from_client_ips(device, ips)

        log_kv(device_map)
        return devices

    @trace
    async def get_device(self, user_id: str, device_id: str) -> JsonDict:
        """Retrieve the given device

        Args:
            user_id: The user to get the device from
            device_id: The device to fetch.

        Returns:
            info on the device
        Raises:
            errors.NotFoundError: if the device was not found
        """
        device = await self.store.get_device(user_id, device_id)
        if device is None:
            raise errors.NotFoundError()

        ips = await self.store.get_last_client_ip_by_device(user_id, device_id)
        _update_device_from_client_ips(device, ips)

        set_tag("device", device)
        set_tag("ips", ips)

        return device

    @trace
    @measure_func("device.get_user_ids_changed")
    async def get_user_ids_changed(
        self, user_id: str, from_token: StreamToken
    ) -> JsonDict:
        """Get list of users that have had the devices updated, or have newly
        joined a room, that `user_id` may be interested in.
        """

        set_tag("user_id", user_id)
        set_tag("from_token", from_token)
        now_room_key = self.store.get_room_max_token()

        room_ids = await self.store.get_rooms_for_user(user_id)

        # First we check if any devices have changed for users that we share
        # rooms with.
        users_who_share_room = await self.store.get_users_who_share_room_with_user(
            user_id
        )

        tracked_users = set(users_who_share_room)

        # Always tell the user about their own devices
        tracked_users.add(user_id)

        changed = await self.store.get_users_whose_devices_changed(
            from_token.device_list_key, tracked_users
        )

        # Then work out if any users have since joined
        rooms_changed = self.store.get_rooms_that_changed(room_ids, from_token.room_key)

        member_events = await self.store.get_membership_changes_for_user(
            user_id, from_token.room_key, now_room_key
        )
        rooms_changed.update(event.room_id for event in member_events)

        stream_ordering = from_token.room_key.stream

        possibly_changed = set(changed)
        possibly_left = set()
        for room_id in rooms_changed:
            current_state_ids = await self.store.get_current_state_ids(room_id)

            # The user may have left the room
            # TODO: Check if they actually did or if we were just invited.
            if room_id not in room_ids:
                for etype, state_key in current_state_ids.keys():
                    if etype != EventTypes.Member:
                        continue
                    possibly_left.add(state_key)
                continue

            # Fetch the current state at the time.
            try:
                event_ids = await self.store.get_forward_extremities_for_room_at_stream_ordering(
                    room_id, stream_ordering=stream_ordering
                )
            except errors.StoreError:
                # we have purged the stream_ordering index since the stream
                # ordering: treat it the same as a new room
                event_ids = []

            # special-case for an empty prev state: include all members
            # in the changed list
            if not event_ids:
                log_kv(
                    {"event": "encountered empty previous state", "room_id": room_id}
                )
                for etype, state_key in current_state_ids.keys():
                    if etype != EventTypes.Member:
                        continue
                    possibly_changed.add(state_key)
                continue

            current_member_id = current_state_ids.get((EventTypes.Member, user_id))
            if not current_member_id:
                continue

            # mapping from event_id -> state_dict
            prev_state_ids = await self.state_store.get_state_ids_for_events(event_ids)

            # Check if we've joined the room? If so we just blindly add all the users to
            # the "possibly changed" users.
            for state_dict in prev_state_ids.values():
                member_event = state_dict.get((EventTypes.Member, user_id), None)
                if not member_event or member_event != current_member_id:
                    for etype, state_key in current_state_ids.keys():
                        if etype != EventTypes.Member:
                            continue
                        possibly_changed.add(state_key)
                    break

            # If there has been any change in membership, include them in the
            # possibly changed list. We'll check if they are joined below,
            # and we're not toooo worried about spuriously adding users.
            for key, event_id in current_state_ids.items():
                etype, state_key = key
                if etype != EventTypes.Member:
                    continue

                # check if this member has changed since any of the extremities
                # at the stream_ordering, and add them to the list if so.
                for state_dict in prev_state_ids.values():
                    prev_event_id = state_dict.get(key, None)
                    if not prev_event_id or prev_event_id != event_id:
                        if state_key != user_id:
                            possibly_changed.add(state_key)
                        break

        if possibly_changed or possibly_left:
            # Take the intersection of the users whose devices may have changed
            # and those that actually still share a room with the user
            possibly_joined = possibly_changed & users_who_share_room
            possibly_left = (possibly_changed | possibly_left) - users_who_share_room
        else:
            possibly_joined = set()
            possibly_left = set()

        result = {"changed": list(possibly_joined), "left": list(possibly_left)}

        log_kv(result)

        return result

    async def on_federation_query_user_devices(self, user_id: str) -> JsonDict:
        stream_id, devices = await self.store.get_e2e_device_keys_for_federation_query(
            user_id
        )
        master_key = await self.store.get_e2e_cross_signing_key(user_id, "master")
        self_signing_key = await self.store.get_e2e_cross_signing_key(
            user_id, "self_signing"
        )

        return {
            "user_id": user_id,
            "stream_id": stream_id,
            "devices": devices,
            "master_key": master_key,
            "self_signing_key": self_signing_key,
        }


class DeviceHandler(DeviceWorkerHandler):
    def __init__(self, hs: "HomeServer"):
        super().__init__(hs)

        self.federation_sender = hs.get_federation_sender()

        self.device_list_updater = DeviceListUpdater(hs, self)

        federation_registry = hs.get_federation_registry()

        federation_registry.register_edu_handler(
            "m.device_list_update", self.device_list_updater.incoming_device_list_update
        )

        hs.get_distributor().observe("user_left_room", self.user_left_room)

    def _check_device_name_length(self, name: Optional[str]) -> None:
        """
        Checks whether a device name is longer than the maximum allowed length.

        Args:
            name: The name of the device.

        Raises:
            SynapseError: if the device name is too long.
        """
        if name and len(name) > MAX_DEVICE_DISPLAY_NAME_LEN:
            raise SynapseError(
                400,
                "Device display name is too long (max %i)"
                % (MAX_DEVICE_DISPLAY_NAME_LEN,),
                errcode=Codes.TOO_LARGE,
            )

    async def check_device_registered(
        self,
        user_id: str,
        device_id: Optional[str],
        initial_device_display_name: Optional[str] = None,
        auth_provider_id: Optional[str] = None,
        auth_provider_session_id: Optional[str] = None,
    ) -> str:
        """
        If the given device has not been registered, register it with the
        supplied display name.

        If no device_id is supplied, we make one up.

        Args:
            user_id:  @user:id
            device_id: device id supplied by client
            initial_device_display_name: device display name from client
            auth_provider_id: The SSO IdP the user used, if any.
            auth_provider_session_id: The session ID (sid) got from the SSO IdP.
        Returns:
            device id (generated if none was supplied)
        """

        self._check_device_name_length(initial_device_display_name)

        if device_id is not None:
            new_device = await self.store.store_device(
                user_id=user_id,
                device_id=device_id,
                initial_device_display_name=initial_device_display_name,
                auth_provider_id=auth_provider_id,
                auth_provider_session_id=auth_provider_session_id,
            )
            if new_device:
                await self.notify_device_update(user_id, [device_id])
            return device_id

        # if the device id is not specified, we'll autogen one, but loop a few
        # times in case of a clash.
        attempts = 0
        while attempts < 5:
            new_device_id = stringutils.random_string(10).upper()
            new_device = await self.store.store_device(
                user_id=user_id,
                device_id=new_device_id,
                initial_device_display_name=initial_device_display_name,
                auth_provider_id=auth_provider_id,
                auth_provider_session_id=auth_provider_session_id,
            )
            if new_device:
                await self.notify_device_update(user_id, [new_device_id])
                return new_device_id
            attempts += 1

        raise errors.StoreError(500, "Couldn't generate a device ID.")

    @trace
    async def delete_device(self, user_id: str, device_id: str) -> None:
        """Delete the given device

        Args:
            user_id: The user to delete the device from.
            device_id: The device to delete.
        """

        try:
            await self.store.delete_device(user_id, device_id)
        except errors.StoreError as e:
            if e.code == 404:
                # no match
                set_tag("error", True)
                log_kv(
                    {"reason": "User doesn't have device id.", "device_id": device_id}
                )
                pass
            else:
                raise

        await self._auth_handler.delete_access_tokens_for_user(
            user_id, device_id=device_id
        )

        await self.store.delete_e2e_keys_by_device(user_id=user_id, device_id=device_id)

        await self.notify_device_update(user_id, [device_id])

    @trace
    async def delete_all_devices_for_user(
        self, user_id: str, except_device_id: Optional[str] = None
    ) -> None:
        """Delete all of the user's devices

        Args:
            user_id: The user to remove all devices from
            except_device_id: optional device id which should not be deleted
        """
        device_map = await self.store.get_devices_by_user(user_id)
        device_ids = list(device_map)
        if except_device_id is not None:
            device_ids = [d for d in device_ids if d != except_device_id]
        await self.delete_devices(user_id, device_ids)

    async def delete_devices(self, user_id: str, device_ids: List[str]) -> None:
        """Delete several devices

        Args:
            user_id: The user to delete devices from.
            device_ids: The list of device IDs to delete
        """

        try:
            await self.store.delete_devices(user_id, device_ids)
        except errors.StoreError as e:
            if e.code == 404:
                # no match
                set_tag("error", True)
                set_tag("reason", "User doesn't have that device id.")
                pass
            else:
                raise

        # Delete access tokens and e2e keys for each device. Not optimised as it is not
        # considered as part of a critical path.
        for device_id in device_ids:
            await self._auth_handler.delete_access_tokens_for_user(
                user_id, device_id=device_id
            )
            await self.store.delete_e2e_keys_by_device(
                user_id=user_id, device_id=device_id
            )

        await self.notify_device_update(user_id, device_ids)

    async def update_device(self, user_id: str, device_id: str, content: dict) -> None:
        """Update the given device

        Args:
            user_id: The user to update devices of.
            device_id: The device to update.
            content: body of update request
        """

        # Reject a new displayname which is too long.
        new_display_name = content.get("display_name")

        self._check_device_name_length(new_display_name)

        try:
            await self.store.update_device(
                user_id, device_id, new_display_name=new_display_name
            )
            await self.notify_device_update(user_id, [device_id])
        except errors.StoreError as e:
            if e.code == 404:
                raise errors.NotFoundError()
            else:
                raise

    @trace
    @measure_func("notify_device_update")
    async def notify_device_update(
        self, user_id: str, device_ids: Collection[str]
    ) -> None:
        """Notify that a user's device(s) has changed. Pokes the notifier, and
        remote servers if the user is local.

        Args:
            user_id: The Matrix ID of the user who's device list has been updated.
            device_ids: The device IDs that have changed.
        """
        if not device_ids:
            # No changes to notify about, so this is a no-op.
            return

        users_who_share_room = await self.store.get_users_who_share_room_with_user(
            user_id
        )

        hosts: Set[str] = set()
        if self.hs.is_mine_id(user_id):
            hosts.update(get_domain_from_id(u) for u in users_who_share_room)
            hosts.discard(self.server_name)

        set_tag("target_hosts", hosts)

        position = await self.store.add_device_change_to_streams(
            user_id, device_ids, list(hosts)
        )

        if not position:
            # This should only happen if there are no updates, so we bail.
            return

        for device_id in device_ids:
            logger.debug(
                "Notifying about update %r/%r, ID: %r", user_id, device_id, position
            )

        room_ids = await self.store.get_rooms_for_user(user_id)

        # specify the user ID too since the user should always get their own device list
        # updates, even if they aren't in any rooms.
        self.notifier.on_new_event(
            "device_list_key", position, users=[user_id], rooms=room_ids
        )

        if hosts:
            logger.info(
                "Sending device list update notif for %r to: %r", user_id, hosts
            )
            for host in hosts:
                self.federation_sender.send_device_messages(host)
                log_kv({"message": "sent device update to host", "host": host})

    async def notify_user_signature_update(
        self, from_user_id: str, user_ids: List[str]
    ) -> None:
        """Notify a user that they have made new signatures of other users.

        Args:
            from_user_id: the user who made the signature
            user_ids: the users IDs that have new signatures
        """

        position = await self.store.add_user_signature_change_to_streams(
            from_user_id, user_ids
        )

        self.notifier.on_new_event("device_list_key", position, users=[from_user_id])

    async def user_left_room(self, user: UserID, room_id: str) -> None:
        user_id = user.to_string()
        room_ids = await self.store.get_rooms_for_user(user_id)
        if not room_ids:
            # We no longer share rooms with this user, so we'll no longer
            # receive device updates. Mark this in DB.
            await self.store.mark_remote_user_device_list_as_unsubscribed(user_id)

    async def store_dehydrated_device(
        self,
        user_id: str,
        device_data: JsonDict,
        initial_device_display_name: Optional[str] = None,
    ) -> str:
        """Store a dehydrated device for a user.  If the user had a previous
        dehydrated device, it is removed.

        Args:
            user_id: the user that we are storing the device for
            device_data: the dehydrated device information
            initial_device_display_name: The display name to use for the device
        Returns:
            device id of the dehydrated device
        """
        device_id = await self.check_device_registered(
            user_id,
            None,
            initial_device_display_name,
        )
        old_device_id = await self.store.store_dehydrated_device(
            user_id, device_id, device_data
        )
        if old_device_id is not None:
            await self.delete_device(user_id, old_device_id)
        return device_id

    async def get_dehydrated_device(
        self, user_id: str
    ) -> Optional[Tuple[str, JsonDict]]:
        """Retrieve the information for a dehydrated device.

        Args:
            user_id: the user whose dehydrated device we are looking for
        Returns:
            a tuple whose first item is the device ID, and the second item is
            the dehydrated device information
        """
        return await self.store.get_dehydrated_device(user_id)

    async def rehydrate_device(
        self, user_id: str, access_token: str, device_id: str
    ) -> dict:
        """Process a rehydration request from the user.

        Args:
            user_id: the user who is rehydrating the device
            access_token: the access token used for the request
            device_id: the ID of the device that will be rehydrated
        Returns:
            a dict containing {"success": True}
        """
        success = await self.store.remove_dehydrated_device(user_id, device_id)

        if not success:
            raise errors.NotFoundError()

        # If the dehydrated device was successfully deleted (the device ID
        # matched the stored dehydrated device), then modify the access
        # token to use the dehydrated device's ID and copy the old device
        # display name to the dehydrated device, and destroy the old device
        # ID
        old_device_id = await self.store.set_device_for_access_token(
            access_token, device_id
        )
        old_device = await self.store.get_device(user_id, old_device_id)
        if old_device is None:
            raise errors.NotFoundError()
        await self.store.update_device(user_id, device_id, old_device["display_name"])
        # can't call self.delete_device because that will clobber the
        # access token so call the storage layer directly
        await self.store.delete_device(user_id, old_device_id)
        await self.store.delete_e2e_keys_by_device(
            user_id=user_id, device_id=old_device_id
        )

        # tell everyone that the old device is gone and that the dehydrated
        # device has a new display name
        await self.notify_device_update(user_id, [old_device_id, device_id])

        return {"success": True}


def _update_device_from_client_ips(
    device: JsonDict, client_ips: Mapping[Tuple[str, str], Mapping[str, Any]]
) -> None:
    ip = client_ips.get((device["user_id"], device["device_id"]), {})
    device.update({"last_seen_ts": ip.get("last_seen"), "last_seen_ip": ip.get("ip")})


class DeviceListUpdater:
    "Handles incoming device list updates from federation and updates the DB"

    def __init__(self, hs: "HomeServer", device_handler: DeviceHandler):
        self.store = hs.get_datastore()
        self.federation = hs.get_federation_client()
        self.clock = hs.get_clock()
        self.device_handler = device_handler

        self._remote_edu_linearizer = Linearizer(name="remote_device_list")

        # user_id -> list of updates waiting to be handled.
        self._pending_updates: Dict[
            str, List[Tuple[str, str, Iterable[str], JsonDict]]
        ] = {}

        # Recently seen stream ids. We don't bother keeping these in the DB,
        # but they're useful to have them about to reduce the number of spurious
        # resyncs.
        self._seen_updates: ExpiringCache[str, Set[str]] = ExpiringCache(
            cache_name="device_update_edu",
            clock=self.clock,
            max_len=10000,
            expiry_ms=30 * 60 * 1000,
            iterable=True,
        )

        # Attempt to resync out of sync device lists every 30s.
        self._resync_retry_in_progress = False
        self.clock.looping_call(
            run_as_background_process,
            30 * 1000,
            func=self._maybe_retry_device_resync,
            desc="_maybe_retry_device_resync",
        )

    @trace
    async def incoming_device_list_update(
        self, origin: str, edu_content: JsonDict
    ) -> None:
        """Called on incoming device list update from federation. Responsible
        for parsing the EDU and adding to pending updates list.
        """

        set_tag("origin", origin)
        set_tag("edu_content", edu_content)
        user_id = edu_content.pop("user_id")
        device_id = edu_content.pop("device_id")
        stream_id = str(edu_content.pop("stream_id"))  # They may come as ints
        prev_ids = edu_content.pop("prev_id", [])
        prev_ids = [str(p) for p in prev_ids]  # They may come as ints

        if get_domain_from_id(user_id) != origin:
            # TODO: Raise?
            logger.warning(
                "Got device list update edu for %r/%r from %r",
                user_id,
                device_id,
                origin,
            )

            set_tag("error", True)
            log_kv(
                {
                    "message": "Got a device list update edu from a user and "
                    "device which does not match the origin of the request.",
                    "user_id": user_id,
                    "device_id": device_id,
                }
            )
            return

        room_ids = await self.store.get_rooms_for_user(user_id)
        if not room_ids:
            # We don't share any rooms with this user. Ignore update, as we
            # probably won't get any further updates.
            set_tag("error", True)
            log_kv(
                {
                    "message": "Got an update from a user for which "
                    "we don't share any rooms",
                    "other user_id": user_id,
                }
            )
            logger.warning(
                "Got device list update edu for %r/%r, but don't share a room",
                user_id,
                device_id,
            )
            return

        logger.debug("Received device list update for %r/%r", user_id, device_id)

        self._pending_updates.setdefault(user_id, []).append(
            (device_id, stream_id, prev_ids, edu_content)
        )

        await self._handle_device_updates(user_id)

    @measure_func("_incoming_device_list_update")
    async def _handle_device_updates(self, user_id: str) -> None:
        "Actually handle pending updates."

        with (await self._remote_edu_linearizer.queue(user_id)):
            pending_updates = self._pending_updates.pop(user_id, [])
            if not pending_updates:
                # This can happen since we batch updates
                return

            for device_id, stream_id, prev_ids, _ in pending_updates:
                logger.debug(
                    "Handling update %r/%r, ID: %r, prev: %r ",
                    user_id,
                    device_id,
                    stream_id,
                    prev_ids,
                )

            # Given a list of updates we check if we need to resync. This
            # happens if we've missed updates.
            resync = await self._need_to_do_resync(user_id, pending_updates)

            if logger.isEnabledFor(logging.INFO):
                logger.info(
                    "Received device list update for %s, requiring resync: %s. Devices: %s",
                    user_id,
                    resync,
                    ", ".join(u[0] for u in pending_updates),
                )

            if resync:
                await self.user_device_resync(user_id)
            else:
                # Simply update the single device, since we know that is the only
                # change (because of the single prev_id matching the current cache)
                for device_id, stream_id, _, content in pending_updates:
                    await self.store.update_remote_device_list_cache_entry(
                        user_id, device_id, content, stream_id
                    )

                await self.device_handler.notify_device_update(
                    user_id, [device_id for device_id, _, _, _ in pending_updates]
                )

                self._seen_updates.setdefault(user_id, set()).update(
                    stream_id for _, stream_id, _, _ in pending_updates
                )

    async def _need_to_do_resync(
        self, user_id: str, updates: Iterable[Tuple[str, str, Iterable[str], JsonDict]]
    ) -> bool:
        """Given a list of updates for a user figure out if we need to do a full
        resync, or whether we have enough data that we can just apply the delta.
        """
        seen_updates: Set[str] = self._seen_updates.get(user_id, set())

        extremity = await self.store.get_device_list_last_stream_id_for_remote(user_id)

        logger.debug("Current extremity for %r: %r", user_id, extremity)

        stream_id_in_updates = set()  # stream_ids in updates list
        for _, stream_id, prev_ids, _ in updates:
            if not prev_ids:
                # We always do a resync if there are no previous IDs
                return True

            for prev_id in prev_ids:
                if prev_id == extremity:
                    continue
                elif prev_id in seen_updates:
                    continue
                elif prev_id in stream_id_in_updates:
                    continue
                else:
                    return True

            stream_id_in_updates.add(stream_id)

        return False

    @trace
    async def _maybe_retry_device_resync(self) -> None:
        """Retry to resync device lists that are out of sync, except if another retry is
        in progress.
        """
        if self._resync_retry_in_progress:
            return

        try:
            # Prevent another call of this function to retry resyncing device lists so
            # we don't send too many requests.
            self._resync_retry_in_progress = True
            # Get all of the users that need resyncing.
            need_resync = await self.store.get_user_ids_requiring_device_list_resync()
            # Iterate over the set of user IDs.
            for user_id in need_resync:
                try:
                    # Try to resync the current user's devices list.
                    result = await self.user_device_resync(
                        user_id=user_id,
                        mark_failed_as_stale=False,
                    )

                    # user_device_resync only returns a result if it managed to
                    # successfully resync and update the database. Updating the table
                    # of users requiring resync isn't necessary here as
                    # user_device_resync already does it (through
                    # self.store.update_remote_device_list_cache).
                    if result:
                        logger.debug(
                            "Successfully resynced the device list for %s",
                            user_id,
                        )
                except Exception as e:
                    # If there was an issue resyncing this user, e.g. if the remote
                    # server sent a malformed result, just log the error instead of
                    # aborting all the subsequent resyncs.
                    logger.debug(
                        "Could not resync the device list for %s: %s",
                        user_id,
                        e,
                    )
        finally:
            # Allow future calls to retry resyncinc out of sync device lists.
            self._resync_retry_in_progress = False

    async def user_device_resync(
        self, user_id: str, mark_failed_as_stale: bool = True
    ) -> Optional[JsonDict]:
        """Fetches all devices for a user and updates the device cache with them.

        Args:
            user_id: The user's id whose device_list will be updated.
            mark_failed_as_stale: Whether to mark the user's device list as stale
                if the attempt to resync failed.
        Returns:
            A dict with device info as under the "devices" in the result of this
            request:
            https://matrix.org/docs/spec/server_server/r0.1.2#get-matrix-federation-v1-user-devices-userid
        """
        logger.debug("Attempting to resync the device list for %s", user_id)
        log_kv({"message": "Doing resync to update device list."})
        # Fetch all devices for the user.
        origin = get_domain_from_id(user_id)
        try:
            result = await self.federation.query_user_devices(origin, user_id)
        except NotRetryingDestination:
            if mark_failed_as_stale:
                # Mark the remote user's device list as stale so we know we need to retry
                # it later.
                await self.store.mark_remote_user_device_cache_as_stale(user_id)

            return None
        except (RequestSendFailed, HttpResponseException) as e:
            logger.warning(
                "Failed to handle device list update for %s: %s",
                user_id,
                e,
            )

            if mark_failed_as_stale:
                # Mark the remote user's device list as stale so we know we need to retry
                # it later.
                await self.store.mark_remote_user_device_cache_as_stale(user_id)

            # We abort on exceptions rather than accepting the update
            # as otherwise synapse will 'forget' that its device list
            # is out of date. If we bail then we will retry the resync
            # next time we get a device list update for this user_id.
            # This makes it more likely that the device lists will
            # eventually become consistent.
            return None
        except FederationDeniedError as e:
            set_tag("error", True)
            log_kv({"reason": "FederationDeniedError"})
            logger.info(e)
            return None
        except Exception as e:
            set_tag("error", True)
            log_kv(
                {"message": "Exception raised by federation request", "exception": e}
            )
            logger.exception("Failed to handle device list update for %s", user_id)

            if mark_failed_as_stale:
                # Mark the remote user's device list as stale so we know we need to retry
                # it later.
                await self.store.mark_remote_user_device_cache_as_stale(user_id)

            return None
        log_kv({"result": result})
        stream_id = result["stream_id"]
        devices = result["devices"]

        # Get the master key and the self-signing key for this user if provided in the
        # response (None if not in the response).
        # The response will not contain the user signing key, as this key is only used by
        # its owner, thus it doesn't make sense to send it over federation.
        master_key = result.get("master_key")
        self_signing_key = result.get("self_signing_key")

        ignore_devices = False
        # If the remote server has more than ~1000 devices for this user
        # we assume that something is going horribly wrong (e.g. a bot
        # that logs in and creates a new device every time it tries to
        # send a message).  Maintaining lots of devices per user in the
        # cache can cause serious performance issues as if this request
        # takes more than 60s to complete, internal replication from the
        # inbound federation worker to the synapse master may time out
        # causing the inbound federation to fail and causing the remote
        # server to retry, causing a DoS.  So in this scenario we give
        # up on storing the total list of devices and only handle the
        # delta instead.
        if len(devices) > 1000:
            logger.warning(
                "Ignoring device list snapshot for %s as it has >1K devs (%d)",
                user_id,
                len(devices),
            )
            devices = []
            ignore_devices = True
        else:
            prev_stream_id = await self.store.get_device_list_last_stream_id_for_remote(
                user_id
            )
            cached_devices = await self.store.get_cached_devices_for_user(user_id)

            # To ensure that a user with no devices is cached, we skip the resync only
            # if we have a stream_id from previously writing a cache entry.
            if prev_stream_id is not None and cached_devices == {
                d["device_id"]: d for d in devices
            }:
                logging.info(
                    "Skipping device list resync for %s, as our cache matches already",
                    user_id,
                )
                devices = []
                ignore_devices = True

        for device in devices:
            logger.debug(
                "Handling resync update %r/%r, ID: %r",
                user_id,
                device["device_id"],
                stream_id,
            )

        if not ignore_devices:
            await self.store.update_remote_device_list_cache(
                user_id, devices, stream_id
            )
        # mark the cache as valid, whether or not we actually processed any device
        # list updates.
        await self.store.mark_remote_user_device_cache_as_valid(user_id)
        device_ids = [device["device_id"] for device in devices]

        # Handle cross-signing keys.
        cross_signing_device_ids = await self.process_cross_signing_key_update(
            user_id,
            master_key,
            self_signing_key,
        )
        device_ids = device_ids + cross_signing_device_ids

        if device_ids:
            await self.device_handler.notify_device_update(user_id, device_ids)

        # We clobber the seen updates since we've re-synced from a given
        # point.
        self._seen_updates[user_id] = {stream_id}

        return result

    async def process_cross_signing_key_update(
        self,
        user_id: str,
        master_key: Optional[JsonDict],
        self_signing_key: Optional[JsonDict],
    ) -> List[str]:
        """Process the given new master and self-signing key for the given remote user.

        Args:
            user_id: The ID of the user these keys are for.
            master_key: The dict of the cross-signing master key as returned by the
                remote server.
            self_signing_key: The dict of the cross-signing self-signing key as returned
                by the remote server.

        Return:
            The device IDs for the given keys.
        """
        device_ids = []

        current_keys_map = await self.store.get_e2e_cross_signing_keys_bulk([user_id])
        current_keys = current_keys_map.get(user_id) or {}

        if master_key and master_key != current_keys.get("master"):
            await self.store.set_e2e_cross_signing_key(user_id, "master", master_key)
            _, verify_key = get_verify_key_from_cross_signing_key(master_key)
            # verify_key is a VerifyKey from signedjson, which uses
            # .version to denote the portion of the key ID after the
            # algorithm and colon, which is the device ID
            device_ids.append(verify_key.version)
        if self_signing_key and self_signing_key != current_keys.get("self_signing"):
            await self.store.set_e2e_cross_signing_key(
                user_id, "self_signing", self_signing_key
            )
            _, verify_key = get_verify_key_from_cross_signing_key(self_signing_key)
            device_ids.append(verify_key.version)

        return device_ids