text: # Copyright 2019 New Vector 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.
  
  import abc
  import logging
  from collections import OrderedDict
  from typing import (
      TYPE_CHECKING,
      Collection,
      Dict,
      Hashable,
      Iterable,
      List,
      Optional,
      Set,
      Tuple,
  )
  
  import attr
  from prometheus_client import Counter
  from typing_extensions import Literal
  
  from twisted.internet import defer
  from twisted.internet.interfaces import IDelayedCall
  
  import synapse.metrics
  from synapse.api.presence import UserPresenceState
  from synapse.events import EventBase
  from synapse.federation.sender.per_destination_queue import PerDestinationQueue
  from synapse.federation.sender.transaction_manager import TransactionManager
  from synapse.federation.units import Edu
  from synapse.logging.context import make_deferred_yieldable, run_in_background
  from synapse.metrics import (
      LaterGauge,
      event_processing_loop_counter,
      event_processing_loop_room_count,
      events_processed_counter,
  )
  from synapse.metrics.background_process_metrics import (
      run_as_background_process,
      wrap_as_background_process,
  )
  from synapse.types import JsonDict, ReadReceipt, RoomStreamToken
  from synapse.util import Clock
  from synapse.util.metrics import Measure
  
  if TYPE_CHECKING:
      from synapse.events.presence_router import PresenceRouter
      from synapse.server import HomeServer
  
  logger = logging.getLogger(__name__)
  
  sent_pdus_destination_dist_count = Counter(
      "synapse_federation_client_sent_pdu_destinations:count",
      "Number of PDUs queued for sending to one or more destinations",
  )
  
  sent_pdus_destination_dist_total = Counter(
      "synapse_federation_client_sent_pdu_destinations:total",
      "Total number of PDUs queued for sending across all destinations",
  )
  
  # Time (in s) after Synapse's startup that we will begin to wake up destinations
  # that have catch-up outstanding.
  CATCH_UP_STARTUP_DELAY_SEC = 15
  
  # Time (in s) to wait in between waking up each destination, i.e. one destination
  # will be woken up every <x> seconds after Synapse's startup until we have woken
  # every destination has outstanding catch-up.
  CATCH_UP_STARTUP_INTERVAL_SEC = 5
  
  
  class AbstractFederationSender(metaclass=abc.ABCMeta):
      @abc.abstractmethod
      def notify_new_events(self, max_token: RoomStreamToken) -> None:
          """This gets called when we have some new events we might want to
          send out to other servers.
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      async def send_read_receipt(self, receipt: ReadReceipt) -> None:
          """Send a RR to any other servers in the room
  
          Args:
              receipt: receipt to be sent
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      def send_presence_to_destinations(
          self, states: Iterable[UserPresenceState], destinations: Iterable[str]
      ) -> None:
          """Send the given presence states to the given destinations.
  
          Args:
              destinations:
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      def build_and_send_edu(
          self,
          destination: str,
          edu_type: str,
          content: JsonDict,
          key: Optional[Hashable] = None,
      ) -> None:
          """Construct an Edu object, and queue it for sending
  
          Args:
              destination: name of server to send to
              edu_type: type of EDU to send
              content: content of EDU
              key: clobbering key for this edu
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      def send_device_messages(self, destination: str, immediate: bool = True) -> None:
          """Tells the sender that a new device message is ready to be sent to the
          destination. The `immediate` flag specifies whether the messages should
          be tried to be sent immediately, or whether it can be delayed for a
          short while (to aid performance).
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      def wake_destination(self, destination: str) -> None:
          """Called when we want to retry sending transactions to a remote.
  
          This is mainly useful if the remote server has been down and we think it
          might have come back.
          """
          raise NotImplementedError()
  
      @abc.abstractmethod
      def get_current_token(self) -> int:
          raise NotImplementedError()
  
      @abc.abstractmethod
      def federation_ack(self, instance_name: str, token: int) -> None:
          raise NotImplementedError()
  
      @abc.abstractmethod
      async def get_replication_rows(
          self, instance_name: str, from_token: int, to_token: int, target_row_count: int
      ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
          raise NotImplementedError()
  
  
  @attr.s
  class _DestinationWakeupQueue:
      """A queue of destinations that need to be woken up due to new updates.
  
      Staggers waking up of per destination queues to ensure that we don't attempt
      to start TLS connections with many hosts all at once, leading to pinned CPU.
      """
  
      # The maximum duration in seconds between queuing up a destination and it
      # being woken up.
      _MAX_TIME_IN_QUEUE = 30.0
  
      # The maximum duration in seconds between waking up consecutive destination
      # queues.
      _MAX_DELAY = 0.1
  
      sender: "FederationSender" = attr.ib()
      clock: Clock = attr.ib()
      queue: "OrderedDict[str, Literal[None]]" = attr.ib(factory=OrderedDict)
      processing: bool = attr.ib(default=False)
  
      def add_to_queue(self, destination: str) -> None:
          """Add a destination to the queue to be woken up."""
  
          self.queue[destination] = None
  
          if not self.processing:
              self._handle()
  
      @wrap_as_background_process("_DestinationWakeupQueue.handle")
      async def _handle(self) -> None:
          """Background process to drain the queue."""
  
          if not self.queue:
              return
  
          assert not self.processing
          self.processing = True
  
          try:
              # We start with a delay that should drain the queue quickly enough that
              # we process all destinations in the queue in _MAX_TIME_IN_QUEUE
              # seconds.
              #
              # We also add an upper bound to the delay, to gracefully handle the
              # case where the queue only has a few entries in it.
              current_sleep_seconds = min(
                  self._MAX_DELAY, self._MAX_TIME_IN_QUEUE / len(self.queue)
              )
  
              while self.queue:
                  destination, _ = self.queue.popitem(last=False)
  
                  queue = self.sender._get_per_destination_queue(destination)
  
                  if not queue._new_data_to_send:
                      # The per destination queue has already been woken up.
                      continue
  
                  queue.attempt_new_transaction()
  
                  await self.clock.sleep(current_sleep_seconds)
  
                  if not self.queue:
                      break
  
                  # More destinations may have been added to the queue, so we may
                  # need to reduce the delay to ensure everything gets processed
                  # within _MAX_TIME_IN_QUEUE seconds.
                  current_sleep_seconds = min(
                      current_sleep_seconds, self._MAX_TIME_IN_QUEUE / len(self.queue)
                  )
  
          finally:
              self.processing = False
  
  
  class FederationSender(AbstractFederationSender):
      def __init__(self, hs: "HomeServer"):
          self.hs = hs
          self.server_name = hs.hostname
  
          self.store = hs.get_datastores().main
          self.state = hs.get_state_handler()
  
          self._storage_controllers = hs.get_storage_controllers()
  
          self.clock = hs.get_clock()
          self.is_mine_id = hs.is_mine_id
  
          self._presence_router: Optional["PresenceRouter"] = None
          self._transaction_manager = TransactionManager(hs)
  
          self._instance_name = hs.get_instance_name()
          self._federation_shard_config = hs.config.worker.federation_shard_config
  
          # map from destination to PerDestinationQueue
          self._per_destination_queues: Dict[str, PerDestinationQueue] = {}
  
          LaterGauge(
              "synapse_federation_transaction_queue_pending_destinations",
              "",
              [],
              lambda: sum(
                  1
                  for d in self._per_destination_queues.values()
                  if d.transmission_loop_running
              ),
          )
  
          LaterGauge(
              "synapse_federation_transaction_queue_pending_pdus",
              "",
              [],
              lambda: sum(
                  d.pending_pdu_count() for d in self._per_destination_queues.values()
              ),
          )
          LaterGauge(
              "synapse_federation_transaction_queue_pending_edus",
              "",
              [],
              lambda: sum(
                  d.pending_edu_count() for d in self._per_destination_queues.values()
              ),
          )
  
          self._is_processing = False
          self._last_poked_id = -1
  
          # map from room_id to a set of PerDestinationQueues which we believe are
          # awaiting a call to flush_read_receipts_for_room. The presence of an entry
          # here for a given room means that we are rate-limiting RR flushes to that room,
          # and that there is a pending call to _flush_rrs_for_room in the system.
          self._queues_awaiting_rr_flush_by_room: Dict[str, Set[PerDestinationQueue]] = {}
  
          self._rr_txn_interval_per_room_ms = (
              1000.0
              / hs.config.ratelimiting.federation_rr_transactions_per_room_per_second
          )
  
          # wake up destinations that have outstanding PDUs to be caught up
          self._catchup_after_startup_timer: Optional[
              IDelayedCall
          ] = self.clock.call_later(
              CATCH_UP_STARTUP_DELAY_SEC,
              run_as_background_process,
              "wake_destinations_needing_catchup",
              self._wake_destinations_needing_catchup,
          )
  
          self._external_cache = hs.get_external_cache()
  
          self._destination_wakeup_queue = _DestinationWakeupQueue(self, self.clock)
  
      def _get_per_destination_queue(self, destination: str) -> PerDestinationQueue:
          """Get or create a PerDestinationQueue for the given destination
  
          Args:
              destination: server_name of remote server
          """
          queue = self._per_destination_queues.get(destination)
          if not queue:
              queue = PerDestinationQueue(self.hs, self._transaction_manager, destination)
              self._per_destination_queues[destination] = queue
          return queue
  
      def notify_new_events(self, max_token: RoomStreamToken) -> None:
          """This gets called when we have some new events we might want to
          send out to other servers.
          """
          # We just use the minimum stream ordering and ignore the vector clock
          # component. This is safe to do as long as we *always* ignore the vector
          # clock components.
          current_id = max_token.stream
  
          self._last_poked_id = max(current_id, self._last_poked_id)
  
          if self._is_processing:
              return
  
          # fire off a processing loop in the background
          run_as_background_process(
              "process_event_queue_for_federation", self._process_event_queue_loop
          )
  
      async def _process_event_queue_loop(self) -> None:
          try:
              self._is_processing = True
              while True:
                  last_token = await self.store.get_federation_out_pos("events")
                  next_token, events = await self.store.get_all_new_events_stream(
                      last_token, self._last_poked_id, limit=100
                  )
  
                  logger.debug(
                      "Handling %i -> %i: %i events to send (current id %i)",
                      last_token,
                      next_token,
                      len(events),
                      self._last_poked_id,
                  )
  
                  if not events and next_token >= self._last_poked_id:
                      logger.debug("All events processed")
                      break
  
                  async def handle_event(event: EventBase) -> None:
                      # Only send events for this server.
                      send_on_behalf_of = event.internal_metadata.get_send_on_behalf_of()
                      is_mine = self.is_mine_id(event.sender)
                      if not is_mine and send_on_behalf_of is None:
                          logger.debug("Not sending remote-origin event %s", event)
                          return
  
                      # We also want to not send out-of-band membership events.
                      #
                      # OOB memberships are used in three (and a half) situations:
                      #
                      # (1) invite events which we have received over federation. Those
                      #     will have a `sender` on a different server, so will be
                      #     skipped by the "is_mine" test above anyway.
                      #
                      # (2) rejections of invites to federated rooms - either remotely
                      #     or locally generated. (Such rejections are normally
                      #     created via federation, in which case the remote server is
                      #     responsible for sending out the rejection. If that fails,
                      #     we'll create a leave event locally, but that's only really
                      #     for the benefit of the invited user - we don't have enough
                      #     information to send it out over federation).
                      #
                      # (2a) rescinded knocks. These are identical to rejected invites.
                      #
                      # (3) knock events which we have sent over federation. As with
                      #     invite rejections, the remote server should send them out to
                      #     the federation.
                      #
                      # So, in all the above cases, we want to ignore such events.
                      #
                      # OOB memberships are always(?) outliers anyway, so if we *don't*
                      # ignore them, we'll get an exception further down when we try to
                      # fetch the membership list for the room.
                      #
                      # Arguably, we could equivalently ignore all outliers here, since
                      # in theory the only way for an outlier with a local `sender` to
                      # exist is by being an OOB membership (via one of (2), (2a) or (3)
                      # above).
                      #
                      if event.internal_metadata.is_out_of_band_membership():
                          logger.debug("Not sending OOB membership event %s", event)
                          return
  
                      # Finally, there are some other events that we should not send out
                      # until someone asks for them. They are explicitly flagged as such
                      # with `proactively_send: False`.
                      if not event.internal_metadata.should_proactively_send():
                          logger.debug(
                              "Not sending event with proactively_send=false: %s", event
                          )
                          return
  
                      destinations: Optional[Collection[str]] = None
                      if not event.prev_event_ids():
                          # If there are no prev event IDs then the state is empty
                          # and so no remote servers in the room
                          destinations = set()
                      else:
                          # We check the external cache for the destinations, which is
                          # stored per state group.
  
                          sg = await self._external_cache.get(
                              "event_to_prev_state_group", event.event_id
                          )
                          if sg:
                              destinations = await self._external_cache.get(
                                  "get_joined_hosts", str(sg)
                              )
  
                      if destinations is None:
                          try:
                              # Get the state from before the event.
                              # We need to make sure that this is the state from before
                              # the event and not from after it.
                              # Otherwise if the last member on a server in a room is
                              # banned then it won't receive the event because it won't
                              # be in the room after the ban.
                              destinations = await self.state.get_hosts_in_room_at_events(
                                  event.room_id, event_ids=event.prev_event_ids()
                              )
                          except Exception:
                              logger.exception(
                                  "Failed to calculate hosts in room for event: %s",
                                  event.event_id,
                              )
                              return
  
                      sharded_destinations = {
                          d
                          for d in destinations
                          if self._federation_shard_config.should_handle(
                              self._instance_name, d
                          )
                      }
  
                      if send_on_behalf_of is not None:
                          # If we are sending the event on behalf of another server
                          # then it already has the event and there is no reason to
                          # send the event to it.
                          sharded_destinations.discard(send_on_behalf_of)
  
                      logger.debug("Sending %s to %r", event, sharded_destinations)
  
                      if sharded_destinations:
                          await self._send_pdu(event, sharded_destinations)
  
                          now = self.clock.time_msec()
                          ts = await self.store.get_received_ts(event.event_id)
                          assert ts is not None
                          synapse.metrics.event_processing_lag_by_event.labels(
                              "federation_sender"
                          ).observe((now - ts) / 1000)
  
                  async def handle_room_events(events: List[EventBase]) -> None:
                      logger.debug(
                          "Handling %i events in room %s", len(events), events[0].room_id
                      )
                      with Measure(self.clock, "handle_room_events"):
                          for event in events:
                              await handle_event(event)
  
                  events_by_room: Dict[str, List[EventBase]] = {}
                  for event in events:
                      events_by_room.setdefault(event.room_id, []).append(event)
  
                  await make_deferred_yieldable(
                      defer.gatherResults(
                          [
                              run_in_background(handle_room_events, evs)
                              for evs in events_by_room.values()
                          ],
                          consumeErrors=True,
                      )
                  )
  
                  logger.debug("Successfully handled up to %i", next_token)
                  await self.store.update_federation_out_pos("events", next_token)
  
                  if events:
                      now = self.clock.time_msec()
                      ts = await self.store.get_received_ts(events[-1].event_id)
                      assert ts is not None
  
                      synapse.metrics.event_processing_lag.labels(
                          "federation_sender"
                      ).set(now - ts)
                      synapse.metrics.event_processing_last_ts.labels(
                          "federation_sender"
                      ).set(ts)
  
                      events_processed_counter.inc(len(events))
  
                      event_processing_loop_room_count.labels("federation_sender").inc(
                          len(events_by_room)
                      )
  
                  event_processing_loop_counter.labels("federation_sender").inc()
  
                  synapse.metrics.event_processing_positions.labels(
                      "federation_sender"
                  ).set(next_token)
  
          finally:
              self._is_processing = False
  
      async def _send_pdu(self, pdu: EventBase, destinations: Iterable[str]) -> None:
          # We loop through all destinations to see whether we already have
          # a transaction in progress. If we do, stick it in the pending_pdus
          # table and we'll get back to it later.
  
          destinations = set(destinations)
          destinations.discard(self.server_name)
          logger.debug("Sending to: %s", str(destinations))
  
          if not destinations:
              return
  
          sent_pdus_destination_dist_total.inc(len(destinations))
          sent_pdus_destination_dist_count.inc()
  
          assert pdu.internal_metadata.stream_ordering
  
          # track the fact that we have a PDU for these destinations,
          # to allow us to perform catch-up later on if the remote is unreachable
          # for a while.
          await self.store.store_destination_rooms_entries(
              destinations,
              pdu.room_id,
              pdu.internal_metadata.stream_ordering,
          )
  
          for destination in destinations:
              self._get_per_destination_queue(destination).send_pdu(pdu)
  
      async def send_read_receipt(self, receipt: ReadReceipt) -> None:
          """Send a RR to any other servers in the room
  
          Args:
              receipt: receipt to be sent
          """
  
          # Some background on the rate-limiting going on here.
          #
          # It turns out that if we attempt to send out RRs as soon as we get them from
          # a client, then we end up trying to do several hundred Hz of federation
          # transactions. (The number of transactions scales as O(N^2) on the size of a
          # room, since in a large room we have both more RRs coming in, and more servers
          # to send them to.)
          #
          # This leads to a lot of CPU load, and we end up getting behind. The solution
          # currently adopted is as follows:
          #
          # The first receipt in a given room is sent out immediately, at time T0. Any
          # further receipts are, in theory, batched up for N seconds, where N is calculated
          # based on the number of servers in the room to achieve a transaction frequency
          # of around 50Hz. So, for example, if there were 100 servers in the room, then
          # N would be 100 / 50Hz = 2 seconds.
          #
          # Then, after T+N, we flush out any receipts that have accumulated, and restart
          # the timer to flush out more receipts at T+2N, etc. If no receipts accumulate,
          # we stop the cycle and go back to the start.
          #
          # However, in practice, it is often possible to flush out receipts earlier: in
          # particular, if we are sending a transaction to a given server anyway (for
          # example, because we have a PDU or a RR in another room to send), then we may
          # as well send out all of the pending RRs for that server. So it may be that
          # by the time we get to T+N, we don't actually have any RRs left to send out.
          # Nevertheless we continue to buffer up RRs for the room in question until we
          # reach the point that no RRs arrive between timer ticks.
          #
          # For even more background, see https://github.com/matrix-org/synapse/issues/4730.
  
          room_id = receipt.room_id
  
          # Work out which remote servers should be poked and poke them.
          domains_set = await self._storage_controllers.state.get_current_hosts_in_room(
              room_id
          )
          domains = [
              d
              for d in domains_set
              if d != self.server_name
              and self._federation_shard_config.should_handle(self._instance_name, d)
          ]
          if not domains:
              return
  
          queues_pending_flush = self._queues_awaiting_rr_flush_by_room.get(room_id)
  
          # if there is no flush yet scheduled, we will send out these receipts with
          # immediate flushes, and schedule the next flush for this room.
          if queues_pending_flush is not None:
              logger.debug("Queuing receipt for: %r", domains)
          else:
              logger.debug("Sending receipt to: %r", domains)
              self._schedule_rr_flush_for_room(room_id, len(domains))
  
          for domain in domains:
              queue = self._get_per_destination_queue(domain)
              queue.queue_read_receipt(receipt)
  
              # if there is already a RR flush pending for this room, then make sure this
              # destination is registered for the flush
              if queues_pending_flush is not None:
                  queues_pending_flush.add(queue)
              else:
                  queue.flush_read_receipts_for_room(room_id)
  
      def _schedule_rr_flush_for_room(self, room_id: str, n_domains: int) -> None:
          # that is going to cause approximately len(domains) transactions, so now back
          # off for that multiplied by RR_TXN_INTERVAL_PER_ROOM
          backoff_ms = self._rr_txn_interval_per_room_ms * n_domains
  
          logger.debug("Scheduling RR flush in %s in %d ms", room_id, backoff_ms)
          self.clock.call_later(backoff_ms, self._flush_rrs_for_room, room_id)
          self._queues_awaiting_rr_flush_by_room[room_id] = set()
  
      def _flush_rrs_for_room(self, room_id: str) -> None:
          queues = self._queues_awaiting_rr_flush_by_room.pop(room_id)
          logger.debug("Flushing RRs in %s to %s", room_id, queues)
  
          if not queues:
              # no more RRs arrived for this room; we are done.
              return
  
          # schedule the next flush
          self._schedule_rr_flush_for_room(room_id, len(queues))
  
          for queue in queues:
              queue.flush_read_receipts_for_room(room_id)
  
      def send_presence_to_destinations(
          self, states: Iterable[UserPresenceState], destinations: Iterable[str]
      ) -> None:
          """Send the given presence states to the given destinations.
          destinations (list[str])
          """
  
          if not states or not self.hs.config.server.use_presence:
              # No-op if presence is disabled.
              return
  
          # Ensure we only send out presence states for local users.
          for state in states:
              assert self.is_mine_id(state.user_id)
  
          for destination in destinations:
              if destination == self.server_name:
                  continue
              if not self._federation_shard_config.should_handle(
                  self._instance_name, destination
              ):
                  continue
  
              self._get_per_destination_queue(destination).send_presence(
                  states, start_loop=False
              )
  
              self._destination_wakeup_queue.add_to_queue(destination)
  
      def build_and_send_edu(
          self,
          destination: str,
          edu_type: str,
          content: JsonDict,
          key: Optional[Hashable] = None,
      ) -> None:
          """Construct an Edu object, and queue it for sending
  
          Args:
              destination: name of server to send to
              edu_type: type of EDU to send
              content: content of EDU
              key: clobbering key for this edu
          """
          if destination == self.server_name:
              logger.info("Not sending EDU to ourselves")
              return
  
          if not self._federation_shard_config.should_handle(
              self._instance_name, destination
          ):
              return
  
          edu = Edu(
              origin=self.server_name,
              destination=destination,
              edu_type=edu_type,
              content=content,
          )
  
          self.send_edu(edu, key)
  
      def send_edu(self, edu: Edu, key: Optional[Hashable]) -> None:
          """Queue an EDU for sending
  
          Args:
              edu: edu to send
              key: clobbering key for this edu
          """
          if not self._federation_shard_config.should_handle(
              self._instance_name, edu.destination
          ):
              return
  
          queue = self._get_per_destination_queue(edu.destination)
          if key:
              queue.send_keyed_edu(edu, key)
          else:
              queue.send_edu(edu)
  
      def send_device_messages(self, destination: str, immediate: bool = False) -> None:
          if destination == self.server_name:
              logger.warning("Not sending device update to ourselves")
              return
  
          if not self._federation_shard_config.should_handle(
              self._instance_name, destination
          ):
              return
  
          if immediate:
              self._get_per_destination_queue(destination).attempt_new_transaction()
          else:
              self._get_per_destination_queue(destination).mark_new_data()
              self._destination_wakeup_queue.add_to_queue(destination)
  
      def wake_destination(self, destination: str) -> None:
          """Called when we want to retry sending transactions to a remote.
  
          This is mainly useful if the remote server has been down and we think it
          might have come back.
          """
  
          if destination == self.server_name:
              logger.warning("Not waking up ourselves")
              return
  
          if not self._federation_shard_config.should_handle(
              self._instance_name, destination
          ):
              return
  
          self._get_per_destination_queue(destination).attempt_new_transaction()
  
      @staticmethod
      def get_current_token() -> int:
          # Dummy implementation for case where federation sender isn't offloaded
          # to a worker.
          return 0
  
      def federation_ack(self, instance_name: str, token: int) -> None:
          # It is not expected that this gets called on FederationSender.
          raise NotImplementedError()
  
      @staticmethod
      async def get_replication_rows(
          instance_name: str, from_token: int, to_token: int, target_row_count: int
      ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
          # Dummy implementation for case where federation sender isn't offloaded
          # to a worker.
          return [], 0, False
  
      async def _wake_destinations_needing_catchup(self) -> None:
          """
          Wakes up destinations that need catch-up and are not currently being
          backed off from.
  
          In order to reduce load spikes, adds a delay between each destination.
          """
  
          last_processed: Optional[str] = None
  
          while True:
              destinations_to_wake = (
                  await self.store.get_catch_up_outstanding_destinations(last_processed)
              )
  
              if not destinations_to_wake:
                  # finished waking all destinations!
                  self._catchup_after_startup_timer = None
                  break
  
              last_processed = destinations_to_wake[-1]
  
              destinations_to_wake = [
                  d
                  for d in destinations_to_wake
                  if self._federation_shard_config.should_handle(self._instance_name, d)
              ]
  
              for destination in destinations_to_wake:
                  logger.info(
                      "Destination %s has outstanding catch-up, waking up.",
                      last_processed,
                  )
                  self.wake_destination(destination)
                  await self.clock.sleep(CATCH_UP_STARTUP_INTERVAL_SEC)
  