summary refs log tree commit diff
path: root/synapse/federation/sender/__init__.py
blob: 720d7bd74d078617e5d55716a7cc036fb888661f (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
# 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, 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) -> None:
        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 _PresenceQueue:
    """A queue of destinations that need to be woken up due to new presence
    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("_PresenceQueue.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_datastore()
        self.state = hs.get_state_handler()

        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._presence_queue = _PresenceQueue(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 %s -> %s", last_token, next_token)

                if not events and next_token >= self._last_poked_id:
                    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:
                        return

                    if not event.internal_metadata.should_proactively_send():
                        return

                    destinations: Optional[Set[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

                    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.
                        destinations.discard(send_on_behalf_of)

                    logger.debug("Sending %s to %r", event, destinations)

                    if destinations:
                        await self._send_pdu(event, 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: Iterable[EventBase]) -> None:
                    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,
                    )
                )

                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.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._presence_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) -> 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

        self._get_per_destination_queue(destination).attempt_new_transaction()

    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)