referrerpolicy=no-referrer-when-downgrade
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
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

use crate::{
	state_machine::{ConsensusGossip, TopicNotification, PERIODIC_MAINTENANCE_INTERVAL},
	Network, Syncing, Validator,
};

use sc_network::{
	service::traits::{NotificationEvent, ValidationResult},
	types::ProtocolName,
	NotificationService, ReputationChange,
};
use sc_network_sync::SyncEvent;

use futures::{
	channel::mpsc::{channel, Receiver, Sender},
	prelude::*,
};
use log::trace;
use prometheus_endpoint::Registry;
use sc_network_types::PeerId;
use sp_runtime::traits::Block as BlockT;
use std::{
	collections::{HashMap, VecDeque},
	pin::Pin,
	sync::Arc,
	task::{Context, Poll},
};

/// Wraps around an implementation of the [`Network`] trait and provides gossiping capabilities on
/// top of it.
pub struct GossipEngine<B: BlockT> {
	state_machine: ConsensusGossip<B>,
	network: Box<dyn Network<B> + Send>,
	sync: Box<dyn Syncing<B>>,
	periodic_maintenance_interval: futures_timer::Delay,
	protocol: ProtocolName,

	/// Incoming events from the syncing service.
	sync_event_stream: Pin<Box<dyn Stream<Item = SyncEvent> + Send>>,
	/// Handle for polling notification-related events.
	notification_service: Box<dyn NotificationService>,
	/// Outgoing events to the consumer.
	message_sinks: HashMap<B::Hash, Vec<Sender<TopicNotification>>>,
	/// Buffered messages (see [`ForwardingState`]).
	forwarding_state: ForwardingState<B>,

	is_terminated: bool,
}

/// A gossip engine receives messages from the network via the `network_event_stream` and forwards
/// them to upper layers via the `message_sinks`. In the scenario where messages have been received
/// from the network but a subscribed message sink is not yet ready to receive the messages, the
/// messages are buffered. To model this process a gossip engine can be in two states.
enum ForwardingState<B: BlockT> {
	/// The gossip engine is currently not forwarding any messages and will poll the network for
	/// more messages to forward.
	Idle,
	/// The gossip engine is in the progress of forwarding messages and thus will not poll the
	/// network for more messages until it has send all current messages into the subscribed
	/// message sinks.
	Busy(VecDeque<(B::Hash, TopicNotification)>),
}

impl<B: BlockT> Unpin for GossipEngine<B> {}

impl<B: BlockT> GossipEngine<B> {
	/// Create a new instance.
	pub fn new<N, S>(
		network: N,
		sync: S,
		notification_service: Box<dyn NotificationService>,
		protocol: impl Into<ProtocolName>,
		validator: Arc<dyn Validator<B>>,
		metrics_registry: Option<&Registry>,
	) -> Self
	where
		B: 'static,
		N: Network<B> + Send + Clone + 'static,
		S: Syncing<B> + Send + Clone + 'static,
	{
		let protocol = protocol.into();
		let sync_event_stream = sync.event_stream("network-gossip");

		GossipEngine {
			state_machine: ConsensusGossip::new(validator, protocol.clone(), metrics_registry),
			network: Box::new(network),
			sync: Box::new(sync),
			notification_service,
			periodic_maintenance_interval: futures_timer::Delay::new(PERIODIC_MAINTENANCE_INTERVAL),
			protocol,

			sync_event_stream,
			message_sinks: HashMap::new(),
			forwarding_state: ForwardingState::Idle,

			is_terminated: false,
		}
	}

	pub fn report(&self, who: PeerId, reputation: ReputationChange) {
		self.network.report_peer(who, reputation);
	}

	/// Registers a message without propagating it to any peers. The message
	/// becomes available to new peers or when the service is asked to gossip
	/// the message's topic. No validation is performed on the message, if the
	/// message is already expired it should be dropped on the next garbage
	/// collection.
	pub fn register_gossip_message(&mut self, topic: B::Hash, message: Vec<u8>) {
		self.state_machine.register_message(topic, message);
	}

	/// Broadcast all messages with given topic.
	pub fn broadcast_topic(&mut self, topic: B::Hash, force: bool) {
		self.state_machine.broadcast_topic(&mut self.notification_service, topic, force);
	}

	/// Get data of valid, incoming messages for a topic (but might have expired meanwhile).
	pub fn messages_for(&mut self, topic: B::Hash) -> Receiver<TopicNotification> {
		let past_messages = self.state_machine.messages_for(topic).collect::<Vec<_>>();
		// The channel length is not critical for correctness. By the implementation of `channel`
		// each sender is guaranteed a single buffer slot, making it a non-rendezvous channel and
		// thus preventing direct dead-locks. A minimum channel length of 10 is an estimate based on
		// the fact that despite `NotificationsReceived` having a `Vec` of messages, it only ever
		// contains a single message.
		let (mut tx, rx) = channel(usize::max(past_messages.len(), 10));

		for notification in past_messages {
			tx.try_send(notification)
				.expect("receiver known to be live, and buffer size known to suffice; qed");
		}

		self.message_sinks.entry(topic).or_default().push(tx);

		rx
	}

	/// Send all messages with given topic to a peer.
	pub fn send_topic(&mut self, who: &PeerId, topic: B::Hash, force: bool) {
		self.state_machine.send_topic(&mut self.notification_service, who, topic, force)
	}

	/// Multicast a message to all peers.
	pub fn gossip_message(&mut self, topic: B::Hash, message: Vec<u8>, force: bool) {
		self.state_machine
			.multicast(&mut self.notification_service, topic, message, force)
	}

	/// Send addressed message to the given peers. The message is not kept or multicast
	/// later on.
	pub fn send_message(&mut self, who: Vec<PeerId>, data: Vec<u8>) {
		for who in &who {
			self.state_machine
				.send_message(&mut self.notification_service, who, data.clone());
		}
	}

	/// Notify everyone we're connected to that we have the given block.
	///
	/// Note: this method isn't strictly related to gossiping and should eventually be moved
	/// somewhere else.
	pub fn announce(&self, block: B::Hash, associated_data: Option<Vec<u8>>) {
		self.sync.announce_block(block, associated_data);
	}

	/// Consume [`GossipEngine`] and return the notification service.
	pub fn take_notification_service(self) -> Box<dyn NotificationService> {
		self.notification_service
	}
}

impl<B: BlockT> Future for GossipEngine<B> {
	type Output = ();

	fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
		let this = &mut *self;

		'outer: loop {
			match &mut this.forwarding_state {
				ForwardingState::Idle => {
					let next_notification_event =
						this.notification_service.next_event().poll_unpin(cx);
					let sync_event_stream = this.sync_event_stream.poll_next_unpin(cx);

					if next_notification_event.is_pending() && sync_event_stream.is_pending() {
						break
					}

					match next_notification_event {
						Poll::Ready(Some(event)) => match event {
							NotificationEvent::ValidateInboundSubstream {
								peer,
								handshake,
								result_tx,
								..
							} => {
								// only accept peers whose role can be determined
								let result = this
									.network
									.peer_role(peer, handshake)
									.map_or(ValidationResult::Reject, |_| ValidationResult::Accept);
								let _ = result_tx.send(result);
							},
							NotificationEvent::NotificationStreamOpened {
								peer, handshake, ..
							} => {
								let Some(role) = this.network.peer_role(peer, handshake) else {
									log::debug!(target: "gossip", "role for {peer} couldn't be determined");
									continue
								};

								this.state_machine.new_peer(
									&mut this.notification_service,
									peer,
									role,
								);
							},
							NotificationEvent::NotificationStreamClosed { peer } => {
								this.state_machine
									.peer_disconnected(&mut this.notification_service, peer);
							},
							NotificationEvent::NotificationReceived { peer, notification } => {
								let to_forward = this.state_machine.on_incoming(
									&mut *this.network,
									&mut this.notification_service,
									peer,
									vec![notification],
								);
								this.forwarding_state = ForwardingState::Busy(to_forward.into());
							},
						},
						// The network event stream closed. Do the same for [`GossipValidator`].
						Poll::Ready(None) => {
							self.is_terminated = true;
							return Poll::Ready(())
						},
						Poll::Pending => {},
					}

					match sync_event_stream {
						Poll::Ready(Some(event)) => match event {
							SyncEvent::PeerConnected(remote) =>
								this.network.add_set_reserved(remote, this.protocol.clone()),
							SyncEvent::PeerDisconnected(remote) =>
								this.network.remove_set_reserved(remote, this.protocol.clone()),
						},
						// The sync event stream closed. Do the same for [`GossipValidator`].
						Poll::Ready(None) => {
							self.is_terminated = true;
							return Poll::Ready(())
						},
						Poll::Pending => {},
					}
				},
				ForwardingState::Busy(to_forward) => {
					let (topic, notification) = match to_forward.pop_front() {
						Some(n) => n,
						None => {
							this.forwarding_state = ForwardingState::Idle;
							continue
						},
					};

					let sinks = match this.message_sinks.get_mut(&topic) {
						Some(sinks) => sinks,
						None => continue,
					};

					// Make sure all sinks for the given topic are ready.
					for sink in sinks.iter_mut() {
						match sink.poll_ready(cx) {
							Poll::Ready(Ok(())) => {},
							// Receiver has been dropped. Ignore for now, filtered out in (1).
							Poll::Ready(Err(_)) => {},
							Poll::Pending => {
								// Push back onto queue for later.
								to_forward.push_front((topic, notification));
								break 'outer
							},
						}
					}

					// Filter out all closed sinks.
					sinks.retain(|sink| !sink.is_closed()); // (1)

					if sinks.is_empty() {
						this.message_sinks.remove(&topic);
						continue
					}

					trace!(
						target: "gossip",
						"Pushing consensus message to sinks for {}.", topic,
					);

					// Send the notification on each sink.
					for sink in sinks {
						match sink.start_send(notification.clone()) {
							Ok(()) => {},
							Err(e) if e.is_full() => {
								unreachable!("Previously ensured that all sinks are ready; qed.")
							},
							// Receiver got dropped. Will be removed in next iteration (See (1)).
							Err(_) => {},
						}
					}
				},
			}
		}

		while let Poll::Ready(()) = this.periodic_maintenance_interval.poll_unpin(cx) {
			this.periodic_maintenance_interval.reset(PERIODIC_MAINTENANCE_INTERVAL);
			this.state_machine.tick(&mut this.notification_service);

			this.message_sinks.retain(|_, sinks| {
				sinks.retain(|sink| !sink.is_closed());
				!sinks.is_empty()
			});
		}

		Poll::Pending
	}
}

impl<B: BlockT> futures::future::FusedFuture for GossipEngine<B> {
	fn is_terminated(&self) -> bool {
		self.is_terminated
	}
}

#[cfg(test)]
mod tests {
	use super::*;
	use crate::{ValidationResult, ValidatorContext};
	use codec::{DecodeAll, Encode};
	use futures::{
		channel::mpsc::{unbounded, UnboundedReceiver, UnboundedSender},
		executor::{block_on, block_on_stream},
		future::poll_fn,
	};
	use quickcheck::{Arbitrary, Gen, QuickCheck};
	use sc_network::{
		config::MultiaddrWithPeerId,
		service::traits::{Direction, MessageSink, NotificationEvent},
		Event, NetworkBlock, NetworkEventStream, NetworkPeers, NotificationService, Roles,
	};
	use sc_network_common::role::ObservedRole;
	use sc_network_sync::SyncEventStream;
	use sc_network_types::multiaddr::Multiaddr;
	use sp_runtime::{
		testing::H256,
		traits::{Block as BlockT, NumberFor},
	};
	use std::{
		collections::HashSet,
		sync::{Arc, Mutex},
	};
	use substrate_test_runtime_client::runtime::Block;

	#[derive(Clone, Default)]
	struct TestNetwork {}

	#[async_trait::async_trait]
	impl NetworkPeers for TestNetwork {
		fn set_authorized_peers(&self, _peers: HashSet<PeerId>) {
			unimplemented!();
		}

		fn set_authorized_only(&self, _reserved_only: bool) {
			unimplemented!();
		}

		fn add_known_address(&self, _peer_id: PeerId, _addr: Multiaddr) {
			unimplemented!();
		}

		fn report_peer(&self, _peer_id: PeerId, _cost_benefit: ReputationChange) {}

		fn peer_reputation(&self, _peer_id: &PeerId) -> i32 {
			unimplemented!()
		}

		fn disconnect_peer(&self, _peer_id: PeerId, _protocol: ProtocolName) {
			unimplemented!();
		}

		fn accept_unreserved_peers(&self) {
			unimplemented!();
		}

		fn deny_unreserved_peers(&self) {
			unimplemented!();
		}

		fn add_reserved_peer(&self, _peer: MultiaddrWithPeerId) -> Result<(), String> {
			unimplemented!();
		}

		fn remove_reserved_peer(&self, _peer_id: PeerId) {
			unimplemented!();
		}

		fn set_reserved_peers(
			&self,
			_protocol: ProtocolName,
			_peers: HashSet<Multiaddr>,
		) -> Result<(), String> {
			unimplemented!();
		}

		fn add_peers_to_reserved_set(
			&self,
			_protocol: ProtocolName,
			_peers: HashSet<Multiaddr>,
		) -> Result<(), String> {
			unimplemented!();
		}

		fn remove_peers_from_reserved_set(
			&self,
			_protocol: ProtocolName,
			_peers: Vec<PeerId>,
		) -> Result<(), String> {
			unimplemented!();
		}

		fn sync_num_connected(&self) -> usize {
			unimplemented!();
		}

		fn peer_role(&self, _peer_id: PeerId, handshake: Vec<u8>) -> Option<ObservedRole> {
			Roles::decode_all(&mut &handshake[..])
				.ok()
				.and_then(|role| Some(ObservedRole::from(role)))
		}

		async fn reserved_peers(&self) -> Result<Vec<PeerId>, ()> {
			unimplemented!();
		}
	}

	impl NetworkEventStream for TestNetwork {
		fn event_stream(&self, _name: &'static str) -> Pin<Box<dyn Stream<Item = Event> + Send>> {
			unimplemented!();
		}
	}

	impl NetworkBlock<<Block as BlockT>::Hash, NumberFor<Block>> for TestNetwork {
		fn announce_block(&self, _hash: <Block as BlockT>::Hash, _data: Option<Vec<u8>>) {
			unimplemented!();
		}

		fn new_best_block_imported(
			&self,
			_hash: <Block as BlockT>::Hash,
			_number: NumberFor<Block>,
		) {
			unimplemented!();
		}
	}

	#[derive(Clone, Default)]
	struct TestSync {
		inner: Arc<Mutex<TestSyncInner>>,
	}

	#[derive(Clone, Default)]
	struct TestSyncInner {
		event_senders: Vec<UnboundedSender<SyncEvent>>,
	}

	impl SyncEventStream for TestSync {
		fn event_stream(
			&self,
			_name: &'static str,
		) -> Pin<Box<dyn Stream<Item = SyncEvent> + Send>> {
			let (tx, rx) = unbounded();
			self.inner.lock().unwrap().event_senders.push(tx);

			Box::pin(rx)
		}
	}

	impl NetworkBlock<<Block as BlockT>::Hash, NumberFor<Block>> for TestSync {
		fn announce_block(&self, _hash: <Block as BlockT>::Hash, _data: Option<Vec<u8>>) {
			unimplemented!();
		}

		fn new_best_block_imported(
			&self,
			_hash: <Block as BlockT>::Hash,
			_number: NumberFor<Block>,
		) {
			unimplemented!();
		}
	}

	#[derive(Debug)]
	pub(crate) struct TestNotificationService {
		rx: UnboundedReceiver<NotificationEvent>,
	}

	#[async_trait::async_trait]
	impl sc_network::service::traits::NotificationService for TestNotificationService {
		async fn open_substream(&mut self, _peer: PeerId) -> Result<(), ()> {
			unimplemented!();
		}

		async fn close_substream(&mut self, _peer: PeerId) -> Result<(), ()> {
			unimplemented!();
		}

		fn send_sync_notification(&mut self, _peer: &PeerId, _notification: Vec<u8>) {
			unimplemented!();
		}

		async fn send_async_notification(
			&mut self,
			_peer: &PeerId,
			_notification: Vec<u8>,
		) -> Result<(), sc_network::error::Error> {
			unimplemented!();
		}

		async fn set_handshake(&mut self, _handshake: Vec<u8>) -> Result<(), ()> {
			unimplemented!();
		}

		fn try_set_handshake(&mut self, _handshake: Vec<u8>) -> Result<(), ()> {
			unimplemented!();
		}

		async fn next_event(&mut self) -> Option<NotificationEvent> {
			self.rx.next().await
		}

		fn clone(&mut self) -> Result<Box<dyn NotificationService>, ()> {
			unimplemented!();
		}

		fn protocol(&self) -> &ProtocolName {
			unimplemented!();
		}

		fn message_sink(&self, _peer: &PeerId) -> Option<Box<dyn MessageSink>> {
			unimplemented!();
		}
	}

	struct AllowAll;
	impl Validator<Block> for AllowAll {
		fn validate(
			&self,
			_context: &mut dyn ValidatorContext<Block>,
			_sender: &PeerId,
			_data: &[u8],
		) -> ValidationResult<H256> {
			ValidationResult::ProcessAndKeep(H256::default())
		}
	}

	/// Regression test for the case where the `GossipEngine.network_event_stream` closes. One
	/// should not ignore a `Poll::Ready(None)` as `poll_next_unpin` will panic on subsequent calls.
	///
	/// See https://github.com/paritytech/substrate/issues/5000 for details.
	#[test]
	fn returns_when_network_event_stream_closes() {
		let network = TestNetwork::default();
		let sync = Arc::new(TestSync::default());
		let (tx, rx) = unbounded();
		let notification_service = Box::new(TestNotificationService { rx });
		let mut gossip_engine = GossipEngine::<Block>::new(
			network.clone(),
			sync,
			notification_service,
			"/my_protocol",
			Arc::new(AllowAll {}),
			None,
		);

		// drop notification service sender side.
		drop(tx);

		block_on(poll_fn(move |ctx| {
			if let Poll::Pending = gossip_engine.poll_unpin(ctx) {
				panic!(
					"Expected gossip engine to finish on first poll, given that \
					 `GossipEngine.network_event_stream` closes right away."
				)
			}
			Poll::Ready(())
		}))
	}

	#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
	async fn keeps_multiple_subscribers_per_topic_updated_with_both_old_and_new_messages() {
		let topic = H256::default();
		let protocol = ProtocolName::from("/my_protocol");
		let remote_peer = PeerId::random();
		let network = TestNetwork::default();
		let sync = Arc::new(TestSync::default());
		let (mut tx, rx) = unbounded();
		let notification_service = Box::new(TestNotificationService { rx });

		let mut gossip_engine = GossipEngine::<Block>::new(
			network.clone(),
			sync.clone(),
			notification_service,
			protocol.clone(),
			Arc::new(AllowAll {}),
			None,
		);

		// Register the remote peer.
		tx.send(NotificationEvent::NotificationStreamOpened {
			peer: remote_peer,
			direction: Direction::Inbound,
			negotiated_fallback: None,
			handshake: Roles::FULL.encode(),
		})
		.await
		.unwrap();

		let messages = vec![vec![1], vec![2]];

		// Send first event before subscribing.
		tx.send(NotificationEvent::NotificationReceived {
			peer: remote_peer,
			notification: messages[0].clone().into(),
		})
		.await
		.unwrap();

		let mut subscribers = vec![];
		for _ in 0..2 {
			subscribers.push(gossip_engine.messages_for(topic));
		}

		// Send second event after subscribing.
		tx.send(NotificationEvent::NotificationReceived {
			peer: remote_peer,
			notification: messages[1].clone().into(),
		})
		.await
		.unwrap();

		tokio::spawn(gossip_engine);

		// Note: `block_on_stream()`-derived iterator will block the current thread,
		//       so we need a `multi_thread` `tokio::test` runtime flavor.
		let mut subscribers =
			subscribers.into_iter().map(|s| block_on_stream(s)).collect::<Vec<_>>();

		// Expect each subscriber to receive both events.
		for message in messages {
			for subscriber in subscribers.iter_mut() {
				assert_eq!(
					subscriber.next(),
					Some(TopicNotification { message: message.clone(), sender: Some(remote_peer) }),
				);
			}
		}
	}

	#[test]
	fn forwarding_to_different_size_and_topic_channels() {
		#[derive(Clone, Debug)]
		struct ChannelLengthAndTopic {
			length: usize,
			topic: H256,
		}

		impl Arbitrary for ChannelLengthAndTopic {
			fn arbitrary(g: &mut Gen) -> Self {
				let possible_length = (0..100).collect::<Vec<usize>>();
				let possible_topics = (0..10).collect::<Vec<u64>>();
				Self {
					length: *g.choose(&possible_length).unwrap(),
					// Make sure channel topics and message topics overlap by choosing a small
					// range.
					topic: H256::from_low_u64_ne(*g.choose(&possible_topics).unwrap()),
				}
			}
		}

		#[derive(Clone, Debug)]
		struct Message {
			topic: H256,
		}

		impl Arbitrary for Message {
			fn arbitrary(g: &mut Gen) -> Self {
				let possible_topics = (0..10).collect::<Vec<u64>>();
				Self {
					// Make sure channel topics and message topics overlap by choosing a small
					// range.
					topic: H256::from_low_u64_ne(*g.choose(&possible_topics).unwrap()),
				}
			}
		}

		/// Validator that always returns `ProcessAndKeep` interpreting the first 32 bytes of data
		/// as the message topic.
		struct TestValidator;

		impl Validator<Block> for TestValidator {
			fn validate(
				&self,
				_context: &mut dyn ValidatorContext<Block>,
				_sender: &PeerId,
				data: &[u8],
			) -> ValidationResult<H256> {
				ValidationResult::ProcessAndKeep(H256::from_slice(&data[0..32]))
			}
		}

		fn prop(channels: Vec<ChannelLengthAndTopic>, notifications: Vec<Vec<Message>>) {
			let protocol = ProtocolName::from("/my_protocol");
			let remote_peer = PeerId::random();
			let network = TestNetwork::default();
			let sync = Arc::new(TestSync::default());
			let (mut tx, rx) = unbounded();
			let notification_service = Box::new(TestNotificationService { rx });

			let num_channels_per_topic = channels.iter().fold(
				HashMap::new(),
				|mut acc, ChannelLengthAndTopic { topic, .. }| {
					acc.entry(topic).and_modify(|e| *e += 1).or_insert(1);
					acc
				},
			);

			let expected_msgs_per_topic_all_chan = notifications
				.iter()
				.fold(HashMap::new(), |mut acc, messages| {
					for message in messages {
						acc.entry(message.topic).and_modify(|e| *e += 1).or_insert(1);
					}
					acc
				})
				.into_iter()
				// Messages are cloned for each channel with the corresponding topic, thus multiply
				// with the amount of channels per topic. If there is no channel for a given topic,
				// don't expect any messages for the topic to be received.
				.map(|(topic, num)| (topic, num_channels_per_topic.get(&topic).unwrap_or(&0) * num))
				.collect::<HashMap<H256, _>>();

			let mut gossip_engine = GossipEngine::<Block>::new(
				network.clone(),
				sync.clone(),
				notification_service,
				protocol.clone(),
				Arc::new(TestValidator {}),
				None,
			);

			// Create channels.
			let (txs, mut rxs) = channels
				.iter()
				.map(|ChannelLengthAndTopic { length, topic }| (*topic, channel(*length)))
				.fold((vec![], vec![]), |mut acc, (topic, (tx, rx))| {
					acc.0.push((topic, tx));
					acc.1.push((topic, rx));
					acc
				});

			// Insert sender sides into `gossip_engine`.
			for (topic, tx) in txs {
				match gossip_engine.message_sinks.get_mut(&topic) {
					Some(entry) => entry.push(tx),
					None => {
						gossip_engine.message_sinks.insert(topic, vec![tx]);
					},
				}
			}

			// Register the remote peer.
			tx.start_send(NotificationEvent::NotificationStreamOpened {
				peer: remote_peer,
				direction: Direction::Inbound,
				negotiated_fallback: None,
				handshake: Roles::FULL.encode(),
			})
			.unwrap();

			// Send messages into the network event stream.
			for (i_notification, messages) in notifications.iter().enumerate() {
				let messages: Vec<Vec<u8>> = messages
					.into_iter()
					.enumerate()
					.map(|(i_message, Message { topic })| {
						// Embed the topic in the first 256 bytes of the message to be extracted by
						// the [`TestValidator`] later on.
						let mut message = topic.as_bytes().to_vec();

						// Make sure the message is unique via `i_notification` and `i_message` to
						// ensure [`ConsensusBridge`] does not deduplicate it.
						message.push(i_notification.try_into().unwrap());
						message.push(i_message.try_into().unwrap());

						message.into()
					})
					.collect();

				for message in messages {
					tx.start_send(NotificationEvent::NotificationReceived {
						peer: remote_peer,
						notification: message,
					})
					.unwrap();
				}
			}

			let mut received_msgs_per_topic_all_chan = HashMap::<H256, _>::new();

			// Poll both gossip engine and each receiver and track the amount of received messages.
			block_on(poll_fn(|cx| {
				loop {
					if let Poll::Ready(()) = gossip_engine.poll_unpin(cx) {
						unreachable!(
							"Event stream sender side is not dropped, thus gossip engine does not \
							 terminate",
						);
					}

					let mut progress = false;

					for (topic, rx) in rxs.iter_mut() {
						match rx.poll_next_unpin(cx) {
							Poll::Ready(Some(_)) => {
								progress = true;
								received_msgs_per_topic_all_chan
									.entry(*topic)
									.and_modify(|e| *e += 1)
									.or_insert(1);
							},
							Poll::Ready(None) => {
								unreachable!("Sender side of channel is never dropped")
							},
							Poll::Pending => {},
						}
					}

					if !progress {
						break
					}
				}
				Poll::Ready(())
			}));

			// Compare amount of expected messages with amount of received messages.
			for (expected_topic, expected_num) in expected_msgs_per_topic_all_chan.iter() {
				assert_eq!(
					received_msgs_per_topic_all_chan.get(&expected_topic).unwrap_or(&0),
					expected_num,
				);
			}
			for (received_topic, received_num) in expected_msgs_per_topic_all_chan.iter() {
				assert_eq!(
					expected_msgs_per_topic_all_chan.get(&received_topic).unwrap_or(&0),
					received_num,
				);
			}
		}

		// Past regressions.
		prop(vec![], vec![vec![Message { topic: H256::default() }]]);
		prop(
			vec![ChannelLengthAndTopic { length: 71, topic: H256::default() }],
			vec![vec![Message { topic: H256::default() }]],
		);

		QuickCheck::new().quickcheck(prop as fn(_, _))
	}
}