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
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
// 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/>.

//! Collection of request-response protocols.
//!
//! The [`RequestResponsesBehaviour`] struct defined in this module provides support for zero or
//! more so-called "request-response" protocols.
//!
//! A request-response protocol works in the following way:
//!
//! - For every emitted request, a new substream is open and the protocol is negotiated. If the
//! remote supports the protocol, the size of the request is sent as a LEB128 number, followed
//! with the request itself. The remote then sends the size of the response as a LEB128 number,
//! followed with the response.
//!
//! - Requests have a certain time limit before they time out. This time includes the time it
//! takes to send/receive the request and response.
//!
//! - If provided, a ["requests processing"](ProtocolConfig::inbound_queue) channel
//! is used to handle incoming requests.

use crate::{
	peer_store::{PeerStoreProvider, BANNED_THRESHOLD},
	types::ProtocolName,
	ReputationChange,
};

use futures::{channel::oneshot, prelude::*};
use libp2p::{
	core::{Endpoint, Multiaddr},
	request_response::{self, Behaviour, Codec, Message, ProtocolSupport, ResponseChannel},
	swarm::{
		behaviour::{ConnectionClosed, FromSwarm},
		handler::multi::MultiHandler,
		ConnectionDenied, ConnectionId, NetworkBehaviour, PollParameters, THandler,
		THandlerInEvent, THandlerOutEvent, ToSwarm,
	},
	PeerId,
};

use std::{
	collections::{hash_map::Entry, HashMap},
	io, iter,
	pin::Pin,
	task::{Context, Poll},
	time::{Duration, Instant},
};

pub use libp2p::request_response::{Config, InboundFailure, OutboundFailure, RequestId};

/// Error in a request.
#[derive(Debug, thiserror::Error)]
#[allow(missing_docs)]
pub enum RequestFailure {
	#[error("We are not currently connected to the requested peer.")]
	NotConnected,
	#[error("Given protocol hasn't been registered.")]
	UnknownProtocol,
	#[error("Remote has closed the substream before answering, thereby signaling that it considers the request as valid, but refused to answer it.")]
	Refused,
	#[error("The remote replied, but the local node is no longer interested in the response.")]
	Obsolete,
	#[error("Problem on the network: {0}")]
	Network(OutboundFailure),
}

/// Configuration for a single request-response protocol.
#[derive(Debug, Clone)]
pub struct ProtocolConfig {
	/// Name of the protocol on the wire. Should be something like `/foo/bar`.
	pub name: ProtocolName,

	/// Fallback on the wire protocol names to support.
	pub fallback_names: Vec<ProtocolName>,

	/// Maximum allowed size, in bytes, of a request.
	///
	/// Any request larger than this value will be declined as a way to avoid allocating too
	/// much memory for it.
	pub max_request_size: u64,

	/// Maximum allowed size, in bytes, of a response.
	///
	/// Any response larger than this value will be declined as a way to avoid allocating too
	/// much memory for it.
	pub max_response_size: u64,

	/// Duration after which emitted requests are considered timed out.
	///
	/// If you expect the response to come back quickly, you should set this to a smaller duration.
	pub request_timeout: Duration,

	/// Channel on which the networking service will send incoming requests.
	///
	/// Every time a peer sends a request to the local node using this protocol, the networking
	/// service will push an element on this channel. The receiving side of this channel then has
	/// to pull this element, process the request, and send back the response to send back to the
	/// peer.
	///
	/// The size of the channel has to be carefully chosen. If the channel is full, the networking
	/// service will discard the incoming request send back an error to the peer. Consequently,
	/// the channel being full is an indicator that the node is overloaded.
	///
	/// You can typically set the size of the channel to `T / d`, where `T` is the
	/// `request_timeout` and `d` is the expected average duration of CPU and I/O it takes to
	/// build a response.
	///
	/// Can be `None` if the local node does not support answering incoming requests.
	/// If this is `None`, then the local node will not advertise support for this protocol towards
	/// other peers. If this is `Some` but the channel is closed, then the local node will
	/// advertise support for this protocol, but any incoming request will lead to an error being
	/// sent back.
	pub inbound_queue: Option<async_channel::Sender<IncomingRequest>>,
}

/// A single request received by a peer on a request-response protocol.
#[derive(Debug)]
pub struct IncomingRequest {
	/// Who sent the request.
	pub peer: PeerId,

	/// Request sent by the remote. Will always be smaller than
	/// [`ProtocolConfig::max_request_size`].
	pub payload: Vec<u8>,

	/// Channel to send back the response.
	///
	/// There are two ways to indicate that handling the request failed:
	///
	/// 1. Drop `pending_response` and thus not changing the reputation of the peer.
	///
	/// 2. Sending an `Err(())` via `pending_response`, optionally including reputation changes for
	/// the given peer.
	pub pending_response: oneshot::Sender<OutgoingResponse>,
}

/// Response for an incoming request to be send by a request protocol handler.
#[derive(Debug)]
pub struct OutgoingResponse {
	/// The payload of the response.
	///
	/// `Err(())` if none is available e.g. due an error while handling the request.
	pub result: Result<Vec<u8>, ()>,

	/// Reputation changes accrued while handling the request. To be applied to the reputation of
	/// the peer sending the request.
	pub reputation_changes: Vec<ReputationChange>,

	/// If provided, the `oneshot::Sender` will be notified when the request has been sent to the
	/// peer.
	///
	/// > **Note**: Operating systems typically maintain a buffer of a few dozen kilobytes of
	/// >			outgoing data for each TCP socket, and it is not possible for a user
	/// >			application to inspect this buffer. This channel here is not actually notified
	/// >			when the response has been fully sent out, but rather when it has fully been
	/// >			written to the buffer managed by the operating system.
	pub sent_feedback: Option<oneshot::Sender<()>>,
}

/// When sending a request, what to do on a disconnected recipient.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum IfDisconnected {
	/// Try to connect to the peer.
	TryConnect,
	/// Just fail if the destination is not yet connected.
	ImmediateError,
}

/// Convenience functions for `IfDisconnected`.
impl IfDisconnected {
	/// Shall we connect to a disconnected peer?
	pub fn should_connect(self) -> bool {
		match self {
			Self::TryConnect => true,
			Self::ImmediateError => false,
		}
	}
}

/// Event generated by the [`RequestResponsesBehaviour`].
#[derive(Debug)]
pub enum Event {
	/// A remote sent a request and either we have successfully answered it or an error happened.
	///
	/// This event is generated for statistics purposes.
	InboundRequest {
		/// Peer which has emitted the request.
		peer: PeerId,
		/// Name of the protocol in question.
		protocol: ProtocolName,
		/// Whether handling the request was successful or unsuccessful.
		///
		/// When successful contains the time elapsed between when we received the request and when
		/// we sent back the response. When unsuccessful contains the failure reason.
		result: Result<Duration, ResponseFailure>,
	},

	/// A request initiated using [`RequestResponsesBehaviour::send_request`] has succeeded or
	/// failed.
	///
	/// This event is generated for statistics purposes.
	RequestFinished {
		/// Peer that we send a request to.
		peer: PeerId,
		/// Name of the protocol in question.
		protocol: ProtocolName,
		/// Duration the request took.
		duration: Duration,
		/// Result of the request.
		result: Result<(), RequestFailure>,
	},

	/// A request protocol handler issued reputation changes for the given peer.
	ReputationChanges {
		/// Peer whose reputation needs to be adjust.
		peer: PeerId,
		/// Reputation changes.
		changes: Vec<ReputationChange>,
	},
}

/// Combination of a protocol name and a request id.
///
/// Uniquely identifies an inbound or outbound request among all handled protocols. Note however
/// that uniqueness is only guaranteed between two inbound and likewise between two outbound
/// requests. There is no uniqueness guarantee in a set of both inbound and outbound
/// [`ProtocolRequestId`]s.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct ProtocolRequestId {
	protocol: ProtocolName,
	request_id: RequestId,
}

impl From<(ProtocolName, RequestId)> for ProtocolRequestId {
	fn from((protocol, request_id): (ProtocolName, RequestId)) -> Self {
		Self { protocol, request_id }
	}
}

/// Implementation of `NetworkBehaviour` that provides support for request-response protocols.
pub struct RequestResponsesBehaviour {
	/// The multiple sub-protocols, by name.
	///
	/// Contains the underlying libp2p request-response [`Behaviour`], plus an optional
	/// "response builder" used to build responses for incoming requests.
	protocols: HashMap<
		ProtocolName,
		(Behaviour<GenericCodec>, Option<async_channel::Sender<IncomingRequest>>),
	>,

	/// Pending requests, passed down to a request-response [`Behaviour`], awaiting a reply.
	pending_requests:
		HashMap<ProtocolRequestId, (Instant, oneshot::Sender<Result<Vec<u8>, RequestFailure>>)>,

	/// Whenever an incoming request arrives, a `Future` is added to this list and will yield the
	/// start time and the response to send back to the remote.
	pending_responses: stream::FuturesUnordered<
		Pin<Box<dyn Future<Output = Option<RequestProcessingOutcome>> + Send>>,
	>,

	/// Whenever an incoming request arrives, the arrival [`Instant`] is recorded here.
	pending_responses_arrival_time: HashMap<ProtocolRequestId, Instant>,

	/// Whenever a response is received on `pending_responses`, insert a channel to be notified
	/// when the request has been sent out.
	send_feedback: HashMap<ProtocolRequestId, oneshot::Sender<()>>,

	/// Primarily used to get a reputation of a node.
	peer_store: Box<dyn PeerStoreProvider>,
}

/// Generated by the response builder and waiting to be processed.
struct RequestProcessingOutcome {
	peer: PeerId,
	request_id: RequestId,
	protocol: ProtocolName,
	inner_channel: ResponseChannel<Result<Vec<u8>, ()>>,
	response: OutgoingResponse,
}

impl RequestResponsesBehaviour {
	/// Creates a new behaviour. Must be passed a list of supported protocols. Returns an error if
	/// the same protocol is passed twice.
	pub fn new(
		list: impl Iterator<Item = ProtocolConfig>,
		peer_store: Box<dyn PeerStoreProvider>,
	) -> Result<Self, RegisterError> {
		let mut protocols = HashMap::new();
		for protocol in list {
			let mut cfg = Config::default();
			cfg.set_connection_keep_alive(Duration::from_secs(10));
			cfg.set_request_timeout(protocol.request_timeout);

			let protocol_support = if protocol.inbound_queue.is_some() {
				ProtocolSupport::Full
			} else {
				ProtocolSupport::Outbound
			};

			let rq_rp = Behaviour::new(
				GenericCodec {
					max_request_size: protocol.max_request_size,
					max_response_size: protocol.max_response_size,
				},
				iter::once(protocol.name.as_bytes().to_vec())
					.chain(protocol.fallback_names.iter().map(|name| name.as_bytes().to_vec()))
					.zip(iter::repeat(protocol_support)),
				cfg,
			);

			match protocols.entry(protocol.name) {
				Entry::Vacant(e) => e.insert((rq_rp, protocol.inbound_queue)),
				Entry::Occupied(e) => return Err(RegisterError::DuplicateProtocol(e.key().clone())),
			};
		}

		Ok(Self {
			protocols,
			pending_requests: Default::default(),
			pending_responses: Default::default(),
			pending_responses_arrival_time: Default::default(),
			send_feedback: Default::default(),
			peer_store,
		})
	}

	/// Initiates sending a request.
	///
	/// If there is no established connection to the target peer, the behavior is determined by the
	/// choice of `connect`.
	///
	/// An error is returned if the protocol doesn't match one that has been registered.
	pub fn send_request(
		&mut self,
		target: &PeerId,
		protocol_name: &str,
		request: Vec<u8>,
		pending_response: oneshot::Sender<Result<Vec<u8>, RequestFailure>>,
		connect: IfDisconnected,
	) {
		log::trace!(target: "sub-libp2p", "send request to {target} ({protocol_name:?}), {} bytes", request.len());

		if let Some((protocol, _)) = self.protocols.get_mut(protocol_name) {
			if protocol.is_connected(target) || connect.should_connect() {
				let request_id = protocol.send_request(target, request);
				let prev_req_id = self.pending_requests.insert(
					(protocol_name.to_string().into(), request_id).into(),
					(Instant::now(), pending_response),
				);
				debug_assert!(prev_req_id.is_none(), "Expect request id to be unique.");
			} else if pending_response.send(Err(RequestFailure::NotConnected)).is_err() {
				log::debug!(
					target: "sub-libp2p",
					"Not connected to peer {:?}. At the same time local \
					 node is no longer interested in the result.",
					target,
				);
			}
		} else if pending_response.send(Err(RequestFailure::UnknownProtocol)).is_err() {
			log::debug!(
				target: "sub-libp2p",
				"Unknown protocol {:?}. At the same time local \
				 node is no longer interested in the result.",
				protocol_name,
			);
		}
	}
}

impl NetworkBehaviour for RequestResponsesBehaviour {
	type ConnectionHandler =
		MultiHandler<String, <Behaviour<GenericCodec> as NetworkBehaviour>::ConnectionHandler>;
	type OutEvent = Event;

	fn handle_pending_inbound_connection(
		&mut self,
		_connection_id: ConnectionId,
		_local_addr: &Multiaddr,
		_remote_addr: &Multiaddr,
	) -> Result<(), ConnectionDenied> {
		Ok(())
	}

	fn handle_pending_outbound_connection(
		&mut self,
		_connection_id: ConnectionId,
		_maybe_peer: Option<PeerId>,
		_addresses: &[Multiaddr],
		_effective_role: Endpoint,
	) -> Result<Vec<Multiaddr>, ConnectionDenied> {
		Ok(Vec::new())
	}

	fn handle_established_inbound_connection(
		&mut self,
		connection_id: ConnectionId,
		peer: PeerId,
		local_addr: &Multiaddr,
		remote_addr: &Multiaddr,
	) -> Result<THandler<Self>, ConnectionDenied> {
		let iter = self.protocols.iter_mut().filter_map(|(p, (r, _))| {
			if let Ok(handler) = r.handle_established_inbound_connection(
				connection_id,
				peer,
				local_addr,
				remote_addr,
			) {
				Some((p.to_string(), handler))
			} else {
				None
			}
		});

		Ok(MultiHandler::try_from_iter(iter).expect(
			"Protocols are in a HashMap and there can be at most one handler per protocol name, \
			 which is the only possible error; qed",
		))
	}

	fn handle_established_outbound_connection(
		&mut self,
		connection_id: ConnectionId,
		peer: PeerId,
		addr: &Multiaddr,
		role_override: Endpoint,
	) -> Result<THandler<Self>, ConnectionDenied> {
		let iter = self.protocols.iter_mut().filter_map(|(p, (r, _))| {
			if let Ok(handler) =
				r.handle_established_outbound_connection(connection_id, peer, addr, role_override)
			{
				Some((p.to_string(), handler))
			} else {
				None
			}
		});

		Ok(MultiHandler::try_from_iter(iter).expect(
			"Protocols are in a HashMap and there can be at most one handler per protocol name, \
			 which is the only possible error; qed",
		))
	}

	fn on_swarm_event(&mut self, event: FromSwarm<Self::ConnectionHandler>) {
		match event {
			FromSwarm::ConnectionEstablished(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ConnectionEstablished(e));
				},
			FromSwarm::ConnectionClosed(ConnectionClosed {
				peer_id,
				connection_id,
				endpoint,
				handler,
				remaining_established,
			}) =>
				for (p_name, p_handler) in handler.into_iter() {
					if let Some((proto, _)) = self.protocols.get_mut(p_name.as_str()) {
						proto.on_swarm_event(FromSwarm::ConnectionClosed(ConnectionClosed {
							peer_id,
							connection_id,
							endpoint,
							handler: p_handler,
							remaining_established,
						}));
					} else {
						log::error!(
						  target: "sub-libp2p",
						  "on_swarm_event/connection_closed: no request-response instance registered for protocol {:?}",
						  p_name,
						)
					}
				},
			FromSwarm::DialFailure(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::DialFailure(e));
				},
			FromSwarm::ListenerClosed(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ListenerClosed(e));
				},
			FromSwarm::ListenFailure(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ListenFailure(e));
				},
			FromSwarm::ListenerError(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ListenerError(e));
				},
			FromSwarm::ExpiredExternalAddr(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ExpiredExternalAddr(e));
				},
			FromSwarm::NewListener(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::NewListener(e));
				},
			FromSwarm::ExpiredListenAddr(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::ExpiredListenAddr(e));
				},
			FromSwarm::NewExternalAddr(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::NewExternalAddr(e));
				},
			FromSwarm::AddressChange(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::AddressChange(e));
				},
			FromSwarm::NewListenAddr(e) =>
				for (p, _) in self.protocols.values_mut() {
					NetworkBehaviour::on_swarm_event(p, FromSwarm::NewListenAddr(e));
				},
		}
	}

	fn on_connection_handler_event(
		&mut self,
		peer_id: PeerId,
		connection_id: ConnectionId,
		event: THandlerOutEvent<Self>,
	) {
		let p_name = event.0;
		if let Some((proto, _)) = self.protocols.get_mut(p_name.as_str()) {
			return proto.on_connection_handler_event(peer_id, connection_id, event.1)
		} else {
			log::warn!(
				target: "sub-libp2p",
				"on_connection_handler_event: no request-response instance registered for protocol {:?}",
				p_name
			);
		}
	}

	fn poll(
		&mut self,
		cx: &mut Context,
		params: &mut impl PollParameters,
	) -> Poll<ToSwarm<Self::OutEvent, THandlerInEvent<Self>>> {
		'poll_all: loop {
			// Poll to see if any response is ready to be sent back.
			while let Poll::Ready(Some(outcome)) = self.pending_responses.poll_next_unpin(cx) {
				let RequestProcessingOutcome {
					peer,
					request_id,
					protocol: protocol_name,
					inner_channel,
					response: OutgoingResponse { result, reputation_changes, sent_feedback },
				} = match outcome {
					Some(outcome) => outcome,
					// The response builder was too busy or handling the request failed. This is
					// later on reported as a `InboundFailure::Omission`.
					None => continue,
				};

				if let Ok(payload) = result {
					if let Some((protocol, _)) = self.protocols.get_mut(&*protocol_name) {
						log::trace!(target: "sub-libp2p", "send response to {peer} ({protocol_name:?}), {} bytes", payload.len());

						if protocol.send_response(inner_channel, Ok(payload)).is_err() {
							// Note: Failure is handled further below when receiving
							// `InboundFailure` event from request-response [`Behaviour`].
							log::debug!(
								target: "sub-libp2p",
								"Failed to send response for {:?} on protocol {:?} due to a \
								 timeout or due to the connection to the peer being closed. \
								 Dropping response",
								request_id, protocol_name,
							);
						} else if let Some(sent_feedback) = sent_feedback {
							self.send_feedback
								.insert((protocol_name, request_id).into(), sent_feedback);
						}
					}
				}

				if !reputation_changes.is_empty() {
					return Poll::Ready(ToSwarm::GenerateEvent(Event::ReputationChanges {
						peer,
						changes: reputation_changes,
					}))
				}
			}

			// Poll request-responses protocols.
			for (protocol, (behaviour, resp_builder)) in &mut self.protocols {
				'poll_protocol: while let Poll::Ready(ev) = behaviour.poll(cx, params) {
					let ev = match ev {
						// Main events we are interested in.
						ToSwarm::GenerateEvent(ev) => ev,

						// Other events generated by the underlying behaviour are transparently
						// passed through.
						ToSwarm::Dial { opts } => {
							if opts.get_peer_id().is_none() {
								log::error!(
									"The request-response isn't supposed to start dialing addresses"
								);
							}
							return Poll::Ready(ToSwarm::Dial { opts })
						},
						ToSwarm::NotifyHandler { peer_id, handler, event } =>
							return Poll::Ready(ToSwarm::NotifyHandler {
								peer_id,
								handler,
								event: ((*protocol).to_string(), event),
							}),
						ToSwarm::ReportObservedAddr { address, score } =>
							return Poll::Ready(ToSwarm::ReportObservedAddr { address, score }),
						ToSwarm::CloseConnection { peer_id, connection } =>
							return Poll::Ready(ToSwarm::CloseConnection { peer_id, connection }),
					};

					match ev {
						// Received a request from a remote.
						request_response::Event::Message {
							peer,
							message: Message::Request { request_id, request, channel, .. },
						} => {
							self.pending_responses_arrival_time
								.insert((protocol.clone(), request_id).into(), Instant::now());

							let reputation = self.peer_store.peer_reputation(&peer);

							if reputation < BANNED_THRESHOLD {
								log::debug!(
									target: "sub-libp2p",
									"Cannot handle requests from a node with a low reputation {}: {}",
									peer,
									reputation,
								);
								continue 'poll_protocol
							}

							let (tx, rx) = oneshot::channel();

							// Submit the request to the "response builder" passed by the user at
							// initialization.
							if let Some(resp_builder) = resp_builder {
								// If the response builder is too busy, silently drop `tx`. This
								// will be reported by the corresponding request-response
								// [`Behaviour`] through an `InboundFailure::Omission` event.
								// Note that we use `async_channel::bounded` and not `mpsc::channel`
								// because the latter allocates an extra slot for every cloned
								// sender.
								let _ = resp_builder.try_send(IncomingRequest {
									peer,
									payload: request,
									pending_response: tx,
								});
							} else {
								debug_assert!(false, "Received message on outbound-only protocol.");
							}

							let protocol = protocol.clone();

							self.pending_responses.push(Box::pin(async move {
								// The `tx` created above can be dropped if we are not capable of
								// processing this request, which is reflected as a
								// `InboundFailure::Omission` event.
								rx.await.map_or(None, |response| {
									Some(RequestProcessingOutcome {
										peer,
										request_id,
										protocol,
										inner_channel: channel,
										response,
									})
								})
							}));

							// This `continue` makes sure that `pending_responses` gets polled
							// after we have added the new element.
							continue 'poll_all
						},

						// Received a response from a remote to one of our requests.
						request_response::Event::Message {
							peer,
							message: Message::Response { request_id, response },
							..
						} => {
							let (started, delivered) = match self
								.pending_requests
								.remove(&(protocol.clone(), request_id).into())
							{
								Some((started, pending_response)) => {
									log::trace!(
										target: "sub-libp2p",
										"received response from {peer} ({protocol:?}), {} bytes",
										response.as_ref().map_or(0usize, |response| response.len()),
									);

									let delivered = pending_response
										.send(response.map_err(|()| RequestFailure::Refused))
										.map_err(|_| RequestFailure::Obsolete);
									(started, delivered)
								},
								None => {
									log::warn!(
										target: "sub-libp2p",
										"Received `RequestResponseEvent::Message` with unexpected request id {:?}",
										request_id,
									);
									debug_assert!(false);
									continue
								},
							};

							let out = Event::RequestFinished {
								peer,
								protocol: protocol.clone(),
								duration: started.elapsed(),
								result: delivered,
							};

							return Poll::Ready(ToSwarm::GenerateEvent(out))
						},

						// One of our requests has failed.
						request_response::Event::OutboundFailure {
							peer,
							request_id,
							error,
							..
						} => {
							let started = match self
								.pending_requests
								.remove(&(protocol.clone(), request_id).into())
							{
								Some((started, pending_response)) => {
									if pending_response
										.send(Err(RequestFailure::Network(error.clone())))
										.is_err()
									{
										log::debug!(
											target: "sub-libp2p",
											"Request with id {:?} failed. At the same time local \
											 node is no longer interested in the result.",
											request_id,
										);
									}
									started
								},
								None => {
									log::warn!(
										target: "sub-libp2p",
										"Received `RequestResponseEvent::Message` with unexpected request id {:?}",
										request_id,
									);
									debug_assert!(false);
									continue
								},
							};

							let out = Event::RequestFinished {
								peer,
								protocol: protocol.clone(),
								duration: started.elapsed(),
								result: Err(RequestFailure::Network(error)),
							};

							return Poll::Ready(ToSwarm::GenerateEvent(out))
						},

						// An inbound request failed, either while reading the request or due to
						// failing to send a response.
						request_response::Event::InboundFailure {
							request_id, peer, error, ..
						} => {
							self.pending_responses_arrival_time
								.remove(&(protocol.clone(), request_id).into());
							self.send_feedback.remove(&(protocol.clone(), request_id).into());
							let out = Event::InboundRequest {
								peer,
								protocol: protocol.clone(),
								result: Err(ResponseFailure::Network(error)),
							};
							return Poll::Ready(ToSwarm::GenerateEvent(out))
						},

						// A response to an inbound request has been sent.
						request_response::Event::ResponseSent { request_id, peer } => {
							let arrival_time = self
								.pending_responses_arrival_time
								.remove(&(protocol.clone(), request_id).into())
								.map(|t| t.elapsed())
								.expect(
									"Time is added for each inbound request on arrival and only \
									 removed on success (`ResponseSent`) or failure \
									 (`InboundFailure`). One can not receive a success event for a \
									 request that either never arrived, or that has previously \
									 failed; qed.",
								);

							if let Some(send_feedback) =
								self.send_feedback.remove(&(protocol.clone(), request_id).into())
							{
								let _ = send_feedback.send(());
							}

							let out = Event::InboundRequest {
								peer,
								protocol: protocol.clone(),
								result: Ok(arrival_time),
							};

							return Poll::Ready(ToSwarm::GenerateEvent(out))
						},
					};
				}
			}

			break Poll::Pending
		}
	}
}

/// Error when registering a protocol.
#[derive(Debug, thiserror::Error)]
pub enum RegisterError {
	/// A protocol has been specified multiple times.
	#[error("{0}")]
	DuplicateProtocol(ProtocolName),
}

/// Error when processing a request sent by a remote.
#[derive(Debug, thiserror::Error)]
pub enum ResponseFailure {
	/// Problem on the network.
	#[error("Problem on the network: {0}")]
	Network(InboundFailure),
}

/// Implements the libp2p [`Codec`] trait. Defines how streams of bytes are turned
/// into requests and responses and vice-versa.
#[derive(Debug, Clone)]
#[doc(hidden)] // Needs to be public in order to satisfy the Rust compiler.
pub struct GenericCodec {
	max_request_size: u64,
	max_response_size: u64,
}

#[async_trait::async_trait]
impl Codec for GenericCodec {
	type Protocol = Vec<u8>;
	type Request = Vec<u8>;
	type Response = Result<Vec<u8>, ()>;

	async fn read_request<T>(
		&mut self,
		_: &Self::Protocol,
		mut io: &mut T,
	) -> io::Result<Self::Request>
	where
		T: AsyncRead + Unpin + Send,
	{
		// Read the length.
		let length = unsigned_varint::aio::read_usize(&mut io)
			.await
			.map_err(|err| io::Error::new(io::ErrorKind::InvalidInput, err))?;
		if length > usize::try_from(self.max_request_size).unwrap_or(usize::MAX) {
			return Err(io::Error::new(
				io::ErrorKind::InvalidInput,
				format!("Request size exceeds limit: {} > {}", length, self.max_request_size),
			))
		}

		// Read the payload.
		let mut buffer = vec![0; length];
		io.read_exact(&mut buffer).await?;
		Ok(buffer)
	}

	async fn read_response<T>(
		&mut self,
		_: &Self::Protocol,
		mut io: &mut T,
	) -> io::Result<Self::Response>
	where
		T: AsyncRead + Unpin + Send,
	{
		// Note that this function returns a `Result<Result<...>>`. Returning an `Err` is
		// considered as a protocol error and will result in the entire connection being closed.
		// Returning `Ok(Err(_))` signifies that a response has successfully been fetched, and
		// that this response is an error.

		// Read the length.
		let length = match unsigned_varint::aio::read_usize(&mut io).await {
			Ok(l) => l,
			Err(unsigned_varint::io::ReadError::Io(err))
				if matches!(err.kind(), io::ErrorKind::UnexpectedEof) =>
				return Ok(Err(())),
			Err(err) => return Err(io::Error::new(io::ErrorKind::InvalidInput, err)),
		};

		if length > usize::try_from(self.max_response_size).unwrap_or(usize::MAX) {
			return Err(io::Error::new(
				io::ErrorKind::InvalidInput,
				format!("Response size exceeds limit: {} > {}", length, self.max_response_size),
			))
		}

		// Read the payload.
		let mut buffer = vec![0; length];
		io.read_exact(&mut buffer).await?;
		Ok(Ok(buffer))
	}

	async fn write_request<T>(
		&mut self,
		_: &Self::Protocol,
		io: &mut T,
		req: Self::Request,
	) -> io::Result<()>
	where
		T: AsyncWrite + Unpin + Send,
	{
		// TODO: check the length?
		// Write the length.
		{
			let mut buffer = unsigned_varint::encode::usize_buffer();
			io.write_all(unsigned_varint::encode::usize(req.len(), &mut buffer)).await?;
		}

		// Write the payload.
		io.write_all(&req).await?;

		io.close().await?;
		Ok(())
	}

	async fn write_response<T>(
		&mut self,
		_: &Self::Protocol,
		io: &mut T,
		res: Self::Response,
	) -> io::Result<()>
	where
		T: AsyncWrite + Unpin + Send,
	{
		// If `res` is an `Err`, we jump to closing the substream without writing anything on it.
		if let Ok(res) = res {
			// TODO: check the length?
			// Write the length.
			{
				let mut buffer = unsigned_varint::encode::usize_buffer();
				io.write_all(unsigned_varint::encode::usize(res.len(), &mut buffer)).await?;
			}

			// Write the payload.
			io.write_all(&res).await?;
		}

		io.close().await?;
		Ok(())
	}
}

#[cfg(test)]
mod tests {
	use super::*;

	use crate::mock::MockPeerStore;
	use futures::{channel::oneshot, executor::LocalPool, task::Spawn};
	use libp2p::{
		core::{
			transport::{MemoryTransport, Transport},
			upgrade,
		},
		identity::Keypair,
		noise,
		swarm::{Executor, Swarm, SwarmBuilder, SwarmEvent},
		Multiaddr,
	};
	use std::{iter, time::Duration};

	struct TokioExecutor(tokio::runtime::Runtime);
	impl Executor for TokioExecutor {
		fn exec(&self, f: Pin<Box<dyn Future<Output = ()> + Send>>) {
			let _ = self.0.spawn(f);
		}
	}

	fn build_swarm(
		list: impl Iterator<Item = ProtocolConfig>,
	) -> (Swarm<RequestResponsesBehaviour>, Multiaddr) {
		let keypair = Keypair::generate_ed25519();

		let transport = MemoryTransport::new()
			.upgrade(upgrade::Version::V1)
			.authenticate(noise::Config::new(&keypair).unwrap())
			.multiplex(libp2p::yamux::Config::default())
			.boxed();

		let behaviour = RequestResponsesBehaviour::new(list, Box::new(MockPeerStore {})).unwrap();

		let runtime = tokio::runtime::Runtime::new().unwrap();
		let mut swarm = SwarmBuilder::with_executor(
			transport,
			behaviour,
			keypair.public().to_peer_id(),
			TokioExecutor(runtime),
		)
		.build();
		let listen_addr: Multiaddr = format!("/memory/{}", rand::random::<u64>()).parse().unwrap();

		swarm.listen_on(listen_addr.clone()).unwrap();
		(swarm, listen_addr)
	}

	#[test]
	fn basic_request_response_works() {
		let protocol_name = "/test/req-resp/1";
		let mut pool = LocalPool::new();

		// Build swarms whose behaviour is [`RequestResponsesBehaviour`].
		let mut swarms = (0..2)
			.map(|_| {
				let (tx, mut rx) = async_channel::bounded::<IncomingRequest>(64);

				pool.spawner()
					.spawn_obj(
						async move {
							while let Some(rq) = rx.next().await {
								let (fb_tx, fb_rx) = oneshot::channel();
								assert_eq!(rq.payload, b"this is a request");
								let _ = rq.pending_response.send(super::OutgoingResponse {
									result: Ok(b"this is a response".to_vec()),
									reputation_changes: Vec::new(),
									sent_feedback: Some(fb_tx),
								});
								fb_rx.await.unwrap();
							}
						}
						.boxed()
						.into(),
					)
					.unwrap();

				let protocol_config = ProtocolConfig {
					name: From::from(protocol_name),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 1024 * 1024,
					request_timeout: Duration::from_secs(30),
					inbound_queue: Some(tx),
				};

				build_swarm(iter::once(protocol_config))
			})
			.collect::<Vec<_>>();

		// Ask `swarm[0]` to dial `swarm[1]`. There isn't any discovery mechanism in place in
		// this test, so they wouldn't connect to each other.
		{
			let dial_addr = swarms[1].1.clone();
			Swarm::dial(&mut swarms[0].0, dial_addr).unwrap();
		}

		let (mut swarm, _) = swarms.remove(0);
		// Running `swarm[0]` in the background.
		pool.spawner()
			.spawn_obj({
				async move {
					loop {
						match swarm.select_next_some().await {
							SwarmEvent::Behaviour(Event::InboundRequest { result, .. }) => {
								result.unwrap();
							},
							_ => {},
						}
					}
				}
				.boxed()
				.into()
			})
			.unwrap();

		// Remove and run the remaining swarm.
		let (mut swarm, _) = swarms.remove(0);
		pool.run_until(async move {
			let mut response_receiver = None;

			loop {
				match swarm.select_next_some().await {
					SwarmEvent::ConnectionEstablished { peer_id, .. } => {
						let (sender, receiver) = oneshot::channel();
						swarm.behaviour_mut().send_request(
							&peer_id,
							protocol_name,
							b"this is a request".to_vec(),
							sender,
							IfDisconnected::ImmediateError,
						);
						assert!(response_receiver.is_none());
						response_receiver = Some(receiver);
					},
					SwarmEvent::Behaviour(Event::RequestFinished { result, .. }) => {
						result.unwrap();
						break
					},
					_ => {},
				}
			}

			assert_eq!(response_receiver.unwrap().await.unwrap().unwrap(), b"this is a response");
		});
	}

	#[test]
	fn max_response_size_exceeded() {
		let protocol_name = "/test/req-resp/1";
		let mut pool = LocalPool::new();

		// Build swarms whose behaviour is [`RequestResponsesBehaviour`].
		let mut swarms = (0..2)
			.map(|_| {
				let (tx, mut rx) = async_channel::bounded::<IncomingRequest>(64);

				pool.spawner()
					.spawn_obj(
						async move {
							while let Some(rq) = rx.next().await {
								assert_eq!(rq.payload, b"this is a request");
								let _ = rq.pending_response.send(super::OutgoingResponse {
									result: Ok(b"this response exceeds the limit".to_vec()),
									reputation_changes: Vec::new(),
									sent_feedback: None,
								});
							}
						}
						.boxed()
						.into(),
					)
					.unwrap();

				let protocol_config = ProtocolConfig {
					name: From::from(protocol_name),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 8, // <-- important for the test
					request_timeout: Duration::from_secs(30),
					inbound_queue: Some(tx),
				};

				build_swarm(iter::once(protocol_config))
			})
			.collect::<Vec<_>>();

		// Ask `swarm[0]` to dial `swarm[1]`. There isn't any discovery mechanism in place in
		// this test, so they wouldn't connect to each other.
		{
			let dial_addr = swarms[1].1.clone();
			Swarm::dial(&mut swarms[0].0, dial_addr).unwrap();
		}

		// Running `swarm[0]` in the background until a `InboundRequest` event happens,
		// which is a hint about the test having ended.
		let (mut swarm, _) = swarms.remove(0);
		pool.spawner()
			.spawn_obj({
				async move {
					loop {
						match swarm.select_next_some().await {
							SwarmEvent::Behaviour(Event::InboundRequest { result, .. }) => {
								assert!(result.is_ok());
								break
							},
							_ => {},
						}
					}
				}
				.boxed()
				.into()
			})
			.unwrap();

		// Remove and run the remaining swarm.
		let (mut swarm, _) = swarms.remove(0);
		pool.run_until(async move {
			let mut response_receiver = None;

			loop {
				match swarm.select_next_some().await {
					SwarmEvent::ConnectionEstablished { peer_id, .. } => {
						let (sender, receiver) = oneshot::channel();
						swarm.behaviour_mut().send_request(
							&peer_id,
							protocol_name,
							b"this is a request".to_vec(),
							sender,
							IfDisconnected::ImmediateError,
						);
						assert!(response_receiver.is_none());
						response_receiver = Some(receiver);
					},
					SwarmEvent::Behaviour(Event::RequestFinished { result, .. }) => {
						assert!(result.is_err());
						break
					},
					_ => {},
				}
			}

			match response_receiver.unwrap().await.unwrap().unwrap_err() {
				RequestFailure::Network(OutboundFailure::ConnectionClosed) => {},
				_ => panic!(),
			}
		});
	}

	/// A [`RequestId`] is a unique identifier among either all inbound or all outbound requests for
	/// a single [`RequestResponsesBehaviour`] behaviour. It is not guaranteed to be unique across
	/// multiple [`RequestResponsesBehaviour`] behaviours. Thus when handling [`RequestId`] in the
	/// context of multiple [`RequestResponsesBehaviour`] behaviours, one needs to couple the
	/// protocol name with the [`RequestId`] to get a unique request identifier.
	///
	/// This test ensures that two requests on different protocols can be handled concurrently
	/// without a [`RequestId`] collision.
	///
	/// See [`ProtocolRequestId`] for additional information.
	#[test]
	fn request_id_collision() {
		let protocol_name_1 = "/test/req-resp-1/1";
		let protocol_name_2 = "/test/req-resp-2/1";
		let mut pool = LocalPool::new();

		let mut swarm_1 = {
			let protocol_configs = vec![
				ProtocolConfig {
					name: From::from(protocol_name_1),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 1024 * 1024,
					request_timeout: Duration::from_secs(30),
					inbound_queue: None,
				},
				ProtocolConfig {
					name: From::from(protocol_name_2),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 1024 * 1024,
					request_timeout: Duration::from_secs(30),
					inbound_queue: None,
				},
			];

			build_swarm(protocol_configs.into_iter()).0
		};

		let (mut swarm_2, mut swarm_2_handler_1, mut swarm_2_handler_2, listen_add_2) = {
			let (tx_1, rx_1) = async_channel::bounded(64);
			let (tx_2, rx_2) = async_channel::bounded(64);

			let protocol_configs = vec![
				ProtocolConfig {
					name: From::from(protocol_name_1),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 1024 * 1024,
					request_timeout: Duration::from_secs(30),
					inbound_queue: Some(tx_1),
				},
				ProtocolConfig {
					name: From::from(protocol_name_2),
					fallback_names: Vec::new(),
					max_request_size: 1024,
					max_response_size: 1024 * 1024,
					request_timeout: Duration::from_secs(30),
					inbound_queue: Some(tx_2),
				},
			];

			let (swarm, listen_addr) = build_swarm(protocol_configs.into_iter());

			(swarm, rx_1, rx_2, listen_addr)
		};

		// Ask swarm 1 to dial swarm 2. There isn't any discovery mechanism in place in this test,
		// so they wouldn't connect to each other.
		swarm_1.dial(listen_add_2).unwrap();

		// Run swarm 2 in the background, receiving two requests.
		pool.spawner()
			.spawn_obj(
				async move {
					loop {
						match swarm_2.select_next_some().await {
							SwarmEvent::Behaviour(Event::InboundRequest { result, .. }) => {
								result.unwrap();
							},
							_ => {},
						}
					}
				}
				.boxed()
				.into(),
			)
			.unwrap();

		// Handle both requests sent by swarm 1 to swarm 2 in the background.
		//
		// Make sure both requests overlap, by answering the first only after receiving the
		// second.
		pool.spawner()
			.spawn_obj(
				async move {
					let protocol_1_request = swarm_2_handler_1.next().await;
					let protocol_2_request = swarm_2_handler_2.next().await;

					protocol_1_request
						.unwrap()
						.pending_response
						.send(OutgoingResponse {
							result: Ok(b"this is a response".to_vec()),
							reputation_changes: Vec::new(),
							sent_feedback: None,
						})
						.unwrap();
					protocol_2_request
						.unwrap()
						.pending_response
						.send(OutgoingResponse {
							result: Ok(b"this is a response".to_vec()),
							reputation_changes: Vec::new(),
							sent_feedback: None,
						})
						.unwrap();
				}
				.boxed()
				.into(),
			)
			.unwrap();

		// Have swarm 1 send two requests to swarm 2 and await responses.
		pool.run_until(async move {
			let mut response_receivers = None;
			let mut num_responses = 0;

			loop {
				match swarm_1.select_next_some().await {
					SwarmEvent::ConnectionEstablished { peer_id, .. } => {
						let (sender_1, receiver_1) = oneshot::channel();
						let (sender_2, receiver_2) = oneshot::channel();
						swarm_1.behaviour_mut().send_request(
							&peer_id,
							protocol_name_1,
							b"this is a request".to_vec(),
							sender_1,
							IfDisconnected::ImmediateError,
						);
						swarm_1.behaviour_mut().send_request(
							&peer_id,
							protocol_name_2,
							b"this is a request".to_vec(),
							sender_2,
							IfDisconnected::ImmediateError,
						);
						assert!(response_receivers.is_none());
						response_receivers = Some((receiver_1, receiver_2));
					},
					SwarmEvent::Behaviour(Event::RequestFinished { result, .. }) => {
						num_responses += 1;
						result.unwrap();
						if num_responses == 2 {
							break
						}
					},
					_ => {},
				}
			}
			let (response_receiver_1, response_receiver_2) = response_receivers.unwrap();
			assert_eq!(response_receiver_1.await.unwrap().unwrap(), b"this is a response");
			assert_eq!(response_receiver_2.await.unwrap().unwrap(), b"this is a response");
		});
	}
}