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
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
//! `BridgeDescMgr` - downloads and caches bridges' router descriptors

use std::borrow::Cow;
use std::cmp::Ordering;
use std::collections::{BinaryHeap, HashMap, HashSet, VecDeque};
use std::fmt::{self, Debug, Display};
use std::num::NonZeroU8;
use std::ops;
use std::panic::AssertUnwindSafe;
use std::sync::{Arc, Mutex, MutexGuard, Weak};
use std::time::{Duration, Instant, SystemTime};

use async_trait::async_trait;
use derive_more::{Deref, DerefMut};
use educe::Educe;
use futures::future;
use futures::select;
use futures::stream::{BoxStream, StreamExt};
use futures::task::{SpawnError, SpawnExt as _};
use futures::FutureExt;
use tracing::{debug, error, info, trace};

use safelog::sensitive;
use tor_basic_utils::retry::RetryDelay;
use tor_basic_utils::BinaryHeapExt as _;
use tor_checkable::{SelfSigned, Timebound};
use tor_circmgr::CircMgr;
use tor_error::{error_report, internal, ErrorKind, HasKind};
use tor_error::{AbsRetryTime, HasRetryTime, RetryTime};
use tor_guardmgr::bridge::{BridgeConfig, BridgeDesc};
use tor_guardmgr::bridge::{BridgeDescError, BridgeDescEvent, BridgeDescList, BridgeDescProvider};
use tor_netdoc::doc::routerdesc::RouterDesc;
use tor_rtcompat::Runtime;

use crate::event::FlagPublisher;
use crate::storage::CachedBridgeDescriptor;
use crate::{DirMgrStore, DynStore};

#[cfg(test)]
mod bdtest;

/// The key we use in all our data structures
///
/// This type saves typing and would make it easier to change the bridge descriptor manager
/// to take and handle another way of identifying the bridges it is working with.
type BridgeKey = BridgeConfig;

/// Active vs dormant state, as far as the bridge descriptor manager is concerned
///
/// This is usually derived in higher layers from `arti_client::DormantMode`,
/// whether `TorClient::bootstrap()` has been called, etc.
#[non_exhaustive]
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
// TODO: These proliferating `Dormancy` enums should be centralized and unified with `TaskHandle`
//     https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/845#note_2853190
pub enum Dormancy {
    /// Dormant (inactive)
    ///
    /// Bridge descriptor downloads, or refreshes, will not be started.
    ///
    /// In-progress downloads will be stopped if possible,
    /// but they may continue until they complete (or fail).
    // TODO async task cancellation: actually cancel these in this case
    ///
    /// So a dormant BridgeDescMgr may still continue to
    /// change the return value from [`bridges()`](BridgeDescProvider::bridges)
    /// and continue to report [`BridgeDescEvent`]s.
    ///
    /// When the BridgeDescMgr is dormant,
    /// `bridges()` may return stale descriptors
    /// (that is, descriptors which ought to have been refetched and may no longer be valid),
    /// or stale errors
    /// (that is, errors which occurred some time ago,
    /// and which would normally have been retried by now).
    Dormant,

    /// Active
    ///
    /// Bridge descriptors will be downloaded as requested.
    ///
    /// When a bridge descriptor manager has been `Dormant`,
    /// it may continue to provide stale data (as described)
    /// for a while after it is made `Active`,
    /// until the required refreshes and retries have taken place (or failed).
    Active,
}

/// **Downloader and cache for bridges' router descriptors**
///
/// This is a handle which is cheap to clone and has internal mutability.
#[derive(Clone)]
pub struct BridgeDescMgr<R: Runtime, M = ()>
where
    M: Mockable<R>,
{
    /// The actual manager
    ///
    /// We have the `Arc` in here, rather than in our callers, because this
    /// makes the API nicer for them, and also because some of our tasks
    /// want a handle they can use to relock and modify the state.
    mgr: Arc<Manager<R, M>>,
}

/// Configuration for the `BridgeDescMgr`
///
/// Currently, the only way to make this is via its `Default` impl.
// TODO: there should be some way to override the defaults.  See #629 for considerations.
#[derive(Debug, Clone)]
pub struct BridgeDescDownloadConfig {
    /// How many bridge descriptor downloads to attempt in parallel?
    parallelism: NonZeroU8,

    /// Default/initial time to retry a failure to download a descriptor
    ///
    /// (This has the semantics of an initial delay for [`RetryDelay`],
    /// and is used unless there is more specific retry information for the particular failure.)
    retry: Duration,

    /// When a downloaded descriptor is going to expire, how soon in advance to refetch it?
    prefetch: Duration,

    /// Minimum interval between successive refetches of the descriptor for the same bridge
    ///
    /// This limits the download activity which can be caused by an errant bridge.
    ///
    /// If the descriptor's validity information is shorter than this, we will use
    /// it after it has expired (rather than treating the bridge as broken).
    min_refetch: Duration,

    /// Maximum interval between successive refetches of the descriptor for the same bridge
    ///
    /// This sets an upper bound on how old a descriptor we are willing to use.
    /// When this time expires, a refetch attempt will be started even if the
    /// descriptor is not going to expire soon.
    //
    // TODO: When this is configurable, we need to make sure we reject
    // configurations with max_refresh < min_refresh, or we may panic.
    max_refetch: Duration,
}

impl Default for BridgeDescDownloadConfig {
    fn default() -> Self {
        let secs = Duration::from_secs;
        BridgeDescDownloadConfig {
            parallelism: 4.try_into().expect("parallelism is zero"),
            retry: secs(30),
            prefetch: secs(1000),
            min_refetch: secs(3600),
            max_refetch: secs(3600 * 3), // matches C Tor behaviour
        }
    }
}

/// Mockable internal methods for within the `BridgeDescMgr`
///
/// Implemented for `()`, meaning "do not use mocks: use the real versions of everything".
///
/// This (`()`) is the default for the type parameter in
/// [`BridgeDescMgr`],
/// and it is the only publicly available implementation,
/// since this trait is sealed.
pub trait Mockable<R>: mockable::MockableAPI<R> {}
impl<R: Runtime> Mockable<R> for () {}

/// Private module which seals [`Mockable`]
/// by containing [`MockableAPI`](mockable::MockableAPI)
mod mockable {
    use super::*;

    /// Defines the actual mockable APIs
    ///
    /// Not nameable (and therefore not implementable)
    /// outside the `bridgedesc` module,
    #[async_trait]
    pub trait MockableAPI<R>: Clone + Send + Sync + 'static {
        /// Circuit manager
        type CircMgr: Send + Sync + 'static;

        /// Download this bridge's descriptor, and return it as a string
        ///
        /// Runs in a task.
        /// Called by `Manager::download_descriptor`, which handles parsing and validation.
        ///
        /// If `if_modified_since` is `Some`,
        /// should tolerate an HTTP 304 Not Modified and return `None` in that case.
        /// If `if_modified_since` is `None`, returning `Ok(None,)` is forbidden.
        async fn download(
            self,
            runtime: &R,
            circmgr: &Self::CircMgr,
            bridge: &BridgeConfig,
            if_modified_since: Option<SystemTime>,
        ) -> Result<Option<String>, Error>;
    }
}
#[async_trait]
impl<R: Runtime> mockable::MockableAPI<R> for () {
    type CircMgr = Arc<CircMgr<R>>;

    /// Actual code for downloading a descriptor document
    async fn download(
        self,
        runtime: &R,
        circmgr: &Self::CircMgr,
        bridge: &BridgeConfig,
        _if_modified_since: Option<SystemTime>,
    ) -> Result<Option<String>, Error> {
        // TODO actually support _if_modified_since
        let circuit = circmgr.get_or_launch_dir_specific(bridge).await?;
        let mut stream = circuit
            .begin_dir_stream()
            .await
            .map_err(Error::StreamFailed)?;
        let request = tor_dirclient::request::RoutersOwnDescRequest::new();
        let response = tor_dirclient::send_request(runtime, &request, &mut stream, None)
            .await
            .map_err(|dce| match dce {
                tor_dirclient::Error::RequestFailed(re) => Error::RequestFailed(re),
                _ => internal!(
                    "tor_dirclient::send_request gave non-RequestFailed {:?}",
                    dce
                )
                .into(),
            })?;
        let output = response.into_output_string()?;
        Ok(Some(output))
    }
}

/// The actual manager.
struct Manager<R: Runtime, M: Mockable<R>> {
    /// The mutable state
    state: Mutex<State>,

    /// Runtime, used for tasks and sleeping
    runtime: R,

    /// Circuit manager, used for creating circuits
    circmgr: M::CircMgr,

    /// Persistent state store
    store: Arc<Mutex<DynStore>>,

    /// Mock for testing, usually `()`
    mockable: M,
}

/// State: our downloaded descriptors (cache), and records of what we're doing
///
/// Various functions (both tasks and public entrypoints),
/// which generally start with a `Manager`,
/// lock the mutex and modify this.
///
/// Generally, the flow is:
///
///  * A public entrypoint, or task, obtains a [`StateGuard`].
///    It modifies the state to represent the callers' new requirements,
///    or things it has done, by updating the state,
///    preserving the invariants but disturbing the "liveness" (see below).
///
///  * [`StateGuard::drop`] calls [`State::process`].
///    This restores the liveness properties.
///
/// ### Possible states of a bridge:
///
/// A bridge can be in one of the following states,
/// represented by its presence in these particular data structures inside `State`:
///
///  * `running`/`queued`: newly added, no outcome yet.
///  * `current` + `running`/`queued`: we are fetching (or going to)
///  * `current = OK` + `refetch_schedule`: fetched OK, will refetch before expiry
///  * `current = Err` + `retry_schedule`: failed, will retry at some point
///
/// ### Invariants:
///
/// Can be disrupted in the middle of a principal function,
/// but should be restored on return.
///
/// * **Tracked**:
///   Each bridge appears at most once in
///   `running`, `queued`, `refetch_schedule` and `retry_schedule`.
///   We call such a bridge Tracked.
///
/// * **Current**
///   Every bridge in `current` is Tracked.
///   (But not every Tracked bridge is necessarily in `current`, yet.)
///
/// * **Schedules**
///   Every bridge in `refetch_schedule` or `retry_schedule` is also in `current`.
///
/// * **Input**:
///   Exactly each bridge that was passed to
///   the last call to [`set_bridges()`](BridgeDescMgr::set_bridges) is Tracked.
///   (If we encountered spawn failures, we treat this as trying to shut down,
///   so we cease attempts to get bridges, and discard the relevant state, violating this.)
///
/// * **Limit**:
///   `running` is capped at the effective parallelism: zero if we are dormant,
///   the configured parallelism otherwise.
///
/// ### Liveness properties:
///
/// These can be disrupted by any function which holds a [`StateGuard`].
/// Will be restored by [`process()`](State::process),
/// which is called when `StateGuard` is dropped.
///
/// Functions that take a `StateGuard` may disturb these invariants
/// and rely on someone else to restore them.
///
/// * **Running**:
///   If `queued` is nonempty, `running` is full.
///
/// * **Timeout**:
///   `earliest_timeout` is the earliest timeout in
///   either `retry_schedule` or `refetch_schedule`.
///   (Disturbances of this property which occur due to system time warps
///   are not necessarily detected and remedied in a timely way,
///   but will be remedied no later than after `max_refetch`.)
struct State {
    /// Our configuration
    config: Arc<BridgeDescDownloadConfig>,

    /// People who will be told when `current` changes.
    subscribers: FlagPublisher<BridgeDescEvent>,

    /// Our current idea of our output, which we give out handles onto.
    current: Arc<BridgeDescList>,

    /// Bridges whose descriptors we are currently downloading.
    running: HashMap<BridgeKey, RunningInfo>,

    /// Bridges which we want to download,
    /// but we're waiting for `running` to be less than `effective_parallelism()`.
    queued: VecDeque<QueuedEntry>,

    /// Are we dormant?
    dormancy: Dormancy,

    /// Bridges that we have a descriptor for,
    /// and when they should be refetched due to validity expiry.
    ///
    /// This is indexed by `SystemTime` because that helps avoids undesirable behaviors
    /// when the system clock changes.
    refetch_schedule: BinaryHeap<RefetchEntry<SystemTime, ()>>,

    /// Bridges that failed earlier, and when they should be retried.
    retry_schedule: BinaryHeap<RefetchEntry<Instant, RetryDelay>>,

    /// Earliest time from either `retry_schedule` or `refetch_schedule`
    ///
    /// `None` means "wait indefinitely".
    earliest_timeout: postage::watch::Sender<Option<Instant>>,
}

impl Debug for State {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        /// Helper to format one bridge entry somewhere
        fn fmt_bridge(
            f: &mut fmt::Formatter,
            b: &BridgeConfig,
            info: &(dyn Display + '_),
        ) -> fmt::Result {
            let info = info.to_string(); // fmt::Formatter doesn't enforce precision, so do this
            writeln!(f, "    {:80.80} | {}", info, b)
        }

        /// Helper to format one of the schedules
        fn fmt_schedule<TT: Ord + Copy + Debug, RD>(
            f: &mut fmt::Formatter,
            summary: &str,
            name: &str,
            schedule: &BinaryHeap<RefetchEntry<TT, RD>>,
        ) -> fmt::Result {
            writeln!(f, "  {}:", name)?;
            for b in schedule {
                fmt_bridge(f, &b.bridge, &format_args!("{} {:?}", summary, &b.when))?;
            }
            Ok(())
        }

        // We are going to have to go multi-line because of the bridge lines,
        // so do completely bespoke formatting rather than `std::fmt::DebugStruct`
        // or a derive.
        writeln!(f, "State {{")?;
        // We'd like to print earliest_timeout but watch::Sender::borrow takes &mut
        writeln!(f, "  earliest_timeout: ???, ..,")?;
        writeln!(f, "  current:")?;
        for (b, v) in &*self.current {
            fmt_bridge(
                f,
                b,
                &match v {
                    Err(e) => Cow::from(format!("C Err {}", e)),
                    Ok(_) => "C Ok".into(),
                },
            )?;
        }
        writeln!(f, "  running:")?;
        for b in self.running.keys() {
            fmt_bridge(f, b, &"R")?;
        }
        writeln!(f, "  queued:")?;
        for qe in &self.queued {
            fmt_bridge(f, &qe.bridge, &"Q")?;
        }
        fmt_schedule(f, "FS", "refetch_schedule", &self.refetch_schedule)?;
        fmt_schedule(f, "TS", "retry_schedule", &self.retry_schedule)?;
        write!(f, "}}")?;

        Ok(())
    }
}

/// Value of the entry in `running`
#[derive(Debug)]
struct RunningInfo {
    /// For cancelling downloads no longer wanted
    join: JoinHandle,

    /// If this previously failed, the persistent retry delay.
    retry_delay: Option<RetryDelay>,
}

/// Entry in `queued`
#[derive(Debug)]
struct QueuedEntry {
    /// The bridge to fetch
    bridge: BridgeKey,

    /// If this previously failed, the persistent retry delay.
    retry_delay: Option<RetryDelay>,
}

/// Entry in one of the `*_schedule`s
///
/// Implements `Ord` and `Eq` but *only looking at the refetch time*.
/// So don't deduplicate by `[Partial]Eq`, or use as a key in a map.
#[derive(Debug)]
struct RefetchEntry<TT, RD> {
    /// When should we requeued this bridge for fetching
    ///
    /// Either [`Instant`] (in `retry_schedule`) or [`SystemTime`] (in `refetch_schedule`).
    when: TT,

    /// The bridge to refetch
    bridge: BridgeKey,

    /// Retry delay
    ///
    /// `RetryDelay` if we previously failed (ie, if this is a retry entry);
    /// otherwise `()`.
    retry_delay: RD,
}

impl<TT: Ord, RD> Ord for RefetchEntry<TT, RD> {
    fn cmp(&self, other: &Self) -> Ordering {
        self.when.cmp(&other.when).reverse()
        // We don't care about the ordering of BridgeConfig or retry_delay.
        // Different BridgeConfig with the same fetch time will be fetched in "some order".
    }
}

impl<TT: Ord, RD> PartialOrd for RefetchEntry<TT, RD> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl<TT: Ord, RD> PartialEq for RefetchEntry<TT, RD> {
    fn eq(&self, other: &Self) -> bool {
        self.cmp(other) == Ordering::Equal
    }
}

impl<TT: Ord, RD> Eq for RefetchEntry<TT, RD> {}

/// Dummy task join handle
///
/// We would like to be able to cancel now-redundant downloads
/// using something like `tokio::task::JoinHandle::abort()`.
/// tor-rtcompat doesn't support that so we stub it for now.
///
/// Providing this stub means the place where the cancellation needs to take place
/// already has the appropriate call to our [`JoinHandle::abort`].
#[derive(Debug)]
struct JoinHandle;

impl JoinHandle {
    /// Would abort this async task, if we could do that.
    fn abort(&self) {}
}

impl<R: Runtime> BridgeDescMgr<R> {
    /// Create a new `BridgeDescMgr`
    ///
    /// This is the public constructor.
    //
    // TODO: That this constructor requires a DirMgr is rather odd.
    // In principle there is little reason why you need a DirMgr to make a BridgeDescMgr.
    // However, BridgeDescMgr needs a Store, and currently that is a private trait, and the
    // implementation is constructible only from the dirmgr's config.  This should probably be
    // tidied up somehow, at some point, perhaps by exposing `Store` and its configuration.
    pub fn new(
        config: &BridgeDescDownloadConfig,
        runtime: R,
        store: DirMgrStore<R>,
        circmgr: Arc<tor_circmgr::CircMgr<R>>,
        dormancy: Dormancy,
    ) -> Result<Self, StartupError> {
        Self::new_internal(runtime, circmgr, store.store, config, dormancy, ())
    }
}

/// If download was successful, what we obtained
///
/// Generated by `process_document`, from a downloaded (or cached) textual descriptor.
#[derive(Debug)]
struct Downloaded {
    /// The bridge descriptor, fully parsed and verified
    desc: BridgeDesc,

    /// When we should start a refresh for this descriptor
    ///
    /// This is derived from the expiry time,
    /// and clamped according to limits in the configuration).
    refetch: SystemTime,
}

impl<R: Runtime, M: Mockable<R>> BridgeDescMgr<R, M> {
    /// Actual constructor, which takes a mockable
    //
    // Allow passing `runtime` by value, which is usual API for this kind of setup function.
    #[allow(clippy::needless_pass_by_value)]
    fn new_internal(
        runtime: R,
        circmgr: M::CircMgr,
        store: Arc<Mutex<DynStore>>,
        config: &BridgeDescDownloadConfig,
        dormancy: Dormancy,
        mockable: M,
    ) -> Result<Self, StartupError> {
        /// Convenience alias
        fn default<T: Default>() -> T {
            Default::default()
        }

        let config = config.clone().into();
        let (earliest_timeout, timeout_update) = postage::watch::channel();

        let state = Mutex::new(State {
            config,
            subscribers: default(),
            current: default(),
            running: default(),
            queued: default(),
            dormancy,
            retry_schedule: default(),
            refetch_schedule: default(),
            earliest_timeout,
        });
        let mgr = Arc::new(Manager {
            state,
            runtime: runtime.clone(),
            circmgr,
            store,
            mockable,
        });

        runtime
            .spawn(timeout_task(
                runtime.clone(),
                Arc::downgrade(&mgr),
                timeout_update,
            ))
            .map_err(|cause| StartupError::Spawn {
                spawning: "timeout task",
                cause: cause.into(),
            })?;

        Ok(BridgeDescMgr { mgr })
    }

    /// Consistency check convenience wrapper
    #[cfg(test)]
    fn check_consistency<'i, I>(&self, input_bridges: Option<I>)
    where
        I: IntoIterator<Item = &'i BridgeKey>,
    {
        self.mgr
            .lock_only()
            .check_consistency(&self.mgr.runtime, input_bridges);
    }

    /// Set whether this `BridgeDescMgr` is active
    // TODO this should instead be handled by a central mechanism; see TODO on Dormancy
    pub fn set_dormancy(&self, dormancy: Dormancy) {
        self.mgr.lock_then_process().dormancy = dormancy;
    }
}

impl<R: Runtime, M: Mockable<R>> BridgeDescProvider for BridgeDescMgr<R, M> {
    fn bridges(&self) -> Arc<BridgeDescList> {
        self.mgr.lock_only().current.clone()
    }

    fn events(&self) -> BoxStream<'static, BridgeDescEvent> {
        let stream = self.mgr.lock_only().subscribers.subscribe();
        Box::pin(stream) as _
    }

    fn set_bridges(&self, new_bridges: &[BridgeConfig]) {
        /// Helper: Called for each bridge that is currently Tracked.
        ///
        /// Checks if `new_bridges` has `bridge`.  If so, removes it from `new_bridges`,
        /// and returns `true`, indicating that this bridge should be kept.
        ///
        /// If not, returns `false`, indicating that this bridge should be removed,
        /// and logs a message.
        fn note_found_keep_p(
            new_bridges: &mut HashSet<BridgeKey>,
            bridge: &BridgeKey,
            was_state: &str,
        ) -> bool {
            let keep = new_bridges.remove(bridge);
            if !keep {
                debug!(r#"forgetting bridge ({}) "{}""#, was_state, bridge);
            }
            keep
        }

        /// Helper: filters `*_schedule` so that it contains only things in `new_bridges`,
        /// removing them as we go.
        fn filter_schedule<TT: Ord + Copy, RD>(
            new_bridges: &mut HashSet<BridgeKey>,
            schedule: &mut BinaryHeap<RefetchEntry<TT, RD>>,
            was_state: &str,
        ) {
            schedule.retain_ext(|b| note_found_keep_p(new_bridges, &b.bridge, was_state));
        }

        let mut state = self.mgr.lock_then_process();
        let state = &mut **state;

        // We go through our own data structures, comparing them with `new_bridges`.
        // Entries in our own structures that aren't in `new_bridges` are removed.
        // Entries that *are* are removed from `new_bridges`.
        // Eventually `new_bridges` is just the list of new bridges to *add*.
        let mut new_bridges: HashSet<_> = new_bridges.iter().cloned().collect();

        // Is there anything in `current` that ought to be deleted?
        if state.current.keys().any(|b| !new_bridges.contains(b)) {
            // Found a bridge In `current` but not `new`
            // We need to remove it (and any others like it) from `current`.
            //
            // Disturbs the invariant *Schedules*:
            // After this maybe the schedules have entries they shouldn't.
            let current: BridgeDescList = state
                .current
                .iter()
                .filter(|(b, _)| new_bridges.contains(&**b))
                .map(|(b, v)| (b.clone(), v.clone()))
                .collect();
            state.set_current_and_notify(current);
        } else {
            // Nothing is being removed, so we can keep `current`.
        }
        // Bridges being newly requested will be added to `current`
        // later, after they have been fetched.

        // Is there anything in running we should abort?
        state.running.retain(|b, ri| {
            let keep = note_found_keep_p(&mut new_bridges, b, "was downloading");
            if !keep {
                ri.join.abort();
            }
            keep
        });

        // Is there anything in queued we should forget about?
        state
            .queued
            .retain(|qe| note_found_keep_p(&mut new_bridges, &qe.bridge, "was queued"));

        // Restore the invariant *Schedules*, that the schedules contain only things in current,
        // by removing the same things from the schedules that we earlier removed from current.
        filter_schedule(
            &mut new_bridges,
            &mut state.retry_schedule,
            "previously failed",
        );
        filter_schedule(
            &mut new_bridges,
            &mut state.refetch_schedule,
            "previously downloaded",
        );

        // OK now we have the list of bridges to add (if any).
        state.queued.extend(new_bridges.into_iter().map(|bridge| {
            debug!(r#" added bridge, queueing for download "{}""#, &bridge);
            QueuedEntry {
                bridge,
                retry_delay: None,
            }
        }));

        // `StateGuard`, from `lock_then_process`, gets dropped here, and runs `process`,
        // to make further progress and restore the liveness properties.
    }
}

impl<R: Runtime, M: Mockable<R>> Manager<R, M> {
    /// Obtain a lock on state, for functions that want to disrupt liveness properties
    ///
    /// When `StateGuard` is dropped, the liveness properties will be restored
    /// by making whatever progress is required.
    ///
    /// See [`State`].
    fn lock_then_process<'s>(self: &'s Arc<Self>) -> StateGuard<'s, R, M> {
        StateGuard {
            state: self.lock_only(),
            mgr: self,
        }
    }

    /// Obtains the lock on state.
    ///
    /// Caller ought not to modify state
    /// so as to invalidate invariants or liveness properties.
    /// Callers which are part of the algorithms in this crate
    /// ought to consider [`lock_then_process`](Manager::lock_then_process) instead.
    fn lock_only(&self) -> MutexGuard<State> {
        self.state.lock().expect("bridge desc manager poisoned")
    }
}

/// Writeable reference to [`State`], entitling the holder to disrupt liveness properties.
///
/// The holder must still maintain the invariants.
///
/// Obtained from [`Manager::lock_then_process`].  See [`State`].
#[derive(Educe, Deref, DerefMut)]
#[educe(Debug)]
struct StateGuard<'s, R: Runtime, M: Mockable<R>> {
    /// Reference to the mutable state
    #[deref]
    #[deref_mut]
    state: MutexGuard<'s, State>,

    /// Reference to the outer container
    ///
    /// Allows the holder to obtain a `'static` (owned) handle `Arc<Manager>`,
    /// for use by spawned tasks.
    #[educe(Debug(ignore))]
    mgr: &'s Arc<Manager<R, M>>,
}

impl<R: Runtime, M: Mockable<R>> Drop for StateGuard<'_, R, M> {
    fn drop(&mut self) {
        self.state.process(self.mgr);
    }
}

impl State {
    /// Ensure progress is made, by restoring all the liveness invariants
    ///
    /// This includes launching circuits as needed.
    fn process<R: Runtime, M: Mockable<R>>(&mut self, mgr: &Arc<Manager<R, M>>) {
        // Restore liveness property *Running*
        self.consider_launching(mgr);

        let now_wall = mgr.runtime.wallclock();

        // Mitigate clock warping
        //
        // If the earliest `SystemTime` is more than `max_refetch` away,
        // the clock must have warped.  If that happens we clamp
        // them all to `max_refetch`.
        //
        // (This is not perfect but will mitigate the worst effects by ensuring
        // that we do *something* at least every `max_refetch`, in the worst case,
        // other than just getting completely stuck.)
        let max_refetch_wall = now_wall + self.config.max_refetch;
        if self
            .refetch_schedule
            .peek()
            .map(|re| re.when > max_refetch_wall)
            == Some(true)
        {
            info!("bridge descriptor manager: clock warped, clamping refetch times");
            self.refetch_schedule = self
                .refetch_schedule
                .drain()
                .map(|mut re| {
                    re.when = max_refetch_wall;
                    re
                })
                .collect();
        }

        // Restore liveness property *Timeout**
        // postage::watch will tell up the timeout task about the new wake-up time.
        let new_earliest_timeout = [
            // First retry.  These are std Instant.
            self.retry_schedule.peek().map(|re| re.when),
            // First refetch.  These are SystemTime, so we must convert them.
            self.refetch_schedule.peek().map(|re| {
                // If duration_since gives Err, that means when is before now,
                // ie we should not be waiting: the wait duration should be 0.
                let wait = re.when.duration_since(now_wall).unwrap_or_default();

                mgr.runtime.now() + wait
            }),
        ]
        .into_iter()
        .flatten()
        .min();
        *self.earliest_timeout.borrow_mut() = new_earliest_timeout;
    }

    /// Launch download attempts if we can
    ///
    /// Specifically: if we have things in `queued`, and `running` is shorter than
    /// `effective_parallelism()`, we launch task(s) to attempt download(s).
    ///
    /// Restores liveness invariant *Running*.
    ///
    /// Idempotent.  Forms part of `process`.
    #[allow(clippy::blocks_in_conditions)]
    fn consider_launching<R: Runtime, M: Mockable<R>>(&mut self, mgr: &Arc<Manager<R, M>>) {
        let mut to_remove = vec![];

        while self.running.len() < self.effective_parallelism() {
            let QueuedEntry {
                bridge,
                retry_delay,
            } = match self.queued.pop_front() {
                Some(qe) => qe,
                None => break,
            };
            match mgr
                .runtime
                .spawn({
                    let config = self.config.clone();
                    let bridge = bridge.clone();
                    let inner = mgr.clone();
                    let mockable = inner.mockable.clone();

                    // The task which actually downloads a descriptor.
                    async move {
                        let got =
                            AssertUnwindSafe(inner.download_descriptor(mockable, &bridge, &config))
                                .catch_unwind()
                                .await
                                .unwrap_or_else(|_| {
                                    Err(internal!("download descriptor task panicked!").into())
                                });
                        match &got {
                            Ok(_) => debug!(r#"download succeeded for "{}""#, bridge),
                            Err(err) => debug!(r#"download failed for "{}": {}"#, bridge, err),
                        };
                        let mut state = inner.lock_then_process();
                        state.record_download_outcome(bridge, got);
                        // `StateGuard`, from `lock_then_process`, gets dropped here, and runs `process`,
                        // to make further progress and restore the liveness properties.
                    }
                })
                .map(|()| JoinHandle)
            {
                Ok(join) => {
                    self.running
                        .insert(bridge, RunningInfo { join, retry_delay });
                }
                Err(_) => {
                    // Spawn failed.
                    //
                    // We are going to forget about this bridge.
                    // And we're going to do that without notifying anyone.
                    // We *do* want to remove it from `current` because simply forgetting
                    // about a refetch could leave expired data there.
                    // We amortize this, so we don't do a lot of O(n^2) work on shutdown.
                    to_remove.push(bridge);
                }
            }
        }

        if !to_remove.is_empty() {
            self.modify_current(|current| {
                for bridge in to_remove {
                    current.remove(&bridge);
                }
            });
        }
    }

    /// Modify `current` and notify subscribers
    ///
    /// Helper function which modifies only `current`, not any of the rest of the state.
    /// it is the caller's responsibility to ensure that the invariants are upheld.
    ///
    /// The implementation actually involves cloning `current`,
    /// so it is best to amortize calls to this function.
    fn modify_current<T, F: FnOnce(&mut BridgeDescList) -> T>(&mut self, f: F) -> T {
        let mut current = (*self.current).clone();
        let r = f(&mut current);
        self.set_current_and_notify(current);
        r
    }

    /// Set `current` to a value and notify
    ///
    /// Helper function which modifies only `current`, not any of the rest of the state.
    /// it is the caller's responsibility to ensure that the invariants are upheld.
    fn set_current_and_notify<BDL: Into<Arc<BridgeDescList>>>(&mut self, new: BDL) {
        self.current = new.into();
        self.subscribers.publish(BridgeDescEvent::SomethingChanged);
    }

    /// Obtain the currently-desired level of parallelism
    ///
    /// Helper function.  The return value depends the mutable state and also the `config`.
    ///
    /// This is how we implement dormancy.
    fn effective_parallelism(&self) -> usize {
        match self.dormancy {
            Dormancy::Active => usize::from(u8::from(self.config.parallelism)),
            Dormancy::Dormant => 0,
        }
    }
}

impl<R: Runtime, M: Mockable<R>> StateGuard<'_, R, M> {
    /// Record a download outcome.
    ///
    /// Final act of the the descriptor download task.
    /// `got` is from [`download_descriptor`](Manager::download_descriptor).
    fn record_download_outcome(&mut self, bridge: BridgeKey, got: Result<Downloaded, Error>) {
        let RunningInfo { retry_delay, .. } = match self.running.remove(&bridge) {
            Some(ri) => ri,
            None => {
                debug!("bridge descriptor download completed for no-longer-configured bridge");
                return;
            }
        };

        let insert = match got {
            Ok(Downloaded { desc, refetch }) => {
                // Successful download.  Schedule the refetch, and we'll insert Ok.

                self.refetch_schedule.push(RefetchEntry {
                    when: refetch,
                    bridge: bridge.clone(),
                    retry_delay: (),
                });

                Ok(desc)
            }
            Err(err) => {
                // Failed.  Schedule the retry, and we'll insert Err.

                let mut retry_delay =
                    retry_delay.unwrap_or_else(|| RetryDelay::from_duration(self.config.retry));

                let retry = err.retry_time();
                // We retry at least as early as
                let now = self.mgr.runtime.now();
                let retry = retry.absolute(now, || retry_delay.next_delay(&mut rand::thread_rng()));
                // Retry at least as early as max_refetch.  That way if a bridge is
                // misconfigured we will see it be fixed eventually.
                let retry = {
                    let earliest = now;
                    let latest = || now + self.config.max_refetch;
                    match retry {
                        AbsRetryTime::Immediate => earliest,
                        AbsRetryTime::Never => latest(),
                        AbsRetryTime::At(i) => i.clamp(earliest, latest()),
                    }
                };
                self.retry_schedule.push(RefetchEntry {
                    when: retry,
                    bridge: bridge.clone(),
                    retry_delay,
                });

                Err(Box::new(err) as _)
            }
        };

        self.modify_current(|current| current.insert(bridge, insert));
    }
}

impl<R: Runtime, M: Mockable<R>> Manager<R, M> {
    /// Downloads a descriptor.
    ///
    /// The core of the descriptor download task
    /// launched by `State::consider_launching`.
    ///
    /// Uses Mockable::download to actually get the document.
    /// So most of this function is parsing and checking.
    ///
    /// The returned value is precisely the `got` input to
    /// [`record_download_outcome`](StateGuard::record_download_outcome).
    async fn download_descriptor(
        &self,
        mockable: M,
        bridge: &BridgeConfig,
        config: &BridgeDescDownloadConfig,
    ) -> Result<Downloaded, Error> {
        // convenience alias, capturing the usual parameters from our variables.
        let process_document = |text| process_document(&self.runtime, config, text);

        let store = || {
            self.store
                .lock()
                .map_err(|_| internal!("bridge descriptor store poisoned"))
        };

        let cache_entry: Option<CachedBridgeDescriptor> = (|| store()?.lookup_bridgedesc(bridge))()
            .unwrap_or_else(|err| {
                error_report!(
                    err,
                    r#"bridge descriptor cache lookup failed, for "{}""#,
                    sensitive(bridge),
                );
                None
            });

        let now = self.runtime.wallclock();
        let cached_good: Option<Downloaded> = if let Some(cached) = &cache_entry {
            if cached.fetched > now {
                // was fetched "in the future"
                None
            } else {
                // let's see if it's any use
                match process_document(&cached.document) {
                    Err(err) => {
                        // We had a doc in the cache but our attempt to use it failed
                        // We wouldn't have written a bad cache entry.
                        // So one of the following must be true:
                        //  * We were buggy or are stricter now or something
                        //  * The document was valid but its validity time has expired
                        // In any case we can't reuse it.
                        // (This happens in normal operation, when a document expires.)
                        trace!(r#"cached document for "{}" invalid: {}"#, &bridge, err);
                        None
                    }
                    Ok(got) => {
                        // The cached document looks valid.
                        // But how long ago did we fetch it?
                        // We need to enforce max_refresh even for still-valid documents.
                        if now.duration_since(cached.fetched).ok() <= Some(config.max_refetch) {
                            // Was fetched recently, too.  We can just reuse it.
                            return Ok(got);
                        }
                        Some(got)
                    }
                }
            }
        } else {
            None
        };

        // If cached_good is Some, we found a plausible cache entry; if we got here, it was
        // past its max_refresh.  So in that case we want to send a request with
        // if-modified-since.  If we get Not Modified, we can reuse it (and update the fetched time).
        let if_modified_since = cached_good
            .as_ref()
            .map(|got| got.desc.as_ref().published());

        debug!(
            r#"starting download for "{}"{}"#,
            bridge,
            match if_modified_since {
                Some(ims) => format!(
                    " if-modified-since {}",
                    humantime::format_rfc3339_seconds(ims),
                ),
                None => "".into(),
            }
        );

        let text = mockable
            .clone()
            .download(&self.runtime, &self.circmgr, bridge, if_modified_since)
            .await?;

        let (document, got) = if let Some(text) = text {
            let got = process_document(&text)?;
            (text, got)
        } else if let Some(cached) = cached_good {
            (
                cache_entry
                    .expect("cached_good but not cache_entry")
                    .document,
                cached,
            )
        } else {
            return Err(internal!("download gave None but no if-modified-since").into());
        };

        // IEFI catches cache store errors, which we log but don't do anything else with
        (|| {
            let cached = CachedBridgeDescriptor {
                document,
                fetched: now, // this is from before we started the fetch, which is correct
            };

            // Calculate when the cache should forget about this.
            // We want to add a bit of slop for the purposes of mild clock skew handling,
            // etc., and the prefetch time is a good proxy for that.
            let until = got
                .refetch
                .checked_add(config.prefetch)
                .unwrap_or(got.refetch /*uh*/);

            store()?.store_bridgedesc(bridge, cached, until)?;
            Ok(())
        })()
        .unwrap_or_else(|err: crate::Error| {
            error_report!(err, "failed to cache downloaded bridge descriptor",);
        });

        Ok(got)
    }
}

/// Processes and analyses a textual descriptor document into a `Downloaded`
///
/// Parses it, checks the signature, checks the document validity times,
/// and if that's all good, calculates when will want to refetch it.
fn process_document<R: Runtime>(
    runtime: &R,
    config: &BridgeDescDownloadConfig,
    text: &str,
) -> Result<Downloaded, Error> {
    let desc = RouterDesc::parse(text)?;

    // We *could* just trust this because we have trustworthy provenance
    // we know that the channel machinery authenticated the identity keys in `bridge`.
    // But let's do some cross-checking anyway.
    // `check_signature` checks the self-signature.
    let desc = desc.check_signature().map_err(Arc::new)?;

    let now = runtime.wallclock();
    desc.is_valid_at(&now)?;

    // Justification that use of "dangerously" is correct:
    // 1. We have checked this just above, so it is valid now.
    // 2. We are extracting the timeout and implement our own refetch logic using expires.
    let (desc, (_, expires)) = desc.dangerously_into_parts();

    // Our refetch schedule, and enforcement of descriptor expiry, is somewhat approximate.
    // The following situations can result in a nominally-expired descriptor being used:
    //
    // 1. We primarily enforce the timeout by looking at the expiry time,
    //    subtracting a configured constant, and scheduling the start of a refetch then.
    //    If it takes us longer to do the retry, than the prefetch constant,
    //    we'll still be providing the old descriptor to consumers in the meantime.
    //
    // 2. We apply a minimum time before we will refetch a descriptor.
    //    So if the validity time is unreasonably short, we'll use it beyond that time.
    //
    // 3. Clock warping could confuse this algorithm.  This is inevitable because we
    //    are relying on calendar times (SystemTime) in the descriptor, and because
    //    we don't have a mechanism for being told about clock warps rather than the
    //    passage of time.
    //
    // We think this is all OK given that a bridge descriptor is used for trying to
    // connect to the bridge itself.  In particular, we don't want to completely trust
    // bridges to control our retry logic.
    let refetch = match expires {
        ops::Bound::Included(expires) | ops::Bound::Excluded(expires) => expires
            .checked_sub(config.prefetch)
            .ok_or(Error::ExtremeValidityTime)?,

        ops::Bound::Unbounded => now
            .checked_add(config.max_refetch)
            .ok_or(Error::ExtremeValidityTime)?,
    };
    let refetch = refetch.clamp(now + config.min_refetch, now + config.max_refetch);

    let desc = BridgeDesc::new(Arc::new(desc));

    Ok(Downloaded { desc, refetch })
}

/// Task which waits for the timeout, and requeues bridges that need to be refetched
///
/// This task's job is to execute the wakeup instructions provided via `updates`.
///
/// `updates` is the receiving end of [`State`]'s `earliest_timeout`,
/// which is maintained to be the earliest time any of the schedules says we should wake up
/// (liveness property *Timeout*).
async fn timeout_task<R: Runtime, M: Mockable<R>>(
    runtime: R,
    inner: Weak<Manager<R, M>>,
    update: postage::watch::Receiver<Option<Instant>>,
) {
    /// Requeue things in `*_schedule` whose time for action has arrived
    ///
    /// `retry_delay_map` converts `retry_delay` from the schedule (`RetryDelay` or `()`)
    /// into the `Option` which appears in [`QueuedEntry`].
    ///
    /// Helper function.  Idempotent.
    fn requeue_as_required<TT: Ord + Copy + Debug, RD, RDM: Fn(RD) -> Option<RetryDelay>>(
        queued: &mut VecDeque<QueuedEntry>,
        schedule: &mut BinaryHeap<RefetchEntry<TT, RD>>,
        now: TT,
        retry_delay_map: RDM,
    ) {
        while let Some(ent) = schedule.peek() {
            if ent.when > now {
                break;
            }
            let re = schedule.pop().expect("schedule became empty!");
            let bridge = re.bridge;
            let retry_delay = retry_delay_map(re.retry_delay);

            queued.push_back(QueuedEntry {
                bridge,
                retry_delay,
            });
        }
    }

    let mut next_wakeup = Some(runtime.now());
    let mut update = update.fuse();
    loop {
        select! {
            // Someone modified the schedules, and sent us a new earliest timeout
            changed = update.next() => {
                // changed is Option<Option< >>.
                // The outer Option is from the Stream impl for watch::Receiver - None means EOF.
                // The inner Option is Some(wakeup_time), or None meaning "wait indefinitely"
                next_wakeup = if let Some(changed) = changed {
                    changed
                } else {
                    // Oh, actually, the watch::Receiver is EOF - we're to shut down
                    break
                }
            },

            // Wait until the specified earliest wakeup time
            () = async {
                if let Some(next_wakeup) = next_wakeup {
                    let now = runtime.now();
                    if next_wakeup > now {
                        let duration = next_wakeup - now;
                        runtime.sleep(duration).await;
                    }
                } else {
                    #[allow(clippy::semicolon_if_nothing_returned)] // rust-clippy/issues/9729
                    { future::pending().await }
                }
            }.fuse() => {
                // We have reached the pre-programmed time.  Check what needs doing.

                let inner = if let Some(i) = inner.upgrade() { i } else { break; };
                let mut state = inner.lock_then_process();
                let state = &mut **state; // Do the DerefMut once so we can borrow fields

                requeue_as_required(
                    &mut state.queued,
                    &mut state.refetch_schedule,
                    runtime.wallclock(),
                    |()| None,
                );

                requeue_as_required(
                    &mut state.queued,
                    &mut state.retry_schedule,
                    runtime.now(),
                    Some,
                );

                // `StateGuard`, from `lock_then_process`, gets dropped here, and runs `process`,
                // to make further progress and restore the liveness properties.
            }
        }
    }
}

/// Error which occurs during bridge descriptor manager startup
#[derive(Clone, Debug, thiserror::Error)]
#[non_exhaustive]
pub enum StartupError {
    /// No circuit manager in the directory manager
    #[error(
        "tried to create bridge descriptor manager from directory manager with no circuit manager"
    )]
    MissingCircMgr,

    /// Unable to spawn task
    //
    // TODO lots of our Errors have a variant exactly like this.
    // Maybe we should make a struct tor_error::SpawnError.
    #[error("Unable to spawn {spawning}")]
    Spawn {
        /// What we were trying to spawn.
        spawning: &'static str,
        /// What happened when we tried to spawn it.
        #[source]
        cause: Arc<SpawnError>,
    },
}

impl HasKind for StartupError {
    fn kind(&self) -> ErrorKind {
        use ErrorKind as EK;
        use StartupError as SE;
        match self {
            SE::MissingCircMgr => EK::Internal,
            SE::Spawn { cause, .. } => cause.kind(),
        }
    }
}

/// An error which occurred trying to obtain the descriptor for a particular bridge
#[derive(Clone, Debug, thiserror::Error)]
#[non_exhaustive]
pub enum Error {
    /// Couldn't establish a circuit to the bridge
    #[error("Failed to establish circuit")]
    CircuitFailed(#[from] tor_circmgr::Error),

    /// Couldn't establish a directory stream to the bridge
    #[error("Failed to establish directory stream")]
    StreamFailed(#[source] tor_proto::Error),

    /// Directory request failed
    #[error("Directory request failed")]
    RequestFailed(#[from] tor_dirclient::RequestFailedError),

    /// Failed to parse descriptor in response
    #[error("Failed to parse descriptor in response")]
    ParseFailed(#[from] tor_netdoc::Error),

    /// Signature check failed
    #[error("Signature check failed")]
    SignatureCheckFailed(#[from] Arc<signature::Error>),

    /// Obtained descriptor but it is outside its validity time
    #[error("Descriptor is outside its validity time, as supplied")]
    BadValidityTime(#[from] tor_checkable::TimeValidityError),

    /// A bridge descriptor has very extreme validity times
    /// such that our refetch time calculations overflow.
    #[error("Descriptor validity time range is too extreme for us to cope with")]
    ExtremeValidityTime,

    /// There was a programming error somewhere in our code, or the calling code.
    #[error("Programming error")]
    Bug(#[from] tor_error::Bug),

    /// Error used for testing
    #[cfg(test)]
    #[error("Error for testing, {0:?}, retry at {1:?}")]
    TestError(&'static str, RetryTime),
}

impl HasKind for Error {
    fn kind(&self) -> ErrorKind {
        use Error as E;
        use ErrorKind as EK;
        let bridge_protocol_violation = EK::TorAccessFailed;
        match self {
            // We trust that tor_circmgr returns TorAccessFailed when it ought to.
            E::CircuitFailed(e) => e.kind(),
            E::StreamFailed(e) => e.kind(),
            E::RequestFailed(e) => e.kind(),
            E::ParseFailed(..) => bridge_protocol_violation,
            E::SignatureCheckFailed(..) => bridge_protocol_violation,
            E::ExtremeValidityTime => bridge_protocol_violation,
            E::BadValidityTime(..) => EK::ClockSkew,
            E::Bug(e) => e.kind(),
            #[cfg(test)]
            E::TestError(..) => EK::Internal,
        }
    }
}

impl HasRetryTime for Error {
    fn retry_time(&self) -> RetryTime {
        use Error as E;
        use RetryTime as R;
        match self {
            // Errors with their own retry times
            E::CircuitFailed(e) => e.retry_time(),

            // Remote misbehavior, maybe the network is being strange?
            E::StreamFailed(..) => R::AfterWaiting,
            E::RequestFailed(..) => R::AfterWaiting,

            // Remote misconfiguration, detected *after* we successfully made the channel
            // (so not a network problem).  We'll say "never" for RetryTime,
            // even though actually we will in fact retry in at most `max_refetch`.
            E::ParseFailed(..) => R::Never,
            E::SignatureCheckFailed(..) => R::Never,
            E::BadValidityTime(..) => R::Never,
            E::ExtremeValidityTime => R::Never,

            // Probably, things are broken here, rather than remotely.
            E::Bug(..) => R::Never,

            #[cfg(test)]
            E::TestError(_, retry) => *retry,
        }
    }
}

impl BridgeDescError for Error {}

impl State {
    /// Consistency check (for testing)
    ///
    /// `input` should be what was passed to `set_bridges` (or `None` if not known).
    ///
    /// Does not make any changes.
    /// Only takes `&mut` because postage::watch::Sender::borrow` wants it.
    #[cfg(test)]
    fn check_consistency<'i, R, I>(&mut self, runtime: &R, input: Option<I>)
    where
        R: Runtime,
        I: IntoIterator<Item = &'i BridgeKey>,
    {
        /// Where we found a thing was Tracked
        #[derive(Debug, Clone, Copy, Eq, PartialEq)]
        enum Where {
            /// Found in `running`
            Running,
            /// Found in `queued`
            Queued,
            /// Found in the schedule `sch`
            Schedule {
                sch_name: &'static str,
                /// Starts out as `false`, set to `true` when we find this in `current`
                found_in_current: bool,
            },
        }

        /// Records the expected input from `input`, and what we have found so far
        struct Tracked {
            /// Were we told what the last `set_bridges` call got as input?
            known_input: bool,
            /// `Some` means we have seen this bridge in one our records (other than `current`)
            tracked: HashMap<BridgeKey, Option<Where>>,
            /// Earliest instant found in any schedule
            earliest: Option<Instant>,
        }

        let mut tracked = if let Some(input) = input {
            let tracked = input.into_iter().map(|b| (b.clone(), None)).collect();
            Tracked {
                tracked,
                known_input: true,
                earliest: None,
            }
        } else {
            Tracked {
                tracked: HashMap::new(),
                known_input: false,
                earliest: None,
            }
        };

        impl Tracked {
            /// Note that `bridge` is Tracked
            fn note(&mut self, where_: Where, b: &BridgeKey) {
                match self.tracked.get(b) {
                    // Invariant *Tracked* - ie appears at most once
                    Some(Some(prev_where)) => {
                        panic!("duplicate {:?} {:?} {:?}", prev_where, where_, b);
                    }
                    // Invariant *Input (every tracked bridge is was in input)*
                    None if self.known_input => {
                        panic!("unexpected {:?} {:?}", where_, b);
                    }
                    // OK, we've not seen it before, note it as being here
                    _ => {
                        self.tracked.insert(b.clone(), Some(where_));
                    }
                }
            }
        }

        /// Walk `schedule` and update `tracked` (including `tracked.earliest`)
        ///
        /// Check invariant *Tracked* and *Schedule* wrt this schedule.
        #[cfg(test)]
        fn walk_sch<TT: Ord + Copy + Debug, RD, CT: Fn(TT) -> Instant>(
            tracked: &mut Tracked,
            sch_name: &'static str,
            schedule: &BinaryHeap<RefetchEntry<TT, RD>>,
            conv_time: CT,
        ) {
            let where_ = Where::Schedule {
                sch_name,
                found_in_current: false,
            };

            if let Some(first) = schedule.peek() {
                // Of course this is a heap, so this ought to be a wasteful scan,
                // but, indirectly,this tests our implementation of `Ord` for `RefetchEntry`.
                for re in schedule {
                    tracked.note(where_, &re.bridge);
                }

                let scanned = schedule
                    .iter()
                    .map(|re| re.when)
                    .min()
                    .expect("schedule empty!");
                assert_eq!(scanned, first.when);
                tracked.earliest = Some(
                    [tracked.earliest, Some(conv_time(scanned))]
                        .into_iter()
                        .flatten()
                        .min()
                        .expect("flatten of chain Some was empty"),
                );
            }
        }

        // *Timeout* (prep)
        //
        // This will fail if there is clock skew, but won't mind if
        // the earliest refetch time is in the past.
        let now_wall = runtime.wallclock();
        let now_mono = runtime.now();
        let adj_wall = |wallclock: SystemTime| {
            // Good grief what a palaver!
            if let Ok(ahead) = wallclock.duration_since(now_wall) {
                now_mono + ahead
            } else if let Ok(behind) = now_wall.duration_since(wallclock) {
                now_mono
                    .checked_sub(behind)
                    .expect("time subtraction underflow")
            } else {
                panic!("times should be totally ordered!")
            }
        };

        // *Tracked*
        //
        // We walk our data structures in turn

        for b in self.running.keys() {
            tracked.note(Where::Running, b);
        }
        for qe in &self.queued {
            tracked.note(Where::Queued, &qe.bridge);
        }

        walk_sch(&mut tracked, "refetch", &self.refetch_schedule, adj_wall);
        walk_sch(&mut tracked, "retry", &self.retry_schedule, |t| t);

        // *Current*
        for b in self.current.keys() {
            let found = tracked
                .tracked
                .get_mut(b)
                .and_then(Option::as_mut)
                .unwrap_or_else(|| panic!("current but untracked {:?}", b));
            if let Where::Schedule {
                found_in_current, ..
            } = found
            {
                *found_in_current = true;
            }
        }

        // *Input (sense: every input bridge is tracked)*
        //
        // (Will not cope if spawn ever failed, since that violates the invariant.)
        for (b, where_) in &tracked.tracked {
            match where_ {
                None => panic!("missing {}", &b),
                Some(Where::Schedule {
                    sch_name,
                    found_in_current,
                }) => {
                    assert!(found_in_current, "not-Schedule {} {}", &b, sch_name);
                }
                _ => {}
            }
        }

        // *Limit*
        let parallelism = self.effective_parallelism();
        assert!(self.running.len() <= parallelism);

        // *Running*
        assert!(self.running.len() == parallelism || self.queued.is_empty());

        // *Timeout* (final)
        assert_eq!(tracked.earliest, *self.earliest_timeout.borrow());
    }
}