calloop/sources/
transient.rs

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
//! Wrapper for a transient Calloop event source.
//!
//! If you have high level event source that you expect to remain in the event
//! loop indefinitely, and another event source nested inside that one that you
//! expect to require removal or disabling from time to time, this module can
//! handle it for you.

/// A [`TransientSource`] wraps a Calloop event source and manages its
/// registration. A user of this type only needs to perform the usual Calloop
/// calls (`process_events()` and `*register()`) and the return value of
/// [`process_events()`](crate::EventSource::process_events).
///
/// Rather than needing to check for the full set of
/// [`PostAction`](crate::PostAction) values returned from `process_events()`,
/// you can just check for `Continue` or `Reregister` and pass that back out
/// through your own `process_events()` implementation. In your registration
/// functions, you then only need to call the same function on this type ie.
/// `register()` inside `register()` etc.
///
/// For example, say you have a source that contains a channel along with some
/// other logic. If the channel's sending end has been dropped, it needs to be
/// removed from the loop. So to manage this, you use this in your struct:
///
/// ```none,actually-rust-but-see-https://github.com/rust-lang/rust/issues/63193
/// struct CompositeSource {
///    // Event source for channel.
///    mpsc_receiver: TransientSource<calloop::channel::Channel<T>>,
///
///    // Any other fields go here...
/// }
/// ```
///
/// To create the transient source, you can simply use the `Into`
/// implementation:
///
/// ```none,actually-rust-but-see-https://github.com/rust-lang/rust/issues/63193
/// let (sender, source) = channel();
/// let mpsc_receiver: TransientSource<Channel> = source.into();
/// ```
///
/// (If you want to start off with an empty `TransientSource`, you can just use
/// `Default::default()` instead.)
///
/// `TransientSource` implements [`EventSource`](crate::EventSource) and passes
/// through `process_events()` calls, so in the parent's `process_events()`
/// implementation you can just do this:
///
/// ```none,actually-rust-but-see-https://github.com/rust-lang/rust/issues/63193
/// fn process_events<F>(
///     &mut self,
///     readiness: calloop::Readiness,
///     token: calloop::Token,
///     callback: F,
/// ) -> Result<calloop::PostAction, Self::Error>
/// where
///     F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
/// {
///     let channel_return = self.mpsc_receiver.process_events(readiness, token, callback)?;
///
///     // Perform other logic here...
///
///     Ok(channel_return)
/// }
/// ```
///
/// Note that:
///
///   - You can call `process_events()` on the `TransientSource<Channel>` even
///     if the channel has been unregistered and dropped. All that will happen
///     is that you won't get any events from it.
///
///   - The [`PostAction`](crate::PostAction) returned from `process_events()`
///     will only ever be `PostAction::Continue` or `PostAction::Reregister`.
///     You will still need to combine this with the result of any other sources
///     (transient or not).
///
/// Once you return `channel_return` from your `process_events()` method (and
/// assuming it propagates all the way up to the event loop itself through any
/// other event sources), the event loop might call `reregister()` on your
/// source. All your source has to do is:
///
/// ```none,actually-rust-but-see-https://github.com/rust-lang/rust/issues/63193
/// fn reregister(
///     &mut self,
///     poll: &mut calloop::Poll,
///     token_factory: &mut calloop::TokenFactory,
/// ) -> crate::Result<()> {
///     self.mpsc_receiver.reregister(poll, token_factory)?;
///
///     // Other registration actions...
///
///     Ok(())
/// }
/// ```
///
/// The `TransientSource` will take care of updating the registration of the
/// inner source, even if it actually needs to be unregistered or initially
/// registered.
///
/// ## Replacing or removing `TransientSource`s
///
/// Not properly removing or replacing `TransientSource`s can cause spurious
/// wakeups of the event loop, and in some cases can leak file descriptors or
/// fail to free entries in Calloop's internal data structures. No unsoundness
/// or undefined behaviour will result, but leaking file descriptors can result
/// in errors or panics.
///
/// If you want to remove a source before it returns `PostAction::Remove`, use
/// the [`TransientSource::remove()`] method. If you want to replace a source
/// with another one, use the [`TransientSource::replace()`] method. Either of
/// these may be called at any time during processing or from outside the event
/// loop. Both require either returning `PostAction::Reregister` from the
/// `process_event()` call that does this, or reregistering the event source
/// some other way eg. via the top-level loop handle.
///
/// If, instead, you directly assign a new source to the variable holding the
/// `TransientSource`, the inner source will be dropped before it can be
/// unregistered. For example:
///
/// ```none,actually-rust-but-see-https://github.com/rust-lang/rust/issues/63193
/// self.mpsc_receiver = Default::default();
/// self.mpsc_receiver = new_channel.into();
/// ```
#[derive(Debug, Default)]
pub struct TransientSource<T> {
    state: TransientSourceState<T>,
}

/// This is the internal state of the [`TransientSource`], as a separate type so
/// it's not exposed.
#[derive(Debug)]
enum TransientSourceState<T> {
    /// The source should be kept in the loop.
    Keep(T),
    /// The source needs to be registered with the loop.
    Register(T),
    /// The source needs to be disabled but kept.
    Disable(T),
    /// The source needs to be removed from the loop.
    Remove(T),
    /// The source is being replaced by another. For most API purposes (eg.
    /// `map()`), this will be treated as the `Register` state enclosing the new
    /// source.
    Replace {
        /// The new source, which will be registered and used from now on.
        new: T,
        /// The old source, which will be unregistered and dropped.
        old: T,
    },
    /// The source has been removed from the loop and dropped (this might also
    /// be observed if there is a panic while changing states).
    None,
}

impl<T> Default for TransientSourceState<T> {
    fn default() -> Self {
        Self::None
    }
}

impl<T> TransientSourceState<T> {
    /// If a caller needs to flag the contained source for removal or
    /// registration, we need to replace the enum variant safely. This requires
    /// having a `None` value in there temporarily while we do the swap.
    ///
    /// If the variant is `None` the value will not change and `replacer` will
    /// not be called. If the variant is `Replace` then `replacer` will be
    /// called **on the new source**, which may cause the old source to leak
    /// registration in the event loop if it has not yet been unregistered.
    ///
    /// The `replacer` function here is expected to be one of the enum variant
    /// constructors eg. `replace(TransientSource::Remove)`.
    fn replace_state<F>(&mut self, replacer: F)
    where
        F: FnOnce(T) -> Self,
    {
        *self = match std::mem::take(self) {
            Self::Keep(source)
            | Self::Register(source)
            | Self::Remove(source)
            | Self::Disable(source)
            | Self::Replace { new: source, .. } => replacer(source),
            Self::None => return,
        };
    }
}

impl<T> TransientSource<T> {
    /// Apply a function to the enclosed source, if it exists and is not about
    /// to be removed.
    pub fn map<F, U>(&mut self, f: F) -> Option<U>
    where
        F: FnOnce(&mut T) -> U,
    {
        match &mut self.state {
            TransientSourceState::Keep(source)
            | TransientSourceState::Register(source)
            | TransientSourceState::Disable(source)
            | TransientSourceState::Replace { new: source, .. } => Some(f(source)),
            TransientSourceState::Remove(_) | TransientSourceState::None => None,
        }
    }

    /// Returns `true` if there is no wrapped event source.
    pub fn is_none(&self) -> bool {
        matches!(self.state, TransientSourceState::None)
    }

    /// Removes the wrapped event source from the event loop and this wrapper.
    ///
    /// If this is called from outside of the event loop, you will need to wake
    /// up the event loop for any changes to take place. If it is called from
    /// within the event loop, you must return `PostAction::Reregister` from
    /// your own event source's `process_events()`, and the source will be
    /// unregistered as needed after it exits.
    pub fn remove(&mut self) {
        self.state.replace_state(TransientSourceState::Remove);
    }

    /// Replace the currently wrapped source with the given one.  No more events
    /// will be generated from the old source after this point. The old source
    /// will not be dropped immediately, it will be kept so that it can be
    /// deregistered.
    ///
    /// If this is called from outside of the event loop, you will need to wake
    /// up the event loop for any changes to take place. If it is called from
    /// within the event loop, you must return `PostAction::Reregister` from
    /// your own event source's `process_events()`, and the sources will be
    /// registered and unregistered as needed after it exits.
    pub fn replace(&mut self, new: T) {
        self.state
            .replace_state(|old| TransientSourceState::Replace { new, old });
    }
}

impl<T: crate::EventSource> From<T> for TransientSource<T> {
    fn from(source: T) -> Self {
        Self {
            state: TransientSourceState::Register(source),
        }
    }
}

impl<T: crate::EventSource> crate::EventSource for TransientSource<T> {
    type Event = T::Event;
    type Metadata = T::Metadata;
    type Ret = T::Ret;
    type Error = T::Error;

    fn process_events<F>(
        &mut self,
        readiness: crate::Readiness,
        token: crate::Token,
        callback: F,
    ) -> Result<crate::PostAction, Self::Error>
    where
        F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
    {
        let reregister = if let TransientSourceState::Keep(source) = &mut self.state {
            let child_post_action = source.process_events(readiness, token, callback)?;

            match child_post_action {
                // Nothing needs to change.
                crate::PostAction::Continue => false,

                // Our child source needs re-registration, therefore this
                // wrapper needs re-registration.
                crate::PostAction::Reregister => true,

                // If our nested source needs to be removed or disabled, we need
                // to swap it out for the "Remove" or "Disable" variant.
                crate::PostAction::Disable => {
                    self.state.replace_state(TransientSourceState::Disable);
                    true
                }

                crate::PostAction::Remove => {
                    self.state.replace_state(TransientSourceState::Remove);
                    true
                }
            }
        } else {
            false
        };

        let post_action = if reregister {
            crate::PostAction::Reregister
        } else {
            crate::PostAction::Continue
        };

        Ok(post_action)
    }

    fn register(
        &mut self,
        poll: &mut crate::Poll,
        token_factory: &mut crate::TokenFactory,
    ) -> crate::Result<()> {
        match &mut self.state {
            TransientSourceState::Keep(source) => {
                source.register(poll, token_factory)?;
            }
            TransientSourceState::Register(source)
            | TransientSourceState::Disable(source)
            | TransientSourceState::Replace { new: source, .. } => {
                source.register(poll, token_factory)?;
                self.state.replace_state(TransientSourceState::Keep);
                // Drops the disposed source in the Replace case.
            }
            TransientSourceState::Remove(_source) => {
                self.state.replace_state(|_| TransientSourceState::None);
            }
            TransientSourceState::None => (),
        }
        Ok(())
    }

    fn reregister(
        &mut self,
        poll: &mut crate::Poll,
        token_factory: &mut crate::TokenFactory,
    ) -> crate::Result<()> {
        match &mut self.state {
            TransientSourceState::Keep(source) => source.reregister(poll, token_factory)?,
            TransientSourceState::Register(source) => {
                source.register(poll, token_factory)?;
                self.state.replace_state(TransientSourceState::Keep);
            }
            TransientSourceState::Disable(source) => {
                source.unregister(poll)?;
            }
            TransientSourceState::Remove(source) => {
                source.unregister(poll)?;
                self.state.replace_state(|_| TransientSourceState::None);
            }
            TransientSourceState::Replace { new, old } => {
                old.unregister(poll)?;
                new.register(poll, token_factory)?;
                self.state.replace_state(TransientSourceState::Keep);
                // Drops 'dispose'.
            }
            TransientSourceState::None => (),
        }
        Ok(())
    }

    fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
        match &mut self.state {
            TransientSourceState::Keep(source)
            | TransientSourceState::Register(source)
            | TransientSourceState::Disable(source) => source.unregister(poll)?,
            TransientSourceState::Remove(source) => {
                source.unregister(poll)?;
                self.state.replace_state(|_| TransientSourceState::None);
            }
            TransientSourceState::Replace { new, old } => {
                old.unregister(poll)?;
                new.unregister(poll)?;
                self.state.replace_state(TransientSourceState::Register);
            }
            TransientSourceState::None => (),
        }
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{
        channel::{channel, Channel, Event},
        ping::{make_ping, PingSource},
        Dispatcher, EventSource, PostAction,
    };
    use std::{
        rc::Rc,
        sync::atomic::{AtomicBool, Ordering},
        time::Duration,
    };

    #[test]
    fn test_transient_drop() {
        // A test source that sets a flag when it's dropped.
        struct TestSource<'a> {
            dropped: &'a AtomicBool,
            ping: PingSource,
        }

        impl<'a> Drop for TestSource<'a> {
            fn drop(&mut self) {
                self.dropped.store(true, Ordering::Relaxed)
            }
        }

        impl<'a> crate::EventSource for TestSource<'a> {
            type Event = ();
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                callback: F,
            ) -> Result<crate::PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                self.ping.process_events(readiness, token, callback)?;
                Ok(PostAction::Remove)
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.ping.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.ping.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.ping.unregister(poll)
            }
        }

        // Test that the inner source is actually dropped when it asks to be
        // removed from the loop, while the TransientSource remains. We use two
        // flags for this:
        // - fired: should be set only when the inner event source has an event
        // - dropped: set by the drop handler for the inner source (it's an
        //   AtomicBool becaues it requires a longer lifetime than the fired
        //   flag)
        let mut fired = false;
        let dropped = false.into();

        // The inner source that should be dropped after the first loop run.
        let (pinger, ping) = make_ping().unwrap();
        let inner = TestSource {
            dropped: &dropped,
            ping,
        };

        // The TransientSource wrapper.
        let outer: TransientSource<_> = inner.into();

        let mut event_loop = crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();

        let _token = handle
            .insert_source(outer, |_, _, fired| {
                *fired = true;
            })
            .unwrap();

        // First loop run: the ping generates an event for the inner source.
        pinger.ping();

        event_loop.dispatch(Duration::ZERO, &mut fired).unwrap();

        assert!(fired);
        assert!(dropped.load(Ordering::Relaxed));

        // Second loop run: the ping does nothing because the receiver has been
        // dropped.
        fired = false;

        pinger.ping();

        event_loop.dispatch(Duration::ZERO, &mut fired).unwrap();
        assert!(!fired);
    }

    #[test]
    fn test_transient_passthrough() {
        // Test that event processing works when a source is nested inside a
        // TransientSource. In particular, we want to ensure that the final
        // event is received even if it corresponds to that same event source
        // returning `PostAction::Remove`.
        let (sender, receiver) = channel();
        let outer: TransientSource<_> = receiver.into();

        let mut event_loop = crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();

        // Our callback puts the receied events in here for us to check later.
        let mut msg_queue = vec![];

        let _token = handle
            .insert_source(outer, |msg, _, queue: &mut Vec<_>| {
                queue.push(msg);
            })
            .unwrap();

        // Send some data and drop the sender. We specifically want to test that
        // we get the "closed" message.
        sender.send(0u32).unwrap();
        sender.send(1u32).unwrap();
        sender.send(2u32).unwrap();
        sender.send(3u32).unwrap();
        drop(sender);

        // Run loop once to process events.
        event_loop.dispatch(Duration::ZERO, &mut msg_queue).unwrap();

        assert!(matches!(
            msg_queue.as_slice(),
            &[
                Event::Msg(0u32),
                Event::Msg(1u32),
                Event::Msg(2u32),
                Event::Msg(3u32),
                Event::Closed
            ]
        ));
    }

    #[test]
    fn test_transient_map() {
        struct IdSource {
            id: u32,
            ping: PingSource,
        }

        impl EventSource for IdSource {
            type Event = u32;
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                mut callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                let id = self.id;
                self.ping
                    .process_events(readiness, token, |_, md| callback(id, md))?;

                let action = if self.id > 2 {
                    PostAction::Remove
                } else {
                    PostAction::Continue
                };

                Ok(action)
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.ping.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.ping.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.ping.unregister(poll)
            }
        }

        struct WrapperSource(TransientSource<IdSource>);

        impl EventSource for WrapperSource {
            type Event = <IdSource as EventSource>::Event;
            type Metadata = <IdSource as EventSource>::Metadata;
            type Ret = <IdSource as EventSource>::Ret;
            type Error = <IdSource as EventSource>::Error;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                let action = self.0.process_events(readiness, token, callback);
                self.0.map(|inner| inner.id += 1);
                action
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.0.map(|inner| inner.id += 1);
                self.0.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.0.map(|inner| inner.id += 1);
                self.0.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.0.map(|inner| inner.id += 1);
                self.0.unregister(poll)
            }
        }

        // To test the id later.
        let mut id = 0;

        // Create our source.
        let (pinger, ping) = make_ping().unwrap();
        let inner = IdSource { id, ping };

        // The TransientSource wrapper.
        let outer: TransientSource<_> = inner.into();

        // The top level source.
        let top = WrapperSource(outer);

        // Create a dispatcher so we can check the source afterwards.
        let dispatcher = Dispatcher::new(top, |got_id, _, test_id| {
            *test_id = got_id;
        });

        let mut event_loop = crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();

        let token = handle.register_dispatcher(dispatcher.clone()).unwrap();

        // First loop run: the ping generates an event for the inner source.
        // The ID should be 1 after the increment in register().
        pinger.ping();
        event_loop.dispatch(Duration::ZERO, &mut id).unwrap();
        assert_eq!(id, 1);

        // Second loop run: the ID should be 2 after the previous
        // process_events().
        pinger.ping();
        event_loop.dispatch(Duration::ZERO, &mut id).unwrap();
        assert_eq!(id, 2);

        // Third loop run: the ID should be 3 after another process_events().
        pinger.ping();
        event_loop.dispatch(Duration::ZERO, &mut id).unwrap();
        assert_eq!(id, 3);

        // Fourth loop run: the callback is no longer called by the inner
        // source, so our local ID is not incremented.
        pinger.ping();
        event_loop.dispatch(Duration::ZERO, &mut id).unwrap();
        assert_eq!(id, 3);

        // Remove the dispatcher so we can inspect the sources.
        handle.remove(token);

        let mut top_after = dispatcher.into_source_inner();

        // I expect the inner source to be dropped, so the TransientSource
        // variant is None (its version of None, not Option::None), so its map()
        // won't call the passed-in function (hence the unreachable!()) and its
        // return value should be Option::None.
        assert!(top_after.0.map(|_| unreachable!()).is_none());
    }

    #[test]
    fn test_transient_disable() {
        // Test that disabling and enabling is handled properly.
        struct DisablingSource(PingSource);

        impl EventSource for DisablingSource {
            type Event = ();
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                self.0.process_events(readiness, token, callback)?;
                Ok(PostAction::Disable)
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.0.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.0.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.0.unregister(poll)
            }
        }

        // Flag for checking when the source fires.
        let mut fired = false;

        // Create our source.
        let (pinger, ping) = make_ping().unwrap();

        let inner = DisablingSource(ping);

        // The TransientSource wrapper.
        let outer: TransientSource<_> = inner.into();

        let mut event_loop = crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();
        let token = handle
            .insert_source(outer, |_, _, fired| {
                *fired = true;
            })
            .unwrap();

        // Ping here and not later, to check that disabling after an event is
        // triggered but not processed does not discard the event.
        pinger.ping();
        event_loop.dispatch(Duration::ZERO, &mut fired).unwrap();
        assert!(fired);

        // Source should now be disabled.
        pinger.ping();
        fired = false;
        event_loop.dispatch(Duration::ZERO, &mut fired).unwrap();
        assert!(!fired);

        // Re-enable the source.
        handle.enable(&token).unwrap();

        // Trigger another event.
        pinger.ping();
        fired = false;
        event_loop.dispatch(Duration::ZERO, &mut fired).unwrap();
        assert!(fired);
    }

    #[test]
    fn test_transient_replace_unregister() {
        // This is a bit of a complex test, but it essentially boils down to:
        // how can a "parent" event source containing a TransientSource replace
        // the "child" source without leaking the source's registration?

        // First, a source that finishes immediately. This is so we cover the
        // edge case of replacing a source as soon as it wants to be removed.
        struct FinishImmediatelySource {
            source: PingSource,
            data: Option<i32>,
            registered: bool,
            dropped: Rc<AtomicBool>,
        }

        impl FinishImmediatelySource {
            // The constructor passes out the drop flag so we can check that
            // this source was or wasn't dropped.
            fn new(source: PingSource, data: i32) -> (Self, Rc<AtomicBool>) {
                let dropped = Rc::new(false.into());

                (
                    Self {
                        source,
                        data: Some(data),
                        registered: false,
                        dropped: Rc::clone(&dropped),
                    },
                    dropped,
                )
            }
        }

        impl EventSource for FinishImmediatelySource {
            type Event = i32;
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                mut callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                let mut data = self.data.take();

                self.source.process_events(readiness, token, |_, _| {
                    if let Some(data) = data.take() {
                        callback(data, &mut ())
                    }
                })?;

                self.data = data;

                Ok(if self.data.is_none() {
                    PostAction::Remove
                } else {
                    PostAction::Continue
                })
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.registered = true;
                self.source.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.source.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.registered = false;
                self.source.unregister(poll)
            }
        }

        // The drop handler sets a flag we can check for debugging (we want to
        // know that the source itself was dropped), and also checks that the
        // source was unregistered. Ultimately neither the source nor its
        // registration should be leaked.

        impl Drop for FinishImmediatelySource {
            fn drop(&mut self) {
                assert!(!self.registered, "source dropped while still registered");
                self.dropped.store(true, Ordering::Relaxed);
            }
        }

        // Our wrapper source handles detecting when the child source finishes,
        // and replacing that child source with another one that will generate
        // more events. This is one intended use case of the TransientSource.

        struct WrapperSource {
            current: TransientSource<FinishImmediatelySource>,
            replacement: Option<FinishImmediatelySource>,
            dropped: Rc<AtomicBool>,
        }

        impl WrapperSource {
            // The constructor passes out the drop flag so we can check that
            // this source was or wasn't dropped.
            fn new(
                first: FinishImmediatelySource,
                second: FinishImmediatelySource,
            ) -> (Self, Rc<AtomicBool>) {
                let dropped = Rc::new(false.into());

                (
                    Self {
                        current: first.into(),
                        replacement: second.into(),
                        dropped: Rc::clone(&dropped),
                    },
                    dropped,
                )
            }
        }

        impl EventSource for WrapperSource {
            type Event = i32;
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                mut callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                // Did our event source generate an event?
                let mut fired = false;

                let post_action = self.current.process_events(readiness, token, |data, _| {
                    callback(data, &mut ());
                    fired = true;
                })?;

                if fired {
                    // The event source will be unregistered after the current
                    // process_events() iteration is finished. The replace()
                    // method will handle doing that even while we've added a
                    // new source.
                    if let Some(replacement) = self.replacement.take() {
                        self.current.replace(replacement);
                    }

                    // Parent source is responsible for flagging this, but it's
                    // already set.
                    assert_eq!(post_action, PostAction::Reregister);
                }

                Ok(post_action)
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.current.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.current.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.current.unregister(poll)
            }
        }

        impl Drop for WrapperSource {
            fn drop(&mut self) {
                self.dropped.store(true, Ordering::Relaxed);
            }
        }

        // Construct the various nested sources - FinishImmediatelySource inside
        // TransientSource inside WrapperSource. The numbers let us verify which
        // event source fires first.
        let (ping0_tx, ping0_rx) = crate::ping::make_ping().unwrap();
        let (ping1_tx, ping1_rx) = crate::ping::make_ping().unwrap();
        let (inner0, inner0_dropped) = FinishImmediatelySource::new(ping0_rx, 0);
        let (inner1, inner1_dropped) = FinishImmediatelySource::new(ping1_rx, 1);
        let (outer, outer_dropped) = WrapperSource::new(inner0, inner1);

        // Now the actual test starts.

        let mut event_loop: crate::EventLoop<(Option<i32>, crate::LoopSignal)> =
            crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();
        let signal = event_loop.get_signal();

        // This is how we communicate with the event sources.
        let mut context = (None, signal);

        let _token = handle
            .insert_source(outer, |data, _, (evt, sig)| {
                *evt = Some(data);
                sig.stop();
            })
            .unwrap();

        // Ensure our sources fire.
        ping0_tx.ping();
        ping1_tx.ping();

        // Use run() rather than dispatch() because it's not strictly part of
        // any API contract as to how many runs of the event loop it takes to
        // replace the nested source.
        event_loop.run(None, &mut context, |_| {}).unwrap();

        // First, make sure the inner source actually did fire.
        assert_eq!(context.0.take(), Some(0), "first inner source did not fire");

        // Make sure that the outer source is still alive.
        assert!(
            !outer_dropped.load(Ordering::Relaxed),
            "outer source already dropped"
        );

        // Make sure that the inner child source IS dropped now.
        assert!(
            inner0_dropped.load(Ordering::Relaxed),
            "first inner source not dropped"
        );

        // Make sure that, in between the first event and second event, the
        // replacement child source still exists.
        assert!(
            !inner1_dropped.load(Ordering::Relaxed),
            "replacement inner source dropped"
        );

        // Run the event loop until we get a second event.
        event_loop.run(None, &mut context, |_| {}).unwrap();

        // Ensure the replacement source fired (which checks that it was
        // registered and is being processed by the TransientSource).
        assert_eq!(context.0.take(), Some(1), "replacement source did not fire");
    }

    #[test]
    fn test_transient_remove() {
        // This tests that calling remove(), even before an event source has
        // requested its own removal, results in the event source being removed.

        const STOP_AT: i32 = 2;

        // A wrapper source to automate the removal of the inner source.
        struct WrapperSource {
            inner: TransientSource<Channel<i32>>,
        }

        impl EventSource for WrapperSource {
            type Event = i32;
            type Metadata = ();
            type Ret = ();
            type Error = Box<dyn std::error::Error + Sync + Send>;

            fn process_events<F>(
                &mut self,
                readiness: crate::Readiness,
                token: crate::Token,
                mut callback: F,
            ) -> Result<PostAction, Self::Error>
            where
                F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
            {
                let mut remove = false;

                let mut post_action = self.inner.process_events(readiness, token, |evt, _| {
                    if let Event::Msg(num) = evt {
                        callback(num, &mut ());
                        remove = num >= STOP_AT;
                    }
                })?;

                if remove {
                    self.inner.remove();
                    post_action |= PostAction::Reregister;
                }

                Ok(post_action)
            }

            fn register(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.inner.register(poll, token_factory)
            }

            fn reregister(
                &mut self,
                poll: &mut crate::Poll,
                token_factory: &mut crate::TokenFactory,
            ) -> crate::Result<()> {
                self.inner.reregister(poll, token_factory)
            }

            fn unregister(&mut self, poll: &mut crate::Poll) -> crate::Result<()> {
                self.inner.unregister(poll)
            }
        }

        // Create our sources and loop.

        let (sender, receiver) = channel();
        let wrapper = WrapperSource {
            inner: receiver.into(),
        };

        let mut event_loop = crate::EventLoop::try_new().unwrap();
        let handle = event_loop.handle();

        handle
            .insert_source(wrapper, |num, _, out: &mut Option<_>| {
                *out = Some(num);
            })
            .unwrap();

        // Storage for callback data.
        let mut out = None;

        // Send some data we expect to get callbacks for.
        for num in 0..=STOP_AT {
            sender.send(num).unwrap();
            event_loop.dispatch(Duration::ZERO, &mut out).unwrap();
            assert_eq!(out.take(), Some(num));
        }

        // Now we expect the receiver to be gone.
        assert!(matches!(
            sender.send(STOP_AT + 1),
            Err(std::sync::mpsc::SendError { .. })
        ));
    }
}