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
//! Runs `!Send` futures on the current thread.
use crate::loom::cell::UnsafeCell;
use crate::loom::sync::{Arc, Mutex};
#[cfg(tokio_unstable)]
use crate::runtime;
use crate::runtime::task::{self, JoinHandle, LocalOwnedTasks, Task, TaskHarnessScheduleHooks};
use crate::runtime::{context, ThreadId, BOX_FUTURE_THRESHOLD};
use crate::sync::AtomicWaker;
use crate::util::RcCell;

use std::cell::Cell;
use std::collections::VecDeque;
use std::fmt;
use std::future::Future;
use std::marker::PhantomData;
use std::pin::Pin;
use std::rc::Rc;
use std::task::Poll;

use pin_project_lite::pin_project;

cfg_rt! {
    /// A set of tasks which are executed on the same thread.
    ///
    /// In some cases, it is necessary to run one or more futures that do not
    /// implement [`Send`] and thus are unsafe to send between threads. In these
    /// cases, a [local task set] may be used to schedule one or more `!Send`
    /// futures to run together on the same thread.
    ///
    /// For example, the following code will not compile:
    ///
    /// ```rust,compile_fail
    /// use std::rc::Rc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     // `Rc` does not implement `Send`, and thus may not be sent between
    ///     // threads safely.
    ///     let nonsend_data = Rc::new("my nonsend data...");
    ///
    ///     let nonsend_data = nonsend_data.clone();
    ///     // Because the `async` block here moves `nonsend_data`, the future is `!Send`.
    ///     // Since `tokio::spawn` requires the spawned future to implement `Send`, this
    ///     // will not compile.
    ///     tokio::spawn(async move {
    ///         println!("{}", nonsend_data);
    ///         // ...
    ///     }).await.unwrap();
    /// }
    /// ```
    ///
    /// # Use with `run_until`
    ///
    /// To spawn `!Send` futures, we can use a local task set to schedule them
    /// on the thread calling [`Runtime::block_on`]. When running inside of the
    /// local task set, we can use [`task::spawn_local`], which can spawn
    /// `!Send` futures. For example:
    ///
    /// ```rust
    /// use std::rc::Rc;
    /// use tokio::task;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let nonsend_data = Rc::new("my nonsend data...");
    ///
    ///     // Construct a local task set that can run `!Send` futures.
    ///     let local = task::LocalSet::new();
    ///
    ///     // Run the local task set.
    ///     local.run_until(async move {
    ///         let nonsend_data = nonsend_data.clone();
    ///         // `spawn_local` ensures that the future is spawned on the local
    ///         // task set.
    ///         task::spawn_local(async move {
    ///             println!("{}", nonsend_data);
    ///             // ...
    ///         }).await.unwrap();
    ///     }).await;
    /// }
    /// ```
    /// **Note:** The `run_until` method can only be used in `#[tokio::main]`,
    /// `#[tokio::test]` or directly inside a call to [`Runtime::block_on`]. It
    /// cannot be used inside a task spawned with `tokio::spawn`.
    ///
    /// ## Awaiting a `LocalSet`
    ///
    /// Additionally, a `LocalSet` itself implements `Future`, completing when
    /// *all* tasks spawned on the `LocalSet` complete. This can be used to run
    /// several futures on a `LocalSet` and drive the whole set until they
    /// complete. For example,
    ///
    /// ```rust
    /// use tokio::{task, time};
    /// use std::rc::Rc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let nonsend_data = Rc::new("world");
    ///     let local = task::LocalSet::new();
    ///
    ///     let nonsend_data2 = nonsend_data.clone();
    ///     local.spawn_local(async move {
    ///         // ...
    ///         println!("hello {}", nonsend_data2)
    ///     });
    ///
    ///     local.spawn_local(async move {
    ///         time::sleep(time::Duration::from_millis(100)).await;
    ///         println!("goodbye {}", nonsend_data)
    ///     });
    ///
    ///     // ...
    ///
    ///     local.await;
    /// }
    /// ```
    /// **Note:** Awaiting a `LocalSet` can only be done inside
    /// `#[tokio::main]`, `#[tokio::test]` or directly inside a call to
    /// [`Runtime::block_on`]. It cannot be used inside a task spawned with
    /// `tokio::spawn`.
    ///
    /// ## Use inside `tokio::spawn`
    ///
    /// The two methods mentioned above cannot be used inside `tokio::spawn`, so
    /// to spawn `!Send` futures from inside `tokio::spawn`, we need to do
    /// something else. The solution is to create the `LocalSet` somewhere else,
    /// and communicate with it using an [`mpsc`] channel.
    ///
    /// The following example puts the `LocalSet` inside a new thread.
    /// ```
    /// use tokio::runtime::Builder;
    /// use tokio::sync::{mpsc, oneshot};
    /// use tokio::task::LocalSet;
    ///
    /// // This struct describes the task you want to spawn. Here we include
    /// // some simple examples. The oneshot channel allows sending a response
    /// // to the spawner.
    /// #[derive(Debug)]
    /// enum Task {
    ///     PrintNumber(u32),
    ///     AddOne(u32, oneshot::Sender<u32>),
    /// }
    ///
    /// #[derive(Clone)]
    /// struct LocalSpawner {
    ///    send: mpsc::UnboundedSender<Task>,
    /// }
    ///
    /// impl LocalSpawner {
    ///     pub fn new() -> Self {
    ///         let (send, mut recv) = mpsc::unbounded_channel();
    ///
    ///         let rt = Builder::new_current_thread()
    ///             .enable_all()
    ///             .build()
    ///             .unwrap();
    ///
    ///         std::thread::spawn(move || {
    ///             let local = LocalSet::new();
    ///
    ///             local.spawn_local(async move {
    ///                 while let Some(new_task) = recv.recv().await {
    ///                     tokio::task::spawn_local(run_task(new_task));
    ///                 }
    ///                 // If the while loop returns, then all the LocalSpawner
    ///                 // objects have been dropped.
    ///             });
    ///
    ///             // This will return once all senders are dropped and all
    ///             // spawned tasks have returned.
    ///             rt.block_on(local);
    ///         });
    ///
    ///         Self {
    ///             send,
    ///         }
    ///     }
    ///
    ///     pub fn spawn(&self, task: Task) {
    ///         self.send.send(task).expect("Thread with LocalSet has shut down.");
    ///     }
    /// }
    ///
    /// // This task may do !Send stuff. We use printing a number as an example,
    /// // but it could be anything.
    /// //
    /// // The Task struct is an enum to support spawning many different kinds
    /// // of operations.
    /// async fn run_task(task: Task) {
    ///     match task {
    ///         Task::PrintNumber(n) => {
    ///             println!("{}", n);
    ///         },
    ///         Task::AddOne(n, response) => {
    ///             // We ignore failures to send the response.
    ///             let _ = response.send(n + 1);
    ///         },
    ///     }
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let spawner = LocalSpawner::new();
    ///
    ///     let (send, response) = oneshot::channel();
    ///     spawner.spawn(Task::AddOne(10, send));
    ///     let eleven = response.await.unwrap();
    ///     assert_eq!(eleven, 11);
    /// }
    /// ```
    ///
    /// [`Send`]: trait@std::marker::Send
    /// [local task set]: struct@LocalSet
    /// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
    /// [`task::spawn_local`]: fn@spawn_local
    /// [`mpsc`]: mod@crate::sync::mpsc
    pub struct LocalSet {
        /// Current scheduler tick.
        tick: Cell<u8>,

        /// State available from thread-local.
        context: Rc<Context>,

        /// This type should not be Send.
        _not_send: PhantomData<*const ()>,
    }
}

/// State available from the thread-local.
struct Context {
    /// State shared between threads.
    shared: Arc<Shared>,

    /// True if a task panicked without being handled and the local set is
    /// configured to shutdown on unhandled panic.
    unhandled_panic: Cell<bool>,
}

/// `LocalSet` state shared between threads.
struct Shared {
    /// # Safety
    ///
    /// This field must *only* be accessed from the thread that owns the
    /// `LocalSet` (i.e., `Thread::current().id() == owner`).
    local_state: LocalState,

    /// Remote run queue sender.
    queue: Mutex<Option<VecDeque<task::Notified<Arc<Shared>>>>>,

    /// Wake the `LocalSet` task.
    waker: AtomicWaker,

    /// How to respond to unhandled task panics.
    #[cfg(tokio_unstable)]
    pub(crate) unhandled_panic: crate::runtime::UnhandledPanic,
}

/// Tracks the `LocalSet` state that must only be accessed from the thread that
/// created the `LocalSet`.
struct LocalState {
    /// The `ThreadId` of the thread that owns the `LocalSet`.
    owner: ThreadId,

    /// Local run queue sender and receiver.
    local_queue: UnsafeCell<VecDeque<task::Notified<Arc<Shared>>>>,

    /// Collection of all active tasks spawned onto this executor.
    owned: LocalOwnedTasks<Arc<Shared>>,
}

pin_project! {
    #[derive(Debug)]
    struct RunUntil<'a, F> {
        local_set: &'a LocalSet,
        #[pin]
        future: F,
    }
}

tokio_thread_local!(static CURRENT: LocalData = const { LocalData {
    ctx: RcCell::new(),
    wake_on_schedule: Cell::new(false),
} });

struct LocalData {
    ctx: RcCell<Context>,
    wake_on_schedule: Cell<bool>,
}

impl LocalData {
    /// Should be called except when we call `LocalSet::enter`.
    /// Especially when we poll a `LocalSet`.
    #[must_use = "dropping this guard will reset the entered state"]
    fn enter(&self, ctx: Rc<Context>) -> LocalDataEnterGuard<'_> {
        let ctx = self.ctx.replace(Some(ctx));
        let wake_on_schedule = self.wake_on_schedule.replace(false);
        LocalDataEnterGuard {
            local_data_ref: self,
            ctx,
            wake_on_schedule,
        }
    }
}

/// A guard for `LocalData::enter()`
struct LocalDataEnterGuard<'a> {
    local_data_ref: &'a LocalData,
    ctx: Option<Rc<Context>>,
    wake_on_schedule: bool,
}

impl<'a> Drop for LocalDataEnterGuard<'a> {
    fn drop(&mut self) {
        self.local_data_ref.ctx.set(self.ctx.take());
        self.local_data_ref
            .wake_on_schedule
            .set(self.wake_on_schedule)
    }
}

cfg_rt! {
    /// Spawns a `!Send` future on the current [`LocalSet`].
    ///
    /// The spawned future will run on the same thread that called `spawn_local`.
    ///
    /// The provided future will start running in the background immediately
    /// when `spawn_local` is called, even if you don't await the returned
    /// `JoinHandle`.
    ///
    /// # Panics
    ///
    /// This function panics if called outside of a [`LocalSet`].
    ///
    /// Note that if [`tokio::spawn`] is used from within a `LocalSet`, the
    /// resulting new task will _not_ be inside the `LocalSet`, so you must use
    /// `spawn_local` if you want to stay within the `LocalSet`.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::rc::Rc;
    /// use tokio::task;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let nonsend_data = Rc::new("my nonsend data...");
    ///
    ///     let local = task::LocalSet::new();
    ///
    ///     // Run the local task set.
    ///     local.run_until(async move {
    ///         let nonsend_data = nonsend_data.clone();
    ///         task::spawn_local(async move {
    ///             println!("{}", nonsend_data);
    ///             // ...
    ///         }).await.unwrap();
    ///     }).await;
    /// }
    /// ```
    ///
    /// [`LocalSet`]: struct@crate::task::LocalSet
    /// [`tokio::spawn`]: fn@crate::task::spawn
    #[track_caller]
    pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        if cfg!(debug_assertions) && std::mem::size_of::<F>() > BOX_FUTURE_THRESHOLD {
            spawn_local_inner(Box::pin(future), None)
        } else {
            spawn_local_inner(future, None)
        }
    }


    #[track_caller]
    pub(super) fn spawn_local_inner<F>(future: F, name: Option<&str>) -> JoinHandle<F::Output>
    where F: Future + 'static,
          F::Output: 'static
    {
        match CURRENT.with(|LocalData { ctx, .. }| ctx.get()) {
            None => panic!("`spawn_local` called from outside of a `task::LocalSet`"),
            Some(cx) => cx.spawn(future, name)
       }
    }
}

/// Initial queue capacity.
const INITIAL_CAPACITY: usize = 64;

/// Max number of tasks to poll per tick.
const MAX_TASKS_PER_TICK: usize = 61;

/// How often it check the remote queue first.
const REMOTE_FIRST_INTERVAL: u8 = 31;

/// Context guard for `LocalSet`
pub struct LocalEnterGuard {
    ctx: Option<Rc<Context>>,

    /// Distinguishes whether the context was entered or being polled.
    /// When we enter it, the value `wake_on_schedule` is set. In this case
    /// `spawn_local` refers the context, whereas it is not being polled now.
    wake_on_schedule: bool,
}

impl Drop for LocalEnterGuard {
    fn drop(&mut self) {
        CURRENT.with(
            |LocalData {
                 ctx,
                 wake_on_schedule,
             }| {
                ctx.set(self.ctx.take());
                wake_on_schedule.set(self.wake_on_schedule);
            },
        );
    }
}

impl fmt::Debug for LocalEnterGuard {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("LocalEnterGuard").finish()
    }
}

impl LocalSet {
    /// Returns a new local task set.
    pub fn new() -> LocalSet {
        let owner = context::thread_id().expect("cannot create LocalSet during thread shutdown");

        LocalSet {
            tick: Cell::new(0),
            context: Rc::new(Context {
                shared: Arc::new(Shared {
                    local_state: LocalState {
                        owner,
                        owned: LocalOwnedTasks::new(),
                        local_queue: UnsafeCell::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
                    },
                    queue: Mutex::new(Some(VecDeque::with_capacity(INITIAL_CAPACITY))),
                    waker: AtomicWaker::new(),
                    #[cfg(tokio_unstable)]
                    unhandled_panic: crate::runtime::UnhandledPanic::Ignore,
                }),
                unhandled_panic: Cell::new(false),
            }),
            _not_send: PhantomData,
        }
    }

    /// Enters the context of this `LocalSet`.
    ///
    /// The [`spawn_local`] method will spawn tasks on the `LocalSet` whose
    /// context you are inside.
    ///
    /// [`spawn_local`]: fn@crate::task::spawn_local
    pub fn enter(&self) -> LocalEnterGuard {
        CURRENT.with(
            |LocalData {
                 ctx,
                 wake_on_schedule,
                 ..
             }| {
                let ctx = ctx.replace(Some(self.context.clone()));
                let wake_on_schedule = wake_on_schedule.replace(true);
                LocalEnterGuard {
                    ctx,
                    wake_on_schedule,
                }
            },
        )
    }

    /// Spawns a `!Send` task onto the local task set.
    ///
    /// This task is guaranteed to be run on the current thread.
    ///
    /// Unlike the free function [`spawn_local`], this method may be used to
    /// spawn local tasks when the `LocalSet` is _not_ running. The provided
    /// future will start running once the `LocalSet` is next started, even if
    /// you don't await the returned `JoinHandle`.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use tokio::task;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let local = task::LocalSet::new();
    ///
    ///     // Spawn a future on the local set. This future will be run when
    ///     // we call `run_until` to drive the task set.
    ///     local.spawn_local(async {
    ///        // ...
    ///     });
    ///
    ///     // Run the local task set.
    ///     local.run_until(async move {
    ///         // ...
    ///     }).await;
    ///
    ///     // When `run` finishes, we can spawn _more_ futures, which will
    ///     // run in subsequent calls to `run_until`.
    ///     local.spawn_local(async {
    ///        // ...
    ///     });
    ///
    ///     local.run_until(async move {
    ///         // ...
    ///     }).await;
    /// }
    /// ```
    /// [`spawn_local`]: fn@spawn_local
    #[track_caller]
    pub fn spawn_local<F>(&self, future: F) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        self.spawn_named(future, None)
    }

    /// Runs a future to completion on the provided runtime, driving any local
    /// futures spawned on this task set on the current thread.
    ///
    /// This runs the given future on the runtime, blocking until it is
    /// complete, and yielding its resolved result. Any tasks or timers which
    /// the future spawns internally will be executed on the runtime. The future
    /// may also call [`spawn_local`] to `spawn_local` additional local futures on the
    /// current thread.
    ///
    /// This method should not be called from an asynchronous context.
    ///
    /// # Panics
    ///
    /// This function panics if the executor is at capacity, if the provided
    /// future panics, or if called within an asynchronous execution context.
    ///
    /// # Notes
    ///
    /// Since this function internally calls [`Runtime::block_on`], and drives
    /// futures in the local task set inside that call to `block_on`, the local
    /// futures may not use [in-place blocking]. If a blocking call needs to be
    /// issued from a local task, the [`spawn_blocking`] API may be used instead.
    ///
    /// For example, this will panic:
    /// ```should_panic
    /// use tokio::runtime::Runtime;
    /// use tokio::task;
    ///
    /// let rt  = Runtime::new().unwrap();
    /// let local = task::LocalSet::new();
    /// local.block_on(&rt, async {
    ///     let join = task::spawn_local(async {
    ///         let blocking_result = task::block_in_place(|| {
    ///             // ...
    ///         });
    ///         // ...
    ///     });
    ///     join.await.unwrap();
    /// })
    /// ```
    /// This, however, will not panic:
    /// ```
    /// use tokio::runtime::Runtime;
    /// use tokio::task;
    ///
    /// let rt  = Runtime::new().unwrap();
    /// let local = task::LocalSet::new();
    /// local.block_on(&rt, async {
    ///     let join = task::spawn_local(async {
    ///         let blocking_result = task::spawn_blocking(|| {
    ///             // ...
    ///         }).await;
    ///         // ...
    ///     });
    ///     join.await.unwrap();
    /// })
    /// ```
    ///
    /// [`spawn_local`]: fn@spawn_local
    /// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
    /// [in-place blocking]: fn@crate::task::block_in_place
    /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
    #[track_caller]
    #[cfg(feature = "rt")]
    #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
    pub fn block_on<F>(&self, rt: &crate::runtime::Runtime, future: F) -> F::Output
    where
        F: Future,
    {
        rt.block_on(self.run_until(future))
    }

    /// Runs a future to completion on the local set, returning its output.
    ///
    /// This returns a future that runs the given future with a local set,
    /// allowing it to call [`spawn_local`] to spawn additional `!Send` futures.
    /// Any local futures spawned on the local set will be driven in the
    /// background until the future passed to `run_until` completes. When the future
    /// passed to `run_until` finishes, any local futures which have not completed
    /// will remain on the local set, and will be driven on subsequent calls to
    /// `run_until` or when [awaiting the local set] itself.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe when `future` is cancel safe.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use tokio::task;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     task::LocalSet::new().run_until(async {
    ///         task::spawn_local(async move {
    ///             // ...
    ///         }).await.unwrap();
    ///         // ...
    ///     }).await;
    /// }
    /// ```
    ///
    /// [`spawn_local`]: fn@spawn_local
    /// [awaiting the local set]: #awaiting-a-localset
    pub async fn run_until<F>(&self, future: F) -> F::Output
    where
        F: Future,
    {
        let run_until = RunUntil {
            future,
            local_set: self,
        };
        run_until.await
    }

    #[track_caller]
    pub(in crate::task) fn spawn_named<F>(
        &self,
        future: F,
        name: Option<&str>,
    ) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        if cfg!(debug_assertions) && std::mem::size_of::<F>() > BOX_FUTURE_THRESHOLD {
            self.spawn_named_inner(Box::pin(future), name)
        } else {
            self.spawn_named_inner(future, name)
        }
    }

    #[track_caller]
    fn spawn_named_inner<F>(&self, future: F, name: Option<&str>) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        let handle = self.context.spawn(future, name);

        // Because a task was spawned from *outside* the `LocalSet`, wake the
        // `LocalSet` future to execute the new task, if it hasn't been woken.
        //
        // Spawning via the free fn `spawn` does not require this, as it can
        // only be called from *within* a future executing on the `LocalSet` —
        // in that case, the `LocalSet` must already be awake.
        self.context.shared.waker.wake();
        handle
    }

    /// Ticks the scheduler, returning whether the local future needs to be
    /// notified again.
    fn tick(&self) -> bool {
        for _ in 0..MAX_TASKS_PER_TICK {
            // Make sure we didn't hit an unhandled panic
            assert!(!self.context.unhandled_panic.get(), "a spawned task panicked and the LocalSet is configured to shutdown on unhandled panic");

            match self.next_task() {
                // Run the task
                //
                // Safety: As spawned tasks are `!Send`, `run_unchecked` must be
                // used. We are responsible for maintaining the invariant that
                // `run_unchecked` is only called on threads that spawned the
                // task initially. Because `LocalSet` itself is `!Send`, and
                // `spawn_local` spawns into the `LocalSet` on the current
                // thread, the invariant is maintained.
                Some(task) => crate::runtime::coop::budget(|| task.run()),
                // We have fully drained the queue of notified tasks, so the
                // local future doesn't need to be notified again — it can wait
                // until something else wakes a task in the local set.
                None => return false,
            }
        }

        true
    }

    fn next_task(&self) -> Option<task::LocalNotified<Arc<Shared>>> {
        let tick = self.tick.get();
        self.tick.set(tick.wrapping_add(1));

        let task = if tick % REMOTE_FIRST_INTERVAL == 0 {
            self.context
                .shared
                .queue
                .lock()
                .as_mut()
                .and_then(|queue| queue.pop_front())
                .or_else(|| self.pop_local())
        } else {
            self.pop_local().or_else(|| {
                self.context
                    .shared
                    .queue
                    .lock()
                    .as_mut()
                    .and_then(VecDeque::pop_front)
            })
        };

        task.map(|task| unsafe {
            // Safety: because the `LocalSet` itself is `!Send`, we know we are
            // on the same thread if we have access to the `LocalSet`, and can
            // therefore access the local run queue.
            self.context.shared.local_state.assert_owner(task)
        })
    }

    fn pop_local(&self) -> Option<task::Notified<Arc<Shared>>> {
        unsafe {
            // Safety: because the `LocalSet` itself is `!Send`, we know we are
            // on the same thread if we have access to the `LocalSet`, and can
            // therefore access the local run queue.
            self.context.shared.local_state.task_pop_front()
        }
    }

    fn with<T>(&self, f: impl FnOnce() -> T) -> T {
        CURRENT.with(|local_data| {
            let _guard = local_data.enter(self.context.clone());
            f()
        })
    }

    /// This method is like `with`, but it just calls `f` without setting the thread-local if that
    /// fails.
    fn with_if_possible<T>(&self, f: impl FnOnce() -> T) -> T {
        let mut f = Some(f);

        let res = CURRENT.try_with(|local_data| {
            let _guard = local_data.enter(self.context.clone());
            (f.take().unwrap())()
        });

        match res {
            Ok(res) => res,
            Err(_access_error) => (f.take().unwrap())(),
        }
    }
}

cfg_unstable! {
    impl LocalSet {
        /// Configure how the `LocalSet` responds to an unhandled panic on a
        /// spawned task.
        ///
        /// By default, an unhandled panic (i.e. a panic not caught by
        /// [`std::panic::catch_unwind`]) has no impact on the `LocalSet`'s
        /// execution. The panic is error value is forwarded to the task's
        /// [`JoinHandle`] and all other spawned tasks continue running.
        ///
        /// The `unhandled_panic` option enables configuring this behavior.
        ///
        /// * `UnhandledPanic::Ignore` is the default behavior. Panics on
        ///   spawned tasks have no impact on the `LocalSet`'s execution.
        /// * `UnhandledPanic::ShutdownRuntime` will force the `LocalSet` to
        ///   shutdown immediately when a spawned task panics even if that
        ///   task's `JoinHandle` has not been dropped. All other spawned tasks
        ///   will immediately terminate and further calls to
        ///   [`LocalSet::block_on`] and [`LocalSet::run_until`] will panic.
        ///
        /// # Panics
        ///
        /// This method panics if called after the `LocalSet` has started
        /// running.
        ///
        /// # Unstable
        ///
        /// This option is currently unstable and its implementation is
        /// incomplete. The API may change or be removed in the future. See
        /// tokio-rs/tokio#4516 for more details.
        ///
        /// # Examples
        ///
        /// The following demonstrates a `LocalSet` configured to shutdown on
        /// panic. The first spawned task panics and results in the `LocalSet`
        /// shutting down. The second spawned task never has a chance to
        /// execute. The call to `run_until` will panic due to the runtime being
        /// forcibly shutdown.
        ///
        /// ```should_panic
        /// use tokio::runtime::UnhandledPanic;
        ///
        /// # #[tokio::main]
        /// # async fn main() {
        /// tokio::task::LocalSet::new()
        ///     .unhandled_panic(UnhandledPanic::ShutdownRuntime)
        ///     .run_until(async {
        ///         tokio::task::spawn_local(async { panic!("boom"); });
        ///         tokio::task::spawn_local(async {
        ///             // This task never completes
        ///         });
        ///
        ///         // Do some work, but `run_until` will panic before it completes
        /// # loop { tokio::task::yield_now().await; }
        ///     })
        ///     .await;
        /// # }
        /// ```
        ///
        /// [`JoinHandle`]: struct@crate::task::JoinHandle
        pub fn unhandled_panic(&mut self, behavior: crate::runtime::UnhandledPanic) -> &mut Self {
            // TODO: This should be set as a builder
            Rc::get_mut(&mut self.context)
                .and_then(|ctx| Arc::get_mut(&mut ctx.shared))
                .expect("Unhandled Panic behavior modified after starting LocalSet")
                .unhandled_panic = behavior;
            self
        }

        /// Returns the [`Id`] of the current `LocalSet` runtime.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use tokio::task;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let local_set = task::LocalSet::new();
        ///     println!("Local set id: {}", local_set.id());
        /// }
        /// ```
        ///
        /// **Note**: This is an [unstable API][unstable]. The public API of this type
        /// may break in 1.x releases. See [the documentation on unstable
        /// features][unstable] for details.
        ///
        /// [unstable]: crate#unstable-features
        /// [`Id`]: struct@crate::runtime::Id
        pub fn id(&self) -> runtime::Id {
            self.context.shared.local_state.owned.id.into()
        }
    }
}

impl fmt::Debug for LocalSet {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("LocalSet").finish()
    }
}

impl Future for LocalSet {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
        // Register the waker before starting to work
        self.context.shared.waker.register_by_ref(cx.waker());

        if self.with(|| self.tick()) {
            // If `tick` returns true, we need to notify the local future again:
            // there are still tasks remaining in the run queue.
            cx.waker().wake_by_ref();
            Poll::Pending

        // Safety: called from the thread that owns `LocalSet`. Because
        // `LocalSet` is `!Send`, this is safe.
        } else if unsafe { self.context.shared.local_state.owned_is_empty() } {
            // If the scheduler has no remaining futures, we're done!
            Poll::Ready(())
        } else {
            // There are still futures in the local set, but we've polled all the
            // futures in the run queue. Therefore, we can just return Pending
            // since the remaining futures will be woken from somewhere else.
            Poll::Pending
        }
    }
}

impl Default for LocalSet {
    fn default() -> LocalSet {
        LocalSet::new()
    }
}

impl Drop for LocalSet {
    fn drop(&mut self) {
        self.with_if_possible(|| {
            // Shut down all tasks in the LocalOwnedTasks and close it to
            // prevent new tasks from ever being added.
            unsafe {
                // Safety: called from the thread that owns `LocalSet`
                self.context.shared.local_state.close_and_shutdown_all();
            }

            // We already called shutdown on all tasks above, so there is no
            // need to call shutdown.

            // Safety: note that this *intentionally* bypasses the unsafe
            // `Shared::local_queue()` method. This is in order to avoid the
            // debug assertion that we are on the thread that owns the
            // `LocalSet`, because on some systems (e.g. at least some macOS
            // versions), attempting to get the current thread ID can panic due
            // to the thread's local data that stores the thread ID being
            // dropped *before* the `LocalSet`.
            //
            // Despite avoiding the assertion here, it is safe for us to access
            // the local queue in `Drop`, because the `LocalSet` itself is
            // `!Send`, so we can reasonably guarantee that it will not be
            // `Drop`ped from another thread.
            let local_queue = unsafe {
                // Safety: called from the thread that owns `LocalSet`
                self.context.shared.local_state.take_local_queue()
            };
            for task in local_queue {
                drop(task);
            }

            // Take the queue from the Shared object to prevent pushing
            // notifications to it in the future.
            let queue = self.context.shared.queue.lock().take().unwrap();
            for task in queue {
                drop(task);
            }

            // Safety: called from the thread that owns `LocalSet`
            assert!(unsafe { self.context.shared.local_state.owned_is_empty() });
        });
    }
}

// === impl Context ===

impl Context {
    #[track_caller]
    fn spawn<F>(&self, future: F, name: Option<&str>) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        let id = crate::runtime::task::Id::next();
        let future = crate::util::trace::task(future, "local", name, id.as_u64());

        // Safety: called from the thread that owns the `LocalSet`
        let (handle, notified) = {
            self.shared.local_state.assert_called_from_owner_thread();
            self.shared
                .local_state
                .owned
                .bind(future, self.shared.clone(), id)
        };

        if let Some(notified) = notified {
            self.shared.schedule(notified);
        }

        handle
    }
}

// === impl LocalFuture ===

impl<T: Future> Future for RunUntil<'_, T> {
    type Output = T::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
        let me = self.project();

        me.local_set.with(|| {
            me.local_set
                .context
                .shared
                .waker
                .register_by_ref(cx.waker());

            let _no_blocking = crate::runtime::context::disallow_block_in_place();
            let f = me.future;

            if let Poll::Ready(output) = f.poll(cx) {
                return Poll::Ready(output);
            }

            if me.local_set.tick() {
                // If `tick` returns `true`, we need to notify the local future again:
                // there are still tasks remaining in the run queue.
                cx.waker().wake_by_ref();
            }

            Poll::Pending
        })
    }
}

impl Shared {
    /// Schedule the provided task on the scheduler.
    fn schedule(&self, task: task::Notified<Arc<Self>>) {
        CURRENT.with(|localdata| {
            match localdata.ctx.get() {
                // If the current `LocalSet` is being polled, we don't need to wake it.
                // When we `enter` it, then the value `wake_on_schedule` is set to be true.
                // In this case it is not being polled, so we need to wake it.
                Some(cx) if cx.shared.ptr_eq(self) && !localdata.wake_on_schedule.get() => unsafe {
                    // Safety: if the current `LocalSet` context points to this
                    // `LocalSet`, then we are on the thread that owns it.
                    cx.shared.local_state.task_push_back(task);
                },

                // We are on the thread that owns the `LocalSet`, so we can
                // wake to the local queue.
                _ if context::thread_id().ok() == Some(self.local_state.owner) => {
                    unsafe {
                        // Safety: we just checked that the thread ID matches
                        // the localset's owner, so this is safe.
                        self.local_state.task_push_back(task);
                    }
                    // We still have to wake the `LocalSet`, because it isn't
                    // currently being polled.
                    self.waker.wake();
                }

                // We are *not* on the thread that owns the `LocalSet`, so we
                // have to wake to the remote queue.
                _ => {
                    // First, check whether the queue is still there (if not, the
                    // LocalSet is dropped). Then push to it if so, and if not,
                    // do nothing.
                    let mut lock = self.queue.lock();

                    if let Some(queue) = lock.as_mut() {
                        queue.push_back(task);
                        drop(lock);
                        self.waker.wake();
                    }
                }
            }
        });
    }

    fn ptr_eq(&self, other: &Shared) -> bool {
        std::ptr::eq(self, other)
    }
}

// This is safe because (and only because) we *pinky pwomise* to never touch the
// local run queue except from the thread that owns the `LocalSet`.
unsafe impl Sync for Shared {}

impl task::Schedule for Arc<Shared> {
    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
        // Safety, this is always called from the thread that owns `LocalSet`
        unsafe { self.local_state.task_remove(task) }
    }

    fn schedule(&self, task: task::Notified<Self>) {
        Shared::schedule(self, task);
    }

    // localset does not currently support task hooks
    fn hooks(&self) -> TaskHarnessScheduleHooks {
        TaskHarnessScheduleHooks {
            task_terminate_callback: None,
        }
    }

    cfg_unstable! {
        fn unhandled_panic(&self) {
            use crate::runtime::UnhandledPanic;

            match self.unhandled_panic {
                UnhandledPanic::Ignore => {
                    // Do nothing
                }
                UnhandledPanic::ShutdownRuntime => {
                    // This hook is only called from within the runtime, so
                    // `CURRENT` should match with `&self`, i.e. there is no
                    // opportunity for a nested scheduler to be called.
                    CURRENT.with(|LocalData { ctx, .. }| match ctx.get() {
                        Some(cx) if Arc::ptr_eq(self, &cx.shared) => {
                            cx.unhandled_panic.set(true);
                            // Safety: this is always called from the thread that owns `LocalSet`
                            unsafe { cx.shared.local_state.close_and_shutdown_all(); }
                        }
                        _ => unreachable!("runtime core not set in CURRENT thread-local"),
                    })
                }
            }
        }
    }
}

impl LocalState {
    unsafe fn task_pop_front(&self) -> Option<task::Notified<Arc<Shared>>> {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.local_queue.with_mut(|ptr| (*ptr).pop_front())
    }

    unsafe fn task_push_back(&self, task: task::Notified<Arc<Shared>>) {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.local_queue.with_mut(|ptr| (*ptr).push_back(task));
    }

    unsafe fn take_local_queue(&self) -> VecDeque<task::Notified<Arc<Shared>>> {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.local_queue.with_mut(|ptr| std::mem::take(&mut (*ptr)))
    }

    unsafe fn task_remove(&self, task: &Task<Arc<Shared>>) -> Option<Task<Arc<Shared>>> {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.owned.remove(task)
    }

    /// Returns true if the `LocalSet` does not have any spawned tasks
    unsafe fn owned_is_empty(&self) -> bool {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.owned.is_empty()
    }

    unsafe fn assert_owner(
        &self,
        task: task::Notified<Arc<Shared>>,
    ) -> task::LocalNotified<Arc<Shared>> {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.owned.assert_owner(task)
    }

    unsafe fn close_and_shutdown_all(&self) {
        // The caller ensures it is called from the same thread that owns
        // the LocalSet.
        self.assert_called_from_owner_thread();

        self.owned.close_and_shutdown_all();
    }

    #[track_caller]
    fn assert_called_from_owner_thread(&self) {
        // FreeBSD has some weirdness around thread-local destruction.
        // TODO: remove this hack when thread id is cleaned up
        #[cfg(not(any(target_os = "openbsd", target_os = "freebsd")))]
        debug_assert!(
            // if we couldn't get the thread ID because we're dropping the local
            // data, skip the assertion --- the `Drop` impl is not going to be
            // called from another thread, because `LocalSet` is `!Send`
            context::thread_id()
                .map(|id| id == self.owner)
                .unwrap_or(true),
            "`LocalSet`'s local run queue must not be accessed by another thread!"
        );
    }
}

// This is `Send` because it is stored in `Shared`. It is up to the caller to
// ensure they are on the same thread that owns the `LocalSet`.
unsafe impl Send for LocalState {}

#[cfg(all(test, not(loom)))]
mod tests {
    use super::*;

    // Does a `LocalSet` running on a current-thread runtime...basically work?
    //
    // This duplicates a test in `tests/task_local_set.rs`, but because this is
    // a lib test, it will run under Miri, so this is necessary to catch stacked
    // borrows violations in the `LocalSet` implementation.
    #[test]
    fn local_current_thread_scheduler() {
        let f = async {
            LocalSet::new()
                .run_until(async {
                    spawn_local(async {}).await.unwrap();
                })
                .await;
        };
        crate::runtime::Builder::new_current_thread()
            .build()
            .expect("rt")
            .block_on(f)
    }

    // Tests that when a task on a `LocalSet` is woken by an io driver on the
    // same thread, the task is woken to the localset's local queue rather than
    // its remote queue.
    //
    // This test has to be defined in the `local.rs` file as a lib test, rather
    // than in `tests/`, because it makes assertions about the local set's
    // internal state.
    #[test]
    fn wakes_to_local_queue() {
        use super::*;
        use crate::sync::Notify;
        let rt = crate::runtime::Builder::new_current_thread()
            .build()
            .expect("rt");
        rt.block_on(async {
            let local = LocalSet::new();
            let notify = Arc::new(Notify::new());
            let task = local.spawn_local({
                let notify = notify.clone();
                async move {
                    notify.notified().await;
                }
            });
            let mut run_until = Box::pin(local.run_until(async move {
                task.await.unwrap();
            }));

            // poll the run until future once
            crate::future::poll_fn(|cx| {
                let _ = run_until.as_mut().poll(cx);
                Poll::Ready(())
            })
            .await;

            notify.notify_one();
            let task = unsafe { local.context.shared.local_state.task_pop_front() };
            // TODO(eliza): it would be nice to be able to assert that this is
            // the local task.
            assert!(
                task.is_some(),
                "task should have been notified to the LocalSet's local queue"
            );
        })
    }
}