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
//! Runs `!Send` futures on the current thread.
use crate::loom::sync::{Arc, Mutex};
use crate::runtime::task::{self, JoinHandle, LocalOwnedTasks, Task};
use crate::sync::AtomicWaker;
use crate::util::VecDequeCell;

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::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 unsend_data = Rc::new("my unsend data...");
    ///
    ///     let unsend_data = unsend_data.clone();
    ///     // Because the `async` block here moves `unsend_data`, the future is `!Send`.
    ///     // Since `tokio::spawn` requires the spawned future to implement `Send`, this
    ///     // will not compile.
    ///     tokio::spawn(async move {
    ///         println!("{}", unsend_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 unsend_data = Rc::new("my unsend 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 unsend_data = unsend_data.clone();
    ///         // `spawn_local` ensures that the future is spawned on the local
    ///         // task set.
    ///         task::spawn_local(async move {
    ///             println!("{}", unsend_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 unsend_data = Rc::new("world");
    ///     let local = task::LocalSet::new();
    ///
    ///     let unsend_data2 = unsend_data.clone();
    ///     local.spawn_local(async move {
    ///         // ...
    ///         println!("hello {}", unsend_data2)
    ///     });
    ///
    ///     local.spawn_local(async move {
    ///         time::sleep(time::Duration::from_millis(100)).await;
    ///         println!("goodbye {}", unsend_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 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: Context,

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

/// State available from the thread-local.
struct Context {
    /// Collection of all active tasks spawned onto this executor.
    owned: LocalOwnedTasks<Arc<Shared>>,

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

    /// State shared between threads.
    shared: Arc<Shared>,
}

/// LocalSet state shared between threads.
struct Shared {
    /// Remote run queue sender.
    queue: Mutex<Option<VecDeque<task::Notified<Arc<Shared>>>>>,

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

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

scoped_thread_local!(static CURRENT: Context);

cfg_rt! {
    /// Spawns a `!Send` future on the local task set.
    ///
    /// The spawned future will be run on the same thread that called `spawn_local.`
    /// This may only be called from the context of a local task set.
    ///
    /// # Panics
    ///
    /// - This function panics if called outside of a local task set.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::rc::Rc;
    /// use tokio::task;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let unsend_data = Rc::new("my unsend data...");
    ///
    ///     let local = task::LocalSet::new();
    ///
    ///     // Run the local task set.
    ///     local.run_until(async move {
    ///         let unsend_data = unsend_data.clone();
    ///         task::spawn_local(async move {
    ///             println!("{}", unsend_data);
    ///             // ...
    ///         }).await.unwrap();
    ///     }).await;
    /// }
    /// ```
    #[track_caller]
    pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
    where
        F: Future + 'static,
        F::Output: 'static,
    {
        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
    {
        CURRENT.with(|maybe_cx| {
            let cx = maybe_cx
                .expect("`spawn_local` called from outside of a `task::LocalSet`");

            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;

impl LocalSet {
    /// Returns a new local task set.
    pub fn new() -> LocalSet {
        LocalSet {
            tick: Cell::new(0),
            context: Context {
                owned: LocalOwnedTasks::new(),
                queue: VecDequeCell::with_capacity(INITIAL_CAPACITY),
                shared: Arc::new(Shared {
                    queue: Mutex::new(Some(VecDeque::with_capacity(INITIAL_CAPACITY))),
                    waker: AtomicWaker::new(),
                }),
            },
            _not_send: PhantomData,
        }
    }

    /// 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 task set is _not_ running. For example:
    /// ```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
    #[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` 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.
    ///
    /// # 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
    }

    pub(in crate::task) fn spawn_named<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 {
            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::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.context.queue.pop_front())
        } else {
            self.context.queue.pop_front().or_else(|| {
                self.context
                    .shared
                    .queue
                    .lock()
                    .as_mut()
                    .and_then(|queue| queue.pop_front())
            })
        };

        task.map(|task| self.context.owned.assert_owner(task))
    }

    fn with<T>(&self, f: impl FnOnce() -> T) -> T {
        CURRENT.set(&self.context, f)
    }
}

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
        } else if self.context.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(|| {
            // Shut down all tasks in the LocalOwnedTasks and close it to
            // prevent new tasks from ever being added.
            self.context.owned.close_and_shutdown_all();

            // We already called shutdown on all tasks above, so there is no
            // need to call shutdown.
            for task in self.context.queue.take() {
                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);
            }

            assert!(self.context.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());

        let (handle, notified) = self.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::enter::disallow_blocking();
            let f = me.future;

            if let Poll::Ready(output) = crate::coop::budget(|| 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(|maybe_cx| match maybe_cx {
            Some(cx) if cx.shared.ptr_eq(self) => {
                cx.queue.push_back(task);
            }
            _ => {
                // 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)
    }
}

impl task::Schedule for Arc<Shared> {
    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
        CURRENT.with(|maybe_cx| {
            let cx = maybe_cx.expect("scheduler context missing");
            assert!(cx.shared.ptr_eq(self));
            cx.owned.remove(task)
        })
    }

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