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
use core::fmt;
use core::future::Future;
use core::marker::PhantomData;
use core::mem;
use core::ptr::NonNull;
use core::sync::atomic::Ordering;
use core::task::Waker;

use alloc::boxed::Box;

use crate::header::Header;
use crate::raw::RawTask;
use crate::state::*;
use crate::Task;

mod sealed {
    use super::*;
    pub trait Sealed<M> {}

    impl<M, F> Sealed<M> for F where F: Fn(Runnable<M>) {}

    impl<M, F> Sealed<M> for WithInfo<F> where F: Fn(Runnable<M>, ScheduleInfo) {}
}

/// A builder that creates a new task.
#[derive(Debug)]
pub struct Builder<M> {
    /// The metadata associated with the task.
    pub(crate) metadata: M,

    /// Whether or not a panic that occurs in the task should be propagated.
    #[cfg(feature = "std")]
    pub(crate) propagate_panic: bool,
}

impl<M: Default> Default for Builder<M> {
    fn default() -> Self {
        Builder::new().metadata(M::default())
    }
}

/// Extra scheduling information that can be passed to the scheduling function.
///
/// The data source of this struct is directly from the actual implementation
/// of the crate itself, different from [`Runnable`]'s metadata, which is
/// managed by the caller.
///
/// # Examples
///
/// ```
/// use async_task::{Runnable, ScheduleInfo, WithInfo};
/// use std::sync::{Arc, Mutex};
///
/// // The future inside the task.
/// let future = async {
///     println!("Hello, world!");
/// };
///
/// // If the task gets woken up while running, it will be sent into this channel.
/// let (s, r) = flume::unbounded();
/// // Otherwise, it will be placed into this slot.
/// let lifo_slot = Arc::new(Mutex::new(None));
/// let schedule = move |runnable: Runnable, info: ScheduleInfo| {
///     if info.woken_while_running {
///         s.send(runnable).unwrap()
///     } else {
///         let last = lifo_slot.lock().unwrap().replace(runnable);
///         if let Some(last) = last {
///             s.send(last).unwrap()
///         }
///     }
/// };
///
/// // Create the actual scheduler to be spawned with some future.
/// let scheduler = WithInfo(schedule);
/// // Create a task with the future and the scheduler.
/// let (runnable, task) = async_task::spawn(future, scheduler);
/// ```
#[derive(Debug, Copy, Clone)]
#[non_exhaustive]
pub struct ScheduleInfo {
    /// Indicates whether the task gets woken up while running.
    ///
    /// It is set to true usually because the task has yielded itself to the
    /// scheduler.
    pub woken_while_running: bool,
}

impl ScheduleInfo {
    pub(crate) fn new(woken_while_running: bool) -> Self {
        ScheduleInfo {
            woken_while_running,
        }
    }
}

/// The trait for scheduling functions.
pub trait Schedule<M = ()>: sealed::Sealed<M> {
    /// The actual scheduling procedure.
    fn schedule(&self, runnable: Runnable<M>, info: ScheduleInfo);
}

impl<M, F> Schedule<M> for F
where
    F: Fn(Runnable<M>),
{
    fn schedule(&self, runnable: Runnable<M>, _: ScheduleInfo) {
        self(runnable)
    }
}

/// Pass a scheduling function with more scheduling information - a.k.a.
/// [`ScheduleInfo`].
///
/// Sometimes, it's useful to pass the runnable's state directly to the
/// scheduling function, such as whether it's woken up while running. The
/// scheduler can thus use the information to determine its scheduling
/// strategy.
///
/// The data source of [`ScheduleInfo`] is directly from the actual
/// implementation of the crate itself, different from [`Runnable`]'s metadata,
/// which is managed by the caller.
///
/// # Examples
///
/// ```
/// use async_task::{ScheduleInfo, WithInfo};
/// use std::sync::{Arc, Mutex};
///
/// // The future inside the task.
/// let future = async {
///     println!("Hello, world!");
/// };
///
/// // If the task gets woken up while running, it will be sent into this channel.
/// let (s, r) = flume::unbounded();
/// // Otherwise, it will be placed into this slot.
/// let lifo_slot = Arc::new(Mutex::new(None));
/// let schedule = move |runnable, info: ScheduleInfo| {
///     if info.woken_while_running {
///         s.send(runnable).unwrap()
///     } else {
///         let last = lifo_slot.lock().unwrap().replace(runnable);
///         if let Some(last) = last {
///             s.send(last).unwrap()
///         }
///     }
/// };
///
/// // Create a task with the future and the schedule function.
/// let (runnable, task) = async_task::spawn(future, WithInfo(schedule));
/// ```
#[derive(Debug)]
pub struct WithInfo<F>(pub F);

impl<F> From<F> for WithInfo<F> {
    fn from(value: F) -> Self {
        WithInfo(value)
    }
}

impl<M, F> Schedule<M> for WithInfo<F>
where
    F: Fn(Runnable<M>, ScheduleInfo),
{
    fn schedule(&self, runnable: Runnable<M>, info: ScheduleInfo) {
        (self.0)(runnable, info)
    }
}

impl Builder<()> {
    /// Creates a new task builder.
    ///
    /// By default, this task builder has no metadata. Use the [`metadata`] method to
    /// set the metadata.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_task::Builder;
    ///
    /// let (runnable, task) = Builder::new().spawn(|()| async {}, |_| {});
    /// ```
    pub fn new() -> Builder<()> {
        Builder {
            metadata: (),
            #[cfg(feature = "std")]
            propagate_panic: false,
        }
    }

    /// Adds metadata to the task.
    ///
    /// In certain cases, it may be useful to associate some metadata with a task. For instance,
    /// you may want to associate a name with a task, or a priority for a priority queue. This
    /// method allows the user to attach arbitrary metadata to a task that is available through
    /// the [`Runnable`] or the [`Task`].
    ///
    /// # Examples
    ///
    /// This example creates an executor that associates a "priority" number with each task, and
    /// then runs the tasks in order of priority.
    ///
    /// ```
    /// use async_task::{Builder, Runnable};
    /// use once_cell::sync::Lazy;
    /// use std::cmp;
    /// use std::collections::BinaryHeap;
    /// use std::sync::Mutex;
    ///
    /// # smol::future::block_on(async {
    /// /// A wrapper around a `Runnable<usize>` that implements `Ord` so that it can be used in a
    /// /// priority queue.
    /// struct TaskWrapper(Runnable<usize>);
    ///
    /// impl PartialEq for TaskWrapper {
    ///     fn eq(&self, other: &Self) -> bool {
    ///         self.0.metadata() == other.0.metadata()
    ///     }
    /// }
    ///
    /// impl Eq for TaskWrapper {}
    ///
    /// impl PartialOrd for TaskWrapper {
    ///    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
    ///       Some(self.cmp(other))
    ///    }
    /// }
    ///
    /// impl Ord for TaskWrapper {
    ///    fn cmp(&self, other: &Self) -> cmp::Ordering {
    ///        self.0.metadata().cmp(other.0.metadata())
    ///    }
    /// }
    ///
    /// static EXECUTOR: Lazy<Mutex<BinaryHeap<TaskWrapper>>> = Lazy::new(|| {
    ///     Mutex::new(BinaryHeap::new())
    /// });
    ///
    /// let schedule = |runnable| {
    ///     EXECUTOR.lock().unwrap().push(TaskWrapper(runnable));
    /// };
    ///
    /// // Spawn a few tasks with different priorities.
    /// let spawn_task = move |priority| {
    ///     let (runnable, task) = Builder::new().metadata(priority).spawn(
    ///         move |_| async move { priority },
    ///         schedule,
    ///     );
    ///     runnable.schedule();
    ///     task
    /// };
    ///
    /// let t1 = spawn_task(1);
    /// let t2 = spawn_task(2);
    /// let t3 = spawn_task(3);
    ///
    /// // Run the tasks in order of priority.
    /// let mut metadata_seen = vec![];
    /// while let Some(TaskWrapper(runnable)) = EXECUTOR.lock().unwrap().pop() {
    ///     metadata_seen.push(*runnable.metadata());
    ///     runnable.run();
    /// }
    ///
    /// assert_eq!(metadata_seen, vec![3, 2, 1]);
    /// assert_eq!(t1.await, 1);
    /// assert_eq!(t2.await, 2);
    /// assert_eq!(t3.await, 3);
    /// # });
    /// ```
    pub fn metadata<M>(self, metadata: M) -> Builder<M> {
        Builder {
            metadata,
            #[cfg(feature = "std")]
            propagate_panic: self.propagate_panic,
        }
    }
}

impl<M> Builder<M> {
    /// Propagates panics that occur in the task.
    ///
    /// When this is `true`, panics that occur in the task will be propagated to the caller of
    /// the [`Task`]. When this is false, no special action is taken when a panic occurs in the
    /// task, meaning that the caller of [`Runnable::run`] will observe a panic.
    ///
    /// This is only available when the `std` feature is enabled. By default, this is `false`.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_task::Builder;
    /// use futures_lite::future::poll_fn;
    /// use std::future::Future;
    /// use std::panic;
    /// use std::pin::Pin;
    /// use std::task::{Context, Poll};
    ///
    /// fn did_panic<F: FnOnce()>(f: F) -> bool {
    ///     panic::catch_unwind(panic::AssertUnwindSafe(f)).is_err()
    /// }
    ///
    /// # smol::future::block_on(async {
    /// let (runnable1, mut task1) = Builder::new()
    ///    .propagate_panic(true)
    ///    .spawn(|()| async move { panic!() }, |_| {});
    ///
    /// let (runnable2, mut task2) = Builder::new()
    ///    .propagate_panic(false)
    ///    .spawn(|()| async move { panic!() }, |_| {});
    ///
    /// assert!(!did_panic(|| { runnable1.run(); }));
    /// assert!(did_panic(|| { runnable2.run(); }));
    ///
    /// let waker = poll_fn(|cx| Poll::Ready(cx.waker().clone())).await;
    /// let mut cx = Context::from_waker(&waker);
    /// assert!(did_panic(|| { let _ = Pin::new(&mut task1).poll(&mut cx); }));
    /// assert!(did_panic(|| { let _ = Pin::new(&mut task2).poll(&mut cx); }));
    /// # });
    /// ```
    #[cfg(feature = "std")]
    pub fn propagate_panic(self, propagate_panic: bool) -> Builder<M> {
        Builder {
            metadata: self.metadata,
            propagate_panic,
        }
    }

    /// Creates a new task.
    ///
    /// The returned [`Runnable`] is used to poll the `future`, and the [`Task`] is used to await its
    /// output.
    ///
    /// Method [`run()`][`Runnable::run()`] polls the task's future once. Then, the [`Runnable`]
    /// vanishes and only reappears when its [`Waker`] wakes the task, thus scheduling it to be run
    /// again.
    ///
    /// When the task is woken, its [`Runnable`] is passed to the `schedule` function.
    /// The `schedule` function should not attempt to run the [`Runnable`] nor to drop it. Instead, it
    /// should push it into a task queue so that it can be processed later.
    ///
    /// If you need to spawn a future that does not implement [`Send`] or isn't `'static`, consider
    /// using [`spawn_local()`] or [`spawn_unchecked()`] instead.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_task::Builder;
    ///
    /// // The future inside the task.
    /// let future = async {
    ///     println!("Hello, world!");
    /// };
    ///
    /// // A function that schedules the task when it gets woken up.
    /// let (s, r) = flume::unbounded();
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with the future and the schedule function.
    /// let (runnable, task) = Builder::new().spawn(|()| future, schedule);
    /// ```
    pub fn spawn<F, Fut, S>(self, future: F, schedule: S) -> (Runnable<M>, Task<Fut::Output, M>)
    where
        F: FnOnce(&M) -> Fut,
        Fut: Future + Send + 'static,
        Fut::Output: Send + 'static,
        S: Schedule<M> + Send + Sync + 'static,
    {
        unsafe { self.spawn_unchecked(future, schedule) }
    }

    /// Creates a new thread-local task.
    ///
    /// This function is same as [`spawn()`], except it does not require [`Send`] on `future`. If the
    /// [`Runnable`] is used or dropped on another thread, a panic will occur.
    ///
    /// This function is only available when the `std` feature for this crate is enabled.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_task::{Builder, Runnable};
    /// use flume::{Receiver, Sender};
    /// use std::rc::Rc;
    ///
    /// thread_local! {
    ///     // A queue that holds scheduled tasks.
    ///     static QUEUE: (Sender<Runnable>, Receiver<Runnable>) = flume::unbounded();
    /// }
    ///
    /// // Make a non-Send future.
    /// let msg: Rc<str> = "Hello, world!".into();
    /// let future = async move {
    ///     println!("{}", msg);
    /// };
    ///
    /// // A function that schedules the task when it gets woken up.
    /// let s = QUEUE.with(|(s, _)| s.clone());
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with the future and the schedule function.
    /// let (runnable, task) = Builder::new().spawn_local(move |()| future, schedule);
    /// ```
    #[cfg(feature = "std")]
    pub fn spawn_local<F, Fut, S>(
        self,
        future: F,
        schedule: S,
    ) -> (Runnable<M>, Task<Fut::Output, M>)
    where
        F: FnOnce(&M) -> Fut,
        Fut: Future + 'static,
        Fut::Output: 'static,
        S: Schedule<M> + Send + Sync + 'static,
    {
        use std::mem::ManuallyDrop;
        use std::pin::Pin;
        use std::task::{Context, Poll};
        use std::thread::{self, ThreadId};

        #[inline]
        fn thread_id() -> ThreadId {
            std::thread_local! {
                static ID: ThreadId = thread::current().id();
            }
            ID.try_with(|id| *id)
                .unwrap_or_else(|_| thread::current().id())
        }

        struct Checked<F> {
            id: ThreadId,
            inner: ManuallyDrop<F>,
        }

        impl<F> Drop for Checked<F> {
            fn drop(&mut self) {
                assert!(
                    self.id == thread_id(),
                    "local task dropped by a thread that didn't spawn it"
                );
                unsafe {
                    ManuallyDrop::drop(&mut self.inner);
                }
            }
        }

        impl<F: Future> Future for Checked<F> {
            type Output = F::Output;

            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
                assert!(
                    self.id == thread_id(),
                    "local task polled by a thread that didn't spawn it"
                );
                unsafe { self.map_unchecked_mut(|c| &mut *c.inner).poll(cx) }
            }
        }

        // Wrap the future into one that checks which thread it's on.
        let future = move |meta| {
            let future = future(meta);

            Checked {
                id: thread_id(),
                inner: ManuallyDrop::new(future),
            }
        };

        unsafe { self.spawn_unchecked(future, schedule) }
    }

    /// Creates a new task without [`Send`], [`Sync`], and `'static` bounds.
    ///
    /// This function is same as [`spawn()`], except it does not require [`Send`], [`Sync`], and
    /// `'static` on `future` and `schedule`.
    ///
    /// # Safety
    ///
    /// - If `Fut` is not [`Send`], its [`Runnable`] must be used and dropped on the original
    ///   thread.
    /// - If `Fut` is not `'static`, borrowed non-metadata variables must outlive its [`Runnable`].
    /// - If `schedule` is not [`Send`] and [`Sync`], all instances of the [`Runnable`]'s [`Waker`]
    ///   must be used and dropped on the original thread.
    /// - If `schedule` is not `'static`, borrowed variables must outlive all instances of the
    ///   [`Runnable`]'s [`Waker`].
    ///
    /// # Examples
    ///
    /// ```
    /// use async_task::Builder;
    ///
    /// // The future inside the task.
    /// let future = async {
    ///     println!("Hello, world!");
    /// };
    ///
    /// // If the task gets woken up, it will be sent into this channel.
    /// let (s, r) = flume::unbounded();
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with the future and the schedule function.
    /// let (runnable, task) = unsafe { Builder::new().spawn_unchecked(move |()| future, schedule) };
    /// ```
    pub unsafe fn spawn_unchecked<'a, F, Fut, S>(
        self,
        future: F,
        schedule: S,
    ) -> (Runnable<M>, Task<Fut::Output, M>)
    where
        F: FnOnce(&'a M) -> Fut,
        Fut: Future + 'a,
        S: Schedule<M>,
        M: 'a,
    {
        // Allocate large futures on the heap.
        let ptr = if mem::size_of::<Fut>() >= 2048 {
            let future = |meta| {
                let future = future(meta);
                Box::pin(future)
            };

            RawTask::<_, Fut::Output, S, M>::allocate(future, schedule, self)
        } else {
            RawTask::<Fut, Fut::Output, S, M>::allocate(future, schedule, self)
        };

        let runnable = Runnable::from_raw(ptr);
        let task = Task {
            ptr,
            _marker: PhantomData,
        };
        (runnable, task)
    }
}

/// Creates a new task.
///
/// The returned [`Runnable`] is used to poll the `future`, and the [`Task`] is used to await its
/// output.
///
/// Method [`run()`][`Runnable::run()`] polls the task's future once. Then, the [`Runnable`]
/// vanishes and only reappears when its [`Waker`] wakes the task, thus scheduling it to be run
/// again.
///
/// When the task is woken, its [`Runnable`] is passed to the `schedule` function.
/// The `schedule` function should not attempt to run the [`Runnable`] nor to drop it. Instead, it
/// should push it into a task queue so that it can be processed later.
///
/// If you need to spawn a future that does not implement [`Send`] or isn't `'static`, consider
/// using [`spawn_local()`] or [`spawn_unchecked()`] instead.
///
/// # Examples
///
/// ```
/// // The future inside the task.
/// let future = async {
///     println!("Hello, world!");
/// };
///
/// // A function that schedules the task when it gets woken up.
/// let (s, r) = flume::unbounded();
/// let schedule = move |runnable| s.send(runnable).unwrap();
///
/// // Create a task with the future and the schedule function.
/// let (runnable, task) = async_task::spawn(future, schedule);
/// ```
pub fn spawn<F, S>(future: F, schedule: S) -> (Runnable, Task<F::Output>)
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,
    S: Schedule + Send + Sync + 'static,
{
    unsafe { spawn_unchecked(future, schedule) }
}

/// Creates a new thread-local task.
///
/// This function is same as [`spawn()`], except it does not require [`Send`] on `future`. If the
/// [`Runnable`] is used or dropped on another thread, a panic will occur.
///
/// This function is only available when the `std` feature for this crate is enabled.
///
/// # Examples
///
/// ```
/// use async_task::Runnable;
/// use flume::{Receiver, Sender};
/// use std::rc::Rc;
///
/// thread_local! {
///     // A queue that holds scheduled tasks.
///     static QUEUE: (Sender<Runnable>, Receiver<Runnable>) = flume::unbounded();
/// }
///
/// // Make a non-Send future.
/// let msg: Rc<str> = "Hello, world!".into();
/// let future = async move {
///     println!("{}", msg);
/// };
///
/// // A function that schedules the task when it gets woken up.
/// let s = QUEUE.with(|(s, _)| s.clone());
/// let schedule = move |runnable| s.send(runnable).unwrap();
///
/// // Create a task with the future and the schedule function.
/// let (runnable, task) = async_task::spawn_local(future, schedule);
/// ```
#[cfg(feature = "std")]
pub fn spawn_local<F, S>(future: F, schedule: S) -> (Runnable, Task<F::Output>)
where
    F: Future + 'static,
    F::Output: 'static,
    S: Schedule + Send + Sync + 'static,
{
    Builder::new().spawn_local(move |()| future, schedule)
}

/// Creates a new task without [`Send`], [`Sync`], and `'static` bounds.
///
/// This function is same as [`spawn()`], except it does not require [`Send`], [`Sync`], and
/// `'static` on `future` and `schedule`.
///
/// # Safety
///
/// - If `future` is not [`Send`], its [`Runnable`] must be used and dropped on the original
///   thread.
/// - If `future` is not `'static`, borrowed variables must outlive its [`Runnable`].
/// - If `schedule` is not [`Send`] and [`Sync`], all instances of the [`Runnable`]'s [`Waker`]
///   must be used and dropped on the original thread.
/// - If `schedule` is not `'static`, borrowed variables must outlive all instances of the
///   [`Runnable`]'s [`Waker`].
///
/// # Examples
///
/// ```
/// // The future inside the task.
/// let future = async {
///     println!("Hello, world!");
/// };
///
/// // If the task gets woken up, it will be sent into this channel.
/// let (s, r) = flume::unbounded();
/// let schedule = move |runnable| s.send(runnable).unwrap();
///
/// // Create a task with the future and the schedule function.
/// let (runnable, task) = unsafe { async_task::spawn_unchecked(future, schedule) };
/// ```
pub unsafe fn spawn_unchecked<F, S>(future: F, schedule: S) -> (Runnable, Task<F::Output>)
where
    F: Future,
    S: Schedule,
{
    Builder::new().spawn_unchecked(move |()| future, schedule)
}

/// A handle to a runnable task.
///
/// Every spawned task has a single [`Runnable`] handle, which only exists when the task is
/// scheduled for running.
///
/// Method [`run()`][`Runnable::run()`] polls the task's future once. Then, the [`Runnable`]
/// vanishes and only reappears when its [`Waker`] wakes the task, thus scheduling it to be run
/// again.
///
/// Dropping a [`Runnable`] cancels the task, which means its future won't be polled again, and
/// awaiting the [`Task`] after that will result in a panic.
///
/// # Examples
///
/// ```
/// use async_task::Runnable;
/// use once_cell::sync::Lazy;
/// use std::{panic, thread};
///
/// // A simple executor.
/// static QUEUE: Lazy<flume::Sender<Runnable>> = Lazy::new(|| {
///     let (sender, receiver) = flume::unbounded::<Runnable>();
///     thread::spawn(|| {
///         for runnable in receiver {
///             let _ignore_panic = panic::catch_unwind(|| runnable.run());
///         }
///     });
///     sender
/// });
///
/// // Create a task with a simple future.
/// let schedule = |runnable| QUEUE.send(runnable).unwrap();
/// let (runnable, task) = async_task::spawn(async { 1 + 2 }, schedule);
///
/// // Schedule the task and await its output.
/// runnable.schedule();
/// assert_eq!(smol::future::block_on(task), 3);
/// ```
pub struct Runnable<M = ()> {
    /// A pointer to the heap-allocated task.
    pub(crate) ptr: NonNull<()>,

    /// A marker capturing generic type `M`.
    pub(crate) _marker: PhantomData<M>,
}

unsafe impl<M: Send + Sync> Send for Runnable<M> {}
unsafe impl<M: Send + Sync> Sync for Runnable<M> {}

#[cfg(feature = "std")]
impl<M> std::panic::UnwindSafe for Runnable<M> {}
#[cfg(feature = "std")]
impl<M> std::panic::RefUnwindSafe for Runnable<M> {}

impl<M> Runnable<M> {
    /// Get the metadata associated with this task.
    ///
    /// Tasks can be created with a metadata object associated with them; by default, this
    /// is a `()` value. See the [`Builder::metadata()`] method for more information.
    pub fn metadata(&self) -> &M {
        &self.header().metadata
    }

    /// Schedules the task.
    ///
    /// This is a convenience method that passes the [`Runnable`] to the schedule function.
    ///
    /// # Examples
    ///
    /// ```
    /// // A function that schedules the task when it gets woken up.
    /// let (s, r) = flume::unbounded();
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with a simple future and the schedule function.
    /// let (runnable, task) = async_task::spawn(async {}, schedule);
    ///
    /// // Schedule the task.
    /// assert_eq!(r.len(), 0);
    /// runnable.schedule();
    /// assert_eq!(r.len(), 1);
    /// ```
    pub fn schedule(self) {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header<M>;
        mem::forget(self);

        unsafe {
            ((*header).vtable.schedule)(ptr, ScheduleInfo::new(false));
        }
    }

    /// Runs the task by polling its future.
    ///
    /// Returns `true` if the task was woken while running, in which case the [`Runnable`] gets
    /// rescheduled at the end of this method invocation. Otherwise, returns `false` and the
    /// [`Runnable`] vanishes until the task is woken.
    /// The return value is just a hint: `true` usually indicates that the task has yielded, i.e.
    /// it woke itself and then gave the control back to the executor.
    ///
    /// If the [`Task`] handle was dropped or if [`cancel()`][`Task::cancel()`] was called, then
    /// this method simply destroys the task.
    ///
    /// If the polled future panics, this method propagates the panic, and awaiting the [`Task`]
    /// after that will also result in a panic.
    ///
    /// # Examples
    ///
    /// ```
    /// // A function that schedules the task when it gets woken up.
    /// let (s, r) = flume::unbounded();
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with a simple future and the schedule function.
    /// let (runnable, task) = async_task::spawn(async { 1 + 2 }, schedule);
    ///
    /// // Run the task and check its output.
    /// runnable.run();
    /// assert_eq!(smol::future::block_on(task), 3);
    /// ```
    pub fn run(self) -> bool {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header<M>;
        mem::forget(self);

        unsafe { ((*header).vtable.run)(ptr) }
    }

    /// Returns a waker associated with this task.
    ///
    /// # Examples
    ///
    /// ```
    /// use smol::future;
    ///
    /// // A function that schedules the task when it gets woken up.
    /// let (s, r) = flume::unbounded();
    /// let schedule = move |runnable| s.send(runnable).unwrap();
    ///
    /// // Create a task with a simple future and the schedule function.
    /// let (runnable, task) = async_task::spawn(future::pending::<()>(), schedule);
    ///
    /// // Take a waker and run the task.
    /// let waker = runnable.waker();
    /// runnable.run();
    ///
    /// // Reschedule the task by waking it.
    /// assert_eq!(r.len(), 0);
    /// waker.wake();
    /// assert_eq!(r.len(), 1);
    /// ```
    pub fn waker(&self) -> Waker {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header<M>;

        unsafe {
            let raw_waker = ((*header).vtable.clone_waker)(ptr);
            Waker::from_raw(raw_waker)
        }
    }

    fn header(&self) -> &Header<M> {
        unsafe { &*(self.ptr.as_ptr() as *const Header<M>) }
    }

    /// Converts this task into a raw pointer.
    ///
    /// To avoid a memory leak the pointer must be converted back to a Runnable using [`Runnable<M>::from_raw`][from_raw].
    ///
    /// `into_raw` does not change the state of the [`Task`], but there is no guarantee that it will be in the same state after calling [`Runnable<M>::from_raw`][from_raw],
    /// as the corresponding [`Task`] might have been dropped or cancelled.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use async_task::{Runnable, spawn};

    /// let (runnable, task) = spawn(async {}, |_| {});
    /// let runnable_pointer = runnable.into_raw();
    ///
    /// unsafe {
    ///     // Convert back to an `Runnable` to prevent leak.
    ///     let runnable = Runnable::<()>::from_raw(runnable_pointer);
    ///     runnable.run();
    ///     // Further calls to `Runnable::from_raw(runnable_pointer)` would be memory-unsafe.
    /// }
    /// // The memory was freed when `x` went out of scope above, so `runnable_pointer` is now dangling!
    /// ```
    /// [from_raw]: #method.from_raw
    pub fn into_raw(self) -> NonNull<()> {
        let ptr = self.ptr;
        mem::forget(self);
        ptr
    }

    /// Converts a raw pointer into a Runnable.
    ///
    /// # Safety
    ///
    /// This method should only be used with raw pointers returned from [`Runnable<M>::into_raw`][into_raw].
    /// It is not safe to use the provided pointer once it is passed to `from_raw`.
    /// Crucially, it is unsafe to call `from_raw` multiple times with the same pointer - even if the resulting [`Runnable`] is not used -
    /// as internally `async-task` uses reference counting.
    ///
    /// It is however safe to call [`Runnable<M>::into_raw`][into_raw] on a [`Runnable`] created with `from_raw` or
    /// after the [`Task`] associated with a given Runnable has been dropped or cancelled.
    ///
    /// The state of the [`Runnable`] created with `from_raw` is not specified.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use async_task::{Runnable, spawn};

    /// let (runnable, task) = spawn(async {}, |_| {});
    /// let runnable_pointer = runnable.into_raw();
    ///
    /// drop(task);
    /// unsafe {
    ///     // Convert back to an `Runnable` to prevent leak.
    ///     let runnable = Runnable::<()>::from_raw(runnable_pointer);
    ///     let did_poll = runnable.run();
    ///     assert!(!did_poll);
    ///     // Further calls to `Runnable::from_raw(runnable_pointer)` would be memory-unsafe.
    /// }
    /// // The memory was freed when `x` went out of scope above, so `runnable_pointer` is now dangling!
    /// ```

    /// [into_raw]: #method.into_raw
    pub unsafe fn from_raw(ptr: NonNull<()>) -> Self {
        Self {
            ptr,
            _marker: Default::default(),
        }
    }
}

impl<M> Drop for Runnable<M> {
    fn drop(&mut self) {
        let ptr = self.ptr.as_ptr();
        let header = self.header();

        unsafe {
            let mut state = header.state.load(Ordering::Acquire);

            loop {
                // If the task has been completed or closed, it can't be canceled.
                if state & (COMPLETED | CLOSED) != 0 {
                    break;
                }

                // Mark the task as closed.
                match header.state.compare_exchange_weak(
                    state,
                    state | CLOSED,
                    Ordering::AcqRel,
                    Ordering::Acquire,
                ) {
                    Ok(_) => break,
                    Err(s) => state = s,
                }
            }

            // Drop the future.
            (header.vtable.drop_future)(ptr);

            // Mark the task as unscheduled.
            let state = header.state.fetch_and(!SCHEDULED, Ordering::AcqRel);

            // Notify the awaiter that the future has been dropped.
            if state & AWAITER != 0 {
                (*header).notify(None);
            }

            // Drop the task reference.
            (header.vtable.drop_ref)(ptr);
        }
    }
}

impl<M: fmt::Debug> fmt::Debug for Runnable<M> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header<M>;

        f.debug_struct("Runnable")
            .field("header", unsafe { &(*header) })
            .finish()
    }
}