1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]

//! A one-shot channel is used for sending a single message between
//! asynchronous tasks. The [`channel`] function is used to create a
//! [`Sender`] and [`Receiver`] handle pair that form the channel.
//!
//! The `Sender` handle is used by the producer to send the value.
//! The `Receiver` handle is used by the consumer to receive the value.
//!
//! Each handle can be used on separate tasks.
//!
//! Since the `send` method is not async, it can be used anywhere. This includes
//! sending between two runtimes, and using it from non-async code.
//!
//! If the [`Receiver`] is closed before receiving a message which has already
//! been sent, the message will remain in the channel until the receiver is
//! dropped, at which point the message will be dropped immediately.
//!
//! # Examples
//!
//! ```
//! use tokio::sync::oneshot;
//!
//! #[tokio::main]
//! async fn main() {
//!     let (tx, rx) = oneshot::channel();
//!
//!     tokio::spawn(async move {
//!         if let Err(_) = tx.send(3) {
//!             println!("the receiver dropped");
//!         }
//!     });
//!
//!     match rx.await {
//!         Ok(v) => println!("got = {:?}", v),
//!         Err(_) => println!("the sender dropped"),
//!     }
//! }
//! ```
//!
//! If the sender is dropped without sending, the receiver will fail with
//! [`error::RecvError`]:
//!
//! ```
//! use tokio::sync::oneshot;
//!
//! #[tokio::main]
//! async fn main() {
//!     let (tx, rx) = oneshot::channel::<u32>();
//!
//!     tokio::spawn(async move {
//!         drop(tx);
//!     });
//!
//!     match rx.await {
//!         Ok(_) => panic!("This doesn't happen"),
//!         Err(_) => println!("the sender dropped"),
//!     }
//! }
//! ```
//!
//! To use a oneshot channel in a `tokio::select!` loop, add `&mut` in front of
//! the channel.
//!
//! ```
//! use tokio::sync::oneshot;
//! use tokio::time::{interval, sleep, Duration};
//!
//! #[tokio::main]
//! # async fn _doc() {}
//! # #[tokio::main(flavor = "current_thread", start_paused = true)]
//! async fn main() {
//!     let (send, mut recv) = oneshot::channel();
//!     let mut interval = interval(Duration::from_millis(100));
//!
//!     # let handle =
//!     tokio::spawn(async move {
//!         sleep(Duration::from_secs(1)).await;
//!         send.send("shut down").unwrap();
//!     });
//!
//!     loop {
//!         tokio::select! {
//!             _ = interval.tick() => println!("Another 100ms"),
//!             msg = &mut recv => {
//!                 println!("Got message: {}", msg.unwrap());
//!                 break;
//!             }
//!         }
//!     }
//!     # handle.await.unwrap();
//! }
//! ```
//!
//! To use a `Sender` from a destructor, put it in an [`Option`] and call
//! [`Option::take`].
//!
//! ```
//! use tokio::sync::oneshot;
//!
//! struct SendOnDrop {
//!     sender: Option<oneshot::Sender<&'static str>>,
//! }
//! impl Drop for SendOnDrop {
//!     fn drop(&mut self) {
//!         if let Some(sender) = self.sender.take() {
//!             // Using `let _ =` to ignore send errors.
//!             let _ = sender.send("I got dropped!");
//!         }
//!     }
//! }
//!
//! #[tokio::main]
//! # async fn _doc() {}
//! # #[tokio::main(flavor = "current_thread")]
//! async fn main() {
//!     let (send, recv) = oneshot::channel();
//!
//!     let send_on_drop = SendOnDrop { sender: Some(send) };
//!     drop(send_on_drop);
//!
//!     assert_eq!(recv.await, Ok("I got dropped!"));
//! }
//! ```

use crate::loom::cell::UnsafeCell;
use crate::loom::sync::atomic::AtomicUsize;
use crate::loom::sync::Arc;
#[cfg(all(tokio_unstable, feature = "tracing"))]
use crate::util::trace;

use std::fmt;
use std::future::Future;
use std::mem::MaybeUninit;
use std::pin::Pin;
use std::sync::atomic::Ordering::{self, AcqRel, Acquire};
use std::task::Poll::{Pending, Ready};
use std::task::{Context, Poll, Waker};

/// Sends a value to the associated [`Receiver`].
///
/// A pair of both a [`Sender`] and a [`Receiver`]  are created by the
/// [`channel`](fn@channel) function.
///
/// # Examples
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, rx) = oneshot::channel();
///
///     tokio::spawn(async move {
///         if let Err(_) = tx.send(3) {
///             println!("the receiver dropped");
///         }
///     });
///
///     match rx.await {
///         Ok(v) => println!("got = {:?}", v),
///         Err(_) => println!("the sender dropped"),
///     }
/// }
/// ```
///
/// If the sender is dropped without sending, the receiver will fail with
/// [`error::RecvError`]:
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, rx) = oneshot::channel::<u32>();
///
///     tokio::spawn(async move {
///         drop(tx);
///     });
///
///     match rx.await {
///         Ok(_) => panic!("This doesn't happen"),
///         Err(_) => println!("the sender dropped"),
///     }
/// }
/// ```
///
/// To use a `Sender` from a destructor, put it in an [`Option`] and call
/// [`Option::take`].
///
/// ```
/// use tokio::sync::oneshot;
///
/// struct SendOnDrop {
///     sender: Option<oneshot::Sender<&'static str>>,
/// }
/// impl Drop for SendOnDrop {
///     fn drop(&mut self) {
///         if let Some(sender) = self.sender.take() {
///             // Using `let _ =` to ignore send errors.
///             let _ = sender.send("I got dropped!");
///         }
///     }
/// }
///
/// #[tokio::main]
/// # async fn _doc() {}
/// # #[tokio::main(flavor = "current_thread")]
/// async fn main() {
///     let (send, recv) = oneshot::channel();
///
///     let send_on_drop = SendOnDrop { sender: Some(send) };
///     drop(send_on_drop);
///
///     assert_eq!(recv.await, Ok("I got dropped!"));
/// }
/// ```
///
/// [`Option`]: std::option::Option
/// [`Option::take`]: std::option::Option::take
#[derive(Debug)]
pub struct Sender<T> {
    inner: Option<Arc<Inner<T>>>,
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    resource_span: tracing::Span,
}

/// Receives a value from the associated [`Sender`].
///
/// A pair of both a [`Sender`] and a [`Receiver`]  are created by the
/// [`channel`](fn@channel) function.
///
/// This channel has no `recv` method because the receiver itself implements the
/// [`Future`] trait. To receive a `Result<T, `[`error::RecvError`]`>`, `.await` the `Receiver` object directly.
///
/// The `poll` method on the `Future` trait is allowed to spuriously return
/// `Poll::Pending` even if the message has been sent. If such a spurious
/// failure happens, then the caller will be woken when the spurious failure has
/// been resolved so that the caller can attempt to receive the message again.
/// Note that receiving such a wakeup does not guarantee that the next call will
/// succeed — it could fail with another spurious failure. (A spurious failure
/// does not mean that the message is lost. It is just delayed.)
///
/// [`Future`]: trait@std::future::Future
///
/// # Examples
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, rx) = oneshot::channel();
///
///     tokio::spawn(async move {
///         if let Err(_) = tx.send(3) {
///             println!("the receiver dropped");
///         }
///     });
///
///     match rx.await {
///         Ok(v) => println!("got = {:?}", v),
///         Err(_) => println!("the sender dropped"),
///     }
/// }
/// ```
///
/// If the sender is dropped without sending, the receiver will fail with
/// [`error::RecvError`]:
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, rx) = oneshot::channel::<u32>();
///
///     tokio::spawn(async move {
///         drop(tx);
///     });
///
///     match rx.await {
///         Ok(_) => panic!("This doesn't happen"),
///         Err(_) => println!("the sender dropped"),
///     }
/// }
/// ```
///
/// To use a `Receiver` in a `tokio::select!` loop, add `&mut` in front of the
/// channel.
///
/// ```
/// use tokio::sync::oneshot;
/// use tokio::time::{interval, sleep, Duration};
///
/// #[tokio::main]
/// # async fn _doc() {}
/// # #[tokio::main(flavor = "current_thread", start_paused = true)]
/// async fn main() {
///     let (send, mut recv) = oneshot::channel();
///     let mut interval = interval(Duration::from_millis(100));
///
///     # let handle =
///     tokio::spawn(async move {
///         sleep(Duration::from_secs(1)).await;
///         send.send("shut down").unwrap();
///     });
///
///     loop {
///         tokio::select! {
///             _ = interval.tick() => println!("Another 100ms"),
///             msg = &mut recv => {
///                 println!("Got message: {}", msg.unwrap());
///                 break;
///             }
///         }
///     }
///     # handle.await.unwrap();
/// }
/// ```
#[derive(Debug)]
pub struct Receiver<T> {
    inner: Option<Arc<Inner<T>>>,
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    resource_span: tracing::Span,
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    async_op_span: tracing::Span,
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    async_op_poll_span: tracing::Span,
}

pub mod error {
    //! Oneshot error types.

    use std::fmt;

    /// Error returned by the `Future` implementation for `Receiver`.
    ///
    /// This error is returned by the receiver when the sender is dropped without sending.
    #[derive(Debug, Eq, PartialEq, Clone)]
    pub struct RecvError(pub(super) ());

    /// Error returned by the `try_recv` function on `Receiver`.
    #[derive(Debug, Eq, PartialEq, Clone)]
    pub enum TryRecvError {
        /// The send half of the channel has not yet sent a value.
        Empty,

        /// The send half of the channel was dropped without sending a value.
        Closed,
    }

    // ===== impl RecvError =====

    impl fmt::Display for RecvError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "channel closed")
        }
    }

    impl std::error::Error for RecvError {}

    // ===== impl TryRecvError =====

    impl fmt::Display for TryRecvError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self {
                TryRecvError::Empty => write!(fmt, "channel empty"),
                TryRecvError::Closed => write!(fmt, "channel closed"),
            }
        }
    }

    impl std::error::Error for TryRecvError {}
}

use self::error::*;

struct Inner<T> {
    /// Manages the state of the inner cell.
    state: AtomicUsize,

    /// The value. This is set by `Sender` and read by `Receiver`. The state of
    /// the cell is tracked by `state`.
    value: UnsafeCell<Option<T>>,

    /// The task to notify when the receiver drops without consuming the value.
    ///
    /// ## Safety
    ///
    /// The `TX_TASK_SET` bit in the `state` field is set if this field is
    /// initialized. If that bit is unset, this field may be uninitialized.
    tx_task: Task,

    /// The task to notify when the value is sent.
    ///
    /// ## Safety
    ///
    /// The `RX_TASK_SET` bit in the `state` field is set if this field is
    /// initialized. If that bit is unset, this field may be uninitialized.
    rx_task: Task,
}

struct Task(UnsafeCell<MaybeUninit<Waker>>);

impl Task {
    unsafe fn will_wake(&self, cx: &mut Context<'_>) -> bool {
        self.with_task(|w| w.will_wake(cx.waker()))
    }

    unsafe fn with_task<F, R>(&self, f: F) -> R
    where
        F: FnOnce(&Waker) -> R,
    {
        self.0.with(|ptr| {
            let waker: *const Waker = (*ptr).as_ptr();
            f(&*waker)
        })
    }

    unsafe fn drop_task(&self) {
        self.0.with_mut(|ptr| {
            let ptr: *mut Waker = (*ptr).as_mut_ptr();
            ptr.drop_in_place();
        });
    }

    unsafe fn set_task(&self, cx: &mut Context<'_>) {
        self.0.with_mut(|ptr| {
            let ptr: *mut Waker = (*ptr).as_mut_ptr();
            ptr.write(cx.waker().clone());
        });
    }
}

#[derive(Clone, Copy)]
struct State(usize);

/// Creates a new one-shot channel for sending single values across asynchronous
/// tasks.
///
/// The function returns separate "send" and "receive" handles. The `Sender`
/// handle is used by the producer to send the value. The `Receiver` handle is
/// used by the consumer to receive the value.
///
/// Each handle can be used on separate tasks.
///
/// # Examples
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, rx) = oneshot::channel();
///
///     tokio::spawn(async move {
///         if let Err(_) = tx.send(3) {
///             println!("the receiver dropped");
///         }
///     });
///
///     match rx.await {
///         Ok(v) => println!("got = {:?}", v),
///         Err(_) => println!("the sender dropped"),
///     }
/// }
/// ```
#[track_caller]
pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    let resource_span = {
        let location = std::panic::Location::caller();

        let resource_span = tracing::trace_span!(
            "runtime.resource",
            concrete_type = "Sender|Receiver",
            kind = "Sync",
            loc.file = location.file(),
            loc.line = location.line(),
            loc.col = location.column(),
        );

        resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            tx_dropped = false,
            tx_dropped.op = "override",
            )
        });

        resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            rx_dropped = false,
            rx_dropped.op = "override",
            )
        });

        resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            value_sent = false,
            value_sent.op = "override",
            )
        });

        resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            value_received = false,
            value_received.op = "override",
            )
        });

        resource_span
    };

    let inner = Arc::new(Inner {
        state: AtomicUsize::new(State::new().as_usize()),
        value: UnsafeCell::new(None),
        tx_task: Task(UnsafeCell::new(MaybeUninit::uninit())),
        rx_task: Task(UnsafeCell::new(MaybeUninit::uninit())),
    });

    let tx = Sender {
        inner: Some(inner.clone()),
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        resource_span: resource_span.clone(),
    };

    #[cfg(all(tokio_unstable, feature = "tracing"))]
    let async_op_span = resource_span
        .in_scope(|| tracing::trace_span!("runtime.resource.async_op", source = "Receiver::await"));

    #[cfg(all(tokio_unstable, feature = "tracing"))]
    let async_op_poll_span =
        async_op_span.in_scope(|| tracing::trace_span!("runtime.resource.async_op.poll"));

    let rx = Receiver {
        inner: Some(inner),
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        resource_span,
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        async_op_span,
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        async_op_poll_span,
    };

    (tx, rx)
}

impl<T> Sender<T> {
    /// Attempts to send a value on this channel, returning it back if it could
    /// not be sent.
    ///
    /// This method consumes `self` as only one value may ever be sent on a oneshot
    /// channel. It is not marked async because sending a message to an oneshot
    /// channel never requires any form of waiting.  Because of this, the `send`
    /// method can be used in both synchronous and asynchronous code without
    /// problems.
    ///
    /// A successful send occurs when it is determined that the other end of the
    /// channel has not hung up already. An unsuccessful send would be one where
    /// the corresponding receiver has already been deallocated. Note that a
    /// return value of `Err` means that the data will never be received, but
    /// a return value of `Ok` does *not* mean that the data will be received.
    /// It is possible for the corresponding receiver to hang up immediately
    /// after this function returns `Ok`.
    ///
    /// # Examples
    ///
    /// Send a value to another task
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel();
    ///
    ///     tokio::spawn(async move {
    ///         if let Err(_) = tx.send(3) {
    ///             println!("the receiver dropped");
    ///         }
    ///     });
    ///
    ///     match rx.await {
    ///         Ok(v) => println!("got = {:?}", v),
    ///         Err(_) => println!("the sender dropped"),
    ///     }
    /// }
    /// ```
    pub fn send(mut self, t: T) -> Result<(), T> {
        let inner = self.inner.take().unwrap();

        inner.value.with_mut(|ptr| unsafe {
            // SAFETY: The receiver will not access the `UnsafeCell` unless the
            // channel has been marked as "complete" (the `VALUE_SENT` state bit
            // is set).
            // That bit is only set by the sender later on in this method, and
            // calling this method consumes `self`. Therefore, if it was possible to
            // call this method, we know that the `VALUE_SENT` bit is unset, and
            // the receiver is not currently accessing the `UnsafeCell`.
            *ptr = Some(t);
        });

        if !inner.complete() {
            unsafe {
                // SAFETY: The receiver will not access the `UnsafeCell` unless
                // the channel has been marked as "complete". Calling
                // `complete()` will return true if this bit is set, and false
                // if it is not set. Thus, if `complete()` returned false, it is
                // safe for us to access the value, because we know that the
                // receiver will not.
                return Err(inner.consume_value().unwrap());
            }
        }

        #[cfg(all(tokio_unstable, feature = "tracing"))]
        self.resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            value_sent = true,
            value_sent.op = "override",
            )
        });

        Ok(())
    }

    /// Waits for the associated [`Receiver`] handle to close.
    ///
    /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
    /// [`Receiver`] value is dropped.
    ///
    /// This function is useful when paired with `select!` to abort a
    /// computation when the receiver is no longer interested in the result.
    ///
    /// # Return
    ///
    /// Returns a `Future` which must be awaited on.
    ///
    /// [`Receiver`]: Receiver
    /// [`close`]: Receiver::close
    ///
    /// # Examples
    ///
    /// Basic usage
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (mut tx, rx) = oneshot::channel::<()>();
    ///
    ///     tokio::spawn(async move {
    ///         drop(rx);
    ///     });
    ///
    ///     tx.closed().await;
    ///     println!("the receiver dropped");
    /// }
    /// ```
    ///
    /// Paired with select
    ///
    /// ```
    /// use tokio::sync::oneshot;
    /// use tokio::time::{self, Duration};
    ///
    /// async fn compute() -> String {
    ///     // Complex computation returning a `String`
    /// # "hello".to_string()
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (mut tx, rx) = oneshot::channel();
    ///
    ///     tokio::spawn(async move {
    ///         tokio::select! {
    ///             _ = tx.closed() => {
    ///                 // The receiver dropped, no need to do any further work
    ///             }
    ///             value = compute() => {
    ///                 // The send can fail if the channel was closed at the exact same
    ///                 // time as when compute() finished, so just ignore the failure.
    ///                 let _ = tx.send(value);
    ///             }
    ///         }
    ///     });
    ///
    ///     // Wait for up to 10 seconds
    ///     let _ = time::timeout(Duration::from_secs(10), rx).await;
    /// }
    /// ```
    pub async fn closed(&mut self) {
        use crate::future::poll_fn;

        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let resource_span = self.resource_span.clone();
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let closed = trace::async_op(
            || poll_fn(|cx| self.poll_closed(cx)),
            resource_span,
            "Sender::closed",
            "poll_closed",
            false,
        );
        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
        let closed = poll_fn(|cx| self.poll_closed(cx));

        closed.await
    }

    /// Returns `true` if the associated [`Receiver`] handle has been dropped.
    ///
    /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
    /// [`Receiver`] value is dropped.
    ///
    /// If `true` is returned, a call to `send` will always result in an error.
    ///
    /// [`Receiver`]: Receiver
    /// [`close`]: Receiver::close
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel();
    ///
    ///     assert!(!tx.is_closed());
    ///
    ///     drop(rx);
    ///
    ///     assert!(tx.is_closed());
    ///     assert!(tx.send("never received").is_err());
    /// }
    /// ```
    pub fn is_closed(&self) -> bool {
        let inner = self.inner.as_ref().unwrap();

        let state = State::load(&inner.state, Acquire);
        state.is_closed()
    }

    /// Checks whether the oneshot channel has been closed, and if not, schedules the
    /// `Waker` in the provided `Context` to receive a notification when the channel is
    /// closed.
    ///
    /// A [`Receiver`] is closed by either calling [`close`] explicitly, or when the
    /// [`Receiver`] value is dropped.
    ///
    /// Note that on multiple calls to poll, only the `Waker` from the `Context` passed
    /// to the most recent call will be scheduled to receive a wakeup.
    ///
    /// [`Receiver`]: struct@crate::sync::oneshot::Receiver
    /// [`close`]: fn@crate::sync::oneshot::Receiver::close
    ///
    /// # Return value
    ///
    /// This function returns:
    ///
    ///  * `Poll::Pending` if the channel is still open.
    ///  * `Poll::Ready(())` if the channel is closed.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// use futures::future::poll_fn;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (mut tx, mut rx) = oneshot::channel::<()>();
    ///
    ///     tokio::spawn(async move {
    ///         rx.close();
    ///     });
    ///
    ///     poll_fn(|cx| tx.poll_closed(cx)).await;
    ///
    ///     println!("the receiver dropped");
    /// }
    /// ```
    pub fn poll_closed(&mut self, cx: &mut Context<'_>) -> Poll<()> {
        // Keep track of task budget
        let coop = ready!(crate::runtime::coop::poll_proceed(cx));

        let inner = self.inner.as_ref().unwrap();

        let mut state = State::load(&inner.state, Acquire);

        if state.is_closed() {
            coop.made_progress();
            return Poll::Ready(());
        }

        if state.is_tx_task_set() {
            let will_notify = unsafe { inner.tx_task.will_wake(cx) };

            if !will_notify {
                state = State::unset_tx_task(&inner.state);

                if state.is_closed() {
                    // Set the flag again so that the waker is released in drop
                    State::set_tx_task(&inner.state);
                    coop.made_progress();
                    return Ready(());
                } else {
                    unsafe { inner.tx_task.drop_task() };
                }
            }
        }

        if !state.is_tx_task_set() {
            // Attempt to set the task
            unsafe {
                inner.tx_task.set_task(cx);
            }

            // Update the state
            state = State::set_tx_task(&inner.state);

            if state.is_closed() {
                coop.made_progress();
                return Ready(());
            }
        }

        Pending
    }
}

impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        if let Some(inner) = self.inner.as_ref() {
            inner.complete();
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            self.resource_span.in_scope(|| {
                tracing::trace!(
                target: "runtime::resource::state_update",
                tx_dropped = true,
                tx_dropped.op = "override",
                )
            });
        }
    }
}

impl<T> Receiver<T> {
    /// Prevents the associated [`Sender`] handle from sending a value.
    ///
    /// Any `send` operation which happens after calling `close` is guaranteed
    /// to fail. After calling `close`, [`try_recv`] should be called to
    /// receive a value if one was sent **before** the call to `close`
    /// completed.
    ///
    /// This function is useful to perform a graceful shutdown and ensure that a
    /// value will not be sent into the channel and never received.
    ///
    /// `close` is no-op if a message is already received or the channel
    /// is already closed.
    ///
    /// [`Sender`]: Sender
    /// [`try_recv`]: Receiver::try_recv
    ///
    /// # Examples
    ///
    /// Prevent a value from being sent
    ///
    /// ```
    /// use tokio::sync::oneshot;
    /// use tokio::sync::oneshot::error::TryRecvError;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = oneshot::channel();
    ///
    ///     assert!(!tx.is_closed());
    ///
    ///     rx.close();
    ///
    ///     assert!(tx.is_closed());
    ///     assert!(tx.send("never received").is_err());
    ///
    ///     match rx.try_recv() {
    ///         Err(TryRecvError::Closed) => {}
    ///         _ => unreachable!(),
    ///     }
    /// }
    /// ```
    ///
    /// Receive a value sent **before** calling `close`
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = oneshot::channel();
    ///
    ///     assert!(tx.send("will receive").is_ok());
    ///
    ///     rx.close();
    ///
    ///     let msg = rx.try_recv().unwrap();
    ///     assert_eq!(msg, "will receive");
    /// }
    /// ```
    pub fn close(&mut self) {
        if let Some(inner) = self.inner.as_ref() {
            inner.close();
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            self.resource_span.in_scope(|| {
                tracing::trace!(
                target: "runtime::resource::state_update",
                rx_dropped = true,
                rx_dropped.op = "override",
                )
            });
        }
    }

    /// Attempts to receive a value.
    ///
    /// If a pending value exists in the channel, it is returned. If no value
    /// has been sent, the current task **will not** be registered for
    /// future notification.
    ///
    /// This function is useful to call from outside the context of an
    /// asynchronous task.
    ///
    /// Note that unlike the `poll` method, the `try_recv` method cannot fail
    /// spuriously. Any send or close event that happens before this call to
    /// `try_recv` will be correctly returned to the caller.
    ///
    /// # Return
    ///
    /// - `Ok(T)` if a value is pending in the channel.
    /// - `Err(TryRecvError::Empty)` if no value has been sent yet.
    /// - `Err(TryRecvError::Closed)` if the sender has dropped without sending
    ///   a value, or if the message has already been received.
    ///
    /// # Examples
    ///
    /// `try_recv` before a value is sent, then after.
    ///
    /// ```
    /// use tokio::sync::oneshot;
    /// use tokio::sync::oneshot::error::TryRecvError;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = oneshot::channel();
    ///
    ///     match rx.try_recv() {
    ///         // The channel is currently empty
    ///         Err(TryRecvError::Empty) => {}
    ///         _ => unreachable!(),
    ///     }
    ///
    ///     // Send a value
    ///     tx.send("hello").unwrap();
    ///
    ///     match rx.try_recv() {
    ///         Ok(value) => assert_eq!(value, "hello"),
    ///         _ => unreachable!(),
    ///     }
    /// }
    /// ```
    ///
    /// `try_recv` when the sender dropped before sending a value
    ///
    /// ```
    /// use tokio::sync::oneshot;
    /// use tokio::sync::oneshot::error::TryRecvError;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = oneshot::channel::<()>();
    ///
    ///     drop(tx);
    ///
    ///     match rx.try_recv() {
    ///         // The channel will never receive a value.
    ///         Err(TryRecvError::Closed) => {}
    ///         _ => unreachable!(),
    ///     }
    /// }
    /// ```
    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
        let result = if let Some(inner) = self.inner.as_ref() {
            let state = State::load(&inner.state, Acquire);

            if state.is_complete() {
                // SAFETY: If `state.is_complete()` returns true, then the
                // `VALUE_SENT` bit has been set and the sender side of the
                // channel will no longer attempt to access the inner
                // `UnsafeCell`. Therefore, it is now safe for us to access the
                // cell.
                match unsafe { inner.consume_value() } {
                    Some(value) => {
                        #[cfg(all(tokio_unstable, feature = "tracing"))]
                        self.resource_span.in_scope(|| {
                            tracing::trace!(
                            target: "runtime::resource::state_update",
                            value_received = true,
                            value_received.op = "override",
                            )
                        });
                        Ok(value)
                    }
                    None => Err(TryRecvError::Closed),
                }
            } else if state.is_closed() {
                Err(TryRecvError::Closed)
            } else {
                // Not ready, this does not clear `inner`
                return Err(TryRecvError::Empty);
            }
        } else {
            Err(TryRecvError::Closed)
        };

        self.inner = None;
        result
    }

    /// Blocking receive to call outside of asynchronous contexts.
    ///
    /// # Panics
    ///
    /// This function panics if called within an asynchronous execution
    /// context.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel::<u8>();
    ///
    ///     let sync_code = thread::spawn(move || {
    ///         assert_eq!(Ok(10), rx.blocking_recv());
    ///     });
    ///
    ///     let _ = tx.send(10);
    ///     sync_code.join().unwrap();
    /// }
    /// ```
    #[track_caller]
    #[cfg(feature = "sync")]
    #[cfg_attr(docsrs, doc(alias = "recv_blocking"))]
    pub fn blocking_recv(self) -> Result<T, RecvError> {
        crate::future::block_on(self)
    }
}

impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        if let Some(inner) = self.inner.as_ref() {
            inner.close();
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            self.resource_span.in_scope(|| {
                tracing::trace!(
                target: "runtime::resource::state_update",
                rx_dropped = true,
                rx_dropped.op = "override",
                )
            });
        }
    }
}

impl<T> Future for Receiver<T> {
    type Output = Result<T, RecvError>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // If `inner` is `None`, then `poll()` has already completed.
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let _res_span = self.resource_span.clone().entered();
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let _ao_span = self.async_op_span.clone().entered();
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let _ao_poll_span = self.async_op_poll_span.clone().entered();

        let ret = if let Some(inner) = self.as_ref().get_ref().inner.as_ref() {
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            let res = ready!(trace_poll_op!("poll_recv", inner.poll_recv(cx)))?;

            #[cfg(any(not(tokio_unstable), not(feature = "tracing")))]
            let res = ready!(inner.poll_recv(cx))?;

            res
        } else {
            panic!("called after complete");
        };

        self.inner = None;
        Ready(Ok(ret))
    }
}

impl<T> Inner<T> {
    fn complete(&self) -> bool {
        let prev = State::set_complete(&self.state);

        if prev.is_closed() {
            return false;
        }

        if prev.is_rx_task_set() {
            // TODO: Consume waker?
            unsafe {
                self.rx_task.with_task(Waker::wake_by_ref);
            }
        }

        true
    }

    fn poll_recv(&self, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
        // Keep track of task budget
        let coop = ready!(crate::runtime::coop::poll_proceed(cx));

        // Load the state
        let mut state = State::load(&self.state, Acquire);

        if state.is_complete() {
            coop.made_progress();
            match unsafe { self.consume_value() } {
                Some(value) => Ready(Ok(value)),
                None => Ready(Err(RecvError(()))),
            }
        } else if state.is_closed() {
            coop.made_progress();
            Ready(Err(RecvError(())))
        } else {
            if state.is_rx_task_set() {
                let will_notify = unsafe { self.rx_task.will_wake(cx) };

                // Check if the task is still the same
                if !will_notify {
                    // Unset the task
                    state = State::unset_rx_task(&self.state);
                    if state.is_complete() {
                        // Set the flag again so that the waker is released in drop
                        State::set_rx_task(&self.state);

                        coop.made_progress();
                        // SAFETY: If `state.is_complete()` returns true, then the
                        // `VALUE_SENT` bit has been set and the sender side of the
                        // channel will no longer attempt to access the inner
                        // `UnsafeCell`. Therefore, it is now safe for us to access the
                        // cell.
                        return match unsafe { self.consume_value() } {
                            Some(value) => Ready(Ok(value)),
                            None => Ready(Err(RecvError(()))),
                        };
                    } else {
                        unsafe { self.rx_task.drop_task() };
                    }
                }
            }

            if !state.is_rx_task_set() {
                // Attempt to set the task
                unsafe {
                    self.rx_task.set_task(cx);
                }

                // Update the state
                state = State::set_rx_task(&self.state);

                if state.is_complete() {
                    coop.made_progress();
                    match unsafe { self.consume_value() } {
                        Some(value) => Ready(Ok(value)),
                        None => Ready(Err(RecvError(()))),
                    }
                } else {
                    Pending
                }
            } else {
                Pending
            }
        }
    }

    /// Called by `Receiver` to indicate that the value will never be received.
    fn close(&self) {
        let prev = State::set_closed(&self.state);

        if prev.is_tx_task_set() && !prev.is_complete() {
            unsafe {
                self.tx_task.with_task(Waker::wake_by_ref);
            }
        }
    }

    /// Consumes the value. This function does not check `state`.
    ///
    /// # Safety
    ///
    /// Calling this method concurrently on multiple threads will result in a
    /// data race. The `VALUE_SENT` state bit is used to ensure that only the
    /// sender *or* the receiver will call this method at a given point in time.
    /// If `VALUE_SENT` is not set, then only the sender may call this method;
    /// if it is set, then only the receiver may call this method.
    unsafe fn consume_value(&self) -> Option<T> {
        self.value.with_mut(|ptr| (*ptr).take())
    }
}

unsafe impl<T: Send> Send for Inner<T> {}
unsafe impl<T: Send> Sync for Inner<T> {}

fn mut_load(this: &mut AtomicUsize) -> usize {
    this.with_mut(|v| *v)
}

impl<T> Drop for Inner<T> {
    fn drop(&mut self) {
        let state = State(mut_load(&mut self.state));

        if state.is_rx_task_set() {
            unsafe {
                self.rx_task.drop_task();
            }
        }

        if state.is_tx_task_set() {
            unsafe {
                self.tx_task.drop_task();
            }
        }
    }
}

impl<T: fmt::Debug> fmt::Debug for Inner<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        use std::sync::atomic::Ordering::Relaxed;

        fmt.debug_struct("Inner")
            .field("state", &State::load(&self.state, Relaxed))
            .finish()
    }
}

/// Indicates that a waker for the receiving task has been set.
///
/// # Safety
///
/// If this bit is not set, the `rx_task` field may be uninitialized.
const RX_TASK_SET: usize = 0b00001;
/// Indicates that a value has been stored in the channel's inner `UnsafeCell`.
///
/// # Safety
///
/// This bit controls which side of the channel is permitted to access the
/// `UnsafeCell`. If it is set, the `UnsafeCell` may ONLY be accessed by the
/// receiver. If this bit is NOT set, the `UnsafeCell` may ONLY be accessed by
/// the sender.
const VALUE_SENT: usize = 0b00010;
const CLOSED: usize = 0b00100;

/// Indicates that a waker for the sending task has been set.
///
/// # Safety
///
/// If this bit is not set, the `tx_task` field may be uninitialized.
const TX_TASK_SET: usize = 0b01000;

impl State {
    fn new() -> State {
        State(0)
    }

    fn is_complete(self) -> bool {
        self.0 & VALUE_SENT == VALUE_SENT
    }

    fn set_complete(cell: &AtomicUsize) -> State {
        // This method is a compare-and-swap loop rather than a fetch-or like
        // other `set_$WHATEVER` methods on `State`. This is because we must
        // check if the state has been closed before setting the `VALUE_SENT`
        // bit.
        //
        // We don't want to set both the `VALUE_SENT` bit if the `CLOSED`
        // bit is already set, because `VALUE_SENT` will tell the receiver that
        // it's okay to access the inner `UnsafeCell`. Immediately after calling
        // `set_complete`, if the channel was closed, the sender will _also_
        // access the `UnsafeCell` to take the value back out, so if a
        // `poll_recv` or `try_recv` call is occurring concurrently, both
        // threads may try to access the `UnsafeCell` if we were to set the
        // `VALUE_SENT` bit on a closed channel.
        let mut state = cell.load(Ordering::Relaxed);
        loop {
            if State(state).is_closed() {
                break;
            }
            // TODO: This could be `Release`, followed by an `Acquire` fence *if*
            // the `RX_TASK_SET` flag is set. However, `loom` does not support
            // fences yet.
            match cell.compare_exchange_weak(
                state,
                state | VALUE_SENT,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => break,
                Err(actual) => state = actual,
            }
        }
        State(state)
    }

    fn is_rx_task_set(self) -> bool {
        self.0 & RX_TASK_SET == RX_TASK_SET
    }

    fn set_rx_task(cell: &AtomicUsize) -> State {
        let val = cell.fetch_or(RX_TASK_SET, AcqRel);
        State(val | RX_TASK_SET)
    }

    fn unset_rx_task(cell: &AtomicUsize) -> State {
        let val = cell.fetch_and(!RX_TASK_SET, AcqRel);
        State(val & !RX_TASK_SET)
    }

    fn is_closed(self) -> bool {
        self.0 & CLOSED == CLOSED
    }

    fn set_closed(cell: &AtomicUsize) -> State {
        // Acquire because we want all later writes (attempting to poll) to be
        // ordered after this.
        let val = cell.fetch_or(CLOSED, Acquire);
        State(val)
    }

    fn set_tx_task(cell: &AtomicUsize) -> State {
        let val = cell.fetch_or(TX_TASK_SET, AcqRel);
        State(val | TX_TASK_SET)
    }

    fn unset_tx_task(cell: &AtomicUsize) -> State {
        let val = cell.fetch_and(!TX_TASK_SET, AcqRel);
        State(val & !TX_TASK_SET)
    }

    fn is_tx_task_set(self) -> bool {
        self.0 & TX_TASK_SET == TX_TASK_SET
    }

    fn as_usize(self) -> usize {
        self.0
    }

    fn load(cell: &AtomicUsize, order: Ordering) -> State {
        let val = cell.load(order);
        State(val)
    }
}

impl fmt::Debug for State {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("State")
            .field("is_complete", &self.is_complete())
            .field("is_closed", &self.is_closed())
            .field("is_rx_task_set", &self.is_rx_task_set())
            .field("is_tx_task_set", &self.is_tx_task_set())
            .finish()
    }
}