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
use crate::sync::batch_semaphore::{self as semaphore, TryAcquireError};
use crate::sync::mpsc::chan;
use crate::sync::mpsc::error::{SendError, TrySendError};

cfg_time! {
    use crate::sync::mpsc::error::SendTimeoutError;
    use crate::time::Duration;
}

use std::fmt;
use std::task::{Context, Poll};

/// Send values to the associated `Receiver`.
///
/// Instances are created by the [`channel`](channel) function.
///
/// To use the `Sender` in a poll function, you can use the [`PollSender`]
/// utility.
///
/// [`PollSender`]: https://docs.rs/tokio-util/0.6/tokio_util/sync/struct.PollSender.html
pub struct Sender<T> {
    chan: chan::Tx<T, Semaphore>,
}

/// Permit to send one value into the channel.
///
/// `Permit` values are returned by [`Sender::reserve()`] and [`Sender::try_reserve()`]
/// and are used to guarantee channel capacity before generating a message to send.
///
/// [`Sender::reserve()`]: Sender::reserve
/// [`Sender::try_reserve()`]: Sender::try_reserve
pub struct Permit<'a, T> {
    chan: &'a chan::Tx<T, Semaphore>,
}

/// Receive values from the associated `Sender`.
///
/// Instances are created by the [`channel`](channel) function.
///
/// This receiver can be turned into a `Stream` using [`ReceiverStream`].
///
/// [`ReceiverStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.ReceiverStream.html
pub struct Receiver<T> {
    /// The channel receiver
    chan: chan::Rx<T, Semaphore>,
}

/// Creates a bounded mpsc channel for communicating between asynchronous tasks
/// with backpressure.
///
/// The channel will buffer up to the provided number of messages.  Once the
/// buffer is full, attempts to `send` new messages will wait until a message is
/// received from the channel. The provided buffer capacity must be at least 1.
///
/// All data sent on `Sender` will become available on `Receiver` in the same
/// order as it was sent.
///
/// The `Sender` can be cloned to `send` to the same channel from multiple code
/// locations. Only one `Receiver` is supported.
///
/// If the `Receiver` is disconnected while trying to `send`, the `send` method
/// will return a `SendError`. Similarly, if `Sender` is disconnected while
/// trying to `recv`, the `recv` method will return a `RecvError`.
///
/// # Panics
///
/// Panics if the buffer capacity is 0.
///
/// # Examples
///
/// ```rust
/// use tokio::sync::mpsc;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx, mut rx) = mpsc::channel(100);
///
///     tokio::spawn(async move {
///         for i in 0..10 {
///             if let Err(_) = tx.send(i).await {
///                 println!("receiver dropped");
///                 return;
///             }
///         }
///     });
///
///     while let Some(i) = rx.recv().await {
///         println!("got = {}", i);
///     }
/// }
/// ```
pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
    assert!(buffer > 0, "mpsc bounded channel requires buffer > 0");
    let semaphore = (semaphore::Semaphore::new(buffer), buffer);
    let (tx, rx) = chan::channel(semaphore);

    let tx = Sender::new(tx);
    let rx = Receiver::new(rx);

    (tx, rx)
}

/// Channel semaphore is a tuple of the semaphore implementation and a `usize`
/// representing the channel bound.
type Semaphore = (semaphore::Semaphore, usize);

impl<T> Receiver<T> {
    pub(crate) fn new(chan: chan::Rx<T, Semaphore>) -> Receiver<T> {
        Receiver { chan }
    }

    /// Receives the next value for this receiver.
    ///
    /// This method returns `None` if the channel has been closed and there are
    /// no remaining messages in the channel's buffer. This indicates that no
    /// further values can ever be received from this `Receiver`. The channel is
    /// closed when all senders have been dropped, or when [`close`] is called.
    ///
    /// If there are no messages in the channel's buffer, but the channel has
    /// not yet been closed, this method will sleep until a message is sent or
    /// the channel is closed.
    ///
    /// Note that if [`close`] is called, but there are still outstanding
    /// [`Permits`] from before it was closed, the channel is not considered
    /// closed by `recv` until the permits are released.
    ///
    /// [`close`]: Self::close
    /// [`Permits`]: struct@crate::sync::mpsc::Permit
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(100);
    ///
    ///     tokio::spawn(async move {
    ///         tx.send("hello").await.unwrap();
    ///     });
    ///
    ///     assert_eq!(Some("hello"), rx.recv().await);
    ///     assert_eq!(None, rx.recv().await);
    /// }
    /// ```
    ///
    /// Values are buffered:
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(100);
    ///
    ///     tx.send("hello").await.unwrap();
    ///     tx.send("world").await.unwrap();
    ///
    ///     assert_eq!(Some("hello"), rx.recv().await);
    ///     assert_eq!(Some("world"), rx.recv().await);
    /// }
    /// ```
    pub async fn recv(&mut self) -> Option<T> {
        use crate::future::poll_fn;
        poll_fn(|cx| self.chan.recv(cx)).await
    }

    /// Blocking receive to call outside of asynchronous contexts.
    ///
    /// This method returns `None` if the channel has been closed and there are
    /// no remaining messages in the channel's buffer. This indicates that no
    /// further values can ever be received from this `Receiver`. The channel is
    /// closed when all senders have been dropped, or when [`close`] is called.
    ///
    /// If there are no messages in the channel's buffer, but the channel has
    /// not yet been closed, this method will block until a message is sent or
    /// the channel is closed.
    ///
    /// This method is intended for use cases where you are sending from
    /// asynchronous code to synchronous code, and will work even if the sender
    /// is not using [`blocking_send`] to send the message.
    ///
    /// Note that if [`close`] is called, but there are still outstanding
    /// [`Permits`] from before it was closed, the channel is not considered
    /// closed by `blocking_recv` until the permits are released.
    ///
    /// [`close`]: Self::close
    /// [`Permits`]: struct@crate::sync::mpsc::Permit
    /// [`blocking_send`]: fn@crate::sync::mpsc::Sender::blocking_send
    ///
    /// # Panics
    ///
    /// This function panics if called within an asynchronous execution
    /// context.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use tokio::runtime::Runtime;
    /// use tokio::sync::mpsc;
    ///
    /// fn main() {
    ///     let (tx, mut rx) = mpsc::channel::<u8>(10);
    ///
    ///     let sync_code = thread::spawn(move || {
    ///         assert_eq!(Some(10), rx.blocking_recv());
    ///     });
    ///
    ///     Runtime::new()
    ///         .unwrap()
    ///         .block_on(async move {
    ///             let _ = tx.send(10).await;
    ///         });
    ///     sync_code.join().unwrap()
    /// }
    /// ```
    #[cfg(feature = "sync")]
    pub fn blocking_recv(&mut self) -> Option<T> {
        crate::future::block_on(self.recv())
    }

    /// Closes the receiving half of a channel without dropping it.
    ///
    /// This prevents any further messages from being sent on the channel while
    /// still enabling the receiver to drain messages that are buffered. Any
    /// outstanding [`Permit`] values will still be able to send messages.
    ///
    /// To guarantee that no messages are dropped, after calling `close()`,
    /// `recv()` must be called until `None` is returned. If there are
    /// outstanding [`Permit`] values, the `recv` method will not return `None`
    /// until those are released.
    ///
    /// [`Permit`]: Permit
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(20);
    ///
    ///     tokio::spawn(async move {
    ///         let mut i = 0;
    ///         while let Ok(permit) = tx.reserve().await {
    ///             permit.send(i);
    ///             i += 1;
    ///         }
    ///     });
    ///
    ///     rx.close();
    ///
    ///     while let Some(msg) = rx.recv().await {
    ///         println!("got {}", msg);
    ///     }
    ///
    ///     // Channel closed and no messages are lost.
    /// }
    /// ```
    pub fn close(&mut self) {
        self.chan.close();
    }

    /// Polls to receive the next message on this channel.
    ///
    /// This method returns:
    ///
    ///  * `Poll::Pending` if no messages are available but the channel is not
    ///    closed.
    ///  * `Poll::Ready(Some(message))` if a message is available.
    ///  * `Poll::Ready(None)` if the channel has been closed and all messages
    ///    sent before it was closed have been received.
    ///
    /// When the method returns `Poll::Pending`, the `Waker` in the provided
    /// `Context` is scheduled to receive a wakeup when a message is sent on any
    /// receiver, or when the channel is closed.  Note that on multiple calls to
    /// `poll_recv`, only the `Waker` from the `Context` passed to the most
    /// recent call is scheduled to receive a wakeup.
    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
        self.chan.recv(cx)
    }
}

impl<T> fmt::Debug for Receiver<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Receiver")
            .field("chan", &self.chan)
            .finish()
    }
}

impl<T> Unpin for Receiver<T> {}

impl<T> Sender<T> {
    pub(crate) fn new(chan: chan::Tx<T, Semaphore>) -> Sender<T> {
        Sender { chan }
    }

    /// Sends a value, waiting until there is capacity.
    ///
    /// 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 closed. 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`.
    ///
    /// # Errors
    ///
    /// If the receive half of the channel is closed, either due to [`close`]
    /// being called or the [`Receiver`] handle dropping, the function returns
    /// an error. The error includes the value passed to `send`.
    ///
    /// [`close`]: Receiver::close
    /// [`Receiver`]: Receiver
    ///
    /// # Examples
    ///
    /// In the following example, each call to `send` will block until the
    /// previously sent value was received.
    ///
    /// ```rust
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     tokio::spawn(async move {
    ///         for i in 0..10 {
    ///             if let Err(_) = tx.send(i).await {
    ///                 println!("receiver dropped");
    ///                 return;
    ///             }
    ///         }
    ///     });
    ///
    ///     while let Some(i) = rx.recv().await {
    ///         println!("got = {}", i);
    ///     }
    /// }
    /// ```
    pub async fn send(&self, value: T) -> Result<(), SendError<T>> {
        match self.reserve().await {
            Ok(permit) => {
                permit.send(value);
                Ok(())
            }
            Err(_) => Err(SendError(value)),
        }
    }

    /// Completes when the receiver has dropped.
    ///
    /// This allows the producers to get notified when interest in the produced
    /// values is canceled and immediately stop doing work.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx1, rx) = mpsc::channel::<()>(1);
    ///     let tx2 = tx1.clone();
    ///     let tx3 = tx1.clone();
    ///     let tx4 = tx1.clone();
    ///     let tx5 = tx1.clone();
    ///     tokio::spawn(async move {
    ///         drop(rx);
    ///     });
    ///
    ///     futures::join!(
    ///         tx1.closed(),
    ///         tx2.closed(),
    ///         tx3.closed(),
    ///         tx4.closed(),
    ///         tx5.closed()
    ///     );
    ///     println!("Receiver dropped");
    /// }
    /// ```
    pub async fn closed(&self) {
        self.chan.closed().await
    }

    /// Attempts to immediately send a message on this `Sender`
    ///
    /// This method differs from [`send`] by returning immediately if the channel's
    /// buffer is full or no receiver is waiting to acquire some data. Compared
    /// with [`send`], this function has two failure cases instead of one (one for
    /// disconnection, one for a full buffer).
    ///
    /// # Errors
    ///
    /// If the channel capacity has been reached, i.e., the channel has `n`
    /// buffered values where `n` is the argument passed to [`channel`], then an
    /// error is returned.
    ///
    /// If the receive half of the channel is closed, either due to [`close`]
    /// being called or the [`Receiver`] handle dropping, the function returns
    /// an error. The error includes the value passed to `send`.
    ///
    /// [`send`]: Sender::send
    /// [`channel`]: channel
    /// [`close`]: Receiver::close
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     // Create a channel with buffer size 1
    ///     let (tx1, mut rx) = mpsc::channel(1);
    ///     let tx2 = tx1.clone();
    ///
    ///     tokio::spawn(async move {
    ///         tx1.send(1).await.unwrap();
    ///         tx1.send(2).await.unwrap();
    ///         // task waits until the receiver receives a value.
    ///     });
    ///
    ///     tokio::spawn(async move {
    ///         // This will return an error and send
    ///         // no message if the buffer is full
    ///         let _ = tx2.try_send(3);
    ///     });
    ///
    ///     let mut msg;
    ///     msg = rx.recv().await.unwrap();
    ///     println!("message {} received", msg);
    ///
    ///     msg = rx.recv().await.unwrap();
    ///     println!("message {} received", msg);
    ///
    ///     // Third message may have never been sent
    ///     match rx.recv().await {
    ///         Some(msg) => println!("message {} received", msg),
    ///         None => println!("the third message was never sent"),
    ///     }
    /// }
    /// ```
    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
        match self.chan.semaphore().0.try_acquire(1) {
            Ok(_) => {}
            Err(TryAcquireError::Closed) => return Err(TrySendError::Closed(message)),
            Err(TryAcquireError::NoPermits) => return Err(TrySendError::Full(message)),
        }

        // Send the message
        self.chan.send(message);
        Ok(())
    }

    /// Sends a value, waiting until there is capacity, but only for a limited time.
    ///
    /// Shares the same success and error conditions as [`send`], adding one more
    /// condition for an unsuccessful send, which is when the provided timeout has
    /// elapsed, and there is no capacity available.
    ///
    /// [`send`]: Sender::send
    ///
    /// # Errors
    ///
    /// If the receive half of the channel is closed, either due to [`close`]
    /// being called or the [`Receiver`] having been dropped,
    /// the function returns an error. The error includes the value passed to `send`.
    ///
    /// [`close`]: Receiver::close
    /// [`Receiver`]: Receiver
    ///
    /// # Examples
    ///
    /// In the following example, each call to `send_timeout` will block until the
    /// previously sent value was received, unless the timeout has elapsed.
    ///
    /// ```rust
    /// use tokio::sync::mpsc;
    /// use tokio::time::{sleep, Duration};
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     tokio::spawn(async move {
    ///         for i in 0..10 {
    ///             if let Err(e) = tx.send_timeout(i, Duration::from_millis(100)).await {
    ///                 println!("send error: #{:?}", e);
    ///                 return;
    ///             }
    ///         }
    ///     });
    ///
    ///     while let Some(i) = rx.recv().await {
    ///         println!("got = {}", i);
    ///         sleep(Duration::from_millis(200)).await;
    ///     }
    /// }
    /// ```
    #[cfg(feature = "time")]
    #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
    pub async fn send_timeout(
        &self,
        value: T,
        timeout: Duration,
    ) -> Result<(), SendTimeoutError<T>> {
        let permit = match crate::time::timeout(timeout, self.reserve()).await {
            Err(_) => {
                return Err(SendTimeoutError::Timeout(value));
            }
            Ok(Err(_)) => {
                return Err(SendTimeoutError::Closed(value));
            }
            Ok(Ok(permit)) => permit,
        };

        permit.send(value);
        Ok(())
    }

    /// Blocking send to call outside of asynchronous contexts.
    ///
    /// This method is intended for use cases where you are sending from
    /// synchronous code to asynchronous code, and will work even if the
    /// receiver is not using [`blocking_recv`] to receive the message.
    ///
    /// [`blocking_recv`]: fn@crate::sync::mpsc::Receiver::blocking_recv
    ///
    /// # Panics
    ///
    /// This function panics if called within an asynchronous execution
    /// context.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use tokio::runtime::Runtime;
    /// use tokio::sync::mpsc;
    ///
    /// fn main() {
    ///     let (tx, mut rx) = mpsc::channel::<u8>(1);
    ///
    ///     let sync_code = thread::spawn(move || {
    ///         tx.blocking_send(10).unwrap();
    ///     });
    ///
    ///     Runtime::new().unwrap().block_on(async move {
    ///         assert_eq!(Some(10), rx.recv().await);
    ///     });
    ///     sync_code.join().unwrap()
    /// }
    /// ```
    #[cfg(feature = "sync")]
    pub fn blocking_send(&self, value: T) -> Result<(), SendError<T>> {
        crate::future::block_on(self.send(value))
    }

    /// Checks if the channel has been closed. This happens when the
    /// [`Receiver`] is dropped, or when the [`Receiver::close`] method is
    /// called.
    ///
    /// [`Receiver`]: crate::sync::mpsc::Receiver
    /// [`Receiver::close`]: crate::sync::mpsc::Receiver::close
    ///
    /// ```
    /// let (tx, rx) = tokio::sync::mpsc::channel::<()>(42);
    /// assert!(!tx.is_closed());
    ///
    /// let tx2 = tx.clone();
    /// assert!(!tx2.is_closed());
    ///
    /// drop(rx);
    /// assert!(tx.is_closed());
    /// assert!(tx2.is_closed());
    /// ```
    pub fn is_closed(&self) -> bool {
        self.chan.is_closed()
    }

    /// Wait for channel capacity. Once capacity to send one message is
    /// available, it is reserved for the caller.
    ///
    /// If the channel is full, the function waits for the number of unreceived
    /// messages to become less than the channel capacity. Capacity to send one
    /// message is reserved for the caller. A [`Permit`] is returned to track
    /// the reserved capacity. The [`send`] function on [`Permit`] consumes the
    /// reserved capacity.
    ///
    /// Dropping [`Permit`] without sending a message releases the capacity back
    /// to the channel.
    ///
    /// [`Permit`]: Permit
    /// [`send`]: Permit::send
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     // Reserve capacity
    ///     let permit = tx.reserve().await.unwrap();
    ///
    ///     // Trying to send directly on the `tx` will fail due to no
    ///     // available capacity.
    ///     assert!(tx.try_send(123).is_err());
    ///
    ///     // Sending on the permit succeeds
    ///     permit.send(456);
    ///
    ///     // The value sent on the permit is received
    ///     assert_eq!(rx.recv().await.unwrap(), 456);
    /// }
    /// ```
    pub async fn reserve(&self) -> Result<Permit<'_, T>, SendError<()>> {
        match self.chan.semaphore().0.acquire(1).await {
            Ok(_) => {}
            Err(_) => return Err(SendError(())),
        }

        Ok(Permit { chan: &self.chan })
    }

    /// Try to acquire a slot in the channel without waiting for the slot to become
    /// available.
    ///
    /// If the channel is full this function will return [`TrySendError`], otherwise
    /// if there is a slot available it will return a [`Permit`] that will then allow you
    /// to [`send`] on the channel with a guaranteed slot. This function is similar to
    /// [`reserve`] except it does not await for the slot to become available.
    ///
    /// Dropping [`Permit`] without sending a message releases the capacity back
    /// to the channel.
    ///
    /// [`Permit`]: Permit
    /// [`send`]: Permit::send
    /// [`reserve`]: Sender::reserve
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     // Reserve capacity
    ///     let permit = tx.try_reserve().unwrap();
    ///
    ///     // Trying to send directly on the `tx` will fail due to no
    ///     // available capacity.
    ///     assert!(tx.try_send(123).is_err());
    ///
    ///     // Trying to reserve an additional slot on the `tx` will
    ///     // fail because there is no capacity.
    ///     assert!(tx.try_reserve().is_err());
    ///
    ///     // Sending on the permit succeeds
    ///     permit.send(456);
    ///
    ///     // The value sent on the permit is received
    ///     assert_eq!(rx.recv().await.unwrap(), 456);
    ///
    /// }
    /// ```
    pub fn try_reserve(&self) -> Result<Permit<'_, T>, TrySendError<()>> {
        match self.chan.semaphore().0.try_acquire(1) {
            Ok(_) => {}
            Err(_) => return Err(TrySendError::Full(())),
        }

        Ok(Permit { chan: &self.chan })
    }

    /// Returns `true` if senders belong to the same channel.
    ///
    /// # Examples
    ///
    /// ```
    /// let (tx, rx) = tokio::sync::mpsc::channel::<()>(1);
    /// let  tx2 = tx.clone();
    /// assert!(tx.same_channel(&tx2));
    ///
    /// let (tx3, rx3) = tokio::sync::mpsc::channel::<()>(1);
    /// assert!(!tx3.same_channel(&tx2));
    /// ```
    pub fn same_channel(&self, other: &Self) -> bool {
        self.chan.same_channel(&other.chan)
    }

    /// Returns the current capacity of the channel.
    ///
    /// The capacity goes down when sending a value by calling [`send`] or by reserving capacity
    /// with [`reserve`]. The capacity goes up when values are received by the [`Receiver`].
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel::<()>(5);
    ///
    ///     assert_eq!(tx.capacity(), 5);
    ///
    ///     // Making a reservation drops the capacity by one.
    ///     let permit = tx.reserve().await.unwrap();
    ///     assert_eq!(tx.capacity(), 4);
    ///
    ///     // Sending and receiving a value increases the caapcity by one.
    ///     permit.send(());
    ///     rx.recv().await.unwrap();
    ///     assert_eq!(tx.capacity(), 5);
    /// }
    /// ```
    ///
    /// [`send`]: Sender::send
    /// [`reserve`]: Sender::reserve
    pub fn capacity(&self) -> usize {
        self.chan.semaphore().0.available_permits()
    }
}

impl<T> Clone for Sender<T> {
    fn clone(&self) -> Self {
        Sender {
            chan: self.chan.clone(),
        }
    }
}

impl<T> fmt::Debug for Sender<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Sender")
            .field("chan", &self.chan)
            .finish()
    }
}

// ===== impl Permit =====

impl<T> Permit<'_, T> {
    /// Sends a value using the reserved capacity.
    ///
    /// Capacity for the message has already been reserved. The message is sent
    /// to the receiver and the permit is consumed. The operation will succeed
    /// even if the receiver half has been closed. See [`Receiver::close`] for
    /// more details on performing a clean shutdown.
    ///
    /// [`Receiver::close`]: Receiver::close
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     // Reserve capacity
    ///     let permit = tx.reserve().await.unwrap();
    ///
    ///     // Trying to send directly on the `tx` will fail due to no
    ///     // available capacity.
    ///     assert!(tx.try_send(123).is_err());
    ///
    ///     // Send a message on the permit
    ///     permit.send(456);
    ///
    ///     // The value sent on the permit is received
    ///     assert_eq!(rx.recv().await.unwrap(), 456);
    /// }
    /// ```
    pub fn send(self, value: T) {
        use std::mem;

        self.chan.send(value);

        // Avoid the drop logic
        mem::forget(self);
    }
}

impl<T> Drop for Permit<'_, T> {
    fn drop(&mut self) {
        use chan::Semaphore;

        let semaphore = self.chan.semaphore();

        // Add the permit back to the semaphore
        semaphore.add_permit();

        if semaphore.is_closed() && semaphore.is_idle() {
            self.chan.wake_rx();
        }
    }
}

impl<T> fmt::Debug for Permit<'_, T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Permit")
            .field("chan", &self.chan)
            .finish()
    }
}