      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

    use crate::sync::batch_semaphore::{self as semaphore, TryAcquireError};
    use crate::sync::mpsc::chan;
    #[cfg(unix)]
    #[cfg(any(feature = "signal", feature = "process"))]
    use crate::sync::mpsc::error::TryRecvError;
    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.
    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())
        }

        /// Attempts to return a pending value on this receiver without blocking.
        ///
        /// This method will never block the caller in order to wait for data to
        /// become available. Instead, this will always return immediately with
        /// a possible option of pending data on the channel.
        ///
        /// This is useful for a flavor of "optimistic check" before deciding to
        /// block on a receiver.
        ///
        /// Compared with recv, this function has two failure cases instead of
        /// one (one for disconnection, one for an empty buffer).
        #[cfg(unix)]
        #[cfg(any(feature = "signal", feature = "process"))]
        pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
            self.chan.try_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)
        }
    }

    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()
        }
    }