#
local_channel/
mpsc.rs

    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

    //! A non-thread-safe multi-producer, single-consumer, futures-aware, FIFO queue.

    use alloc::{collections::VecDeque, rc::Rc};
    use core::{
        cell::RefCell,
        fmt,
        future::poll_fn,
        pin::Pin,
        task::{Context, Poll},
    };
    use std::error::Error;

    use futures_core::stream::Stream;
    use futures_sink::Sink;
    use local_waker::LocalWaker;

    /// Creates a unbounded in-memory channel with buffered storage.
    ///
    /// [Sender]s and [Receiver]s are `!Send`.
    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let shared = Rc::new(RefCell::new(Shared {
            has_receiver: true,
            buffer: VecDeque::new(),
            blocked_recv: LocalWaker::new(),
        }));

        let sender = Sender {
            shared: shared.clone(),
        };

        let receiver = Receiver { shared };

        (sender, receiver)
    }

    #[derive(Debug)]
    struct Shared<T> {
        buffer: VecDeque<T>,
        blocked_recv: LocalWaker,
        has_receiver: bool,
    }

    /// The transmission end of a channel.
    ///
    /// This is created by the `channel` function.
    #[derive(Debug)]
    pub struct Sender<T> {
        shared: Rc<RefCell<Shared<T>>>,
    }

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

    impl<T> Sender<T> {
        /// Sends the provided message along this channel.
        pub fn send(&self, item: T) -> Result<(), SendError<T>> {
            let mut shared = self.shared.borrow_mut();

            if !shared.has_receiver {
                // receiver was dropped
                return Err(SendError(item));
            };

            shared.buffer.push_back(item);
            shared.blocked_recv.wake();

            Ok(())
        }

        /// Closes the sender half.
        ///
        /// This prevents any further messages from being sent on the channel, by any sender, while
        /// still enabling the receiver to drain messages that are already buffered.
        pub fn close(&mut self) {
            self.shared.borrow_mut().has_receiver = false;
        }
    }

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

    impl<T> Sink<T> for Sender<T> {
        type Error = SendError<T>;

        fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            Poll::Ready(Ok(()))
        }

        fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), SendError<T>> {
            self.send(item)
        }

        fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), SendError<T>>> {
            Poll::Ready(Ok(()))
        }

        fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            Poll::Ready(Ok(()))
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            let count = Rc::strong_count(&self.shared);
            let shared = self.shared.borrow_mut();

            // check is last sender is about to drop
            if shared.has_receiver && count == 2 {
                // Wake up receiver as its stream has ended
                shared.blocked_recv.wake();
            }
        }
    }

    /// The receiving end of a channel which implements the `Stream` trait.
    ///
    /// This is created by the [`channel`] function.
    #[derive(Debug)]
    pub struct Receiver<T> {
        shared: Rc<RefCell<Shared<T>>>,
    }

    impl<T> Receiver<T> {
        /// Receive the next value.
        ///
        /// Returns `None` if the channel is empty and has been [closed](Sender::close) explicitly or
        /// when all senders have been dropped and, therefore, no more values can ever be sent though
        /// this channel.
        pub async fn recv(&mut self) -> Option<T> {
            let mut this = Pin::new(self);
            poll_fn(|cx| this.as_mut().poll_next(cx)).await
        }

        /// Create an associated [Sender].
        pub fn sender(&self) -> Sender<T> {
            Sender {
                shared: self.shared.clone(),
            }
        }
    }

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

    impl<T> Stream for Receiver<T> {
        type Item = T;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let mut shared = self.shared.borrow_mut();

            if Rc::strong_count(&self.shared) == 1 {
                // All senders have been dropped, so drain the buffer and end the stream.
                return Poll::Ready(shared.buffer.pop_front());
            }

            if let Some(msg) = shared.buffer.pop_front() {
                Poll::Ready(Some(msg))
            } else {
                shared.blocked_recv.register(cx.waker());
                Poll::Pending
            }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            let mut shared = self.shared.borrow_mut();
            shared.buffer.clear();
            shared.has_receiver = false;
        }
    }

    /// Error returned when attempting to send after the channels' [Receiver] is dropped or closed.
    ///
    /// Allows access to message that failed to send with [`into_inner`](Self::into_inner).
    pub struct SendError<T>(pub T);

    impl<T> SendError<T> {
        /// Returns the message that was attempted to be sent but failed.
        pub fn into_inner(self) -> T {
            self.0
        }
    }

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

    impl<T> fmt::Display for SendError<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "send failed because receiver is gone")
        }
    }

    impl<T> Error for SendError<T> {}

    #[cfg(test)]
    mod tests {
        use futures_util::{future::lazy, StreamExt as _};

        use super::*;

        #[tokio::test]
        async fn test_mpsc() {
            let (tx, mut rx) = channel();
            tx.send("test").unwrap();
            assert_eq!(rx.next().await.unwrap(), "test");

            let tx2 = tx.clone();
            tx2.send("test2").unwrap();
            assert_eq!(rx.next().await.unwrap(), "test2");

            assert_eq!(
                lazy(|cx| Pin::new(&mut rx).poll_next(cx)).await,
                Poll::Pending
            );
            drop(tx2);
            assert_eq!(
                lazy(|cx| Pin::new(&mut rx).poll_next(cx)).await,
                Poll::Pending
            );
            drop(tx);
            assert_eq!(rx.next().await, None);

            let (tx, rx) = channel();
            tx.send("test").unwrap();
            drop(rx);
            assert!(tx.send("test").is_err());

            let (mut tx, _) = channel();
            let tx2 = tx.clone();
            tx.close();
            assert!(tx.send("test").is_err());
            assert!(tx2.send("test").is_err());
        }

        #[tokio::test]
        async fn test_recv() {
            let (tx, mut rx) = channel();
            tx.send("test").unwrap();
            assert_eq!(rx.recv().await.unwrap(), "test");
            drop(tx);

            let (tx, mut rx) = channel();
            tx.send("test").unwrap();
            assert_eq!(rx.recv().await.unwrap(), "test");
            drop(tx);
            assert!(rx.recv().await.is_none());
        }
    }