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    //! A single-producer, multi-consumer channel that only retains the *last* sent
    //! value.
    //!
    //! This channel is useful for watching for changes to a value from multiple
    //! points in the code base, for example, changes to configuration values.
    //!
    //! # Usage
    //!
    //! [`channel`] returns a [`Sender`] / [`Receiver`] pair. These are the producer
    //! and sender halves of the channel. The channel is created with an initial
    //! value. The **latest** value stored in the channel is accessed with
    //! [`Receiver::borrow()`]. Awaiting [`Receiver::changed()`] waits for a new
    //! value to sent by the [`Sender`] half.
    //!
    //! # Examples
    //!
    //! ```
    //! use tokio::sync::watch;
    //!
    //! # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    //!     let (tx, mut rx) = watch::channel("hello");
    //!
    //!     tokio::spawn(async move {
    //!         while rx.changed().await.is_ok() {
    //!             println!("received = {:?}", *rx.borrow());
    //!         }
    //!     });
    //!
    //!     tx.send("world")?;
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! # Closing
    //!
    //! [`Sender::is_closed`] and [`Sender::closed`] allow the producer to detect
    //! when all [`Receiver`] handles have been dropped. This indicates that there
    //! is no further interest in the values being produced and work can be stopped.
    //!
    //! # Thread safety
    //!
    //! Both [`Sender`] and [`Receiver`] are thread safe. They can be moved to other
    //! threads and can be used in a concurrent environment. Clones of [`Receiver`]
    //! handles may be moved to separate threads and also used concurrently.
    //!
    //! [`Sender`]: crate::sync::watch::Sender
    //! [`Receiver`]: crate::sync::watch::Receiver
    //! [`Receiver::changed()`]: crate::sync::watch::Receiver::changed
    //! [`Receiver::borrow()`]: crate::sync::watch::Receiver::borrow
    //! [`channel`]: crate::sync::watch::channel
    //! [`Sender::is_closed`]: crate::sync::watch::Sender::is_closed
    //! [`Sender::closed`]: crate::sync::watch::Sender::closed

    use crate::sync::Notify;

    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::atomic::Ordering::{Relaxed, SeqCst};
    use crate::loom::sync::{Arc, RwLock, RwLockReadGuard};
    use std::ops;

    /// Receives values from the associated [`Sender`](struct@Sender).
    ///
    /// Instances are created by the [`channel`](fn@channel) function.
    ///
    /// To turn this receiver into a `Stream`, you can use the [`WatchStream`]
    /// wrapper.
    ///
    /// [`WatchStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.WatchStream.html
    #[derive(Debug)]
    pub struct Receiver<T> {
        /// Pointer to the shared state
        shared: Arc<Shared<T>>,

        /// Last observed version
        version: usize,
    }

    /// Sends values to the associated [`Receiver`](struct@Receiver).
    ///
    /// Instances are created by the [`channel`](fn@channel) function.
    #[derive(Debug)]
    pub struct Sender<T> {
        shared: Arc<Shared<T>>,
    }

    /// Returns a reference to the inner value
    ///
    /// Outstanding borrows hold a read lock on the inner value. This means that
    /// long lived borrows could cause the produce half to block. It is recommended
    /// to keep the borrow as short lived as possible.
    #[derive(Debug)]
    pub struct Ref<'a, T> {
        inner: RwLockReadGuard<'a, T>,
    }

    #[derive(Debug)]
    struct Shared<T> {
        /// The most recent value
        value: RwLock<T>,

        /// The current version
        ///
        /// The lowest bit represents a "closed" state. The rest of the bits
        /// represent the current version.
        version: AtomicUsize,

        /// Tracks the number of `Receiver` instances
        ref_count_rx: AtomicUsize,

        /// Notifies waiting receivers that the value changed.
        notify_rx: Notify,

        /// Notifies any task listening for `Receiver` dropped events
        notify_tx: Notify,
    }

    pub mod error {
        //! Watch error types

        use std::fmt;

        /// Error produced when sending a value fails.
        #[derive(Debug)]
        pub struct SendError<T> {
            pub(crate) inner: T,
        }

        // ===== impl SendError =====

        impl<T: fmt::Debug> fmt::Display for SendError<T> {
            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(fmt, "channel closed")
            }
        }

        impl<T: fmt::Debug> std::error::Error for SendError<T> {}

        /// Error produced when receiving a change notification.
        #[derive(Debug)]
        pub struct RecvError(pub(super) ());

        // ===== 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 {}
    }

    const CLOSED: usize = 1;

    /// Creates a new watch channel, returning the "send" and "receive" handles.
    ///
    /// All values sent by [`Sender`] will become visible to the [`Receiver`] handles.
    /// Only the last value sent is made available to the [`Receiver`] half. All
    /// intermediate values are dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::watch;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    ///     let (tx, mut rx) = watch::channel("hello");
    ///
    ///     tokio::spawn(async move {
    ///         while rx.changed().await.is_ok() {
    ///             println!("received = {:?}", *rx.borrow());
    ///         }
    ///     });
    ///
    ///     tx.send("world")?;
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// [`Sender`]: struct@Sender
    /// [`Receiver`]: struct@Receiver
    pub fn channel<T>(init: T) -> (Sender<T>, Receiver<T>) {
        let shared = Arc::new(Shared {
            value: RwLock::new(init),
            version: AtomicUsize::new(0),
            ref_count_rx: AtomicUsize::new(1),
            notify_rx: Notify::new(),
            notify_tx: Notify::new(),
        });

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

        let rx = Receiver { shared, version: 0 };

        (tx, rx)
    }

    impl<T> Receiver<T> {
        /// Returns a reference to the most recently sent value
        ///
        /// Outstanding borrows hold a read lock. This means that long lived borrows
        /// could cause the send half to block. It is recommended to keep the borrow
        /// as short lived as possible.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::watch;
        ///
        /// let (_, rx) = watch::channel("hello");
        /// assert_eq!(*rx.borrow(), "hello");
        /// ```
        pub fn borrow(&self) -> Ref<'_, T> {
            let inner = self.shared.value.read().unwrap();
            Ref { inner }
        }

        /// Wait for a change notification
        ///
        /// Returns when a new value has been sent by the [`Sender`] since the last
        /// time `changed()` was called. When the `Sender` half is dropped, `Err` is
        /// returned.
        ///
        /// [`Sender`]: struct@Sender
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::watch;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = watch::channel("hello");
        ///
        ///     tokio::spawn(async move {
        ///         tx.send("goodbye").unwrap();
        ///     });
        ///
        ///     assert!(rx.changed().await.is_ok());
        ///     assert_eq!(*rx.borrow(), "goodbye");
        ///
        ///     // The `tx` handle has been dropped
        ///     assert!(rx.changed().await.is_err());
        /// }
        /// ```
        pub async fn changed(&mut self) -> Result<(), error::RecvError> {
            loop {
                // In order to avoid a race condition, we first request a notification,
                // **then** check the current value's version. If a new version exists,
                // the notification request is dropped.
                let notified = self.shared.notify_rx.notified();

                if let Some(ret) = maybe_changed(&self.shared, &mut self.version) {
                    return ret;
                }

                notified.await;
                // loop around again in case the wake-up was spurious
            }
        }
    }

    fn maybe_changed<T>(
        shared: &Shared<T>,
        version: &mut usize,
    ) -> Option<Result<(), error::RecvError>> {
        // Load the version from the state
        let state = shared.version.load(SeqCst);
        let new_version = state & !CLOSED;

        if *version != new_version {
            // Observe the new version and return
            *version = new_version;
            return Some(Ok(()));
        }

        if CLOSED == state & CLOSED {
            // All receivers have dropped.
            return Some(Err(error::RecvError(())));
        }

        None
    }

    impl<T> Clone for Receiver<T> {
        fn clone(&self) -> Self {
            let version = self.version;
            let shared = self.shared.clone();

            // No synchronization necessary as this is only used as a counter and
            // not memory access.
            shared.ref_count_rx.fetch_add(1, Relaxed);

            Receiver { version, shared }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            // No synchronization necessary as this is only used as a counter and
            // not memory access.
            if 1 == self.shared.ref_count_rx.fetch_sub(1, Relaxed) {
                // This is the last `Receiver` handle, tasks waiting on `Sender::closed()`
                self.shared.notify_tx.notify_waiters();
            }
        }
    }

    impl<T> Sender<T> {
        /// Sends a new value via the channel, notifying all receivers.
        pub fn send(&self, value: T) -> Result<(), error::SendError<T>> {
            // This is pretty much only useful as a hint anyway, so synchronization isn't critical.
            if 0 == self.shared.ref_count_rx.load(Relaxed) {
                return Err(error::SendError { inner: value });
            }

            *self.shared.value.write().unwrap() = value;

            // Update the version. 2 is used so that the CLOSED bit is not set.
            self.shared.version.fetch_add(2, SeqCst);

            // Notify all watchers
            self.shared.notify_rx.notify_waiters();

            Ok(())
        }

        /// Returns a reference to the most recently sent value
        ///
        /// Outstanding borrows hold a read lock. This means that long lived borrows
        /// could cause the send half to block. It is recommended to keep the borrow
        /// as short lived as possible.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::watch;
        ///
        /// let (tx, _) = watch::channel("hello");
        /// assert_eq!(*tx.borrow(), "hello");
        /// ```
        pub fn borrow(&self) -> Ref<'_, T> {
            let inner = self.shared.value.read().unwrap();
            Ref { inner }
        }

        /// Checks if the channel has been closed. This happens when all receivers
        /// have dropped.
        ///
        /// # Examples
        ///
        /// ```
        /// let (tx, rx) = tokio::sync::watch::channel(());
        /// assert!(!tx.is_closed());
        ///
        /// drop(rx);
        /// assert!(tx.is_closed());
        /// ```
        pub fn is_closed(&self) -> bool {
            self.shared.ref_count_rx.load(Relaxed) == 0
        }

        /// Completes when all receivers have dropped.
        ///
        /// This allows the producer to get notified when interest in the produced
        /// values is canceled and immediately stop doing work.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::watch;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, rx) = watch::channel("hello");
        ///
        ///     tokio::spawn(async move {
        ///         // use `rx`
        ///         drop(rx);
        ///     });
        ///
        ///     // Waits for `rx` to drop
        ///     tx.closed().await;
        ///     println!("the `rx` handles dropped")
        /// }
        /// ```
        pub async fn closed(&self) {
            let notified = self.shared.notify_tx.notified();

            if self.shared.ref_count_rx.load(Relaxed) == 0 {
                return;
            }

            notified.await;
            debug_assert_eq!(0, self.shared.ref_count_rx.load(Relaxed));
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            self.shared.version.fetch_or(CLOSED, SeqCst);
            self.shared.notify_rx.notify_waiters();
        }
    }

    // ===== impl Ref =====

    impl<T> ops::Deref for Ref<'_, T> {
        type Target = T;

        fn deref(&self) -> &T {
            self.inner.deref()
        }
    }

    #[cfg(all(test, loom))]
    mod tests {
        use futures::future::FutureExt;
        use loom::thread;

        // test for https://github.com/tokio-rs/tokio/issues/3168
        #[test]
        fn watch_spurious_wakeup() {
            loom::model(|| {
                let (send, mut recv) = crate::sync::watch::channel(0i32);

                send.send(1).unwrap();

                let send_thread = thread::spawn(move || {
                    send.send(2).unwrap();
                    send
                });

                recv.changed().now_or_never();

                let send = send_thread.join().unwrap();
                let recv_thread = thread::spawn(move || {
                    recv.changed().now_or_never();
                    recv.changed().now_or_never();
                    recv
                });

                send.send(3).unwrap();

                let mut recv = recv_thread.join().unwrap();
                let send_thread = thread::spawn(move || {
                    send.send(2).unwrap();
                });

                recv.changed().now_or_never();

                send_thread.join().unwrap();
            });
        }

        #[test]
        fn watch_borrow() {
            loom::model(|| {
                let (send, mut recv) = crate::sync::watch::channel(0i32);

                assert!(send.borrow().eq(&0));
                assert!(recv.borrow().eq(&0));

                send.send(1).unwrap();
                assert!(send.borrow().eq(&1));

                let send_thread = thread::spawn(move || {
                    send.send(2).unwrap();
                    send
                });

                recv.changed().now_or_never();

                let send = send_thread.join().unwrap();
                let recv_thread = thread::spawn(move || {
                    recv.changed().now_or_never();
                    recv.changed().now_or_never();
                    recv
                });

                send.send(3).unwrap();

                let recv = recv_thread.join().unwrap();
                assert!(recv.borrow().eq(&3));
                assert!(send.borrow().eq(&3));

                send.send(2).unwrap();

                thread::spawn(move || {
                    assert!(recv.borrow().eq(&2));
                });
                assert!(send.borrow().eq(&2));
            });
        }
    }