# broadcast.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.4.0/src/sync/broadcast.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A multi-producer, multi-consumer broadcast queue. Each sent value is seen by
    //! all consumers.
    //!
    //! A [`Sender`] is used to broadcast values to **all** connected [`Receiver`]
    //! values. [`Sender`] handles are clone-able, allowing concurrent send and
    //! receive actions. [`Sender`] and [`Receiver`] are both `Send` and `Sync` as
    //! long as `T` is also `Send` or `Sync` respectively.
    //!
    //! When a value is sent, **all** [`Receiver`] handles are notified and will
    //! receive the value. The value is stored once inside the channel and cloned on
    //! demand for each receiver. Once all receivers have received a clone of the
    //! value, the value is released from the channel.
    //!
    //! A channel is created by calling [`channel`], specifying the maximum number
    //! of messages the channel can retain at any given time.
    //!
    //! New [`Receiver`] handles are created by calling [`Sender::subscribe`]. The
    //! returned [`Receiver`] will receive values sent **after** the call to
    //! `subscribe`.
    //!
    //! ## Lagging
    //!
    //! As sent messages must be retained until **all** [`Receiver`] handles receive
    //! a clone, broadcast channels are susceptible to the "slow receiver" problem.
    //! In this case, all but one receiver are able to receive values at the rate
    //! they are sent. Because one receiver is stalled, the channel starts to fill
    //! up.
    //!
    //! This broadcast channel implementation handles this case by setting a hard
    //! upper bound on the number of values the channel may retain at any given
    //! time. This upper bound is passed to the [`channel`] function as an argument.
    //!
    //! If a value is sent when the channel is at capacity, the oldest value
    //! currently held by the channel is released. This frees up space for the new
    //! value. Any receiver that has not yet seen the released value will return
    //! [`RecvError::Lagged`] the next time [`recv`] is called.
    //!
    //! Once [`RecvError::Lagged`] is returned, the lagging receiver's position is
    //! updated to the oldest value contained by the channel. The next call to
    //! [`recv`] will return this value.
    //!
    //! This behavior enables a receiver to detect when it has lagged so far behind
    //! that data has been dropped. The caller may decide how to respond to this:
    //! either by aborting its task or by tolerating lost messages and resuming
    //! consumption of the channel.
    //!
    //! ## Closing
    //!
    //! When **all** [`Sender`] handles have been dropped, no new values may be
    //! sent. At this point, the channel is "closed". Once a receiver has received
    //! all values retained by the channel, the next call to [`recv`] will return
    //! with [`RecvError::Closed`].
    //!
    //! [`Sender`]: crate::sync::broadcast::Sender
    //! [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
    //! [`Receiver`]: crate::sync::broadcast::Receiver
    //! [`channel`]: crate::sync::broadcast::channel
    //! [`RecvError::Lagged`]: crate::sync::broadcast::error::RecvError::Lagged
    //! [`RecvError::Closed`]: crate::sync::broadcast::error::RecvError::Closed
    //! [`recv`]: crate::sync::broadcast::Receiver::recv
    //!
    //! # Examples
    //!
    //! Basic usage
    //!
    //! ```
    //! use tokio::sync::broadcast;
    //!
    //! #[tokio::main]
    //! async fn main() {
    //!     let (tx, mut rx1) = broadcast::channel(16);
    //!     let mut rx2 = tx.subscribe();
    //!
    //!     tokio::spawn(async move {
    //!         assert_eq!(rx1.recv().await.unwrap(), 10);
    //!         assert_eq!(rx1.recv().await.unwrap(), 20);
    //!     });
    //!
    //!     tokio::spawn(async move {
    //!         assert_eq!(rx2.recv().await.unwrap(), 10);
    //!         assert_eq!(rx2.recv().await.unwrap(), 20);
    //!     });
    //!
    //!     tx.send(10).unwrap();
    //!     tx.send(20).unwrap();
    //! }
    //! ```
    //!
    //! Handling lag
    //!
    //! ```
    //! use tokio::sync::broadcast;
    //!
    //! #[tokio::main]
    //! async fn main() {
    //!     let (tx, mut rx) = broadcast::channel(2);
    //!
    //!     tx.send(10).unwrap();
    //!     tx.send(20).unwrap();
    //!     tx.send(30).unwrap();
    //!
    //!     // The receiver lagged behind
    //!     assert!(rx.recv().await.is_err());
    //!
    //!     // At this point, we can abort or continue with lost messages
    //!
    //!     assert_eq!(20, rx.recv().await.unwrap());
    //!     assert_eq!(30, rx.recv().await.unwrap());
    //! }
    //! ```

    use crate::loom::cell::UnsafeCell;
    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::{Arc, Mutex, RwLock, RwLockReadGuard};
    use crate::util::linked_list::{self, LinkedList};

    use std::fmt;
    use std::future::Future;
    use std::marker::PhantomPinned;
    use std::pin::Pin;
    use std::ptr::NonNull;
    use std::sync::atomic::Ordering::SeqCst;
    use std::task::{Context, Poll, Waker};
    use std::usize;

    /// Sending-half of the [`broadcast`] channel.
    ///
    /// May be used from many threads. Messages can be sent with
    /// [`send`][Sender::send].
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::broadcast;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx1) = broadcast::channel(16);
    ///     let mut rx2 = tx.subscribe();
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
    ///         assert_eq!(rx1.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
    ///         assert_eq!(rx2.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tx.send(10).unwrap();
    ///     tx.send(20).unwrap();
    /// }
    /// ```
    ///
    /// [`broadcast`]: crate::sync::broadcast
    pub struct Sender<T> {
        shared: Arc<Shared<T>>,
    }

    /// Receiving-half of the [`broadcast`] channel.
    ///
    /// Must not be used concurrently. Messages may be retrieved using
    /// [`recv`][Receiver::recv].
    ///
    /// To turn this receiver into a `Stream`, you can use the [`BroadcastStream`]
    /// wrapper.
    ///
    /// [`BroadcastStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.BroadcastStream.html
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::broadcast;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx1) = broadcast::channel(16);
    ///     let mut rx2 = tx.subscribe();
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
    ///         assert_eq!(rx1.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
    ///         assert_eq!(rx2.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tx.send(10).unwrap();
    ///     tx.send(20).unwrap();
    /// }
    /// ```
    ///
    /// [`broadcast`]: crate::sync::broadcast
    pub struct Receiver<T> {
        /// State shared with all receivers and senders.
        shared: Arc<Shared<T>>,

        /// Next position to read from
        next: u64,
    }

    pub mod error {
        //! Broadcast error types

        use std::fmt;

        /// Error returned by from the [`send`] function on a [`Sender`].
        ///
        /// A **send** operation can only fail if there are no active receivers,
        /// implying that the message could never be received. The error contains the
        /// message being sent as a payload so it can be recovered.
        ///
        /// [`send`]: crate::sync::broadcast::Sender::send
        /// [`Sender`]: crate::sync::broadcast::Sender
        #[derive(Debug)]
        pub struct SendError<T>(pub T);

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

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

        /// An error returned from the [`recv`] function on a [`Receiver`].
        ///
        /// [`recv`]: crate::sync::broadcast::Receiver::recv
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        #[derive(Debug, PartialEq)]
        pub enum RecvError {
            /// There are no more active senders implying no further messages will ever
            /// be sent.
            Closed,

            /// The receiver lagged too far behind. Attempting to receive again will
            /// return the oldest message still retained by the channel.
            ///
            /// Includes the number of skipped messages.
            Lagged(u64),
        }

        impl fmt::Display for RecvError {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                match self {
                    RecvError::Closed => write!(f, "channel closed"),
                    RecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
                }
            }
        }

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

        /// An error returned from the [`try_recv`] function on a [`Receiver`].
        ///
        /// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        #[derive(Debug, PartialEq)]
        pub enum TryRecvError {
            /// The channel is currently empty. There are still active
            /// [`Sender`] handles, so data may yet become available.
            ///
            /// [`Sender`]: crate::sync::broadcast::Sender
            Empty,

            /// There are no more active senders implying no further messages will ever
            /// be sent.
            Closed,

            /// The receiver lagged too far behind and has been forcibly disconnected.
            /// Attempting to receive again will return the oldest message still
            /// retained by the channel.
            ///
            /// Includes the number of skipped messages.
            Lagged(u64),
        }

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

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

    use self::error::*;

    /// Data shared between senders and receivers
    struct Shared<T> {
        /// slots in the channel
        buffer: Box<[RwLock<Slot<T>>]>,

        /// Mask a position -> index
        mask: usize,

        /// Tail of the queue. Includes the rx wait list.
        tail: Mutex<Tail>,

        /// Number of outstanding Sender handles
        num_tx: AtomicUsize,
    }

    /// Next position to write a value
    struct Tail {
        /// Next position to write to
        pos: u64,

        /// Number of active receivers
        rx_cnt: usize,

        /// True if the channel is closed
        closed: bool,

        /// Receivers waiting for a value
        waiters: LinkedList<Waiter, <Waiter as linked_list::Link>::Target>,
    }

    /// Slot in the buffer
    struct Slot<T> {
        /// Remaining number of receivers that are expected to see this value.
        ///
        /// When this goes to zero, the value is released.
        ///
        /// An atomic is used as it is mutated concurrently with the slot read lock
        /// acquired.
        rem: AtomicUsize,

        /// Uniquely identifies the `send` stored in the slot
        pos: u64,

        /// True signals the channel is closed.
        closed: bool,

        /// The value being broadcast.
        ///
        /// The value is set by `send` when the write lock is held. When a reader
        /// drops, `rem` is decremented. When it hits zero, the value is dropped.
        val: UnsafeCell<Option<T>>,
    }

    /// An entry in the wait queue
    struct Waiter {
        /// True if queued
        queued: bool,

        /// Task waiting on the broadcast channel.
        waker: Option<Waker>,

        /// Intrusive linked-list pointers.
        pointers: linked_list::Pointers<Waiter>,

        /// Should not be `Unpin`.
        _p: PhantomPinned,
    }

    struct RecvGuard<'a, T> {
        slot: RwLockReadGuard<'a, Slot<T>>,
    }

    /// Receive a value future
    struct Recv<'a, T> {
        /// Receiver being waited on
        receiver: &'a mut Receiver<T>,

        /// Entry in the waiter `LinkedList`
        waiter: UnsafeCell<Waiter>,
    }

    unsafe impl<'a, T: Send> Send for Recv<'a, T> {}
    unsafe impl<'a, T: Send> Sync for Recv<'a, T> {}

    /// Max number of receivers. Reserve space to lock.
    const MAX_RECEIVERS: usize = usize::MAX >> 2;

    /// Create a bounded, multi-producer, multi-consumer channel where each sent
    /// value is broadcasted to all active receivers.
    ///
    /// All data sent on [`Sender`] will become available on every active
    /// [`Receiver`] in the same order as it was sent.
    ///
    /// The `Sender` can be cloned to `send` to the same channel from multiple
    /// points in the process or it can be used concurrently from an `Arc`. New
    /// `Receiver` handles are created by calling [`Sender::subscribe`].
    ///
    /// If all [`Receiver`] handles are dropped, the `send` method will return a
    /// [`SendError`]. Similarly, if all [`Sender`] handles are dropped, the [`recv`]
    /// method will return a [`RecvError`].
    ///
    /// [`Sender`]: crate::sync::broadcast::Sender
    /// [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
    /// [`Receiver`]: crate::sync::broadcast::Receiver
    /// [`recv`]: crate::sync::broadcast::Receiver::recv
    /// [`SendError`]: crate::sync::broadcast::error::SendError
    /// [`RecvError`]: crate::sync::broadcast::error::RecvError
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::broadcast;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx1) = broadcast::channel(16);
    ///     let mut rx2 = tx.subscribe();
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
    ///         assert_eq!(rx1.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tokio::spawn(async move {
    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
    ///         assert_eq!(rx2.recv().await.unwrap(), 20);
    ///     });
    ///
    ///     tx.send(10).unwrap();
    ///     tx.send(20).unwrap();
    /// }
    /// ```
    pub fn channel<T: Clone>(mut capacity: usize) -> (Sender<T>, Receiver<T>) {
        assert!(capacity > 0, "capacity is empty");
        assert!(capacity <= usize::MAX >> 1, "requested capacity too large");

        // Round to a power of two
        capacity = capacity.next_power_of_two();

        let mut buffer = Vec::with_capacity(capacity);

        for i in 0..capacity {
            buffer.push(RwLock::new(Slot {
                rem: AtomicUsize::new(0),
                pos: (i as u64).wrapping_sub(capacity as u64),
                closed: false,
                val: UnsafeCell::new(None),
            }));
        }

        let shared = Arc::new(Shared {
            buffer: buffer.into_boxed_slice(),
            mask: capacity - 1,
            tail: Mutex::new(Tail {
                pos: 0,
                rx_cnt: 1,
                closed: false,
                waiters: LinkedList::new(),
            }),
            num_tx: AtomicUsize::new(1),
        });

        let rx = Receiver {
            shared: shared.clone(),
            next: 0,
        };

        let tx = Sender { shared };

        (tx, rx)
    }

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

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

    impl<T> Sender<T> {
        /// Attempts to send a value to all active [`Receiver`] handles, returning
        /// it back if it could not be sent.
        ///
        /// A successful send occurs when there is at least one active [`Receiver`]
        /// handle. An unsuccessful send would be one where all associated
        /// [`Receiver`] handles have already been dropped.
        ///
        /// # Return
        ///
        /// On success, the number of subscribed [`Receiver`] handles is returned.
        /// This does not mean that this number of receivers will see the message as
        /// a receiver may drop before receiving the message.
        ///
        /// # Note
        ///
        /// A return value of `Ok` **does not** mean that the sent value will be
        /// observed by all or any of the active [`Receiver`] handles. [`Receiver`]
        /// handles may be dropped before receiving the sent message.
        ///
        /// A return value of `Err` **does not** mean that future calls to `send`
        /// will fail. New [`Receiver`] handles may be created by calling
        /// [`subscribe`].
        ///
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        /// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx1) = broadcast::channel(16);
        ///     let mut rx2 = tx.subscribe();
        ///
        ///     tokio::spawn(async move {
        ///         assert_eq!(rx1.recv().await.unwrap(), 10);
        ///         assert_eq!(rx1.recv().await.unwrap(), 20);
        ///     });
        ///
        ///     tokio::spawn(async move {
        ///         assert_eq!(rx2.recv().await.unwrap(), 10);
        ///         assert_eq!(rx2.recv().await.unwrap(), 20);
        ///     });
        ///
        ///     tx.send(10).unwrap();
        ///     tx.send(20).unwrap();
        /// }
        /// ```
        pub fn send(&self, value: T) -> Result<usize, SendError<T>> {
            self.send2(Some(value))
                .map_err(|SendError(maybe_v)| SendError(maybe_v.unwrap()))
        }

        /// Creates a new [`Receiver`] handle that will receive values sent **after**
        /// this call to `subscribe`.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, _rx) = broadcast::channel(16);
        ///
        ///     // Will not be seen
        ///     tx.send(10).unwrap();
        ///
        ///     let mut rx = tx.subscribe();
        ///
        ///     tx.send(20).unwrap();
        ///
        ///     let value = rx.recv().await.unwrap();
        ///     assert_eq!(20, value);
        /// }
        /// ```
        pub fn subscribe(&self) -> Receiver<T> {
            let shared = self.shared.clone();
            new_receiver(shared)
        }

        /// Returns the number of active receivers
        ///
        /// An active receiver is a [`Receiver`] handle returned from [`channel`] or
        /// [`subscribe`]. These are the handles that will receive values sent on
        /// this [`Sender`].
        ///
        /// # Note
        ///
        /// It is not guaranteed that a sent message will reach this number of
        /// receivers. Active receivers may never call [`recv`] again before
        /// dropping.
        ///
        /// [`recv`]: crate::sync::broadcast::Receiver::recv
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        /// [`Sender`]: crate::sync::broadcast::Sender
        /// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
        /// [`channel`]: crate::sync::broadcast::channel
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, _rx1) = broadcast::channel(16);
        ///
        ///     assert_eq!(1, tx.receiver_count());
        ///
        ///     let mut _rx2 = tx.subscribe();
        ///
        ///     assert_eq!(2, tx.receiver_count());
        ///
        ///     tx.send(10).unwrap();
        /// }
        /// ```
        pub fn receiver_count(&self) -> usize {
            let tail = self.shared.tail.lock();
            tail.rx_cnt
        }

        fn send2(&self, value: Option<T>) -> Result<usize, SendError<Option<T>>> {
            let mut tail = self.shared.tail.lock();

            if tail.rx_cnt == 0 {
                return Err(SendError(value));
            }

            // Position to write into
            let pos = tail.pos;
            let rem = tail.rx_cnt;
            let idx = (pos & self.shared.mask as u64) as usize;

            // Update the tail position
            tail.pos = tail.pos.wrapping_add(1);

            // Get the slot
            let mut slot = self.shared.buffer[idx].write().unwrap();

            // Track the position
            slot.pos = pos;

            // Set remaining receivers
            slot.rem.with_mut(|v| *v = rem);

            // Set the closed bit if the value is `None`; otherwise write the value
            if value.is_none() {
                tail.closed = true;
                slot.closed = true;
            } else {
                slot.val.with_mut(|ptr| unsafe { *ptr = value });
            }

            // Release the slot lock before notifying the receivers.
            drop(slot);

            tail.notify_rx();

            // Release the mutex. This must happen after the slot lock is released,
            // otherwise the writer lock bit could be cleared while another thread
            // is in the critical section.
            drop(tail);

            Ok(rem)
        }
    }

    fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
        let mut tail = shared.tail.lock();

        if tail.rx_cnt == MAX_RECEIVERS {
            panic!("max receivers");
        }

        tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");

        let next = tail.pos;

        drop(tail);

        Receiver { shared, next }
    }

    impl Tail {
        fn notify_rx(&mut self) {
            while let Some(mut waiter) = self.waiters.pop_back() {
                // Safety: `waiters` lock is still held.
                let waiter = unsafe { waiter.as_mut() };

                assert!(waiter.queued);
                waiter.queued = false;

                let waker = waiter.waker.take().unwrap();
                waker.wake();
            }
        }
    }

    impl<T> Clone for Sender<T> {
        fn clone(&self) -> Sender<T> {
            let shared = self.shared.clone();
            shared.num_tx.fetch_add(1, SeqCst);

            Sender { shared }
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            if 1 == self.shared.num_tx.fetch_sub(1, SeqCst) {
                let _ = self.send2(None);
            }
        }
    }

    impl<T> Receiver<T> {
        /// Locks the next value if there is one.
        fn recv_ref(
            &mut self,
            waiter: Option<(&UnsafeCell<Waiter>, &Waker)>,
        ) -> Result<RecvGuard<'_, T>, TryRecvError> {
            let idx = (self.next & self.shared.mask as u64) as usize;

            // The slot holding the next value to read
            let mut slot = self.shared.buffer[idx].read().unwrap();

            if slot.pos != self.next {
                let next_pos = slot.pos.wrapping_add(self.shared.buffer.len() as u64);

                // The receiver has read all current values in the channel and there
                // is no waiter to register
                if waiter.is_none() && next_pos == self.next {
                    return Err(TryRecvError::Empty);
                }

                // Release the `slot` lock before attempting to acquire the `tail`
                // lock. This is required because `send2` acquires the tail lock
                // first followed by the slot lock. Acquiring the locks in reverse
                // order here would result in a potential deadlock: `recv_ref`
                // acquires the `slot` lock and attempts to acquire the `tail` lock
                // while `send2` acquired the `tail` lock and attempts to acquire
                // the slot lock.
                drop(slot);

                let mut tail = self.shared.tail.lock();

                // Acquire slot lock again
                slot = self.shared.buffer[idx].read().unwrap();

                // Make sure the position did not change. This could happen in the
                // unlikely event that the buffer is wrapped between dropping the
                // read lock and acquiring the tail lock.
                if slot.pos != self.next {
                    let next_pos = slot.pos.wrapping_add(self.shared.buffer.len() as u64);

                    if next_pos == self.next {
                        // Store the waker
                        if let Some((waiter, waker)) = waiter {
                            // Safety: called while locked.
                            unsafe {
                                // Only queue if not already queued
                                waiter.with_mut(|ptr| {
                                    // If there is no waker **or** if the currently
                                    // stored waker references a **different** task,
                                    // track the tasks' waker to be notified on
                                    // receipt of a new value.
                                    match (*ptr).waker {
                                        Some(ref w) if w.will_wake(waker) => {}
                                        _ => {
                                            (*ptr).waker = Some(waker.clone());
                                        }
                                    }

                                    if !(*ptr).queued {
                                        (*ptr).queued = true;
                                        tail.waiters.push_front(NonNull::new_unchecked(&mut *ptr));
                                    }
                                });
                            }
                        }

                        return Err(TryRecvError::Empty);
                    }

                    // At this point, the receiver has lagged behind the sender by
                    // more than the channel capacity. The receiver will attempt to
                    // catch up by skipping dropped messages and setting the
                    // internal cursor to the **oldest** message stored by the
                    // channel.
                    //
                    // However, finding the oldest position is a bit more
                    // complicated than `tail-position - buffer-size`. When
                    // the channel is closed, the tail position is incremented to
                    // signal a new `None` message, but `None` is not stored in the
                    // channel itself (see issue #2425 for why).
                    //
                    // To account for this, if the channel is closed, the tail
                    // position is decremented by `buffer-size + 1`.
                    let mut adjust = 0;
                    if tail.closed {
                        adjust = 1
                    }
                    let next = tail
                        .pos
                        .wrapping_sub(self.shared.buffer.len() as u64 + adjust);

                    let missed = next.wrapping_sub(self.next);

                    drop(tail);

                    // The receiver is slow but no values have been missed
                    if missed == 0 {
                        self.next = self.next.wrapping_add(1);

                        return Ok(RecvGuard { slot });
                    }

                    self.next = next;

                    return Err(TryRecvError::Lagged(missed));
                }
            }

            self.next = self.next.wrapping_add(1);

            if slot.closed {
                return Err(TryRecvError::Closed);
            }

            Ok(RecvGuard { slot })
        }
    }

    impl<T: Clone> Receiver<T> {
        /// Receives the next value for this receiver.
        ///
        /// Each [`Receiver`] handle will receive a clone of all values sent
        /// **after** it has subscribed.
        ///
        /// `Err(RecvError::Closed)` is returned when all `Sender` halves have
        /// dropped, indicating that no further values can be sent on the channel.
        ///
        /// If the [`Receiver`] handle falls behind, once the channel is full, newly
        /// sent values will overwrite old values. At this point, a call to [`recv`]
        /// will return with `Err(RecvError::Lagged)` and the [`Receiver`]'s
        /// internal cursor is updated to point to the oldest value still held by
        /// the channel. A subsequent call to [`recv`] will return this value
        /// **unless** it has been since overwritten.
        ///
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        /// [`recv`]: crate::sync::broadcast::Receiver::recv
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx1) = broadcast::channel(16);
        ///     let mut rx2 = tx.subscribe();
        ///
        ///     tokio::spawn(async move {
        ///         assert_eq!(rx1.recv().await.unwrap(), 10);
        ///         assert_eq!(rx1.recv().await.unwrap(), 20);
        ///     });
        ///
        ///     tokio::spawn(async move {
        ///         assert_eq!(rx2.recv().await.unwrap(), 10);
        ///         assert_eq!(rx2.recv().await.unwrap(), 20);
        ///     });
        ///
        ///     tx.send(10).unwrap();
        ///     tx.send(20).unwrap();
        /// }
        /// ```
        ///
        /// Handling lag
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = broadcast::channel(2);
        ///
        ///     tx.send(10).unwrap();
        ///     tx.send(20).unwrap();
        ///     tx.send(30).unwrap();
        ///
        ///     // The receiver lagged behind
        ///     assert!(rx.recv().await.is_err());
        ///
        ///     // At this point, we can abort or continue with lost messages
        ///
        ///     assert_eq!(20, rx.recv().await.unwrap());
        ///     assert_eq!(30, rx.recv().await.unwrap());
        /// }
        /// ```
        pub async fn recv(&mut self) -> Result<T, RecvError> {
            let fut = Recv::new(self);
            fut.await
        }

        /// Attempts to return a pending value on this receiver without awaiting.
        ///
        /// This is useful for a flavor of "optimistic check" before deciding to
        /// await on a receiver.
        ///
        /// Compared with [`recv`], this function has three failure cases instead of two
        /// (one for closed, one for an empty buffer, one for a lagging receiver).
        ///
        /// `Err(TryRecvError::Closed)` is returned when all `Sender` halves have
        /// dropped, indicating that no further values can be sent on the channel.
        ///
        /// If the [`Receiver`] handle falls behind, once the channel is full, newly
        /// sent values will overwrite old values. At this point, a call to [`recv`]
        /// will return with `Err(TryRecvError::Lagged)` and the [`Receiver`]'s
        /// internal cursor is updated to point to the oldest value still held by
        /// the channel. A subsequent call to [`try_recv`] will return this value
        /// **unless** it has been since overwritten. If there are no values to
        /// receive, `Err(TryRecvError::Empty)` is returned.
        ///
        /// [`recv`]: crate::sync::broadcast::Receiver::recv
        /// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
        /// [`Receiver`]: crate::sync::broadcast::Receiver
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::broadcast;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = broadcast::channel(16);
        ///
        ///     assert!(rx.try_recv().is_err());
        ///
        ///     tx.send(10).unwrap();
        ///
        ///     let value = rx.try_recv().unwrap();
        ///     assert_eq!(10, value);
        /// }
        /// ```
        pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
            let guard = self.recv_ref(None)?;
            guard.clone_value().ok_or(TryRecvError::Closed)
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            let mut tail = self.shared.tail.lock();

            tail.rx_cnt -= 1;
            let until = tail.pos;

            drop(tail);

            while self.next < until {
                match self.recv_ref(None) {
                    Ok(_) => {}
                    // The channel is closed
                    Err(TryRecvError::Closed) => break,
                    // Ignore lagging, we will catch up
                    Err(TryRecvError::Lagged(..)) => {}
                    // Can't be empty
                    Err(TryRecvError::Empty) => panic!("unexpected empty broadcast channel"),
                }
            }
        }
    }

    impl<'a, T> Recv<'a, T> {
        fn new(receiver: &'a mut Receiver<T>) -> Recv<'a, T> {
            Recv {
                receiver,
                waiter: UnsafeCell::new(Waiter {
                    queued: false,
                    waker: None,
                    pointers: linked_list::Pointers::new(),
                    _p: PhantomPinned,
                }),
            }
        }

        /// A custom `project` implementation is used in place of `pin-project-lite`
        /// as a custom drop implementation is needed.
        fn project(self: Pin<&mut Self>) -> (&mut Receiver<T>, &UnsafeCell<Waiter>) {
            unsafe {
                // Safety: Receiver is Unpin
                is_unpin::<&mut Receiver<T>>();

                let me = self.get_unchecked_mut();
                (me.receiver, &me.waiter)
            }
        }
    }

    impl<'a, T> Future for Recv<'a, T>
    where
        T: Clone,
    {
        type Output = Result<T, RecvError>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
            let (receiver, waiter) = self.project();

            let guard = match receiver.recv_ref(Some((waiter, cx.waker()))) {
                Ok(value) => value,
                Err(TryRecvError::Empty) => return Poll::Pending,
                Err(TryRecvError::Lagged(n)) => return Poll::Ready(Err(RecvError::Lagged(n))),
                Err(TryRecvError::Closed) => return Poll::Ready(Err(RecvError::Closed)),
            };

            Poll::Ready(guard.clone_value().ok_or(RecvError::Closed))
        }
    }

    impl<'a, T> Drop for Recv<'a, T> {
        fn drop(&mut self) {
            // Acquire the tail lock. This is required for safety before accessing
            // the waiter node.
            let mut tail = self.receiver.shared.tail.lock();

            // safety: tail lock is held
            let queued = self.waiter.with(|ptr| unsafe { (*ptr).queued });

            if queued {
                // Remove the node
                //
                // safety: tail lock is held and the wait node is verified to be in
                // the list.
                unsafe {
                    self.waiter.with_mut(|ptr| {
                        tail.waiters.remove((&mut *ptr).into());
                    });
                }
            }
        }
    }

    /// # Safety
    ///
    /// `Waiter` is forced to be !Unpin.
    unsafe impl linked_list::Link for Waiter {
        type Handle = NonNull<Waiter>;
        type Target = Waiter;

        fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> {
            *handle
        }

        unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
            ptr
        }

        unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
            NonNull::from(&mut target.as_mut().pointers)
        }
    }

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

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

    impl<'a, T> RecvGuard<'a, T> {
        fn clone_value(&self) -> Option<T>
        where
            T: Clone,
        {
            self.slot.val.with(|ptr| unsafe { (*ptr).clone() })
        }
    }

    impl<'a, T> Drop for RecvGuard<'a, T> {
        fn drop(&mut self) {
            // Decrement the remaining counter
            if 1 == self.slot.rem.fetch_sub(1, SeqCst) {
                // Safety: Last receiver, drop the value
                self.slot.val.with_mut(|ptr| unsafe { *ptr = None });
            }
        }
    }

    fn is_unpin<T: Unpin>() {}