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    //! A shutdown channel.
    //!
    //! Each worker holds the `Sender` half. When all the `Sender` halves are
    //! dropped, the `Receiver` receives a notification.

    use crate::loom::sync::Arc;
    use crate::sync::oneshot;

    use std::time::Duration;

    #[derive(Debug, Clone)]
    pub(super) struct Sender {
        tx: Arc<oneshot::Sender<()>>,
    }

    #[derive(Debug)]
    pub(super) struct Receiver {
        rx: oneshot::Receiver<()>,
    }

    pub(super) fn channel() -> (Sender, Receiver) {
        let (tx, rx) = oneshot::channel();
        let tx = Sender { tx: Arc::new(tx) };
        let rx = Receiver { rx };

        (tx, rx)
    }

    impl Receiver {
        /// Blocks the current thread until all `Sender` handles drop.
        ///
        /// If `timeout` is `Some`, the thread is blocked for **at most** `timeout`
        /// duration. If `timeout` is `None`, then the thread is blocked until the
        /// shutdown signal is received.
        ///
        /// If the timeout has elapsed, it returns `false`, otherwise it returns `true`.
        pub(crate) fn wait(&mut self, timeout: Option<Duration>) -> bool {
            use crate::runtime::enter::try_enter;

            if timeout == Some(Duration::from_nanos(0)) {
                return false;
            }

            let mut e = match try_enter(false) {
                Some(enter) => enter,
                _ => {
                    if std::thread::panicking() {
                        // Don't panic in a panic
                        return false;
                    } else {
                        panic!(
                            "Cannot drop a runtime in a context where blocking is not allowed. \
                            This happens when a runtime is dropped from within an asynchronous context."
                        );
                    }
                }
            };

            // The oneshot completes with an Err
            //
            // If blocking fails to wait, this indicates a problem parking the
            // current thread (usually, shutting down a runtime stored in a
            // thread-local).
            if let Some(timeout) = timeout {
                e.block_on_timeout(&mut self.rx, timeout).is_ok()
            } else {
                let _ = e.block_on(&mut self.rx);
                true
            }
        }
    }