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    //! A one-shot, futures-aware channel
    //!
    //! This channel is similar to that in `sync::oneshot` but cannot be sent across
    //! threads.

    use std::cell::{Cell, RefCell};
    use std::fmt;
    use std::rc::{Rc, Weak};

    use {Future, Poll, Async};
    use future::{Executor, IntoFuture, Lazy, lazy};
    use task::{self, Task};

    /// Creates a new futures-aware, one-shot channel.
    ///
    /// This function is the same as `sync::oneshot::channel` except that the
    /// returned values cannot be sent across threads.
    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let inner = Rc::new(RefCell::new(Inner {
            value: None,
            tx_task: None,
            rx_task: None,
        }));
        let tx = Sender {
            inner: Rc::downgrade(&inner),
        };
        let rx = Receiver {
            state: State::Open(inner),
        };
        (tx, rx)
    }

    /// Represents the completion half of a oneshot through which the result of a
    /// computation is signaled.
    ///
    /// This is created by the `unsync::oneshot::channel` function and is equivalent
    /// in functionality to `sync::oneshot::Sender` except that it cannot be sent
    /// across threads.
    #[derive(Debug)]
    pub struct Sender<T> {
        inner: Weak<RefCell<Inner<T>>>,
    }

    /// A future representing the completion of a computation happening elsewhere in
    /// memory.
    ///
    /// This is created by the `unsync::oneshot::channel` function and is equivalent
    /// in functionality to `sync::oneshot::Receiver` except that it cannot be sent
    /// across threads.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Receiver<T> {
        state: State<T>,
    }

    #[derive(Debug)]
    enum State<T> {
        Open(Rc<RefCell<Inner<T>>>),
        Closed(Option<T>),
    }

    pub use sync::oneshot::Canceled;

    #[derive(Debug)]
    struct Inner<T> {
        value: Option<T>,
        tx_task: Option<Task>,
        rx_task: Option<Task>,
    }

    impl<T> Sender<T> {
        /// Completes this oneshot with a successful result.
        ///
        /// This function will consume `self` and indicate to the other end, the
        /// `Receiver`, that the error provided is the result of the computation this
        /// represents.
        ///
        /// If the value is successfully enqueued for the remote end to receive,
        /// then `Ok(())` is returned. If the receiving end was deallocated before
        /// this function was called, however, then `Err` is returned with the value
        /// provided.
        pub fn send(self, val: T) -> Result<(), T> {
            if let Some(inner) = self.inner.upgrade() {
                inner.borrow_mut().value = Some(val);
                Ok(())
            } else {
                Err(val)
            }
        }

        /// Polls this `Sender` half to detect whether the `Receiver` this has
        /// paired with has gone away.
        ///
        /// This function can be used to learn about when the `Receiver` (consumer)
        /// half has gone away and nothing will be able to receive a message sent
        /// from `complete`.
        ///
        /// Like `Future::poll`, this function will panic if it's not called from
        /// within the context of a task. In other words, this should only ever be
        /// called from inside another future.
        ///
        /// If `Ready` is returned then it means that the `Receiver` has disappeared
        /// and the result this `Sender` would otherwise produce should no longer
        /// be produced.
        ///
        /// If `NotReady` is returned then the `Receiver` is still alive and may be
        /// able to receive a message if sent. The current task, however, is
        /// scheduled to receive a notification if the corresponding `Receiver` goes
        /// away.
        pub fn poll_cancel(&mut self) -> Poll<(), ()> {
            match self.inner.upgrade() {
                Some(inner) => {
                    inner.borrow_mut().tx_task = Some(task::current());
                    Ok(Async::NotReady)
                }
                None => Ok(().into()),
            }
        }

        /// Tests to see whether this `Sender`'s corresponding `Receiver`
        /// has gone away.
        ///
        /// This function can be used to learn about when the `Receiver` (consumer)
        /// half has gone away and nothing will be able to receive a message sent
        /// from `send`.
        ///
        /// Note that this function is intended to *not* be used in the context of a
        /// future. If you're implementing a future you probably want to call the
        /// `poll_cancel` function which will block the current task if the
        /// cancellation hasn't happened yet. This can be useful when working on a
        /// non-futures related thread, though, which would otherwise panic if
        /// `poll_cancel` were called.
        pub fn is_canceled(&self) -> bool {
            !self.inner.upgrade().is_some()
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            let inner = match self.inner.upgrade() {
                Some(inner) => inner,
                None => return,
            };
            let rx_task = {
                let mut borrow = inner.borrow_mut();
                borrow.tx_task.take();
                borrow.rx_task.take()
            };
            if let Some(task) = rx_task {
                task.notify();
            }
        }
    }

    impl<T> Receiver<T> {
        /// Gracefully close this receiver, preventing sending any future messages.
        ///
        /// Any `send` operation which happens after this method returns is
        /// guaranteed to fail. Once this method is called the normal `poll` method
        /// can be used to determine whether a message was actually sent or not. If
        /// `Canceled` is returned from `poll` then no message was sent.
        pub fn close(&mut self) {
            let (item, task) = match self.state {
                State::Open(ref inner) => {
                    let mut inner = inner.borrow_mut();
                    drop(inner.rx_task.take());
                    (inner.value.take(), inner.tx_task.take())
                }
                State::Closed(_) => return,
            };
            self.state = State::Closed(item);
            if let Some(task) = task {
                task.notify();
            }
        }
    }

    impl<T> Future for Receiver<T> {
        type Item = T;
        type Error = Canceled;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            let inner = match self.state {
                State::Open(ref mut inner) => inner,
                State::Closed(ref mut item) => {
                    match item.take() {
                        Some(item) => return Ok(item.into()),
                        None => return Err(Canceled),
                    }
                }
            };

            // If we've got a value, then skip the logic below as we're done.
            if let Some(val) = inner.borrow_mut().value.take() {
                return Ok(Async::Ready(val))
            }

            // If we can get mutable access, then the sender has gone away. We
            // didn't see a value above, so we're canceled. Otherwise we park
            // our task and wait for a value to come in.
            if Rc::get_mut(inner).is_some() {
                Err(Canceled)
            } else {
                inner.borrow_mut().rx_task = Some(task::current());
                Ok(Async::NotReady)
            }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            self.close();
        }
    }

    /// Handle returned from the `spawn` function.
    ///
    /// This handle is a future representing the completion of a different future on
    /// a separate executor. Created through the `oneshot::spawn` function this
    /// handle will resolve when the future provided to `spawn` resolves on the
    /// `Executor` instance provided to that function.
    ///
    /// If this handle is dropped then the future will automatically no longer be
    /// polled and is scheduled to be dropped. This can be canceled with the
    /// `forget` function, however.
    pub struct SpawnHandle<T, E> {
        rx: Receiver<Result<T, E>>,
        keep_running: Rc<Cell<bool>>,
    }

    /// Type of future which `Spawn` instances below must be able to spawn.
    pub struct Execute<F: Future> {
        future: F,
        tx: Option<Sender<Result<F::Item, F::Error>>>,
        keep_running: Rc<Cell<bool>>,
    }

    /// Spawns a `future` onto the instance of `Executor` provided, `executor`,
    /// returning a handle representing the completion of the future.
    ///
    /// The `SpawnHandle` returned is a future that is a proxy for `future` itself.
    /// When `future` completes on `executor` then the `SpawnHandle` will itself be
    /// resolved.  Internally `SpawnHandle` contains a `oneshot` channel and is
    /// thus not safe to send across threads.
    ///
    /// The `future` will be canceled if the `SpawnHandle` is dropped. If this is
    /// not desired then the `SpawnHandle::forget` function can be used to continue
    /// running the future to completion.
    ///
    /// # Panics
    ///
    /// This function will panic if the instance of `Spawn` provided is unable to
    /// spawn the `future` provided.
    ///
    /// If the provided instance of `Spawn` does not actually run `future` to
    /// completion, then the returned handle may panic when polled. Typically this
    /// is not a problem, though, as most instances of `Spawn` will run futures to
    /// completion.
    pub fn spawn<F, E>(future: F, executor: &E) -> SpawnHandle<F::Item, F::Error>
        where F: Future,
              E: Executor<Execute<F>>,
    {
        let flag = Rc::new(Cell::new(false));
        let (tx, rx) = channel();
        executor.execute(Execute {
            future: future,
            tx: Some(tx),
            keep_running: flag.clone(),
        }).expect("failed to spawn future");
        SpawnHandle {
            rx: rx,
            keep_running: flag,
        }
    }

    /// Spawns a function `f` onto the `Spawn` instance provided `s`.
    ///
    /// For more information see the `spawn` function in this module. This function
    /// is just a thin wrapper around `spawn` which will execute the closure on the
    /// executor provided and then complete the future that the closure returns.
    pub fn spawn_fn<F, R, E>(f: F, executor: &E) -> SpawnHandle<R::Item, R::Error>
        where F: FnOnce() -> R,
              R: IntoFuture,
              E: Executor<Execute<Lazy<F, R>>>,
    {
        spawn(lazy(f), executor)
    }

    impl<T, E> SpawnHandle<T, E> {
        /// Drop this future without canceling the underlying future.
        ///
        /// When `SpawnHandle` is dropped, the spawned future will be canceled as
        /// well if the future hasn't already resolved. This function can be used
        /// when to drop this future but keep executing the underlying future.
        pub fn forget(self) {
            self.keep_running.set(true);
        }
    }

    impl<T, E> Future for SpawnHandle<T, E> {
        type Item = T;
        type Error = E;

        fn poll(&mut self) -> Poll<T, E> {
            match self.rx.poll() {
                Ok(Async::Ready(Ok(t))) => Ok(t.into()),
                Ok(Async::Ready(Err(e))) => Err(e),
                Ok(Async::NotReady) => Ok(Async::NotReady),
                Err(_) => panic!("future was canceled before completion"),
            }
        }
    }

    impl<T: fmt::Debug, E: fmt::Debug> fmt::Debug for SpawnHandle<T, E> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("SpawnHandle")
             .finish()
        }
    }

    impl<F: Future> Future for Execute<F> {
        type Item = ();
        type Error = ();

        fn poll(&mut self) -> Poll<(), ()> {
            // If we're canceled then we may want to bail out early.
            //
            // If the `forget` function was called, though, then we keep going.
            if self.tx.as_mut().unwrap().poll_cancel().unwrap().is_ready() {
                if !self.keep_running.get() {
                    return Ok(().into())
                }
            }

            let result = match self.future.poll() {
                Ok(Async::NotReady) => return Ok(Async::NotReady),
                Ok(Async::Ready(t)) => Ok(t),
                Err(e) => Err(e),
            };
            drop(self.tx.take().unwrap().send(result));
            Ok(().into())
        }
    }

    impl<F: Future + fmt::Debug> fmt::Debug for Execute<F> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Execute")
             .field("future", &self.future)
             .finish()
        }
    }

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