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    //! A one-shot, futures-aware channel

    use std::sync::Arc;
    use std::sync::atomic::AtomicBool;
    use std::sync::atomic::Ordering::SeqCst;
    use std::error::Error;
    use std::fmt;

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

    /// A future representing the completion of a computation happening elsewhere in
    /// memory.
    ///
    /// This is created by the `oneshot::channel` function.
    #[must_use = "futures do nothing unless polled"]
    #[derive(Debug)]
    pub struct Receiver<T> {
        inner: Arc<Inner<T>>,
    }

    /// Represents the completion half of a oneshot through which the result of a
    /// computation is signaled.
    ///
    /// This is created by the `oneshot::channel` function.
    #[derive(Debug)]
    pub struct Sender<T> {
        inner: Arc<Inner<T>>,
    }

    /// Internal state of the `Receiver`/`Sender` pair above. This is all used as
    /// the internal synchronization between the two for send/recv operations.
    #[derive(Debug)]
    struct Inner<T> {
        /// Indicates whether this oneshot is complete yet. This is filled in both
        /// by `Sender::drop` and by `Receiver::drop`, and both sides interpret it
        /// appropriately.
        ///
        /// For `Receiver`, if this is `true`, then it's guaranteed that `data` is
        /// unlocked and ready to be inspected.
        ///
        /// For `Sender` if this is `true` then the oneshot has gone away and it
        /// can return ready from `poll_cancel`.
        complete: AtomicBool,

        /// The actual data being transferred as part of this `Receiver`. This is
        /// filled in by `Sender::complete` and read by `Receiver::poll`.
        ///
        /// Note that this is protected by `Lock`, but it is in theory safe to
        /// replace with an `UnsafeCell` as it's actually protected by `complete`
        /// above. I wouldn't recommend doing this, however, unless someone is
        /// supremely confident in the various atomic orderings here and there.
        data: Lock<Option<T>>,

        /// Field to store the task which is blocked in `Receiver::poll`.
        ///
        /// This is filled in when a oneshot is polled but not ready yet. Note that
        /// the `Lock` here, unlike in `data` above, is important to resolve races.
        /// Both the `Receiver` and the `Sender` halves understand that if they
        /// can't acquire the lock then some important interference is happening.
        rx_task: Lock<Option<Task>>,

        /// Like `rx_task` above, except for the task blocked in
        /// `Sender::poll_cancel`. Additionally, `Lock` cannot be `UnsafeCell`.
        tx_task: Lock<Option<Task>>,
    }

    /// Creates a new futures-aware, one-shot channel.
    ///
    /// This function is similar to Rust's channels found in the standard library.
    /// Two halves are returned, the first of which is a `Sender` handle, used to
    /// signal the end of a computation and provide its value. The second half is a
    /// `Receiver` which implements the `Future` trait, resolving to the value that
    /// was given to the `Sender` handle.
    ///
    /// Each half can be separately owned and sent across threads/tasks.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use futures::sync::oneshot;
    /// use futures::*;
    ///
    /// let (p, c) = oneshot::channel::<i32>();
    ///
    /// thread::spawn(|| {
    ///     c.map(|i| {
    ///         println!("got: {}", i);
    ///     }).wait();
    /// });
    ///
    /// p.send(3).unwrap();
    /// ```
    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let inner = Arc::new(Inner::new());
        let receiver = Receiver {
            inner: inner.clone(),
        };
        let sender = Sender {
            inner: inner,
        };
        (sender, receiver)
    }

    impl<T> Inner<T> {
        fn new() -> Inner<T> {
            Inner {
                complete: AtomicBool::new(false),
                data: Lock::new(None),
                rx_task: Lock::new(None),
                tx_task: Lock::new(None),
            }
        }

        fn send(&self, t: T) -> Result<(), T> {
            if self.complete.load(SeqCst) {
                return Err(t)
            }

            // Note that this lock acquisition may fail if the receiver
            // is closed and sets the `complete` flag to true, whereupon
            // the receiver may call `poll()`.
            if let Some(mut slot) = self.data.try_lock() {
                assert!(slot.is_none());
                *slot = Some(t);
                drop(slot);

                // If the receiver called `close()` between the check at the
                // start of the function, and the lock being released, then
                // the receiver may not be around to receive it, so try to
                // pull it back out.
                if self.complete.load(SeqCst) {
                    // If lock acquisition fails, then receiver is actually
                    // receiving it, so we're good.
                    if let Some(mut slot) = self.data.try_lock() {
                        if let Some(t) = slot.take() {
                            return Err(t);
                        }
                    }
                }
                Ok(())
            } else {
                // Must have been closed
                Err(t)
            }
        }

        fn poll_cancel(&self) -> Poll<(), ()> {
            // Fast path up first, just read the flag and see if our other half is
            // gone. This flag is set both in our destructor and the oneshot
            // destructor, but our destructor hasn't run yet so if it's set then the
            // oneshot is gone.
            if self.complete.load(SeqCst) {
                return Ok(Async::Ready(()))
            }

            // If our other half is not gone then we need to park our current task
            // and move it into the `notify_cancel` slot to get notified when it's
            // actually gone.
            //
            // If `try_lock` fails, then the `Receiver` is in the process of using
            // it, so we can deduce that it's now in the process of going away and
            // hence we're canceled. If it succeeds then we just store our handle.
            //
            // Crucially we then check `oneshot_gone` *again* before we return.
            // While we were storing our handle inside `notify_cancel` the `Receiver`
            // may have been dropped. The first thing it does is set the flag, and
            // if it fails to acquire the lock it assumes that we'll see the flag
            // later on. So... we then try to see the flag later on!
            let handle = task::current();
            match self.tx_task.try_lock() {
                Some(mut p) => *p = Some(handle),
                None => return Ok(Async::Ready(())),
            }
            if self.complete.load(SeqCst) {
                Ok(Async::Ready(()))
            } else {
                Ok(Async::NotReady)
            }
        }

        fn is_canceled(&self) -> bool {
            self.complete.load(SeqCst)
        }

        fn drop_tx(&self) {
            // Flag that we're a completed `Sender` and try to wake up a receiver.
            // Whether or not we actually stored any data will get picked up and
            // translated to either an item or cancellation.
            //
            // Note that if we fail to acquire the `rx_task` lock then that means
            // we're in one of two situations:
            //
            // 1. The receiver is trying to block in `poll`
            // 2. The receiver is being dropped
            //
            // In the first case it'll check the `complete` flag after it's done
            // blocking to see if it succeeded. In the latter case we don't need to
            // wake up anyone anyway. So in both cases it's ok to ignore the `None`
            // case of `try_lock` and bail out.
            //
            // The first case crucially depends on `Lock` using `SeqCst` ordering
            // under the hood. If it instead used `Release` / `Acquire` ordering,
            // then it would not necessarily synchronize with `inner.complete`
            // and deadlock might be possible, as was observed in
            // https://github.com/rust-lang-nursery/futures-rs/pull/219.
            self.complete.store(true, SeqCst);
            if let Some(mut slot) = self.rx_task.try_lock() {
                if let Some(task) = slot.take() {
                    drop(slot);
                    task.notify();
                }
            }
        }

        fn close_rx(&self) {
            // Flag our completion and then attempt to wake up the sender if it's
            // blocked. See comments in `drop` below for more info
            self.complete.store(true, SeqCst);
            if let Some(mut handle) = self.tx_task.try_lock() {
                if let Some(task) = handle.take() {
                    drop(handle);
                    task.notify()
                }
            }
        }

        fn recv(&self) -> Poll<T, Canceled> {
            let mut done = false;

            // Check to see if some data has arrived. If it hasn't then we need to
            // block our task.
            //
            // Note that the acquisition of the `rx_task` lock might fail below, but
            // the only situation where this can happen is during `Sender::drop`
            // when we are indeed completed already. If that's happening then we
            // know we're completed so keep going.
            if self.complete.load(SeqCst) {
                done = true;
            } else {
                let task = task::current();
                match self.rx_task.try_lock() {
                    Some(mut slot) => *slot = Some(task),
                    None => done = true,
                }
            }

            // If we're `done` via one of the paths above, then look at the data and
            // figure out what the answer is. If, however, we stored `rx_task`
            // successfully above we need to check again if we're completed in case
            // a message was sent while `rx_task` was locked and couldn't notify us
            // otherwise.
            //
            // If we're not done, and we're not complete, though, then we've
            // successfully blocked our task and we return `NotReady`.
            if done || self.complete.load(SeqCst) {
                // If taking the lock fails, the sender will realise that the we're
                // `done` when it checks the `complete` flag on the way out, and will
                // treat the send as a failure.
                if let Some(mut slot) = self.data.try_lock() {
                    if let Some(data) = slot.take() {
                        return Ok(data.into());
                    }
                }
                Err(Canceled)
            } else {
                Ok(Async::NotReady)
            }
        }

        fn drop_rx(&self) {
            // Indicate to the `Sender` that we're done, so any future calls to
            // `poll_cancel` are weeded out.
            self.complete.store(true, SeqCst);

            // If we've blocked a task then there's no need for it to stick around,
            // so we need to drop it. If this lock acquisition fails, though, then
            // it's just because our `Sender` is trying to take the task, so we
            // let them take care of that.
            if let Some(mut slot) = self.rx_task.try_lock() {
                let task = slot.take();
                drop(slot);
                drop(task);
            }

            // Finally, if our `Sender` wants to get notified of us going away, it
            // would have stored something in `tx_task`. Here we try to peel that
            // out and unpark it.
            //
            // Note that the `try_lock` here may fail, but only if the `Sender` is
            // in the process of filling in the task. If that happens then we
            // already flagged `complete` and they'll pick that up above.
            if let Some(mut handle) = self.tx_task.try_lock() {
                if let Some(task) = handle.take() {
                    drop(handle);
                    task.notify()
                }
            }
        }
    }

    impl<T> Sender<T> {
        #[deprecated(note = "renamed to `send`", since = "0.1.11")]
        #[doc(hidden)]
        #[cfg(feature = "with-deprecated")]
        pub fn complete(self, t: T) {
            drop(self.send(t));
        }

        /// Completes this oneshot with a successful result.
        ///
        /// This function will consume `self` and indicate to the other end, the
        /// `Receiver`, that the value 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, t: T) -> Result<(), T> {
            self.inner.send(t)
        }

        /// 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 `send`.
        ///
        /// 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.
        ///
        /// # Panics
        ///
        /// 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 you're calling this function from a context that does not have a
        /// task, then you can use the `is_canceled` API instead.
        pub fn poll_cancel(&mut self) -> Poll<(), ()> {
            self.inner.poll_cancel()
        }

        /// 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.is_canceled()
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            self.inner.drop_tx()
        }
    }

    /// Error returned from a `Receiver<T>` whenever the corresponding `Sender<T>`
    /// is dropped.
    #[derive(Clone, Copy, PartialEq, Eq, Debug)]
    pub struct Canceled;

    impl fmt::Display for Canceled {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            write!(fmt, "oneshot canceled")
        }
    }

    impl Error for Canceled {
        fn description(&self) -> &str {
            "oneshot canceled"
        }
    }

    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) {
            self.inner.close_rx()
        }
    }

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

        fn poll(&mut self) -> Poll<T, Canceled> {
            self.inner.recv()
        }
    }

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

    /// 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: Arc<ExecuteInner<Result<T, E>>>,
    }

    struct ExecuteInner<T> {
        inner: Inner<T>,
        keep_running: AtomicBool,
    }

    /// Type of future which `Execute` instances below must be able to spawn.
    pub struct Execute<F: Future> {
        future: F,
        tx: Arc<ExecuteInner<Result<F::Item, F::Error>>>,
    }

    /// 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 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.
    ///
    /// Note that the returned future will likely panic if the `futures` provided
    /// panics. If a future running on an executor panics that typically means that
    /// the executor drops the future, which falls into the above case of not
    /// running the future to completion essentially.
    pub fn spawn<F, E>(future: F, executor: &E) -> SpawnHandle<F::Item, F::Error>
        where F: Future,
              E: Executor<Execute<F>>,
    {
        let data = Arc::new(ExecuteInner {
            inner: Inner::new(),
            keep_running: AtomicBool::new(false),
        });
        executor.execute(Execute {
            future: future,
            tx: data.clone(),
        }).expect("failed to spawn future");
        SpawnHandle { rx: data }
    }

    /// 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.rx.keep_running.store(true, SeqCst);
        }
    }

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

        fn poll(&mut self) -> Poll<T, E> {
            match self.rx.inner.recv() {
                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<T, E> Drop for SpawnHandle<T, E> {
        fn drop(&mut self) {
            self.rx.inner.drop_rx();
        }
    }

    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.inner.poll_cancel().unwrap().is_ready() {
                if !self.tx.keep_running.load(SeqCst) {
                    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.inner.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()
        }
    }

    impl<F: Future> Drop for Execute<F> {
        fn drop(&mut self) {
            self.tx.inner.drop_tx();
        }
    }

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