pin-project 0.1.1

#![feature(futures_api)]
#![deny(warnings)]

// These codes copied from https://github.com/Pauan/rust-signals/blob/master/src/future.rs

// MIT License

// Copyright (c) 2018

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

use std::{
    future::Future,
    marker::Unpin,
    pin::Pin,
    sync::{
        atomic::{AtomicBool, Ordering},
        Arc, Mutex,
    },
    task::{LocalWaker, Poll, Waker},
};

use pin_project::unsafe_project;

#[derive(Debug)]
struct CancelableFutureState {
    is_cancelled: AtomicBool,
    waker: Mutex<Option<Waker>>,
}

#[unsafe_project]
#[derive(Debug)]
#[must_use = "Futures do nothing unless polled"]
pub struct CancelableFuture<A, B> {
    state: Arc<CancelableFutureState>,
    #[pin]
    future: Option<A>,
    when_cancelled: Option<B>,
}

impl<A, B> Unpin for CancelableFuture<A, B> where A: Unpin {}

impl<A, B> Future for CancelableFuture<A, B>
where
    A: Future,
    B: FnOnce() -> A::Output,
{
    type Output = A::Output;

    fn poll(self: Pin<&mut Self>, waker: &LocalWaker) -> Poll<Self::Output> {
        let this = self.project();
        if this.state.is_cancelled.load(Ordering::SeqCst) {
            Pin::set(this.future, None);
            let callback = this.when_cancelled.take().unwrap();
            Poll::Ready(callback())
        } else {
            match this.future.as_pin_mut().unwrap().poll(waker) {
                Poll::Pending => {
                    *this.state.waker.lock().unwrap() = Some(waker.clone().into_waker());
                    Poll::Pending
                }
                a => a,
            }
        }
    }
}