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future.rs
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future.rs
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use std::pin::Pin;
use std::marker::Unpin;
// TODO use parking_lot ?
use std::sync::{Arc, Weak, Mutex};
// TODO use parking_lot ?
use std::sync::atomic::{AtomicBool, Ordering};
use futures_core::task::{LocalWaker, Waker};
use futures_core::Poll;
use futures_core::future::Future;
use discard::{Discard, DiscardOnDrop};
#[derive(Debug)]
struct CancelableFutureState {
is_cancelled: AtomicBool,
waker: Mutex<Option<Waker>>,
}
#[derive(Debug)]
pub struct CancelableFutureHandle {
state: Weak<CancelableFutureState>,
}
impl Discard for CancelableFutureHandle {
fn discard(self) {
if let Some(state) = self.state.upgrade() {
let mut lock = state.waker.lock().unwrap();
// TODO verify that this is correct
state.is_cancelled.store(true, Ordering::SeqCst);
if let Some(waker) = lock.take() {
drop(lock);
waker.wake();
}
}
}
}
#[derive(Debug)]
#[must_use = "Futures do nothing unless polled"]
pub struct CancelableFuture<A, B> {
state: Arc<CancelableFutureState>,
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;
// TODO should this inline ?
fn poll(self: Pin<&mut Self>, waker: &LocalWaker) -> Poll<Self::Output> {
unsafe_project!(self => {
mut state,
pin future,
mut when_cancelled,
});
// TODO is this correct ?
if state.is_cancelled.load(Ordering::SeqCst) {
// This is necessary in order to prevent the future from calling `waker.wake()` later
Pin::set(future, None);
let callback = when_cancelled.take().unwrap();
// TODO figure out how to call the callback immediately when discard is called, e.g. using two Arc<Mutex<>>
Poll::Ready(callback())
} else {
match future.as_pin_mut().unwrap().poll(waker) {
Poll::Pending => {
// TODO is this correct ?
*state.waker.lock().unwrap() = Some(waker.clone().into_waker());
Poll::Pending
},
a => a,
}
}
}
}
// TODO figure out a more efficient way to implement this
// TODO replace with futures_util::abortable ?
pub fn cancelable_future<A, B>(future: A, when_cancelled: B) -> (DiscardOnDrop<CancelableFutureHandle>, CancelableFuture<A, B>)
where A: Future,
B: FnOnce() -> A::Output {
let state = Arc::new(CancelableFutureState {
is_cancelled: AtomicBool::new(false),
waker: Mutex::new(None),
});
let cancel_handle = DiscardOnDrop::new(CancelableFutureHandle {
state: Arc::downgrade(&state),
});
let cancel_future = CancelableFuture {
state,
future: Some(future),
when_cancelled: Some(when_cancelled),
};
(cancel_handle, cancel_future)
}