async_std/future/future/
try_race.rs

1use std::pin::Pin;
2
3use crate::future::MaybeDone;
4use pin_project_lite::pin_project;
5
6use crate::task::{Context, Poll};
7use std::future::Future;
8
9pin_project! {
10    #[allow(missing_docs)]
11    #[allow(missing_debug_implementations)]
12    pub struct TryRace<L, R>
13    where
14        L: Future,
15        R: Future<Output = L::Output>
16    {
17        #[pin] left: MaybeDone<L>,
18        #[pin] right: MaybeDone<R>,
19    }
20}
21
22impl<L, R> TryRace<L, R>
23where
24    L: Future,
25    R: Future<Output = L::Output>,
26{
27    pub(crate) fn new(left: L, right: R) -> Self {
28        Self {
29            left: MaybeDone::new(left),
30            right: MaybeDone::new(right),
31        }
32    }
33}
34
35impl<L, R, T, E> Future for TryRace<L, R>
36where
37    L: Future<Output = Result<T, E>>,
38    R: Future<Output = L::Output>,
39{
40    type Output = L::Output;
41
42    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
43        let this = self.project();
44        let mut left_errored = false;
45
46        // Check if the left future is ready & successful. Continue if not.
47        let mut left = this.left;
48        if Future::poll(Pin::new(&mut left), cx).is_ready() {
49            if left.as_ref().output().unwrap().is_ok() {
50                return Poll::Ready(left.take().unwrap());
51            } else {
52                left_errored = true;
53            }
54        }
55
56        // Check if the right future is ready & successful. Return err if left
57        // future also resolved to err. Continue if not.
58        let mut right = this.right;
59        let is_ready = Future::poll(Pin::new(&mut right), cx).is_ready();
60        if is_ready && (right.as_ref().output().unwrap().is_ok() || left_errored) {
61            return Poll::Ready(right.take().unwrap());
62        }
63
64        Poll::Pending
65    }
66}
async_std/future/future/try_race.rs

async_std/future/future/
try_race.rs
