tor_rtcompat/
task.rs

1//! Functions for task management that don't belong inside the Runtime
2//! trait.
3
4use std::future::Future;
5use std::pin::Pin;
6use std::task::{Context, Poll};
7
8/// Yield execution back to the runtime temporarily, so that other
9/// tasks can run.
10#[must_use = "yield_now returns a future that must be .awaited on."]
11pub fn yield_now() -> YieldFuture {
12    // TODO: There are functions similar to this in tokio and
13    // async_std and futures_lite.  It would be lovely if futures had
14    // one too.  If it does, we should probably use it.
15    YieldFuture { first_time: true }
16}
17
18/// A future returned by [`yield_now()`].
19///
20/// It returns `Poll::Pending` once, and `Poll::Ready` thereafter.
21#[derive(Debug)]
22#[must_use = "Futures do nothing unless .awaited on."]
23pub struct YieldFuture {
24    /// True if this future has not yet been polled.
25    first_time: bool,
26}
27
28impl Future for YieldFuture {
29    type Output = ();
30    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
31        if self.first_time {
32            self.first_time = false;
33            cx.waker().wake_by_ref();
34            Poll::Pending
35        } else {
36            Poll::Ready(())
37        }
38    }
39}
40
41#[cfg(all(
42    test,
43    any(feature = "native-tls", feature = "rustls"),
44    any(feature = "tokio", feature = "async-std"),
45    not(miri), // this typically results in use of a yield syscall
46))]
47mod test {
48    use super::yield_now;
49    use crate::test_with_all_runtimes;
50
51    use std::sync::atomic::{AtomicBool, Ordering};
52
53    #[test]
54    fn test_yield() {
55        test_with_all_runtimes!(|_| async {
56            let b = AtomicBool::new(false);
57            use Ordering::SeqCst;
58
59            // Both tasks here run in a loop, trying to set 'b' to their
60            // favorite value, and returning once they've done it 10 times.
61            //
62            // Without 'yield_now', one task is likely to monopolize
63            // the scheduler.
64            futures::join!(
65                async {
66                    let mut n = 0_usize;
67                    while n < 10 {
68                        if b.compare_exchange(false, true, SeqCst, SeqCst).is_ok() {
69                            n += 1;
70                        }
71                        yield_now().await;
72                    }
73                },
74                async {
75                    let mut n = 0_usize;
76                    while n < 10 {
77                        if b.compare_exchange(true, false, SeqCst, SeqCst).is_ok() {
78                            n += 1;
79                        }
80                        yield_now().await;
81                    }
82                }
83            );
84            std::io::Result::Ok(())
85        });
86    }
87}
tor_rtcompat/task.rs

tor_rtcompat/
task.rs
