Async Rust: The example of "The Waker Contract" may lose the signal of wake

[xmh0511]

In this example:

use std::task::{Context, Poll, Waker};
use std::pin::Pin;
use std::future::Future;
use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;

/// A future that completes after a delay (toy implementation)
struct Delay {
    completed: Arc<Mutex<bool>>,
    waker_stored: Arc<Mutex<Option<Waker>>>,
    duration: Duration,
    started: bool,
}

impl Delay {
    fn new(duration: Duration) -> Self {
        Delay {
            completed: Arc::new(Mutex::new(false)),
            waker_stored: Arc::new(Mutex::new(None)),
            duration,
            started: false,
        }
    }
}

impl Future for Delay {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        // Check if already completed
        if *self.completed.lock().unwrap() {
            return Poll::Ready(());
        }

        // Store the waker so the background thread can wake us
        *self.waker_stored.lock().unwrap() = Some(cx.waker().clone()); // #1

        // Start the background timer on first poll
        if !self.started {
            self.started = true;
            let completed = Arc::clone(&self.completed);
            let waker = Arc::clone(&self.waker_stored);
            let duration = self.duration;

            thread::spawn(move || {
                thread::sleep(duration);
                *completed.lock().unwrap() = true;

                // CRITICAL: wake the executor so it polls us again
                if let Some(w) = waker.lock().unwrap().take() { // #2
                    w.wake(); // "Hey executor, I'm ready — poll me again!"
                }
            });
        }

        Poll::Pending // Not done yet
    }
}

According to the document of will_wake, the executor can give the new waker such that the saved old waker cannot correctly wake the future. For example, #2 happens-before #1 in the next invocation to poll that passed in a new waker, this would make w.wake() after #2 failure.

For this case, we need to double-check completed at the end of poll.