Miri spurious wake up?

[tiif]

Miri unexpectedly hits _ = &mut write_fut => unreachable!(). This is a case of Miri having different behavior from real system.

#[tokio::test]
async fn poll_fns() {
    let (a, b) = socketpair();
    let afd_a = Arc::new(AsyncFd::new(a).unwrap());
    let afd_b = Arc::new(AsyncFd::new(b).unwrap());

    // Fill up the write side of A
    let mut bytes = 0;
    while let Ok(amt) = afd_a.get_ref().write(&[0; 512]) {
        bytes += amt;
    }

    let waker = TestWaker::new();

    assert_pending!(afd_a.as_ref().poll_read_ready(&mut waker.context()));

    let afd_a_2 = afd_a.clone();
    let r_barrier = Arc::new(tokio::sync::Barrier::new(2));
    let barrier_clone = r_barrier.clone();

    let read_fut = tokio::spawn(async move {
        // Move waker onto this task first
        assert_pending!(poll!(std::future::poll_fn(|cx| afd_a_2
            .as_ref()
            .poll_read_ready(cx))));
        barrier_clone.wait().await;

        let _ = std::future::poll_fn(|cx| afd_a_2.as_ref().poll_read_ready(cx)).await;
    });

    let afd_a_2 = afd_a.clone();
    let w_barrier = Arc::new(tokio::sync::Barrier::new(2));
    let barrier_clone = w_barrier.clone();

    let mut write_fut = tokio::spawn(async move {
        // Move waker onto this task first
        assert_pending!(poll!(std::future::poll_fn(|cx| afd_a_2
            .as_ref()
            .poll_write_ready(cx))));
        barrier_clone.wait().await;

        let _ = std::future::poll_fn(|cx| afd_a_2.as_ref().poll_write_ready(cx)).await;
    });

    r_barrier.wait().await;
    w_barrier.wait().await;

    let readable = afd_a.readable();
    tokio::pin!(readable);

    tokio::select! {
        _ = &mut readable => unreachable!(),
        _ = tokio::task::yield_now() => {}
    }

    // Make A readable. We expect that 'readable' and 'read_fut' will both complete quickly
    afd_b.get_ref().write_all(b"0").unwrap();

    let _ = tokio::join!(readable, read_fut);

    // Our original waker should _not_ be awoken (poll_read_ready retains only the last context)
    assert!(!waker.awoken());

    // The writable side should not be awoken
    tokio::select! {
        _ = &mut write_fut => unreachable!(),
        _ = tokio::time::sleep(Duration::from_millis(5)) => {}
    }

    // Make it writable now
    drain(afd_b.get_ref(), bytes);

    // now we should be writable (ie - the waker for poll_write should still be registered after we wake the read side)
    let _ = write_fut.await;
}

rustc version:

rustc 1.89.0-nightly (5d707b07e 2025-06-02)
binary: rustc
commit-hash: 5d707b07e42766c080c5012869c9988a18dcbb83
commit-date: 2025-06-02
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

[tiif]

I suspect it has something to do with the epoll machinery. Currently looking into this.

@rustbot claim

@rustbot label +A-shims +C-bug