Fix test case since rust-lang/futures-rs#2049

README.md

@@ -65,7 +65,9 @@ and be polled more frequently.
 
 This process can lead to an especially unfortunate cases where a small number of
 tasks can can cause the polling loop of [FuturesUnordered] to
-[spin abnormally]. This issue was [reported by Jon Gjengset].
+[spin abnormally].
+This issue was [reported by Jon Gjengset], and improved on by [limiting the
+amount FuturesUnordered is allowed to spin].
 
 Unicycle addresses this by limiting how frequently a child task may be polled
 per _polling cycle_. This is done by keeping two sets of polling interest and
@@ -75,6 +77,7 @@ wakeups are only registered in the swapped in set which will be polled the next
 cycle. For more details, see the _Architecture_ section below.
 
 [spin abnormally]: https://github.com/udoprog/unicycle/blob/master/tests/spinning_futures_unordered.rs
+[limiting the amount FuturesUnordered is allowed to spin]: https://github.com/rust-lang/futures-rs/pull/2049
 [reported by Jon Gjengset]: https://github.com/rust-lang/futures-rs/issues/2047
 
 ## Architecture

src/lib.rs

@@ -61,7 +61,9 @@
 //!
 //! This process can lead to an especially unfortunate cases where a small number of
 //! tasks can can cause the polling loop of [FuturesUnordered] to
-//! [spin abnormally]. This issue was [reported by Jon Gjengset].
+//! [spin abnormally].
+//! This issue was [reported by Jon Gjengset], and improved on by [limiting the
+//! amount FuturesUnordered is allowed to spin].
 //!
 //! Unicycle addresses this by limiting how frequently a child task may be polled
 //! per _polling cycle_. This is done by keeping two sets of polling interest and
@@ -71,6 +73,7 @@
 //! cycle. For more details, see the _Architecture_ section below.
 //!
 //! [spin abnormally]: https://github.com/udoprog/unicycle/blob/master/tests/spinning_futures_unordered.rs
+//! [limiting the amount FuturesUnordered is allowed to spin]: https://github.com/rust-lang/futures-rs/pull/2049
 //! [reported by Jon Gjengset]: https://github.com/rust-lang/futures-rs/issues/2047
 //!
 //! ## Architecture

tests/spinning_futures_unordered.rs

@@ -3,33 +3,64 @@ use futures::{
     stream::{FuturesUnordered, Stream as _},
 };
 use std::{
+    cell::Cell,
     future::Future,
     pin::Pin,
     task::{Context, Poll},
 };
+use unicycle::Unordered;
 
-struct Spinner;
+struct Spinner<'a>(&'a Cell<usize>);
 
-impl Future for Spinner {
+impl Future for Spinner<'_> {
     type Output = ();
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        println!("Hello!");
+        self.0.set(self.0.get() + 1);
+
+        // Note: this will not be needed once we have a futures release with:
+        // https://github.com/rust-lang/futures-rs/pull/2049
+        if self.0.get() > 16 {
+            return Poll::Ready(());
+        }
+
         cx.waker().wake_by_ref();
         Poll::Pending
     }
 }
 
-#[ignore]
 #[tokio::test]
 async fn test_spinning_futures_unordered() {
+    let count = Cell::new(0);
+
     let futures = FuturesUnordered::new();
-    futures.push(Spinner);
+    futures.push(Spinner(&count));
     pin_utils::pin_mut!(futures);
 
     let _ = poll_fn::<(), _>(move |cx| {
         let _ = Pin::new(&mut futures).poll_next(cx);
-        panic!("We never reach this...");
+        Poll::Ready(())
     })
     .await;
+
+    // Note: FuturesUnordered will spin a bit before yielding.
+    assert!(count.get() > 1);
+}
+
+#[tokio::test]
+async fn test_spinning_unordered() {
+    let count = Cell::new(0);
+
+    let mut futures = Unordered::new();
+    futures.push(Spinner(&count));
+    pin_utils::pin_mut!(futures);
+
+    let _ = poll_fn::<(), _>(move |cx| {
+        let _ = Pin::new(&mut futures).poll_next(cx);
+        Poll::Ready(())
+    })
+    .await;
+
+    // Note: Unicycle guarantees each future is poll at most once.
+    assert_eq!(1, count.get());
 }