FuturesUnordered: Do not poll the same future twice per iteration

futures-util/src/stream/futures_unordered/mod.rs

@@ -30,22 +30,6 @@ use self::task::Task;
 mod ready_to_run_queue;
 use self::ready_to_run_queue::{ReadyToRunQueue, Dequeue};
 
-/// Constant used for a `FuturesUnordered` to determine how many times it is
-/// allowed to poll underlying futures without yielding.
-///
-/// A single call to `poll_next` may potentially do a lot of work before
-/// yielding. This happens in particular if the underlying futures are awoken
-/// frequently but continue to return `Pending`. This is problematic if other
-/// tasks are waiting on the executor, since they do not get to run. This value
-/// caps the number of calls to `poll` on underlying futures a single call to
-/// `poll_next` is allowed to make.
-///
-/// The value itself is chosen somewhat arbitrarily. It needs to be high enough
-/// that amortize wakeup and scheduling costs, but low enough that we do not
-/// starve other tasks for long.
-///
-/// See also https://github.com/rust-lang/futures-rs/issues/2047.
-const YIELD_EVERY: usize = 32;
 
 /// A set of futures which may complete in any order.
 ///
@@ -414,6 +398,22 @@ impl<Fut: Future> Stream for FuturesUnordered<Fut> {
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>)
         -> Poll<Option<Self::Item>>
     {
+        // Variable to determine how many times it is allowed to poll underlying
+        // futures without yielding.
+        //
+        // A single call to `poll_next` may potentially do a lot of work before
+        // yielding. This happens in particular if the underlying futures are awoken
+        // frequently but continue to return `Pending`. This is problematic if other
+        // tasks are waiting on the executor, since they do not get to run. This value
+        // caps the number of calls to `poll` on underlying futures a single call to
+        // `poll_next` is allowed to make.
+        //
+        // The value is the length of FuturesUnordered. This ensures that each
+        // future is polled only once at most per iteration.
+        //
+        // See also https://github.com/rust-lang/futures-rs/issues/2047.
+        let yield_every = self.len();
+
         // Keep track of how many child futures we have polled,
         // in case we want to forcibly yield.
         let mut polled = 0;
@@ -548,7 +548,7 @@ impl<Fut: Future> Stream for FuturesUnordered<Fut> {
                     let task = bomb.task.take().unwrap();
                     bomb.queue.link(task);
 
-                    if polled == YIELD_EVERY {
+                    if polled == yield_every {
                         // We have polled a large number of futures in a row without yielding.
                         // To ensure we do not starve other tasks waiting on the executor,
                         // we yield here, but immediately wake ourselves up to continue.

futures/tests/futures_unordered.rs

@@ -1,3 +1,10 @@
+use futures::future::Future;
+use futures::stream::{FuturesUnordered, StreamExt};
+use futures::task::{Context, Poll};
+use futures_test::task::noop_context;
+use std::iter::FromIterator;
+use std::pin::Pin;
+
 #[test]
 fn is_terminated() {
     use futures::future;
@@ -270,12 +277,13 @@ fn futures_not_moved_after_poll() {
     use futures::future;
     use futures::stream::FuturesUnordered;
     use futures_test::future::FutureTestExt;
-    use futures_test::{assert_stream_done, assert_stream_next};
+    use futures_test::{assert_stream_done, assert_stream_next, assert_stream_pending};
 
     // Future that will be ready after being polled twice,
     // asserting that it does not move.
     let fut = future::ready(()).pending_once().assert_unmoved();
     let mut stream = vec![fut; 3].into_iter().collect::<FuturesUnordered<_>>();
+    assert_stream_pending!(stream);
     assert_stream_next!(stream, ());
     assert_stream_next!(stream, ());
     assert_stream_next!(stream, ());
@@ -326,3 +334,37 @@ fn len_valid_during_out_of_order_completion() {
     assert_eq!(stream.poll_next_unpin(&mut cx), Poll::Ready(Some(Ok(7))));
     assert_eq!(stream.len(), 0);
 }
+
+#[test]
+fn polled_only_once_at_most_per_iteration() {
+    #[derive(Debug, Clone, Copy, Default)]
+    struct F {
+        polled: bool,
+    }
+
+    impl Future for F {
+        type Output = ();
+
+        fn poll(mut self: Pin<&mut Self>, _: &mut Context) -> Poll<Self::Output> {
+            if self.polled {
+                panic!("polled twice")
+            } else {
+                self.polled = true;
+                Poll::Pending
+            }
+        }
+    }
+
+    let cx = &mut noop_context();
+
+    let mut tasks = FuturesUnordered::from_iter(vec![F::default(); 10]);
+    assert!(tasks.poll_next_unpin(cx).is_pending());
+    assert_eq!(10, tasks.iter().filter(|f| f.polled).count());
+
+    let mut tasks = FuturesUnordered::from_iter(vec![F::default(); 33]);
+    assert!(tasks.poll_next_unpin(cx).is_pending());
+    assert_eq!(33, tasks.iter().filter(|f| f.polled).count());
+
+    let mut tasks = FuturesUnordered::<F>::new();
+    assert_eq!(Poll::Ready(None), tasks.poll_next_unpin(cx));
+}