Backport to 0.1: FuturesUnordered: Do not poll the same future twice per iteration

Same as rust-lang#2333. The same issue exists in 0.1, so backporting it there
helps for code that is still using Futures 0.1 in some places.

Author: krallin

src/stream/futures_unordered.rs

@@ -15,23 +15,6 @@ use {task, Stream, Future, Poll, Async};
 use executor::{Notify, UnsafeNotify, NotifyHandle};
 use task_impl::{self, AtomicTask};
 
-/// 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;
-
 /// An unbounded set of futures.
 ///
 /// This "combinator" also serves a special function in this library, providing
@@ -291,6 +274,22 @@ impl<T> Stream for FuturesUnordered<T>
     type Error = T::Error;
 
     fn poll(&mut self) -> Poll<Option<T::Item>, T::Error> {
+        // 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;
@@ -353,7 +352,7 @@ impl<T> Stream for FuturesUnordered<T>
                 // * The future was extracted above (taken ownership). That way
                 //   if it panics we're guaranteed that the future is
                 //   dropped on this thread and doesn't accidentally get
-                //   dropped on a different thread (bad).
+                //   dropped on YIELD_EVERYa different thread (bad).
                 // * We unlink the node from our internal queue to preemptively
                 //   assume it'll panic, in which case we'll want to discard it
                 //   regardless.
@@ -398,7 +397,7 @@ impl<T> Stream for FuturesUnordered<T>
                         *node.future.get() = Some(future);
                         bomb.queue.link(node);
 
-                        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.

tests/futures_unordered.rs

@@ -5,7 +5,8 @@ extern crate futures;
 use std::any::Any;
 
 use futures::sync::oneshot;
-use futures::stream::futures_unordered;
+use std::iter::FromIterator;
+use futures::stream::{futures_unordered, FuturesUnordered};
 use futures::prelude::*;
 
 mod support;
@@ -127,3 +128,40 @@ fn iter_mut_len() {
     assert_eq!(iter_mut.len(), 0);
     assert!(iter_mut.next().is_none());
 }
+
+#[test]
+fn polled_only_once_at_most_per_iteration() {
+    #[derive(Debug, Clone, Copy, Default)]
+    struct F {
+        polled: bool,
+    }
+
+    impl Future for F {
+        type Item = ();
+        type Error = ();
+
+        fn poll(&mut self) -> Result<Async<Self::Item>, Self::Error> {
+            if self.polled {
+                panic!("polled twice")
+            } else {
+                self.polled = true;
+                Ok(Async::NotReady)
+            }
+        }
+    }
+
+
+    let mut tasks = FuturesUnordered::from_iter(vec![F::default(); 10]);
+    let tasks = futures::executor::spawn(tasks);
+    assert!(tasks.poll_stream_notify(&support::notify_noop(), 0).unwrap().is_not_ready());
+    assert_eq!(10, tasks.get_mut().iter_mut().filter(|f| f.polled).count());
+
+    let mut tasks = FuturesUnordered::from_iter(vec![F::default(); 33]);
+    let tasks = futures::executor::spawn(tasks);
+    assert!(tasks.poll_stream_notify(&support::notify_noop(), 0).unwrap().is_not_ready());
+    assert_eq!(33, tasks.get_mut().iter_mut().filter(|f| f.polled).count());
+
+    let mut tasks = FuturesUnordered::<F>::new();
+    let tasks = futures::executor::spawn(tasks);
+    assert!(tasks.poll_stream_notify(&support::notify_noop(), 0).unwrap().is_ready());
+}