Fix incorrect termination of select_with_strategy streams (#2635)

Author: Sushisource

futures-util/src/stream/select_with_strategy.rs

@@ -231,18 +231,23 @@ where
     St1: Stream,
     St2: Stream<Item = St1::Item>,
 {
-    match poll_side(select, side, cx) {
+    let first_done = match poll_side(select, side, cx) {
         Poll::Ready(Some(item)) => return Poll::Ready(Some(item)),
         Poll::Ready(None) => {
             select.internal_state.finish(side);
+            true
         }
-        Poll::Pending => (),
+        Poll::Pending => false,
     };
     let other = side.other();
     match poll_side(select, other, cx) {
         Poll::Ready(None) => {
             select.internal_state.finish(other);
-            Poll::Ready(None)
+            if first_done {
+                Poll::Ready(None)
+            } else {
+                Poll::Pending
+            }
         }
         a => a,
     }

futures/tests/stream.rs

@@ -1,5 +1,9 @@
+use std::cell::Cell;
 use std::iter;
+use std::pin::Pin;
+use std::rc::Rc;
 use std::sync::Arc;
+use std::task::Context;
 
 use futures::channel::mpsc;
 use futures::executor::block_on;
@@ -9,6 +13,7 @@ use futures::sink::SinkExt;
 use futures::stream::{self, StreamExt};
 use futures::task::Poll;
 use futures::{ready, FutureExt};
+use futures_core::Stream;
 use futures_test::task::noop_context;
 
 #[test]
@@ -419,3 +424,40 @@ fn ready_chunks() {
         assert_eq!(s.next().await.unwrap(), vec![4]);
     });
 }
+
+struct SlowStream {
+    times_should_poll: usize,
+    times_polled: Rc<Cell<usize>>,
+}
+impl Stream for SlowStream {
+    type Item = usize;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        self.times_polled.set(self.times_polled.get() + 1);
+        if self.times_polled.get() % 2 == 0 {
+            cx.waker().wake_by_ref();
+            return Poll::Pending;
+        }
+        if self.times_polled.get() >= self.times_should_poll {
+            return Poll::Ready(None);
+        }
+        Poll::Ready(Some(self.times_polled.get()))
+    }
+}
+
+#[test]
+fn select_with_strategy_doesnt_terminate_early() {
+    for side in [stream::PollNext::Left, stream::PollNext::Right] {
+        let times_should_poll = 10;
+        let count = Rc::new(Cell::new(0));
+        let b = stream::iter([10, 20]);
+
+        let mut selected = stream::select_with_strategy(
+            SlowStream { times_should_poll, times_polled: count.clone() },
+            b,
+            |_: &mut ()| side,
+        );
+        block_on(async move { while selected.next().await.is_some() {} });
+        assert_eq!(count.get(), times_should_poll + 1);
+    }
+}