fix(buffers): make LimitedSender/LimitedReceiver wake up correctly

Signed-off-by: Toby Lawrence <toby@nuclearfurnace.com>

lib/vector-buffers/src/topology/channel/limited_queue.rs

@@ -1,5 +1,5 @@
 use crossbeam_queue::ArrayQueue;
-use futures::{ready, task::AtomicWaker, Sink, Stream};
+use futures::{ready, Sink, Stream};
 use std::{
     cmp, fmt,
     pin::Pin,
@@ -32,7 +32,7 @@ struct Inner<T> {
     data: Arc<ArrayQueue<(OwnedSemaphorePermit, T)>>,
     limit: usize,
     limiter: PollSemaphore,
-    read_waker: Arc<AtomicWaker>,
+    read_waker: PollNotify,
     write_waker: PollNotify,
 }
 
@@ -154,7 +154,7 @@ impl<T: Bufferable> LimitedSender<T> {
                         );
 
                         // Don't forget to wake the reader since there's data to consume now. :)
-                        self.inner.read_waker.wake();
+                        self.inner.read_waker.as_ref().notify_one();
 
                         Poll::Ready(Ok(()))
                     }
@@ -220,24 +220,27 @@ impl<T: Bufferable> LimitedReceiver<T> {
     }
 
     pub fn poll_next(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
-        match self.inner.data.pop() {
-            Some((permit, item)) => {
+        loop {
+            if let Some((permit, item)) = self.inner.data.pop() {
                 // We got an item, woohoo! Now, drop the permit which will properly free up permits
                 // in the semaphore, and then also try to notify a pending writer.
                 drop(permit);
                 self.inner.write_waker.as_ref().notify_one();
-                Poll::Ready(Some(item))
+
+                return Poll::Ready(Some(item));
             }
-            // Figure out if we're actually closed or not, to determine if more items might be
-            // coming or if it's time to also close up shop.
-            None => {
-                if self.inner.limiter.is_closed() {
-                    Poll::Ready(None)
-                } else {
-                    self.inner.read_waker.register(cx.waker());
-                    Poll::Pending
-                }
+
+            // There wasn't an item for us to pop, so see if the channel is actually closed.  If so,
+            // then it's time for us to close up shop as well.
+            if self.inner.limiter.is_closed() {
+                return Poll::Ready(None);
             }
+
+            // We're not closed, so we need to wait for a writer to tell us they made some
+            // progress.  This might end up being a spurious wakeup since `Notify` will
+            // store up to one wakeup that gets consumed by the next call to `poll_notify`,
+            // but alas.
+            ready!(self.inner.read_waker.poll_notify(cx));
         }
     }
 
@@ -271,7 +274,7 @@ impl<T> Drop for LimitedSender<T> {
         // If we're the last sender to drop, close the semaphore on our way out the door.
         if self.sender_count.fetch_sub(1, Ordering::SeqCst) == 1 {
             self.inner.limiter.close();
-            self.inner.read_waker.wake();
+            self.inner.read_waker.as_ref().notify_one();
         }
     }
 }
@@ -281,7 +284,7 @@ pub fn limited<T>(limit: usize) -> (LimitedSender<T>, LimitedReceiver<T>) {
         data: Arc::new(ArrayQueue::new(limit)),
         limit,
         limiter: PollSemaphore::new(Arc::new(Semaphore::new(limit))),
-        read_waker: Arc::new(AtomicWaker::new()),
+        read_waker: PollNotify::new(Arc::new(Notify::new())),
         write_waker: PollNotify::new(Arc::new(Notify::new())),
     };