Add push_front and push_back to FuturesOrdered (#2591)

futures-util/src/stream/futures_ordered.rs

@@ -135,11 +135,39 @@ impl<Fut: Future> FuturesOrdered<Fut> {
     /// This function will not call `poll` on the submitted future. The caller
     /// must ensure that `FuturesOrdered::poll` is called in order to receive
     /// task notifications.
+    #[deprecated(note = "use `push_back` instead")]
     pub fn push(&mut self, future: Fut) {
+        self.push_back(future);
+    }
+
+    /// Pushes a future to the back of the queue.
+    ///
+    /// This function submits the given future to the internal set for managing.
+    /// This function will not call `poll` on the submitted future. The caller
+    /// must ensure that `FuturesOrdered::poll` is called in order to receive
+    /// task notifications.
+    pub fn push_back(&mut self, future: Fut) {
         let wrapped = OrderWrapper { data: future, index: self.next_incoming_index };
         self.next_incoming_index += 1;
         self.in_progress_queue.push(wrapped);
     }
+
+    /// Pushes a future to the front of the queue.
+    ///
+    /// This function submits the given future to the internal set for managing.
+    /// This function will not call `poll` on the submitted future. The caller
+    /// must ensure that `FuturesOrdered::poll` is called in order to receive
+    /// task notifications. This future will be the next future to be returned
+    /// complete.
+    pub fn push_front(&mut self, future: Fut) {
+        if self.next_outgoing_index == 0 {
+            self.push_back(future)
+        } else {
+            let wrapped = OrderWrapper { data: future, index: self.next_outgoing_index - 1 };
+            self.next_outgoing_index -= 1;
+            self.in_progress_queue.push(wrapped);
+        }
+    }
 }
 
 impl<Fut: Future> Default for FuturesOrdered<Fut> {
@@ -196,7 +224,7 @@ impl<Fut: Future> FromIterator<Fut> for FuturesOrdered<Fut> {
     {
         let acc = Self::new();
         iter.into_iter().fold(acc, |mut acc, item| {
-            acc.push(item);
+            acc.push_back(item);
             acc
         })
     }
@@ -214,7 +242,7 @@ impl<Fut: Future> Extend<Fut> for FuturesOrdered<Fut> {
         I: IntoIterator<Item = Fut>,
     {
         for item in iter {
-            self.push(item);
+            self.push_back(item);
         }
     }
 }

futures-util/src/stream/stream/buffered.rs

@@ -64,7 +64,7 @@ where
         // our queue of futures.
         while this.in_progress_queue.len() < *this.max {
             match this.stream.as_mut().poll_next(cx) {
-                Poll::Ready(Some(fut)) => this.in_progress_queue.push(fut),
+                Poll::Ready(Some(fut)) => this.in_progress_queue.push_back(fut),
                 Poll::Ready(None) | Poll::Pending => break,
             }
         }

futures-util/src/stream/try_stream/try_buffered.rs

@@ -54,7 +54,7 @@ where
         // our queue of futures. Propagate errors from the stream immediately.
         while this.in_progress_queue.len() < *this.max {
             match this.stream.as_mut().poll_next(cx)? {
-                Poll::Ready(Some(fut)) => this.in_progress_queue.push(fut.into_future()),
+                Poll::Ready(Some(fut)) => this.in_progress_queue.push_back(fut.into_future()),
                 Poll::Ready(None) | Poll::Pending => break,
             }
         }

futures/tests/stream_futures_ordered.rs

@@ -2,6 +2,7 @@ use futures::channel::oneshot;
 use futures::executor::{block_on, block_on_stream};
 use futures::future::{self, join, Future, FutureExt, TryFutureExt};
 use futures::stream::{FuturesOrdered, StreamExt};
+use futures::task::Poll;
 use futures_test::task::noop_context;
 use std::any::Any;
 
@@ -45,6 +46,69 @@ fn works_2() {
     assert!(stream.poll_next_unpin(&mut cx).is_ready());
 }
 
+#[test]
+fn test_push_front() {
+    let (a_tx, a_rx) = oneshot::channel::<i32>();
+    let (b_tx, b_rx) = oneshot::channel::<i32>();
+    let (c_tx, c_rx) = oneshot::channel::<i32>();
+    let (d_tx, d_rx) = oneshot::channel::<i32>();
+
+    let mut stream = FuturesOrdered::new();
+
+    let mut cx = noop_context();
+
+    stream.push_back(a_rx);
+    stream.push_back(b_rx);
+    stream.push_back(c_rx);
+
+    a_tx.send(1).unwrap();
+    b_tx.send(2).unwrap();
+    c_tx.send(3).unwrap();
+
+    // 1 and 2 should be received in order
+    assert_eq!(Poll::Ready(Some(Ok(1))), stream.poll_next_unpin(&mut cx));
+    assert_eq!(Poll::Ready(Some(Ok(2))), stream.poll_next_unpin(&mut cx));
+
+    stream.push_front(d_rx);
+    d_tx.send(4).unwrap();
+
+    // we pushed `d_rx` to the front and sent 4, so we should recieve 4 next
+    // and then 3 after it
+    assert_eq!(Poll::Ready(Some(Ok(4))), stream.poll_next_unpin(&mut cx));
+    assert_eq!(Poll::Ready(Some(Ok(3))), stream.poll_next_unpin(&mut cx));
+}
+
+#[test]
+fn test_push_back() {
+    let (a_tx, a_rx) = oneshot::channel::<i32>();
+    let (b_tx, b_rx) = oneshot::channel::<i32>();
+    let (c_tx, c_rx) = oneshot::channel::<i32>();
+    let (d_tx, d_rx) = oneshot::channel::<i32>();
+
+    let mut stream = FuturesOrdered::new();
+
+    let mut cx = noop_context();
+
+    stream.push_back(a_rx);
+    stream.push_back(b_rx);
+    stream.push_back(c_rx);
+
+    a_tx.send(1).unwrap();
+    b_tx.send(2).unwrap();
+    c_tx.send(3).unwrap();
+
+    // All results should be received in order
+
+    assert_eq!(Poll::Ready(Some(Ok(1))), stream.poll_next_unpin(&mut cx));
+    assert_eq!(Poll::Ready(Some(Ok(2))), stream.poll_next_unpin(&mut cx));
+
+    stream.push_back(d_rx);
+    d_tx.send(4).unwrap();
+
+    assert_eq!(Poll::Ready(Some(Ok(3))), stream.poll_next_unpin(&mut cx));
+    assert_eq!(Poll::Ready(Some(Ok(4))), stream.poll_next_unpin(&mut cx));
+}
+
 #[test]
 fn from_iterator() {
     let stream = vec![future::ready::<i32>(1), future::ready::<i32>(2), future::ready::<i32>(3)]