fix(http2/client): avoid double-polling a Select future

src/proto/h2/client.rs

@@ -6,7 +6,7 @@ use crate::rt::{Read, Write};
 use bytes::Bytes;
 use futures_channel::mpsc::{Receiver, Sender};
 use futures_channel::{mpsc, oneshot};
-use futures_util::future::{self, Either, FutureExt as _, Select};
+use futures_util::future::{Either, Fuse, FutureExt as _};
 use futures_util::stream::{StreamExt as _, StreamFuture};
 use h2::client::{Builder, Connection, SendRequest};
 use h2::SendStream;
@@ -245,10 +245,11 @@ pin_project! {
         T: Unpin,
     {
         #[pin]
-        select: Select<ConnMapErr<T, B>, StreamFuture<Receiver<Never>>>,
+        drop_rx: Fuse<StreamFuture<Receiver<Never>>>,
         #[pin]
         cancel_tx: Option<oneshot::Sender<Never>>,
-        conn: Option<ConnMapErr<T, B>>,
+        #[pin]
+        conn: Fuse<ConnMapErr<T, B>>,
     }
 }
 
@@ -263,9 +264,9 @@ where
         cancel_tx: oneshot::Sender<Never>,
     ) -> Self {
         Self {
-            select: future::select(conn, drop_rx),
+            drop_rx: drop_rx.fuse(),
             cancel_tx: Some(cancel_tx),
-            conn: None,
+            conn: conn.fuse(),
         }
     }
 }
@@ -280,25 +281,20 @@ where
     fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
         let mut this = self.project();
 
-        if let Some(conn) = this.conn {
-            conn.poll_unpin(cx).map(|_| ())
-        } else {
-            match ready!(this.select.poll_unpin(cx)) {
-                Either::Left((_, _)) => {
-                    // ok or err, the `conn` has finished
-                    return Poll::Ready(());
-                }
-                Either::Right((_, b)) => {
-                    // mpsc has been dropped, hopefully polling
-                    // the connection some more should start shutdown
-                    // and then close
-                    trace!("send_request dropped, starting conn shutdown");
-                    drop(this.cancel_tx.take().expect("Future polled twice"));
-                    this.conn = &mut Some(b);
-                    return Poll::Pending;
-                }
-            }
-        }
+        if let Poll::Ready(_) = this.conn.poll_unpin(cx) {
+            // ok or err, the `conn` has finished.
+            return Poll::Ready(());
+        };
+
+        if let Poll::Ready(_) = this.drop_rx.poll_unpin(cx) {
+            // mpsc has been dropped, hopefully polling
+            // the connection some more should start shutdown
+            // and then close.
+            trace!("send_request dropped, starting conn shutdown");
+            drop(this.cancel_tx.take().expect("ConnTask Future polled twice"));
+        };
+
+        Poll::Pending
     }
 }