Close TcpStream when client disconnects.

An attempt at a clean shutdown helps the server to drop its connections
more quickly.

Testing this uncovered a latent bug in DelayQueue wherein `poll_expired`
yields `Pending` when empty. A workaround was added to
`InFlightRequests::poll_expired`: check if there are actually any
outstanding requests before calling `DelayQueue::poll_expired`.

tarpc/src/client.rs

@@ -291,6 +291,9 @@ where
     /// Could not flush the transport.
     #[error("could not flush the transport")]
     Flush(#[source] E),
+    /// Could not close the write end of the transport.
+    #[error("could not close the write end of the transport")]
+    Close(#[source] E),
     /// Could not poll expired requests.
     #[error("could not poll expired requests")]
     Timer(#[source] tokio::time::error::Error),
@@ -335,6 +338,15 @@ where
             .map_err(ChannelError::Flush)
     }
 
+    fn poll_close<'a>(
+        self: &'a mut Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), ChannelError<C::Error>>> {
+        self.transport_pin_mut()
+            .poll_close(cx)
+            .map_err(ChannelError::Close)
+    }
+
     fn canceled_requests_mut<'a>(self: &'a mut Pin<&mut Self>) -> &'a mut CanceledRequests {
         self.as_mut().project().canceled_requests
     }
@@ -394,7 +406,7 @@ where
 
         match (pending_requests_status, canceled_requests_status) {
             (ReceiverStatus::Closed, ReceiverStatus::Closed) => {
-                ready!(self.poll_flush(cx)?);
+                ready!(self.poll_close(cx)?);
                 Poll::Ready(None)
             }
             (ReceiverStatus::Pending, _) | (_, ReceiverStatus::Pending) => {

tarpc/src/server.rs

@@ -333,6 +333,7 @@ where
     type Item = Result<TrackedRequest<Req>, ChannelError<T::Error>>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
+        #[derive(Debug)]
         enum ReceiverStatus {
             Ready,
             Pending,
@@ -388,6 +389,11 @@ where
                 Poll::Pending => Pending,
             };
 
+            tracing::trace!(
+                "Expired requests: {:?}, Inbound: {:?}",
+                expiration_status,
+                request_status
+            );
             match (expiration_status, request_status) {
                 (Ready, _) | (_, Ready) => continue,
                 (Closed, Closed) => return Poll::Ready(None),

tarpc/src/server/in_flight_requests.rs

@@ -98,6 +98,11 @@ impl InFlightRequests {
         &mut self,
         cx: &mut Context,
     ) -> Poll<Option<Result<u64, tokio::time::error::Error>>> {
+        if self.deadlines.is_empty() {
+            // TODO(https://github.com/tokio-rs/tokio/issues/4161)
+            // This is a workaround for DelayQueue not always treating this case correctly.
+            return Poll::Ready(None);
+        }
         self.deadlines.poll_expired(cx).map_ok(|expired| {
             if let Some(RequestData {
                 abort_handle, span, ..
@@ -184,12 +189,31 @@ mod tests {
     #[tokio::test]
     async fn remove_request_doesnt_abort() {
         let mut in_flight_requests = InFlightRequests::default();
+        assert!(in_flight_requests.deadlines.is_empty());
+
         let abort_registration = in_flight_requests
-            .start_request(0, SystemTime::now(), Span::current())
+            .start_request(
+                0,
+                SystemTime::now() + std::time::Duration::from_secs(10),
+                Span::current(),
+            )
             .unwrap();
         let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
 
+        // Precondition: Pending expiration
+        assert_matches!(
+            in_flight_requests.poll_expired(&mut noop_context()),
+            Poll::Pending
+        );
+        assert!(!in_flight_requests.deadlines.is_empty());
+
         assert_matches!(in_flight_requests.remove_request(0), Some(_));
+        // Postcondition: No pending expirations
+        assert!(in_flight_requests.deadlines.is_empty());
+        assert_matches!(
+            in_flight_requests.poll_expired(&mut noop_context()),
+            Poll::Ready(None)
+        );
         assert_matches!(
             abortable_future.poll_unpin(&mut noop_context()),
             Poll::Pending