impr(utils/spsc_fold): add another test case (#10319)

Wondered about the case covered here while investigating #10309.

libs/utils/src/sync/spsc_fold.rs

@@ -453,4 +453,38 @@ mod tests {
         let err = recv_task.await.unwrap().expect_err("should error");
         assert!(matches!(err, RecvError::SenderGone));
     }
+
+    #[tokio::test(start_paused = true)]
+    async fn test_receiver_drop_while_waiting_for_receiver_to_consume_unblocks_sender() {
+        let (mut sender, receiver) = channel();
+
+        let state = receiver.state.clone();
+
+        sender.send((), |_, _| unreachable!()).await.unwrap();
+
+        assert!(matches!(&*state.value.lock().unwrap(), &State::HasData(_)));
+
+        let unmergeable = sender.send((), |_, _| Err(()));
+        let mut unmergeable = std::pin::pin!(unmergeable);
+        tokio::select! {
+            _ = tokio::time::sleep(FOREVER) => {},
+            _ = &mut unmergeable => {
+                panic!("unmergeable should not complete");
+            },
+        }
+
+        assert!(matches!(
+            &*state.value.lock().unwrap(),
+            &State::SenderWaitsForReceiverToConsume(_)
+        ));
+
+        drop(receiver);
+
+        assert!(matches!(
+            &*state.value.lock().unwrap(),
+            &State::ReceiverGone
+        ));
+
+        unmergeable.await.unwrap_err();
+    }
 }