update example

actix-rt/examples/multi_thread_system.rs

@@ -17,44 +17,26 @@ fn main() {
 
 // async main function that acts like #[actix_web::main] or #[tokio::main]
 async fn async_main() {
-    let (tx, rx) = tokio::sync::oneshot::channel();
-
-    // get a handle to system arbiter and spawn async task on it
-    System::current().arbiter().spawn(async {
-        // use tokio::spawn to get inside the context of multi thread tokio runtime
-        let h1 = tokio::spawn(async {
-            println!("thread id is {:?}", std::thread::current().id());
-            std::thread::sleep(std::time::Duration::from_secs(2));
-        });
-
-        // work stealing occurs for this task spawn
-        let h2 = tokio::spawn(async {
-            println!("thread id is {:?}", std::thread::current().id());
-        });
-
-        h1.await.unwrap();
-        h2.await.unwrap();
-        let _ = tx.send(());
-    });
-
-    rx.await.unwrap();
-
-    let (tx, rx) = tokio::sync::oneshot::channel();
-    let now = std::time::Instant::now();
-
-    // without additional tokio::spawn, all spawned tasks run on single thread
-    System::current().arbiter().spawn(async {
+    // use System::spawn to get inside the context of multi thread tokio runtime
+    let h1 = System::spawn(async {
         println!("thread id is {:?}", std::thread::current().id());
         std::thread::sleep(std::time::Duration::from_secs(2));
-        let _ = tx.send(());
     });
 
-    // previous spawn task has blocked the system arbiter thread
-    // so this task will wait for 2 seconds until it can be run
-    System::current().arbiter().spawn(async move {
+    // work stealing occurs for this task spawn
+    let h2 = System::spawn(async {
         println!("thread id is {:?}", std::thread::current().id());
-        assert!(now.elapsed() > std::time::Duration::from_secs(2));
     });
 
-    rx.await.unwrap();
+    h1.await.unwrap();
+    h2.await.unwrap();
+
+    // note actix_rt can not be used in System::spawn
+    let res = System::spawn(async {
+        let _ = actix_rt::spawn(actix_rt::task::yield_now()).await;
+    })
+    .await;
+
+    // result would always be JoinError with panic message logged.
+    assert!(res.is_err());
 }

actix-rt/src/system.rs

@@ -148,24 +148,27 @@ impl System {
     /// Send an async task to [System] and spawn running it.
     /// Returned [JoinHandle](crate::task::JoinHandle) can be used to await for the task's output.
     ///
-    /// # Error
+    /// # Panics
+    /// Panics if no system is registered on the current thread.
+    ///
+    /// # Errors
     /// [actix_rt::spawn](crate::spawn) can not be used inside `System::spawn`.
     pub fn spawn<T>(task: T) -> JoinHandle<T::Output>
     where
         T: Future + Send + 'static,
         T::Output: Send + 'static,
     {
+        let (tx, rx) = oneshot::channel();
+
+        CURRENT.with(|cell| match *cell.borrow() {
+            Some(ref sys) => sys.arbiter_handle.spawn(async {
+                let res = tokio::spawn(task).await;
+                let _ = tx.send(res);
+            }),
+            None => panic!("System is not running"),
+        });
+
         tokio::spawn(async {
-            let (tx, rx) = oneshot::channel();
-
-            CURRENT.with(|cell| match *cell.borrow() {
-                Some(ref sys) => sys.arbiter_handle.spawn(async {
-                    let res = tokio::spawn(task).await;
-                    let _ = tx.send(res);
-                }),
-                None => panic!("System is not running"),
-            });
-
             // unwrap would be caught by tokio and output as JoinError
             rx.await.unwrap().unwrap()
         })