Add cooperative one-at-a-time mode

src/lib.rs

@@ -297,6 +297,7 @@ pub struct Executor {
 pub struct Runner {
     inner: Arc<Inner>,
     state: Arc<Mutex<ThreadState>>,
+    cooperative: bool,
 
     // force Runner to be !Send
     phantom: PhantomData<Rc<()>>,
@@ -330,6 +331,9 @@ impl Runner {
         while let Some(task) = self.pop_front_woken() {
             any_polled = true;
             task.poll(Arc::downgrade(&self.state), waker.clone());
+            if self.cooperative {
+                break;
+            }
         }
 
         any_polled
@@ -490,6 +494,14 @@ impl Executor {
     }
 
     pub fn into_runner(self) -> Runner {
+        self._into_runner(false)
+    }
+
+    pub fn into_runner_cooperative(self) -> Runner {
+        self._into_runner(true)
+    }
+
+    fn _into_runner(self, cooperative: bool) -> Runner {
         let state = ThreadState::new();
         let id = state.handle.id();
 
@@ -503,6 +515,7 @@ impl Executor {
         Runner {
             inner: self.inner,
             state,
+            cooperative,
             phantom: PhantomData,
         }
     }
@@ -552,13 +565,19 @@ impl Executor {
 
         let handle = self.spawn(f);
 
-        RunWith { handle, runner }.await
+        RunWith {
+            handle,
+            runner,
+            cooperative: false,
+        }
+        .await
     }
 }
 
 struct RunWith<T> {
     handle: JoinHandle<T>,
     runner: Runner,
+    cooperative: bool,
 }
 
 impl Future for Runner {
@@ -597,6 +616,11 @@ where
                 if let Poll::Ready(t) = Pin::new(&mut this.handle).poll(cx) {
                     return Poll::Ready(t);
                 }
+
+                if this.cooperative {
+                    cx.waker().wake_by_ref();
+                    return Poll::Pending;
+                }
             }
 
             if let Poll::Ready(t) = Pin::new(&mut this.handle).poll(cx) {