runtime: do not defer yield_now inside block_in_place (#6999)

tokio/src/runtime/context.rs

@@ -183,7 +183,14 @@ cfg_rt! {
     #[track_caller]
     pub(super) fn with_scheduler<R>(f: impl FnOnce(Option<&scheduler::Context>) -> R) -> R {
         let mut f = Some(f);
-        CONTEXT.try_with(|c| c.scheduler.with(f.take().unwrap()))
+        CONTEXT.try_with(|c| {
+            let f = f.take().unwrap();
+            if matches!(c.runtime.get(), EnterRuntime::Entered { .. }) {
+                c.scheduler.with(f)
+            } else {
+                f(None)
+            }
+        })
             .unwrap_or_else(|_| (f.take().unwrap())(None))
     }
 

tokio/tests/task_yield_now.rs

@@ -1,5 +1,5 @@
 #![allow(unknown_lints, unexpected_cfgs)]
-#![cfg(all(feature = "full", tokio_unstable))]
+#![cfg(all(feature = "full", not(target_os = "wasi"), tokio_unstable))]
 
 use tokio::task;
 use tokio_test::task::spawn;
@@ -15,3 +15,11 @@ fn yield_now_outside_of_runtime() {
     assert!(task.is_woken());
     assert!(task.poll().is_ready());
 }
+
+#[tokio::test(flavor = "multi_thread")]
+async fn yield_now_external_executor_and_block_in_place() {
+    let j = tokio::spawn(async {
+        task::block_in_place(|| futures::executor::block_on(task::yield_now()));
+    });
+    j.await.unwrap();
+}