Allow calling thread.yield-to in sync contexts (#12241)

* Add check skip

* Add comments

Author: TartanLlama

crates/wasmtime/src/runtime/component/concurrent.rs

@@ -585,7 +585,10 @@ enum SuspendReason {
     NeedWork,
     /// The fiber is yielding and should be resumed once other tasks have had a
     /// chance to run.
-    Yielding { thread: QualifiedThreadId },
+    Yielding {
+        thread: QualifiedThreadId,
+        skip_may_block_check: bool,
+    },
     /// The fiber was explicitly suspended with a call to `thread.suspend` or `thread.switch-to`.
     ExplicitlySuspending {
         thread: QualifiedThreadId,
@@ -1557,6 +1560,9 @@ impl StoreOpaque {
                 } | SuspendReason::Waiting {
                     skip_may_block_check: true,
                     ..
+                } | SuspendReason::Yielding {
+                    skip_may_block_check: true,
+                    ..
                 }
             ) || old_guest_thread
                 .map(|thread| self.concurrent_state_mut().may_block(thread.task))
@@ -3265,6 +3271,10 @@ impl Instance {
         let reason = if yielding {
             SuspendReason::Yielding {
                 thread: guest_thread,
+                // Tell `StoreOpaque::suspend` it's okay to suspend here since
+                // we're handling a `thread.yield-to` call; otherwise it would
+                // panic if we called it in a non-blocking context.
+                skip_may_block_check: to_thread.is_some(),
             }
         } else {
             SuspendReason::ExplicitlySuspending {
@@ -3477,7 +3487,12 @@ impl Instance {
                         };
                         concurrent_state.push_high_priority(item);
 
-                        store.suspend(SuspendReason::Yielding { thread: caller })?;
+                        store.suspend(SuspendReason::Yielding {
+                            thread: caller,
+                            // `subtask.cancel` is not allowed to be called in a
+                            // sync context, so we cannot skip the may-block check.
+                            skip_may_block_check: false,
+                        })?;
                         break;
                     }
                 }