fixup! Address feedback and refine implementation

src/workerd/api/global-scope.c++

@@ -103,8 +103,13 @@ ServiceWorkerGlobalScope::ServiceWorkerGlobalScope(v8::Isolate* isolate)
                 v8::PromiseRejectEvent event,
                 jsg::V8Ref<v8::Promise> promise,
                 jsg::Value value) {
-          jsg::AsyncContextFrame::Scope scope(js,
-              KJ_ASSERT_NONNULL(jsg::AsyncContextFrame::tryUnwrap(js, promise)));
+          // If async context tracking is enabled, then we need to ensure that we enter the frame
+          // associated with the promise before we invoke the unhandled rejection callback handling.
+          kj::Maybe<jsg::AsyncContextFrame::Scope> maybeScope;
+          if (js.isAsyncContextTrackingEnabled()) {
+            maybeScope.emplace(js,
+                KJ_ASSERT_NONNULL(jsg::AsyncContextFrame::tryUnwrap(js, promise)));
+          }
           auto ev = jsg::alloc<PromiseRejectionEvent>(event, kj::mv(promise), kj::mv(value));
           dispatchEventImpl(js, kj::mv(ev));
         }) {}

src/workerd/jsg/jsg.c++

@@ -160,6 +160,10 @@ void Lock::setAsyncContextTrackingEnabled() {
   IsolateBase::from(v8Isolate).setAsyncContextTrackingEnabled();
 }
 
+bool Lock::isAsyncContextTrackingEnabled() {
+  return IsolateBase::from(v8Isolate).isAsyncContextTrackingEnabled();
+}
+
 void Lock::setCommonJsExportDefault(bool exportDefault) {
   IsolateBase::from(v8Isolate).setCommonJsExportDefault({}, exportDefault);
 }

src/workerd/jsg/jsg.h

@@ -1997,6 +1997,7 @@ class Lock {
   void setLoggerCallback(kj::Function<Logger>&& logger);
 
   void setAsyncContextTrackingEnabled();
+  bool isAsyncContextTrackingEnabled();
 
   // ---------------------------------------------------------------------------
   // Misc. Stuff

src/workerd/jsg/setup.c++

@@ -382,6 +382,8 @@ v8::ModifyCodeGenerationFromStringsResult IsolateBase::modifyCodeGenCallback(
 }
 
 void IsolateBase::setAsyncContextTrackingEnabled() {
+  if (asyncContextTrackingEnabled) return;
+  asyncContextTrackingEnabled = true;
   ptr->SetPromiseHook(&promiseHook);
 }
 

src/workerd/jsg/setup.h

@@ -102,6 +102,7 @@ class IsolateBase {
   }
 
   void setAsyncContextTrackingEnabled();
+  bool isAsyncContextTrackingEnabled() { return asyncContextTrackingEnabled; }
 
 private:
   template <typename TypeWrapper>
@@ -141,6 +142,7 @@ class IsolateBase {
   // and there are a number of async APIs that currently throw. When the captureThrowsAsRejections
   // flag is set, that old behavior is changed to be correct.
   bool exportCommonJsDefault = false;
+  bool asyncContextTrackingEnabled = false;
 
   kj::Maybe<kj::Function<Logger>> maybeLogger;