src: store Local for CallbackScope on stack

This is a requirement of the V8 API, but requires a separate
semver-major change (as it is ABI-breaking) to address.

(There's also a similar requirement for `napi_open_callback_scope`
that would not be easily addressable without breaking ABI compatibility
there as well. In real-world situations, it seems extremely unlikely
that the `CallbackScope` would be the only reference to the resource
object.)

Author: addaleax

src/api/callback.cc

@@ -26,10 +26,10 @@ CallbackScope::CallbackScope(Isolate* isolate,
 CallbackScope::CallbackScope(Environment* env,
                              Local<Object> object,
                              async_context asyncContext)
-  : private_(new InternalCallbackScope(env,
-                                       object,
-                                       asyncContext)),
-    try_catch_(env->isolate()) {
+    : resource_storage_(object),
+      private_(
+          new InternalCallbackScope(env, &resource_storage_, asyncContext)),
+      try_catch_(env->isolate()) {
   try_catch_.SetVerbose(true);
 }
 

src/node.h

@@ -1452,6 +1452,8 @@ class NODE_EXTERN CallbackScope {
   CallbackScope(CallbackScope&&) = delete;
 
  private:
+  void* reserved_;
+  v8::Local<v8::Object> resource_storage_;
   InternalCallbackScope* private_;
   v8::TryCatch try_catch_;
 };