fixup! fixup! src: use Global for storing resource in Node-API callback scope

Author: addaleax

doc/api/n-api.md

@@ -5924,10 +5924,6 @@ the runtime.
 <!-- YAML
 added: v8.6.0
 napiVersion: 1
-changes:
-  - version: REPLACEME
-    pr-url: https://github.com/nodejs/node/pull/59828
-    description: The `async_resource` object will not be held as a strong reference.
 -->
 
 ```c
@@ -5947,19 +5943,23 @@ napi_status napi_async_init(napi_env env,
 
 Returns `napi_ok` if the API succeeded.
 
+The `async_resource` object needs to be kept alive until
+[`napi_async_destroy`][] to keep `async_hooks` related API acts correctly. In
+order to retain ABI compatibility with previous versions, `napi_async_context`s
+are not maintaining the strong reference to the `async_resource` objects to
+avoid introducing causing memory leaks. However, if the `async_resource` is
+garbage collected by JavaScript engine before the `napi_async_context` was
+destroyed by `napi_async_destroy`, calling `napi_async_context` related APIs
+like [`napi_open_callback_scope`][] and [`napi_make_callback`][] can cause
+problems like loss of async context when using the `AsyncLocalStorage` API.
+
 In order to retain ABI compatibility with previous versions, passing `NULL`
 for `async_resource` does not result in an error. However, this is not
 recommended as this will result in undesirable behavior with  `async_hooks`
 [`init` hooks][] and `async_hooks.executionAsyncResource()` as the resource is
 now required by the underlying `async_hooks` implementation in order to provide
 the linkage between async callbacks.
 
-Previous versions of this API were not maintaining a strong reference to
-`async_resource` while the `napi_async_context` object existed and instead
-expected the caller to hold a strong reference. This has been changed, as a
-corresponding call to [`napi_async_destroy`][] for every call to
-`napi_async_init()` is a requirement in any case to avoid memory leaks.
-
 ### `napi_async_destroy`
 
 <!-- YAML

src/node_api.cc

@@ -538,13 +538,19 @@ class AsyncContext {
  public:
   AsyncContext(node_napi_env env,
                v8::Local<v8::Object> resource_object,
-               v8::Local<v8::String> resource_name)
+               const v8::Local<v8::String> resource_name,
+               bool externally_managed_resource)
       : env_(env) {
     async_id_ = node_env()->new_async_id();
     trigger_async_id_ = node_env()->get_default_trigger_async_id();
     v8::Isolate* isolate = node_env()->isolate();
     resource_.Reset(isolate, resource_object);
     context_frame_.Reset(isolate, node::async_context_frame::current(isolate));
+    lost_reference_ = false;
+    if (externally_managed_resource) {
+      resource_.SetWeak(
+          this, AsyncContext::WeakCallback, v8::WeakCallbackType::kParameter);
+    }
 
     node::AsyncWrap::EmitAsyncInit(node_env(),
                                    resource_object,
@@ -553,13 +559,18 @@ class AsyncContext {
                                    trigger_async_id_);
   }
 
-  ~AsyncContext() { node::AsyncWrap::EmitDestroy(node_env(), async_id_); }
+  ~AsyncContext() {
+    resource_.Reset();
+    lost_reference_ = true;
+    node::AsyncWrap::EmitDestroy(node_env(), async_id_);
+  }
 
   inline v8::MaybeLocal<v8::Value> MakeCallback(
       v8::Local<v8::Object> recv,
       const v8::Local<v8::Function> callback,
       int argc,
       v8::Local<v8::Value> argv[]) {
+    EnsureReference();
     return node::InternalMakeCallback(
         node_env(),
         resource(),
@@ -572,11 +583,21 @@ class AsyncContext {
   }
 
   inline napi_callback_scope OpenCallbackScope() {
+    EnsureReference();
     auto scope = new HeapAllocatedCallbackScope(this);
     env_->open_callback_scopes++;
     return scope->to_opaque();
   }
 
+  inline void EnsureReference() {
+    if (lost_reference_) {
+      const v8::HandleScope handle_scope(node_env()->isolate());
+      resource_.Reset(node_env()->isolate(),
+                      v8::Object::New(node_env()->isolate()));
+      lost_reference_ = false;
+    }
+  }
+
   inline node::Environment* node_env() { return env_->node_env(); }
   inline v8::Local<v8::Object> resource() {
     return resource_.Get(node_env()->isolate());
@@ -588,10 +609,15 @@ class AsyncContext {
   static inline void CloseCallbackScope(node_napi_env env,
                                         napi_callback_scope s) {
     delete HeapAllocatedCallbackScope::FromOpaque(s);
-    CHECK_GT(env->open_callback_scopes, 0);
     env->open_callback_scopes--;
   }
 
+  static void WeakCallback(const v8::WeakCallbackInfo<AsyncContext>& data) {
+    AsyncContext* async_context = data.GetParameter();
+    async_context->resource_.Reset();
+    async_context->lost_reference_ = true;
+  }
+
  private:
   class HeapAllocatedCallbackScope final {
    public:
@@ -603,11 +629,15 @@ class AsyncContext {
     }
 
     explicit HeapAllocatedCallbackScope(AsyncContext* async_context)
-        cs_(async_context->node_env(),
-            &async_context->resource_storage_,
-            async_context->async_context()) {}
+        : resource_storage_(async_context->node_env()->isolate(),
+                            async_context->resource_.Get(
+                                async_context->node_env()->isolate())),
+          cs_(async_context->node_env(),
+              &resource_storage_,
+              async_context->async_context()) {}
 
    private:
+    v8::Global<v8::Object> resource_storage_;
     node::CallbackScope cs_;
   };
 
@@ -913,17 +943,23 @@ napi_status NAPI_CDECL napi_async_init(napi_env env,
   v8::Local<v8::Context> context = env->context();
 
   v8::Local<v8::Object> v8_resource;
+  bool externally_managed_resource;
   if (async_resource != nullptr) {
     CHECK_TO_OBJECT(env, context, v8_resource, async_resource);
+    externally_managed_resource = true;
   } else {
     v8_resource = v8::Object::New(isolate);
+    externally_managed_resource = false;
   }
 
   v8::Local<v8::String> v8_resource_name;
   CHECK_TO_STRING(env, context, v8_resource_name, async_resource_name);
 
-  v8impl::AsyncContext* async_context = new v8impl::AsyncContext(
-      reinterpret_cast<node_napi_env>(env), v8_resource, v8_resource_name);
+  v8impl::AsyncContext* async_context =
+      new v8impl::AsyncContext(reinterpret_cast<node_napi_env>(env),
+                               v8_resource,
+                               v8_resource_name,
+                               externally_managed_resource);
 
   *result = reinterpret_cast<napi_async_context>(async_context);