node-api: make reference weak parameter an indirect link to references

As the cancellation of second pass callbacks are not reachable from the current
v8 API, and the second pass callbacks are scheduled with NodePlatform's task
runner, we have to ensure that the weak parameter holds indirect access to the
v8impl::Reference object so that the object can be destroyed on addon env
teardown before the whole node env is able to shutdown.

src/js_native_api_v8.cc

@@ -314,16 +314,38 @@ class RefBase : protected Finalizer, RefTracker {
 };
 
 class Reference : public RefBase {
+  class PersistentWeakParameter {
+   public:
+    explicit PersistentWeakParameter(Reference* reference)
+        : _reference(reference), _second_pass_callback_scheduled(false) {}
+
+    inline Reference* reference() { return _reference; }
+
+    inline void Reset(Reference* reference = nullptr) {
+      _reference = reference;
+    }
+
+    inline bool second_pass_callback_scheduled() {
+      return _second_pass_callback_scheduled;
+    }
+
+    inline void set_second_pass_callback_scheduled() {
+      _second_pass_callback_scheduled = true;
+    }
+
+   private:
+    Reference* _reference;
+    bool _second_pass_callback_scheduled;
+  };
+
  protected:
   template <typename... Args>
-  Reference(napi_env env,
-            v8::Local<v8::Value> value,
-            Args&&... args)
+  Reference(napi_env env, v8::Local<v8::Value> value, Args&&... args)
       : RefBase(env, std::forward<Args>(args)...),
-            _persistent(env->isolate, value) {
+        _persistent(env->isolate, value),
+        _parameter(new PersistentWeakParameter(this)) {
     if (RefCount() == 0) {
-      _persistent.SetWeak(
-          this, FinalizeCallback, v8::WeakCallbackType::kParameter);
+      SetWeak();
     }
   }
 
@@ -344,10 +366,18 @@ class Reference : public RefBase {
                          finalize_hint);
   }
 
+  virtual ~Reference() {
+    if (!_parameter->second_pass_callback_scheduled()) {
+      // The second pass weak callback is not scheduled, there is no pending
+      // weak callbacks to release the weak parameter. Just deleting it.
+      delete _parameter;
+    }
+  }
+
   inline uint32_t Ref() {
     uint32_t refcount = RefBase::Ref();
     if (refcount == 1) {
-      _persistent.ClearWeak();
+      ClearWeak();
     }
     return refcount;
   }
@@ -356,8 +386,7 @@ class Reference : public RefBase {
     uint32_t old_refcount = RefCount();
     uint32_t refcount = RefBase::Unref();
     if (old_refcount == 1 && refcount == 0) {
-      _persistent.SetWeak(
-          this, FinalizeCallback, v8::WeakCallbackType::kParameter);
+      SetWeak();
     }
     return refcount;
   }
@@ -374,38 +403,76 @@ class Reference : public RefBase {
   inline void Finalize(bool is_env_teardown = false) override {
     // During env teardown, `~napi_env()` alone is responsible for finalizing.
     // Thus, we don't want any stray gc passes to trigger a second call to
-    // `Finalize()`, so let's reset the persistent here if nothing is
-    // keeping it alive.
-    if (is_env_teardown && _persistent.IsWeak()) {
-      _persistent.ClearWeak();
+    // `RefBase::Finalize()`, so let's clear the weakness here.
+    if (is_env_teardown) {
+      ClearWeak();
     }
 
     // Chain up to perform the rest of the finalization.
     RefBase::Finalize(is_env_teardown);
   }
 
  private:
+  // The persistent may have been reset in the first pass weak callback.
+  // However a second pass callback may have been scheduled already, we
+  // must ensure that the weak parameter no longer link to this reference
+  // so that the second pass callback is not able to calling into the
+  // `Reference::Finalize()`.
+  inline void ClearWeak() {
+    if (!_persistent.IsEmpty()) {
+      _persistent.ClearWeak();
+    }
+    _parameter->Reset();
+  }
+
+  // Setup the weakness callback and parameter.
+  inline void SetWeak() {
+    _persistent.SetWeak(
+        _parameter, FinalizeCallback, v8::WeakCallbackType::kParameter);
+    _parameter->Reset(this);
+  }
+
   // The N-API finalizer callback may make calls into the engine. V8's heap is
   // not in a consistent state during the weak callback, and therefore it does
   // not support calls back into it. However, it provides a mechanism for adding
   // a finalizer which may make calls back into the engine by allowing us to
   // attach such a second-pass finalizer from the first pass finalizer. Thus,
   // we do that here to ensure that the N-API finalizer callback is free to call
   // into the engine.
-  static void FinalizeCallback(const v8::WeakCallbackInfo<Reference>& data) {
-    Reference* reference = data.GetParameter();
+  static void FinalizeCallback(
+      const v8::WeakCallbackInfo<PersistentWeakParameter>& data) {
+    PersistentWeakParameter* parameter = data.GetParameter();
+    Reference* reference = parameter->reference();
+    if (reference == nullptr) {
+      return;
+    }
 
     // The reference must be reset during the first pass.
     reference->_persistent.Reset();
+    // Mark the parameter not delete-able until the second pass callback is
+    // invoked.
+    parameter->set_second_pass_callback_scheduled();
 
     data.SetSecondPassCallback(SecondPassCallback);
   }
 
-  static void SecondPassCallback(const v8::WeakCallbackInfo<Reference>& data) {
-    data.GetParameter()->Finalize();
+  // Second pass callbacks are scheduled with platform tasks. At env teardown,
+  // the tasks may have already be scheduled and we are unable to cancel the
+  // second pass callback task. So we have to make sure that parameter is kept
+  // alive until the second pass callback is been invoked.
+  static void SecondPassCallback(
+      const v8::WeakCallbackInfo<PersistentWeakParameter>& data) {
+    PersistentWeakParameter* parameter = data.GetParameter();
+    Reference* reference = parameter->reference();
+    delete parameter;
+    if (reference == nullptr) {
+      return;
+    }
+    reference->Finalize();
   }
 
   v8impl::Persistent<v8::Value> _persistent;
+  PersistentWeakParameter* _parameter;
 };
 
 enum UnwrapAction {