Allow delete weak ref from user's side

src/instance_data.h

@@ -126,16 +126,21 @@ class InstanceData {
     return true;
   }
 
+  // DeleteWeakRef returns false if there is no weak ref existing for ptr, which
+  // would only happen when user has removed the weak ref manually.
   template<typename T>
-  void DeleteWeakRef(void* ptr) {
+  bool DeleteWeakRef(void* ptr) {
     WeakRefKey key{internal::TopClass<T>::name, ptr};
     auto it = weak_refs_.find(key);
-    if (it == weak_refs_.end()) {
-      assert(false);
-      return;
-    }
+    if (it == weak_refs_.end())
+      return false;
     if (--it->second.first == 0)
       weak_refs_.erase(it);
+    return true;
+  }
+
+  size_t WeakRefsSize() const {
+    return weak_refs_.size();
   }
 
  private:

src/prototype.h

@@ -44,8 +44,11 @@ napi_status ManagePointerInJSWrapper(napi_env env, T* ptr, napi_value* result) {
   using DataType = decltype(data);
   napi_status s = napi_wrap(env, object, data,
                             [](napi_env env, void* data, void* ptr) {
-    if (internal::CanCachePointer<T>::value)
-      InstanceData::Get(env)->DeleteWeakRef<T>(ptr);
+    if (internal::CanCachePointer<T>::value) {
+      // If the weak ref has already been removed, do not run finalizer.
+      if (!InstanceData::Get(env)->DeleteWeakRef<T>(ptr))
+        return;
+    }
     internal::Finalize<T>::Do(static_cast<DataType>(data));
   }, ptr, nullptr);
   if (s != napi_ok) {

src/prototype_internal.h

@@ -281,8 +281,11 @@ struct DefineClass<T, typename std::enable_if<is_function_pointer<
     using DataType = decltype(data);
     napi_status s = napi_wrap(env, object, data,
                               [](napi_env env, void* data, void* ptr) {
-      if (internal::CanCachePointer<T>::value)
-        InstanceData::Get(env)->DeleteWeakRef<T>(ptr);
+      if (internal::CanCachePointer<T>::value) {
+        // If the weak ref has already been removed, do not run finalizer.
+        if (!InstanceData::Get(env)->DeleteWeakRef<T>(ptr))
+          return;
+      }
       Finalize<T>::Do(static_cast<DataType>(data));
       Destruct<T>::Do(static_cast<T*>(ptr));
     }, ptr.value(), nullptr);