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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Exceptions in constructors causes v8 GC to fail with access violations","articleBody":"Hey folks,\r\nIf an exception is thrown from inside a constructor it seems it causes v8 GC to fail with an access violation in this case below. I am starting to think that this is caused by ```ObjectWrap\u003cT\u003e::FinalizeCallback``` being called when it shouldn't be.\r\n\r\nHere is the example code if you want to try it out quickly https://github.com/blagoev/node-addon-examples/commit/351752fab1b93068aaf84a9cc52d086170778f74\r\n\r\nThe highlights are these.\r\n\r\nThere is a static factory function on the exposed object\r\n\r\n```C++\r\nNapi::Value create(const Napi::CallbackInfo\u0026 info) {\r\n\tNapi::EscapableHandleScope scope(info.Env());\r\n\tNapi::Value val = MyObject::constructor.New({}); //this will throw an exception\r\n\r\n\treturn scope.Escape(info.Env().Undefined());\r\n}\r\n```\r\n\r\nthen the ctor throws some error\r\n\r\n```C++\r\nMyObject::MyObject(const Napi::CallbackInfo\u0026 info)\r\n\t: Napi::ObjectWrap\u003cMyObject\u003e(info) {\r\n\tNapi::Env env = info.Env();\r\n\tNapi::HandleScope scope(env);\r\n\tthrow Napi::Error::New(env, \"TEST\");\r\n......\r\n```\r\n\r\nand we have this private field in the class\r\n```C++\r\nstruct Data {\r\n};\r\n\r\nclass MyObject : public Napi::ObjectWrap\u003cMyObject\u003e {\r\nprivate:\r\n\tstd::shared_ptr\u003cData\u003e m_data;\r\n....\r\n```\r\n\r\nand this is the JS code that tries to call that\r\n```JavaScript\r\nvar addon = require('bindings')('addon');\r\ntry {\r\n    var obj = addon.MyObject.create();\r\n    }\r\n    catch (e) {\r\n        console.error(\"Incorrect expected exception message:\" + e.message);\r\n    }\r\n  \r\n    global.gc()\r\n    global.gc()\r\n    global.gc()\r\n```\r\n\r\nthen there is an exception here \r\n\r\n```\r\n\u003e\taddon.node!std::_Ref_count_base::_Decref() Line 845\tC++\r\n \taddon.node!std::_Ptr_base\u003cData\u003e::_Decref() Line 1122\tC++\r\n \taddon.node!std::shared_ptr\u003cData\u003e::~shared_ptr\u003cData\u003e() Line 1403\tC++\r\n \taddon.node!MyObject::~MyObject()\tC++\r\n \taddon.node!MyObject::`scalar deleting destructor'(unsigned int)\tC++\r\n \taddon.node!Napi::ObjectWrap\u003cMyObject\u003e::FinalizeCallback(napi_env__ * __formal, void * data, void * __formal) Line 3433\tC++\r\n \tnode.exe!`anonymous namespace'::v8impl::Reference::SecondPassCallback(const v8::WeakCallbackInfo\u003c`anonymous namespace'::v8impl::Reference\u003e \u0026 data) Line 496\tC++\r\n \t[Inline Frame] node.exe!v8::internal::GlobalHandles::PendingPhantomCallback::Invoke(v8::internal::Isolate *) Line 905\tC++\r\n \tnode.exe!v8::internal::GlobalHandles::InvokeSecondPassPhantomCallbacks(std::vector\u003cv8::internal::GlobalHandles::PendingPhantomCallback,std::allocator\u003cv8::internal::GlobalHandles::PendingPhantomCallback\u003e\u003e * callbacks, v8::internal::Isolate * isolate) Line 772\tC++\r\n \tnode.exe!v8::internal::GlobalHandles::DispatchPendingPhantomCallbacks(bool synchronous_second_pass) Line 881\tC++\r\n \tnode.exe!v8::internal::GlobalHandles::PostGarbageCollectionProcessing(v8::internal::GarbageCollector collector, const v8::GCCallbackFlags gc_callback_flags) Line 930\tC++\r\n \tnode.exe!v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector collector, const v8::GCCallbackFlags gc_callback_flags) Line 1775\tC++\r\n \tnode.exe!v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace space, v8::internal::GarbageCollectionReason gc_reason, const v8::GCCallbackFlags gc_callback_flags) Line 1392\tC++\r\n \t[Inline Frame] node.exe!v8::internal::Heap::CollectAllGarbage(int) Line 1156\tC++\r\n \tnode.exe!v8::Isolate::RequestGarbageCollectionForTesting(v8::Isolate::GarbageCollectionType type) Line 8273\tC++\r\n \tnode.exe!v8::internal::GCExtension::GC(const v8::FunctionCallbackInfo\u003cv8::Value\u003e \u0026 args) Line 26\tC++\r\n \tnode.exe!v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo * handler) Line 95\tC++\r\n \tnode.exe!v8::internal::`anonymous namespace'::HandleApiCallHelper\u003c0\u003e(v8::internal::Isolate * isolate, v8::internal::Handle\u003cv8::internal::HeapObject\u003e new_target, v8::internal::Handle\u003cv8::internal::HeapObject\u003e fun_data, v8::internal::Handle\u003cv8::internal::FunctionTemplateInfo\u003e receiver, v8::internal::Handle\u003cv8::internal::Object\u003e args, v8::internal::BuiltinArguments) Line 111\tC++\r\n \tnode.exe!v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments args, v8::internal::Isolate * isolate) Line 139\tC++\r\n \tnode.exe!v8::internal::Builtin_HandleApiCall(int args_length, v8::internal::Object * * args_object, v8::internal::Isolate * isolate) Line 127\tC++\r\n\r\n```\r\n\r\n**Note: you need to expose v8 GC for that to work with**\r\n```\r\nnode --expose_gc addon.js\r\n```\r\n\r\n\r\n\r\nIt seems to me this is cause the ```ObjectWrap\u003cT\u003e::FinalizeCallback``` is called but the native object did not get constructed correctly (ie the native object's ctor did not complete). Or is it that my assumption is wrong. \r\n\r\ncheers","author":{"url":"https://github.com/blagoev","@type":"Person","name":"blagoev"},"datePublished":"2019-12-16T18:40:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/635/node-addon-api/issues/635"}