My assumption is that my test app terminates before the GC has a chance to
run.
To test this:
- I run node with the --expose-gc option
- I call global.gc() at the end of my test app
In this case, the destructor of my ObjectWrap is indeed called.
*So it seams the GC doesn't simply "collect everything" when the app
terminates.*
*Why is that?*
It sounds like a pretty bad things to do: some behaviors might expect the
destructor to be called in order to close a socket, etc...
On Friday, November 17, 2017 at 10:27:22 AM UTC+1, Jean-Marc Le Roux wrote:
>
> Thanks for the great help!
> I think it makes a lot of sense now.
>
> So I've extended ObjectWrap like so:
>
> class BSONObject : public node::ObjectWrap
>> {
>> private:
>> bson_t* _bson;
>> public:
>> BSONObject(bson_t* bson, Local<ObjectTemplate> tpl) :
>> _bson(bson)
>> {
>> Local<Object> obj = tpl->NewInstance();
>> initialize(v8::Isolate::GetCurrent(), obj, bson);
>> Wrap(obj);
>> }
>
>
>
> ~BSONObject()
>> {
>> std::cout << "~BSONObject()" << std::endl;
>> bson_destroy(_bson);
>> }
>
>
>
> static
>> Local<ObjectTemplate>
>> create_template(Isolate* isolate, const bson_t* document)
>> {
>> Local<ObjectTemplate> tpl = ObjectTemplate::New(isolate);
>> tpl->SetInternalFieldCount(1);
>
>
> And the "insert" function that I expose in JS and that instanciates the
> BSONObject is written like this:
>
> void
>> insert(const FunctionCallbackInfo<Value>& args)
>> {
>> Isolate* isolate = args.GetIsolate();
>>
>> db->insert(
>> json_stringify(isolate, args[0]),
>> [&args, isolate](const bson_t* document)
>> {
>> if (!args[1]->IsUndefined())
>> {
>> Local<Function> callback = Local<Function>::Cast(args[1]);
>> Local<Object> obj = Object::New(isolate);
>> *Local<Value> argv[] = { (new
>> BSONObject(bson_copy(document)))->handle(isolate) };*
>> callback->Call(isolate->GetCurrentContext()->Global(), 1,
>> argv);
>> }
>> }
>> );
>> }
>
>
> It runs fine.
> But the destructor is never called (I never see "~BSONObject()" on stdout).
>
> Did I miss something?
>
> Thanks !
>
> 2017-11-16 22:01 GMT+01:00 J Decker <[email protected]>:
>
>> You don't manage the instance, the v8 engine does. When it's no longer
>> referenced in the engine, it's garbage collected which triggers the
>> callback set when the perisstent object is made weak.
>>
>> You don't need to keep it anywhere, the object returned into the engine
>> has Internal Fields ( SetInternalFieldCount(1) ) which is used to point to
>> your native class instance. So when the callback is called, it knows what
>> to call to destruct it.
>>
>> You have to 'keep it' in the javascript scrpit if you want to keep it,
>> it's not 'kept' in the native code...
>>
>>
>> --- This is some code I've done that uses setweak directly; but it's
>> fairly scattered....
>>
>> https://github.com/d3x0r/sack.vfs/blob/master/src/jsonParse.cc#L18
>>
>> You can create an instance of your wrapped object with
>>
>> Local<Function> cons = Local<Function>::New( isolate, constructor );
>> cons->NewInstance( isolate->GetCurrentContext(), argc, argv
>> ).ToLocalChecked() ;
>>
>> Hmm that's not the best example source...
>>
>> https://github.com/d3x0r/sack.vfs/blob/master/src/vfs_module.cc#L407
>>
>> This creates a buffer, and makes it weak...
>> https://github.com/d3x0r/sack.vfs/blob/master/src/vfs_module.cc#L438
>>
>> Local<Object> arrayBuffer = ArrayBuffer::New( isolate, buf, len );
>> PARRAY_BUFFER_HOLDER holder = GetHolder();
>> holder->o.Reset( isolate, arrayBuffer );
>> holder->o.SetWeak<ARRAY_BUFFER_HOLDER>( holder, releaseBuffer,
>> WeakCallbackType::kParameter );
>> holder->buffer = buf;
>>
>> PARRAY_BUFFER_HOLDER is just a type that holds the thing to be free (or
>> object containing things to be released), and the persistent reference that
>> has been made weak...
>>
>> struct arrayBufferHolder {
>> void *buffer;
>> Persistent<Object> o;
>> };
>>
>>
>>
>> On Thu, Nov 16, 2017 at 9:56 AM, Jean-Marc Le Roux <
>> [email protected]> wrote:
>>
>>> Node offers a C++ Clss extention ObjectWrap
>>>
>>>
>>> Thanks !
>>> It is related to node. Yet I'm not sure how I'm supposed to use
>>> ObjectWrap.
>>>
>>> I understand I have to create my own class that extends ObjectWrap and
>>> implement a proper destructor.
>>> Then I'll instanciate that class to wrap my Local<Object>. But how do I
>>> manage such instance?
>>> If I don't keep it somewhere, it will be destructed.
>>> If I keep it around, then I'm keeping references to potentially
>>> deallocated memory.
>>>
>>> It feels like I end up with the same problem...
>>>
>>> 2017-11-16 18:36 GMT+01:00 J Decker <[email protected]>:
>>>
>>>>
>>>>
>>>> On Thu, Nov 16, 2017 at 1:24 AM, Jean-Marc Le Roux <
>>>> [email protected]> wrote:
>>>>
>>>>> I assume you're storing references to the objects as Persistent?
>>>>>
>>>>>
>>>>> Nope. You've got the whole code up there.
>>>>> So if I understand correctly:
>>>>>
>>>>> - the objects I need to keep track of the lifecycle should be made
>>>>> Persistent ;
>>>>> - all the other values can be set as Local ;
>>>>> - I should set the weak callback using MakeWeak ;
>>>>> - callback will be called when such object is GCed and I can free
>>>>> my C/C++ memory then.
>>>>>
>>>>> Is that correct ?
>>>>>
>>>>
>>>> Yes. I sthis perhaps related to Node?
>>>> Node offers a C++ Clss extention ObjectWrap
>>>>
>>>> https://nodejs.org/api/addons.html#addons_wrapping_c_objects
>>>> https://github.com/nodejs/node/blob/master/src/node_object_wrap.h
>>>>
>>>> which simplifies that whole process... when the object is GC'd the
>>>> class destructor will get called so you can release values there.
>>>>
>>>>
>>>>
>>>>>
>>>>> 2017-11-15 19:50 GMT+01:00 J Decker <[email protected]>:
>>>>>
>>>>>> I assume you're storing references to the objects as Persistent? All
>>>>>> you need to do then is call SetWeak()
>>>>>> https://v8docs.nodesource.com/node-0.10/d2/d78/classv8_1_1_persistent.html
>>>>>> void MakeWeak (void *parameters, WeakReferenceCallback callback)
>>>>>>
>>>>>> the callback is called when the object is GC'd.
>>>>>>
>>>>>>
>>>>>> On Wed, Nov 15, 2017 at 8:37 AM, Jean-Marc Le Roux <
>>>>>> [email protected]> wrote:
>>>>>>
>>>>>>> So I've found a solution to make things lazy: I set an accessor
>>>>>>> instead of decoding/setting the actual value.
>>>>>>>
>>>>>>> void
>>>>>>> getter(Local<Name> property, const PropertyCallbackInfo<Value>& info)
>>>>>>> {
>>>>>>> Isolate* isolate = info.GetIsolate();
>>>>>>>
>>>>>>> const bson_t* document = reinterpret_cast<bson_t*>(
>>>>>>> info.This()
>>>>>>> ->Get(String::NewFromUtf8(isolate,
>>>>>>> "__bson_document_ptr"))
>>>>>>> ->ToInt32()->Value()
>>>>>>> );
>>>>>>>
>>>>>>> char key[255];
>>>>>>> property->ToString(isolate)->WriteUtf8(key, 255);
>>>>>>>
>>>>>>> bson_iter_t iter;
>>>>>>> bson_iter_init(&iter, document);
>>>>>>> // FIXME: index the property so we don't have to find it
>>>>>>> bson_iter_find(&iter, key);
>>>>>>>
>>>>>>> // FIXME: replace the accessor with the deserialized value
>>>>>>>
>>>>>>> info.GetReturnValue().Set(iterator_to_value(isolate, &iter));
>>>>>>> }
>>>>>>>
>>>>>>> Local<Value>
>>>>>>> fill_object(Isolate* isolate, Local<Object>& obj, const bson_t*
>>>>>>> document)
>>>>>>> {
>>>>>>> obj->Set(
>>>>>>> String::NewFromUtf8(isolate, "__bson_document_ptr"),
>>>>>>> Int32::New(isolate, reinterpret_cast<intptr_t>(document))
>>>>>>> );
>>>>>>>
>>>>>>> bson_iter_t iter;
>>>>>>> if (bson_iter_init(&iter, document))
>>>>>>> {
>>>>>>> while (bson_iter_next(&iter))
>>>>>>> {
>>>>>>> const char* key = bson_iter_key(&iter);
>>>>>>>
>>>>>>> if (!obj->Has(String::NewFromUtf8(isolate,
>>>>>>> key)))
>>>>>>> {
>>>>>>> obj->SetAccessor(
>>>>>>> isolate->GetCurrentContext(),
>>>>>>> String::NewFromUtf8(isolate, key),
>>>>>>> &getter
>>>>>>> );
>>>>>>> }
>>>>>>> }
>>>>>>> }
>>>>>>> }
>>>>>>>
>>>>>>> The secret sauce is to :
>>>>>>>
>>>>>>> - keep the original data (the bson_t) allocated
>>>>>>> - store the corresponding pointer in a v8::Object
>>>>>>>
>>>>>>> But now I have a memory leak because those pointers will never be
>>>>>>> freed.
>>>>>>> How can I know when the Object will be disposed by the GC?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> --
>>>>>>> --
>>>>>>> v8-users mailing list
>>>>>>> [email protected]
>>>>>>> http://groups.google.com/group/v8-users
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>>>>>>>
>>>>>>
>>>>>> --
>>>>>> --
>>>>>> v8-users mailing list
>>>>>> [email protected]
>>>>>> http://groups.google.com/group/v8-users
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>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> *Jean-Marc Le Roux*
>>>>>
>>>>>
>>>>> Founder and CEO of Aerys (http://aerys.in)
>>>>>
>>>>> Blog: http://blogs.aerys.in/jeanmarc-leroux
>>>>> Cell: (+33)6 20 56 45 78
>>>>> Phone: (+33)9 72 40 17 58
>>>>>
>>>>> --
>>>>> --
>>>>> v8-users mailing list
>>>>> [email protected]
>>>>> http://groups.google.com/group/v8-users
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>>>>> an email to [email protected].
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>>>>>
>>>>
>>>> --
>>>> --
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>>>>
>>>
>>>
>>>
>>> --
>>> *Jean-Marc Le Roux*
>>>
>>>
>>> Founder and CEO of Aerys (http://aerys.in)
>>>
>>> Blog: http://blogs.aerys.in/jeanmarc-leroux
>>> Cell: (+33)6 20 56 45 78
>>> Phone: (+33)9 72 40 17 58
>>>
>>> --
>>> --
>>> v8-users mailing list
>>> [email protected]
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>>>
>>
>> --
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>>
>
>
>
> --
> *Jean-Marc Le Roux*
>
>
> Founder and CEO of Aerys (http://aerys.in)
>
> Blog: http://blogs.aerys.in/jeanmarc-leroux
> Cell: (+33)6 20 56 45 78
> Phone: (+33)9 72 40 17 58
>
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