[Development] [Qt-5.6] leak of QtSharedPointer::ExternalRefCountData objects

[Liang Jian]

    The allocated QV4::QObjectWrapper object hold a QPointer which track
'instance', it seems that it should be destroyed with
QObjectWrapper::destroyObject()
to properly destruct the QPointer in it in QV4::MemoryManager::sweep(). But
when sweep() is being called it couldn't find the QObjectWrapper object
created before which will make the QPointer in it not destructed properly.

On Sun, Nov 15, 2015 at 11:58 PM, Liang Jian <jianliang79 at gmail.com> wrote:

>     After some further investigation I can now confirm that one of the
> leak is caused by the following code in QQmlObjectCreator::createInstance(),
> qqmlobjectcreator.cpp:
>
>     *sharedState->allJavaScriptObjects = QV4::QObjectWrapper::wrap(v4,
> instance);
>
>   ++sharedState->allJavaScriptObjects;
>
>
>     But the reason is still unknown
>
>
>
> On Sun, Nov 15, 2015 at 10:19 PM, Liang Jian <jianliang79 at gmail.com>
> wrote:
>
>>       Hello everyone, I checkout qt 5.6 branch yesterday in my windows 10
>> x64 machine, I build it and run my program and see many leaks. Then I use C++
>> Memory Validator to find the source of the leaks, I can now confirm the
>> leaked objects are QtSharedPointer::ExternalRefCountData which are created
>> in QQmlObjectCreator::createInstance(), qqmlobjectcreator.cpp Line 1049,
>> below is the call stack printed by C++ Memory Validator:
>>
>>    Qt5Cored.dll  QtSharedPointer::ExternalRefCountData::getAndRef :
>> [d:\qt5\qtbase\src\corelib\tools\qsharedpointer.cpp Line 1337]
>>
>>    Qt5Qmld.dll  QWeakPointer<QObject>::QWeakPointer<QObject><QObject> :
>> [d:\qt5\qtbase\src\corelib\tools\qsharedpointer_impl.h Line 706]
>>
>>    Qt5Qmld.dll  QPointer<QObject>::QPointer<QObject> :
>> [d:\qt5\qtbase\src\corelib\kernel\qpointer.h Line 65]
>>
>>    Qt5Qmld.dll  QQmlObjectCreator::createInstance :
>> [d:\qt5\qtdeclarative\src\qml\qml\qqmlobjectcreator.cpp Line 1049]
>>     1044 :                 QQmlData *ddata = QQmlData::get(instance,
>> /*create*/true);
>>     1045 :                 ddata->rootObjectInCreation = true;
>>     1046 :
>> sharedState->rootContext->isRootObjectInCreation = false;
>>     1047 :             }
>>     1048 :
>>     1049 :             sharedState->allCreatedObjects.push(instance);
>>     1050 :         } else {
>>     1051 :             Q_ASSERT(typeRef->component);
>>     1052 :             Q_QML_OC_PROFILE(sharedState->profiler,
>> profiler.update(typeRef->component->fileName(),
>>     1053 :                     context->url(), obj->location.line,
>> obj->location.column));
>>     1054 :             if
>> (typeRef->component->compilationUnit->data->isSingleton())
>>    Qt5Qmld.dll  QQmlObjectCreator::create :
>> [d:\qt5\qtdeclarative\src\qml\qml\qqmlobjectcreator.cpp Line 197]
>>
>>    Qt5Qmld.dll  QQmlComponentPrivate::beginCreate :
>> [d:\qt5\qtdeclarative\src\qml\qml\qqmlcomponent.cpp Line 863]
>>
>>    Qt5Qmld.dll  QQmlComponent::beginCreate :
>> [d:\qt5\qtdeclarative\src\qml\qml\qqmlcomponent.cpp Line 812]
>>
>>    Qt5Qmld.dll  QQmlComponent::create :
>> [d:\qt5\qtdeclarative\src\qml\qml\qqmlcomponent.cpp Line 772]
>>
>>    Qt5Quickd.dll  QQuickView::continueExecute :
>> [d:\qt5\qtdeclarative\src\quick\items\qquickview.cpp Line 486]
>>
>>    Qt5Quickd.dll  QQuickViewPrivate::execute :
>> [d:\qt5\qtdeclarative\src\quick\items\qquickview.cpp Line 127]
>>
>>    Qt5Quickd.dll  QQuickView::setSource :
>> [d:\qt5\qtdeclarative\src\quick\items\qquickview.cpp Line 256]
>>
>>
>>     All the leaked QtSharedPointer::ExternalRefCountData objects are
>> allocated in the line
>>     sharedState->allCreatedObjects.push(instance);
>>     then some other code path will also use QPointer (contained in some
>> structures allocated by v4mm) to track the same object 'instance' and I
>> guess the QPointer wasn't properly destructed which will cause the QtSharedPointer::ExternalRefCountData
>> not deleted.
>>     I have investigated this issue for nearly one day, but I can't find
>> which code cause the leak, this is a non-trivial leak, I hope somebody can
>> help to find the source of the leak. thanks!
>>
>>
>
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