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QJSEngine cleanup/destructor

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qjsenginejavascriptqt 5.12destructorcleanup
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  • S SGaist
    13 Apr 2019, 20:28

    Hi and welcome to devnet,

    Do you have a stack trace of the crash ?

    Y Offline
    Y Offline
    yah_nosh
    wrote on 13 Apr 2019, 21:43 last edited by
    #3

    @SGaist Hi, thank you very much.

    Currently, I'm using the below code to try and delete QJSEngine:

    	if (jsEngine)
    	{
    		jsEngine->collectGarbage();
    		jsEngine->deleteLater();
    	}
    

    This crashes the app after it executes. I tried it in a way where I set the parent of the QJSEngine to the main window and do not call deleteLater (i.e I let the window clean it up), but it causes a similar crash. The crash itself is the app triggering a breakpoint is_block_type_valid(header->_block_use)

    This is what I could get out of VS:

    ucrtbased.dll!000007fed011c2f1()    Unknown
    
        ucrtbased.dll!000007fed011f8a5()    Unknown
    
    >    HMARL_Test_Engine.exe!operator delete(void * block) Line 21    C++
    
        HMARL_Test_Engine.exe!operator delete(void * block, unsigned __int64 __formal) Line 16    C++
    
        HMARL_Test_Engine.exe!JSInterface::`scalar deleting destructor'(unsigned int)    C++
    
        Qt5Qmld.dll!QV4::QObjectWrapper::destroyObject(bool lastCall) Line 1107    C++
    
        Qt5Qmld.dll!QV4::MemoryManager::sweep(bool lastSweep, void(*)(const char *) classCountPtr) Line 977    C++
    
        Qt5Qmld.dll!QV4::MemoryManager::~MemoryManager() Line 1225    C++
    
        [External Code]    
    
        Qt5Qmld.dll!QV4::ExecutionEngine::~ExecutionEngine() Line 653    C++
    
        [External Code]    
    
        Qt5Qmld.dll!QJSEngine::~QJSEngine() Line 379    C++
    
        [External Code]    
    
        Qt5Cored.dll!qDeleteInEventHandler(QObject * o) Line 4620    C++
    
        Qt5Cored.dll!QObject::event(QEvent * e) Line 1241    C++
    
        Qt5Widgetsd.dll!QApplicationPrivate::notify_helper(QObject * receiver, QEvent * e) Line 3752    C++
    
        Qt5Widgetsd.dll!QApplication::notify(QObject * receiver, QEvent * e) Line 3109    C++
    
        Qt5Cored.dll!QCoreApplication::notifyInternal2(QObject * receiver, QEvent * event) Line 1061    C++
    
        Qt5Cored.dll!QCoreApplication::sendEvent(QObject * receiver, QEvent * event) Line 1452    C++
    
        Qt5Cored.dll!QCoreApplicationPrivate::sendPostedEvents(QObject * receiver, int event_type, QThreadData * data) Line 1800    C++
    
        Qt5Cored.dll!QEventDispatcherWin32::sendPostedEvents() Line 1093    C++
    
        qwindowsd.dll!QWindowsGuiEventDispatcher::sendPostedEvents() Line 82    C++
    
        Qt5Cored.dll!qt_internal_proc(HWND__ * hwnd, unsigned int message, unsigned __int64 wp, __int64 lp) Line 247    C++
    
        [External Code]    
    
        Qt5Cored.dll!QEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 640    C++
    
        qwindowsd.dll!QWindowsGuiEventDispatcher::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 74    C++
    
        Qt5Cored.dll!QEventLoop::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 139    C++
    
        Qt5Cored.dll!QEventLoop::exec(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 225    C++
    
        Qt5Cored.dll!QCoreApplication::exec() Line 1364    C++
    
        Qt5Guid.dll!QGuiApplication::exec() Line 1780    C++
    
        Qt5Widgetsd.dll!QApplication::exec() Line 2910    C++
    
        HMARL_Test_Engine.exe!main(int argc, char * * argv) Line 10    C++
    
        HMARL_Test_Engine.exe!WinMain(HINSTANCE__ * __formal, HINSTANCE__ * __formal, char * __formal, int __formal) Line 104    C++
    
        [External Code]
    

    Let me know if you need any additional info. Also, apologies if the formatting is suboptimal.

    1 Reply Last reply
    0
    • Y Offline
      Y Offline
      yah_nosh
      wrote on 22 Apr 2019, 23:05 last edited by
      #4

      Anyone have any ideas?

      1 Reply Last reply
      0
      • S Offline
        S Offline
        SGaist
        Lifetime Qt Champion
        wrote on 23 Apr 2019, 20:48 last edited by
        #5

        Did you already check the bug report system for something related ?

        Interested in AI ? www.idiap.ch
        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

        Y 1 Reply Last reply 23 Apr 2019, 21:26
        0
        • S SGaist
          23 Apr 2019, 20:48

          Did you already check the bug report system for something related ?

          Y Offline
          Y Offline
          yah_nosh
          wrote on 23 Apr 2019, 21:26 last edited by
          #6

          @SGaist Yeah, I looked around, nobody seems to have the same problem. Should I report it there?

          Also, just to clarify: I should be able to create and destroy QJSEngine objects this way? So it's not like I'm trying to make the framework do something it's not supposed to?

          1 Reply Last reply
          0
          • S Offline
            S Offline
            SGaist
            Lifetime Qt Champion
            wrote on 24 Apr 2019, 21:59 last edited by
            #7

            Although I haven't used that class extensively, I am not aware of such a limitation. So go on and create a report. Please provide a minimal compilable example.

            Interested in AI ? www.idiap.ch
            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

            Y 1 Reply Last reply 25 Apr 2019, 21:59
            2
            • S SGaist
              24 Apr 2019, 21:59

              Although I haven't used that class extensively, I am not aware of such a limitation. So go on and create a report. Please provide a minimal compilable example.

              Y Offline
              Y Offline
              yah_nosh
              wrote on 25 Apr 2019, 21:59 last edited by
              #8

              @SGaist While making the minimal compilable example, I managed to find the actual source of the problem. It wasn't the destructor at all, at least not directly, but instead me exposing my "interface" to JavaScript. Below is a small example:

              QJSEngine* jsEngine = new QJSEngine();
              
              // Add ourselves to the JS engine
              QJSValue interface = jsEngine->newQObject(interfacePtr);
              
              // Expose interface in the global namespace
              jsEngine->globalObject().setProperty("INTERFACE", interface);
              
              // Do stuff...
              
              delete jsEngine; // <-- error happens here
              
              

              It seems that using newQObject causes trouble once I try destroying the QJSEngine, and the documentation references this as well:

              If the given object is deleted outside of the engine's control, any attempt to access the deleted QObject's members through the JavaScript wrapper object (either by script code or C++) will result in a script exception.

              So I suspect I need to "clean up" this object before deleting the engine, otherwise (presumably) some ownership issues come up when the JS heap is cleaned up. Unfortunately, I have no idea how to fix this. I tried using deleteProperty on the global object, but that did not work.

              JonBJ 1 Reply Last reply 25 Apr 2019, 22:10
              1
              • Y yah_nosh
                25 Apr 2019, 21:59

                @SGaist While making the minimal compilable example, I managed to find the actual source of the problem. It wasn't the destructor at all, at least not directly, but instead me exposing my "interface" to JavaScript. Below is a small example:

                QJSEngine* jsEngine = new QJSEngine();
                
                // Add ourselves to the JS engine
                QJSValue interface = jsEngine->newQObject(interfacePtr);
                
                // Expose interface in the global namespace
                jsEngine->globalObject().setProperty("INTERFACE", interface);
                
                // Do stuff...
                
                delete jsEngine; // <-- error happens here
                
                

                It seems that using newQObject causes trouble once I try destroying the QJSEngine, and the documentation references this as well:

                If the given object is deleted outside of the engine's control, any attempt to access the deleted QObject's members through the JavaScript wrapper object (either by script code or C++) will result in a script exception.

                So I suspect I need to "clean up" this object before deleting the engine, otherwise (presumably) some ownership issues come up when the JS heap is cleaned up. Unfortunately, I have no idea how to fix this. I tried using deleteProperty on the global object, but that did not work.

                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on 25 Apr 2019, 22:10 last edited by
                #9

                @yah_nosh
                Does https://stackoverflow.com/questions/26177693/qjsengine-deletes-my-qobject-how-to-change-ownership-after-qjsenginenewqobjec (QQmlEngine::setObjectOwnership()) help your situation at all?

                Y 1 Reply Last reply 25 Apr 2019, 22:19
                3
                • Y Offline
                  Y Offline
                  yah_nosh
                  wrote on 25 Apr 2019, 22:12 last edited by
                  #10

                  Okay, found the solution. I needed to change the ownership using QQmlEngine::setObjectOwnership, and the JS engine stops trying to delete my objects. So technically there was no bug at all, but the source code and documentation sorely needs an update.

                  1 Reply Last reply
                  3
                  • JonBJ JonB
                    25 Apr 2019, 22:10

                    @yah_nosh
                    Does https://stackoverflow.com/questions/26177693/qjsengine-deletes-my-qobject-how-to-change-ownership-after-qjsenginenewqobjec (QQmlEngine::setObjectOwnership()) help your situation at all?

                    Y Offline
                    Y Offline
                    yah_nosh
                    wrote on 25 Apr 2019, 22:19 last edited by
                    #11

                    @JonB Yeah, sorry, posted my comment right before I noticed yours. It's really unintuitive that the solution is buried in a derived class static function... which apparently uses the base class code anyway.

                    1 Reply Last reply
                    0
                    • S Offline
                      S Offline
                      SGaist
                      Lifetime Qt Champion
                      wrote on 26 Apr 2019, 21:29 last edited by
                      #12

                      @yah_nosh said in QJSEngine cleanup/destructor:

                      Okay, found the solution. I needed to change the ownership using QQmlEngine::setObjectOwnership, and the JS engine stops trying to delete my objects. So technically there was no bug at all, but the source code and documentation sorely needs an update.

                      You should consider improving the documentation and submit a patch for that so everybody can benefit from the outcome of your findings.

                      Interested in AI ? www.idiap.ch
                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                      Y 1 Reply Last reply 3 May 2019, 12:26
                      2
                      • S SGaist
                        26 Apr 2019, 21:29

                        @yah_nosh said in QJSEngine cleanup/destructor:

                        Okay, found the solution. I needed to change the ownership using QQmlEngine::setObjectOwnership, and the JS engine stops trying to delete my objects. So technically there was no bug at all, but the source code and documentation sorely needs an update.

                        You should consider improving the documentation and submit a patch for that so everybody can benefit from the outcome of your findings.

                        Y Offline
                        Y Offline
                        yah_nosh
                        wrote on 3 May 2019, 12:26 last edited by
                        #13

                        @SGaist Yeah, I think as soon as this project is done, I'll append the relevant parts in the QJSEngine docs so others can avoid this headache. Can anyone contribute to the documentation?

                        1 Reply Last reply
                        1
                        • S Offline
                          S Offline
                          SGaist
                          Lifetime Qt Champion
                          wrote on 3 May 2019, 12:55 last edited by
                          #14

                          Anyone can contribute to the documentation, the code, etc. You have to follow the procedures and rules but it's not some walled garden.

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                          1 Reply Last reply
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