Intermittent crash in QGraphicsScene

Chris Hennes

No luck on disabling the BSP tree index, the code still crashes in the same place. If I look at the QGraphicsLineItem object with the broken pen, although pen->d is null, line->d is not, and shows a set of coordinates that cannot actually be a line I have drawn in my code (it's diagonal, all of mine are perfectly vertical or horizontal). I can't tell if that's a clue or if it's an artifact of the same thing that is breaking the pen.

kshegunov

@Chris-Hennes said in Intermittent crash in QGraphicsScene:

No luck on disabling the BSP tree index, the code still crashes in the same place.

I don't see how that's possible, as _q_updateIndex is specific to BSP indexing.

I can't tell if that's a clue or if it's an artifact of the same thing that is breaking the pen.

Probably unreliable. If the pen is null you can't be sure the object holding the pen's valid - the rationale is like with the 13th clock stroke.

Chris Hennes

@kshegunov said in Intermittent crash in QGraphicsScene:

I don't see how that's possible, as _q_updateIndex is specific to BSP indexing.

Sorry I wasn't clear: when I said "in the same place" I was referring to

1  QPen::widthF                                      qpen.cpp                       636

The path to get there is different, but it's still ultimately doing the same thing, and still crashing under the same circumstances. The new call stack is:

1   QPen::widthF                                      qpen.cpp                       636  0x7fff9217711d 
2   QGraphicsLineItem::boundingRect                   qgraphicsitem.cpp              9386 0x55f6dec7     
3   QGraphicsItemPrivate::effectiveBoundingRect       qgraphicsitem.cpp              2960 0x55f77cc6     
4   adjustedItemEffectiveBoundingRect                 qgraphicsscene_p.h             353  0x55fbfe43     
5   QGraphicsScenePrivate::processDirtyItemsRecursive qgraphicsscene.cpp             5205 0x55fba406     
6   QGraphicsScenePrivate::_q_processDirtyItems       qgraphicsscene.cpp             487  0x55fb105e     
7   QGraphicsScene::qt_static_metacall                moc_qgraphicsscene.cpp         184  0x55fa7966     
8   QMetaCallEvent::placeMetaCall                     qobject.cpp                    504  0x55255821     
9   QObject::event                                    qobject.cpp                    1246 0x5524e17f     
10  QGraphicsScene::event                             qgraphicsscene.cpp             3523 0x55fad6f8     
11  QApplicationPrivate::notify_helper                qapplication.cpp               3717 0x559d6f3e     
12  QApplication::notify                              qapplication.cpp               3089 0x559d1973     
13  QCoreApplication::notifyInternal2                 qcoreapplication.cpp           1018 0x551f61f6     
14  QCoreApplication::sendEvent                       qcoreapplication.h             233  0x552010b2     
15  QCoreApplicationPrivate::sendPostedEvents         qcoreapplication.cpp           1678 0x551f7ab2     
16  QEventDispatcherWin32::sendPostedEvents           qeventdispatcher_win.cpp       1063 0x552b1dea     
17  QWindowsGuiEventDispatcher::sendPostedEvents      qwindowsguieventdispatcher.cpp 82   0x7fff9a2dfd74 
18  qt_internal_proc                                  qeventdispatcher_win.cpp       236  0x552afacd     
19  UserCallWinProcCheckWow                           USER32                              0x7fffdb8dbc50 
20  DispatchMessageWorker                             USER32                              0x7fffdb8db5cf 
... <More>

kshegunov

Mind sharing some code - how you clear and create the scene items. I can't tell anything to be plainly wrong by looking at the trace. Perhaps try "delaying" the item reinsertions by some time (e.g. few seconds) to see if it's not some kind event concurrency problem ...?

Chris Hennes

It looks like the following code is the culprit:

    if (_selectionRegion && _scene.items().contains(_selectionRegion)) {
        _selectionRegion->setRect (r);
    } else {
        QPen pen (Qt::black);
        QBrush brush (QColor(0,0,0,70));
        _selectionRegion = _scene.addRect (r, pen, brush);
    }

In particular, something about that first "if" clause. If I just let the thing rebuild the rectangle every time it does not crash (e.g. I comment out the conditional and just execute the second path). This bit of code is called in a number of places, including at the beginning of execution, and at various times pre- and post- clear().

m.sue

Hi @Chris-Hennes

Then selectionRegion is probably deleted (or not yet created) at the moment that you use it. If deleted you can set a breakpoint in the object's destructor and see in the backtrace where this deletion comes from.

-Michael.

Chris Hennes

Well, I know exactly where it's being deleted -- in the calls to clear(). But it seemed to me that those calls would result in _scene.items().contains(_selectionRegion) returning false. And while this seems to be true most of the time, every once in a while it's not. The rectangle is invalid, but is still in the items list.

So, the current workaround (which is being stress-tested as I type) is that the superclass which calls clear() now instead calls a virtual function that calls clear(). In the subclass that owns _selectionRegion I overload this virtual function, call the parent (thereby calling clear()) and then explicitly nullify _selectionRegion. Hopefully this sequence prevents any queued events from calling the _selectionRegion update code in between calls to clear() and the nullification of the member. I've managed to type this whole thing without the code crashing yet, so this looks promising.

J.Hilk

@Chris-Hennes
hi, I think the easiest solution would be setting selectionRegion to a Nullpointer after the clear cmd

selectionRegion->clear();
selectionReadion = Q_NULLPTR;

that way the first part of your if condition will return false and not crash.

Chris Hennes

Right, that's essentially the solution I've got, with the caveat that clear() is called by a superclass that knows nothing about the _selectionRegion so I had to do a bit of refactoring to achieve that effect. The crash seems to be gone with that change, so I'm going to mark this as solved here (and in GitHub). But we all know how this type of bug goes, I might be back tomorrow :/

kshegunov

@Chris-Hennes said in Intermittent crash in QGraphicsScene:

But we all know how this type of bug goes, I might be back tomorrow :/

In either case, please report it to the bugtracker. It might not get fixed soon, or ever, but it's a good idea to be there so others won't struggle with this as much as you had.