crash when switching out of transforms module

[ebrahimebrahim]

The application crashes when switching out of the transforms module:

• start application
• unhide slicer interface
• switch to transforms module
• switch to any other module

[ebrahimebrahim]

It appears to be a segfault as the parent widget of what I assume is a qSlicerTransformsModuleWidget is set to null.

[ebrahimebrahim]

The crash does not occur when we turn off custom QSS

[jcfr]

To try to identify what may be leading the moduleWidget to be invalid, you could also look into having a breakpoint (or qDebug() call) in the destructor of the widget ...

Then when styling is enabled, you could see what lead to destruction of the object in the first place

[ebrahimebrahim]

To try to identify what may be leading the moduleWidget to be invalid, you could also look into having a breakpoint (or qDebug() call) in the destructor of the widget ...

After trying this, it doesn't seem to be invalid. The moduleWidget never gets prematurely destroyed, so that is not the cause of the segfault. Maybe I need to look further down in the backtrace, in the Qt library calls, to see the culprit. But I never figured out how to include Qt debugging symbols, so I have been putting this off.

BTW this a very low priority bug, because the custom app doesn't need the transforms module. But it would still be good to understand what's going on.

[kerim371]

Actually if you look at the picture on Slicer forum then you might notice that the app crashes during QWidget::setParent(). More precisely at QWidgetPrivate::inheritStyle and QWidgetPrivate::setStyle_helper within QWidget::setParent() process.

I believe that QWidget tries to inherit the style from parent to all children or something like that. That is why these two functions are called recursively. Finally at some point QWidgetPrivate::setStyle_helper crashes.

So it is not about qSlicerTransformsModuleWidget is NULL but it is something with style inheritage when we set new parent.

The main questions that I don't have answers:

1. why it crashes only on Linux
2. why it crashes only with qSlicerTransformsModuleWidget

If open qSlicerTransformsModuleWidget.ui in SlicerDesigner then few warnings will appear:

The enumeration-type property contentsFrameShape could not be read.
The set-type property decimalsOption could not be read.
The enumeration-type property TypeOfTransform could not be read.

I googled that and there already been a similar problem with SlicerSALT. And it also was connected with Linux only. @jcfr don't you remember how you solved this error?

I don't know exactly but I guess some CTK widgets have styles incorrectly managed.

[kerim371]

The source of the problem is GlyphPointsNodeComboBox (wich is qMRMLNodeComboBox).
And the crash happens only when any qMRMLNodeComboBox is within AdvancedParameters collapsible group box..

To check that go to qMRMLTransformNodeDisplayWidget and after the UI is setup set its parent to nullptr:

void qMRMLTransformDisplayNodeWidgetPrivate
::init()
{
  Q_Q(qMRMLTransformDisplayNodeWidget);
  this->setupUi(q);

  this->GlyphPointsNodeComboBox->setParent(nullptr);
  /* some other code */
}

Then even you are on Linux and you have StyleSheet aplied then switching to another module from Transforms won't lead to crash.

Though I don't yet understand what is wrong with that widget there...

[kerim371]

Finally I've found what exactly causes the crash.
void qMRMLNodeComboBox::changeEvent(QEvent *event) calls qMRMLNodeComboBoxPrivate::updateDelegate and here new item delegate is created.

The strangest thing is that if you put these three qDebug() lines before that line then everything works as expected:

void qMRMLNodeComboBoxPrivate::updateDelegate(bool force)
{
/* some code */
      if (force ||
          qobject_cast<qMRMLNodeComboBoxDelegate *>(this->ComboBox->itemDelegate()))
        {
        qDebug() << "ONE";
        qDebug() << "TWO";
        qDebug() << "THREE";
        this->ComboBox->setItemDelegate(
            new qMRMLNodeComboBoxMenuDelegate(q->parent(), q->comboBox()));
        }
/* some code */
}

I hope somebody will find a clue why this is happening and how to solve this.

[ebrahimebrahim]

@kerim371 Thanks so much for you investigation and for reporting your observations! Sorry for the late reply I have been traveling. Thanks for all your effort to narrow down the problem.

The strangest thing is that if you put these three qDebug() lines before that line then everything works as expected:

Very weird. I wonder if it is related to compiler optimization; perhaps turning off optimization and removing the debug lines would also prevent the crash.

[kerim371]

Share your results please when you're done testing.
It would be nice if we could solve this.