[Interest] Annoying Bug - Qt 4.8.1 bug?

[Etienne Sandré-Chardonnal]

Dear all,

After debugging a little, it appears that in order:
 - QProgressDialog destructor is called
 - Then QProgressDialog::setValue(slot) is called by the connection from
BackgroundFileWorker::progressChanged(int) signal

This means the slot is called after the dialog was destroyed. It was
automatically destroyed by a WA_DeleteOnClose flag.

This really looks like a Qt bug

I have looked for this bug but only found something python related :
http://srinikom.github.io/pyside-bz-archive/904.html
However this could be a more generic bug not related to QProgressDialog.


Cheers,

Etienne



2014/1/23 Etienne Sandré-Chardonnal <etienne.sandre at m4x.org>

> Dear all,
>
> I'm tracking a bug since a few days, which happens at the end of file
> saving in my app. It saves using a separate thread, while the GUI displays
> a modal progress dialog. The app crashes in the QProgressDialog destructor,
> and this does not happen all the time. I have double checked that the
> destructor is called only once; it's allocated on the heap and I let Qt
> destroy it with the flag WA_DeleteOnClose.
>
> I spent a few hours stripping my app to the bare minimum that keeps it
> crashing. The good news is that it's possible to reproduce it with a small
> project, the bad is the probability for crashing at each click on the
> "save" button decreases when the GUI becomes simple (I had to keep a few
> widgets and UI to keep it at a detectable level) but everything is made
> with .ui's and generated by Qt, so my code is minimal. You may find the
> project attached, I'm running Qt 4.8.1 under win7 64 bit, with mingw32 and
> mingw64 (both crash). *To trigger the crash, you need to click on "Save"
> toolbutton, then press enter. This needs to be repeated several times,
> several varying between 2 and 50.*
>
>
> Here is the only non trivial code that could contain the bug if it's not a
> Qt bug. This is a QThread subclass worker doing the file saving (here it
> does nothing but this crashes anyway on the QProgressDialog destruction)
>
>
> BackgroundFileWorker::BackgroundFileWorker(QObject *parent) :
>
> 	QThread(parent)
>
> {
>
> }
>
>
>
> bool BackgroundFileWorker::startWriting()
>
> {
>
> 	if(isRunning())
>
> 	{
>
> 		return false;
>
> 	}
>
>
> 	_progress =  0;
>
>
> 	//Create a timer for polling progress update
>
> 	QTimer * progressPollingTimer = new QTimer(this);
>
> 	progressPollingTimer->setInterval(200);
>
> 	connect(this, SIGNAL(started()), progressPollingTimer, SLOT(start()));
>
> 	connect(progressPollingTimer, SIGNAL(timeout()), this, SLOT(updateProgress()));
>
> 	connect(this, SIGNAL(finished()), progressPollingTimer, SLOT(stop()));
>
> 	connect(this, SIGNAL(finished()), progressPollingTimer, SLOT(deleteLater()));
>
> 	connect(this, SIGNAL(finished()), this, SLOT(updateProgress()));
>
>
>
> 	//Create an application modal progress dialog
>
> 	QProgressDialog * progress = new QProgressDialog("Please wait while writing the file...", "Abort", 0, 100, dynamic_cast<QWidget*>(parent()));
>
> 	progress->setWindowTitle("Saving File");
>
> 	connect(this, SIGNAL(progressChanged(int)), progress, SLOT(setValue(int)));
>
> 	connect(this, SIGNAL(finished()), progress, SLOT(close()));
>
> 	progress->setWindowModality(Qt::WindowModal);
>
> 	progress->setAttribute(Qt::WA_DeleteOnClose);
>
> 	progress->setMinimumDuration(0);
>
>
> 	emit progressChanged(0);
>
>
> 	start();
>
> 	return true;
>
> }
>
>
> void BackgroundFileWorker::updateProgress()
>
> {
>
> 	if(isRunning() )
>
> 	{
>
> 		_progress+=10;
>
> 		emit progressChanged(_progress);
>
> 	}
>
> 	else if (_progress!=100)
>
> 	{
>
> 			_progress = 100;
>
> 			emit progressChanged(_progress);
>
> 	}
>
> }
>
>
> void BackgroundFileWorker::run()
>
> {
>
> }
>
>
>
>
> Thanks,
>
> Etienne
>
>
>
>
>
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