Deleting QGraphicsItem with QGraphicsEffect leads to segfault

jsulm

@tfm123 Why do you create a QGraphicsRectItem instance in the body of the constructor?

MyRect(QGraphicsItem* parent = nullptr):
		QGraphicsRectItem{QRectF{0.0f, 0.0f, 100.0f, 100.0f}, parent}
{
    setPen(QPen{Qt::white});
    setBrush(QBrush{Qt::white, Qt::SolidPattern});
    new QGraphicsRectItem{this}; // <-- why this?
}

tfm123

@Oleksandr-Malyushytskyy The boundingRect() of the rectangle does not change when the effect is applied to it. But the effect itself has a boundingRect() which changes when the effect is applied. I don't know if this is how it is supposed to be.

tfm123

@Oleksandr-Malyushytskyy This is one of the first things I tried and unfortunately it does not help.

tfm123

@jsulm Like I already wrote, the issue only triggers when MyRect has a child. This is an example. The only reason for things to be like they are is that they are required to be this way to trigger the issue.

aziesemer

@tfm123

I think this bug is described here:
https://bugreports.qt.io/browse/QTBUG-18021

My workaround was to setVisible(false) before removing the item.

JoeCFD

@aziesemer Got random crashes of QLabel with PixMap + QGraphicsEffect on Ubuntu 18.04 + Qt 5.15.2.
Backtrace:
#0 0x00007ffff1510fb7 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1 0x00007ffff1512921 in __GI_abort () at abort.c:79
#2 0x00007ffff155b967 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff1688b0d "%s\n") at ../sysdeps/posix/libc_fatal.c:181
#3 0x00007ffff15629da in malloc_printerr (str=str@entry=0x7ffff1686c3a "corrupted double-linked list") at malloc.c:5342
#4 0x00007ffff1562b94 in malloc_consolidate (av=av@entry=0x7ffff18bdc40 <main_arena>) at malloc.c:4486
#5 0x00007ffff156a12b in _int_free (have_lock=0, p=<optimized out>, av=0x7ffff18bdc40 <main_arena>) at malloc.c:4392
#6 0x00007ffff156a12b in __GI___libc_free (mem=0x5555600bf810) at malloc.c:3134
#7 0x00007ffff3f512ba in QImageData::~QImageData() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#8 0x00007ffff3f517a7 in QImage::~QImage() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#9 0x00007ffff3f8b4dc in QRasterPlatformPixmap::~QRasterPlatformPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#10 0x00007ffff3f8b4f9 in QRasterPlatformPixmap::~QRasterPlatformPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#11 0x00007ffff3f81091 in QPixmap::~QPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#12 0x00007ffff3f858b1 in QPixmapCache::remove(QPixmapCache::Key const&) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#13 0x00007ffff4b66165 in QWidgetPrivate::invalidateGraphicsEffectsRecursively() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#14 0x00007ffff4b661b2 in QWidgetPrivate::setDirtyOpaqueRegion() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#15 0x00007ffff4b79180 in QWidget::~QWidget() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#16 0x00007ffff4c6baa9 in QLabel::~QLabel() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5

Anyone had the same issue?

JoeCFD

@JoeCFD
after I changed the code in the destructor to

    if ( nullptr != m_label ) {
        m_label->setGraphicsEffect( nullptr );
        m_label->setVisible( false );
        m_label->clear();
        delete m_label;
        m_label = nullptr;
    }

backtrace results are different now:

(gdb) backtrace
#0 0x00007ffff418e602 in QPainterPath::~QPainterPath() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
#1 0x00007ffff4bf61f6 in QRenderRule::~QRenderRule() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#2 0x00007ffff279e5c9 in QHashData::free_helper(void ()(QHashData::Node)) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Core.so.5
#3 0x00007ffff279e5c9 in QHashData::free_helper(void ()(QHashData::Node)) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Core.so.5
#4 0x00007ffff4bf50a3 in QHash<QObject const*, QHash<int, QHash<unsigned long long, QRenderRule> > >::remove(QObject const* const&) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#5 0x00007ffff4bd8846 in QStyleSheetStyleCaches::objectDestroyed(QObject*) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#6 0x00007ffff4bd88dc in QStyleSheetStyleCaches::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#7 0x00007ffff2982ddf in void doActivate<false>(QObject*, int, void**) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Core.so.5
#8 0x00007ffff297c50f in QObject::destroyed(QObject*) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Core.so.5
#9 0x00007ffff4b7946b in QWidget::~QWidget() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
#10 0x00007ffff4c6baa9 in QLabel::~QLabel() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5

JoeCFD

@JoeCFD delete m_label; causes the crash. But the crash does not happen every time when the parent of m_label is destroyed.

SGaist

@JoeCFD did you try the workaround mentioned by @Chris-Kawa ?

JoeCFD

@SGaist Thanks for your reply. The random crash may be caused by corrupt memory. Will use valgrind to check the code out.