Segfault probably related to note stacking automation (bug 2)

Hi,

This issue may share the same root cause with issue #6926.

Conditions: I have an instrument (Kicker, but it may not matter), note stacking is active and stacking chord selector is connected to a white noise LFO (yes, this is quite experimental! ^^). The instrument is used in a pattern track.

The issue is not present in the project file I joined to #6926 but it is on my original project, I open the project, open the the pattern editor, open the Kicker instrument, then when I try to go to the Stacking & Arppegios tab, it segfaults. But unlike #6926, it’s not an issue in the main thread (the song isn’t even playing) but in the GUI thread.

I wasn’t able to reproduce starting with an empty project but it happens on the same project (and same instrument) than #6926, systematically too, this is why I suppose both issues may have the same cause.

Backtrace:

```
#0 0x000055555587f893 in lmms::gui::ComboBox::paintEvent(QPaintEvent*) ()
#1 0x00007ffff79a4db8 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#2 0x00007ffff7962fae in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#3 0x00007ffff66b16f8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#4 0x00007ffff799d0d5 in QWidgetPrivate::sendPaintEvent(QRegion const&) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#5 0x00007ffff799d9d1 in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#6 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#7 0x00007ffff799eb39 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#10 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12 0x00007ffff799eb39 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#18 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#19 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#20 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#21 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#23 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#24 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#25 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#26 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
--Type for more, q to quit, c to continue without paging--
#27 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#28 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#29 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#30 0x00007ffff799ecd6 in QWidgetPrivate::paintSiblingsRecursive(QPaintDevice*, QList const&, int, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#31 0x00007ffff799d64c in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags, QPainter*, QWidgetRepaintManager*) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#32 0x00007ffff7974305 in () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#33 0x00007ffff79a5078 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#34 0x00007ffff7962fae in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#35 0x00007ffff66b16f8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#36 0x00007ffff66b4681 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#37 0x00007ffff670a153 in () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#38 0x00007ffff551e7a9 in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#39 0x00007ffff551ea38 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#40 0x00007ffff551eacc in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#41 0x00007ffff6709836 in QEventDispatcherGlib::processEvents(QFlags) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#42 0x00007ffff66b017b in QEventLoop::exec(QFlags) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#43 0x00007ffff66b82d6 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#44 0x000055555568ae83 in main ()
```

The system is Debian 12 / GCC 4.12.2.0-3

I’ll try to provide a project with this bug present but I may not join the full faulty project for legal reason.

Start by reproducing the reported sequence with note stacking enabled, the stacking chord selector connected to a white-noise LFO, and the supplied project or instrument from issue #6926. Inspect the GUI-thread backtrace beginning at lmms::gui::ComboBox::paintEvent(QPaintEvent*). Done means opening the Stacking & Arpeggios tab no longer segfaults under these conditions.

Written by the indexing model from the issue text.