Segfault on external plugin loading

[osrf-migration]

Original report (archived issue) by Andrés Fortier (Bitbucket: andres_fortier).

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Disclaimer: I know this is a weird issue and may be difficult to debug, esp. as I am the only one on a group of 4 devs seeing it. However I'm running out of ideas on what this may be, hence I created the issue. Any help will be much appreciated.

We have an application that defines a set of custom plugins. After updating ign_gui (and other dependencies) the plugin load procedure started to segfault. The details:

• The segfault happens on https://github.com/ignitionrobotics/ign-gui/blob/70f1809c2524f76bf2c69b946567a67f455850e0/src/Plugin.cc#L150 which is called as part of the plugin load procedure.
• Interestingly enough, I seem to be the only affected. Other devs are not experiencing this issue.
• While this->dataPtr is not a nullptr, it seems to have garbage (I think because at no point is dataPtrinitialized to nullptr - https://github.com/ignitionrobotics/ign-gui/blob/70f1809c2524f76bf2c69b946567a67f455850e0/include/ignition/gui/Plugin.hh#L109).
• It seems that Plugin constructor (https://github.com/ignitionrobotics/ign-gui/blob/70f1809c2524f76bf2c69b946567a67f455850e0/src/Plugin.cc#L37) is not being called for our plugin. I tried changing the Plugin source code to:

Plugin::Plugin() : dataPtr(new PluginPrivate)
{
  std::cout << "!!! INIT !!! " << std::endl;
}

and tested with different plugins. The result is that all the plugins shipped with ign_gui call the constructor, but those defined outside (i.e. in an app that use ign_gui as a library) don't call it.

• Same as above happens for the destructors. When a ing_gui plugin is removed from the UI by hitting the X button the destructor is called. However, for "outside" plugins the destructor is not called.
• The constructor not being called would explain dataPtr having garbage, as it is the only place where it is initialized.
• Given that dataPtr is private I can't think on a workaround on our side to fix it.
• Below is the gdb output as stacktrace:

$ gdb visualizer 
[...]
Reading symbols from visualizer...done.
(gdb) r
Starting program: /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/install/bin/visualizer 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Msg] Visualizer 0.1.0
[New Thread 0x7fffde049700 (LWP 24813)]
[New Thread 0x7fffd408d700 (LWP 24814)]
[New Thread 0x7fffd3875700 (LWP 24815)]
[New Thread 0x7fffd3074700 (LWP 24816)]
[GUI] [Msg] Applying stylesheet [:/style.qss]
[GUI] [Msg] Loading plugin [libRenderWidget.so]
[New Thread 0x7fffc23b6700 (LWP 24818)]
[New Thread 0x7fffc1bb5700 (LWP 24819)]
[New Thread 0x7fffc13b4700 (LWP 24820)]
[New Thread 0x7fffc0bb3700 (LWP 24821)]
[New Thread 0x7fffbbfff700 (LWP 24822)]
Thread 1 "visualizer" received signal SIGSEGV, Segmentation fault.
ignition::gui::Plugin::DeleteLaterRequested (this=0x7250b0) at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/ign_gui/src/Plugin.cc:150
150       return this->dataPtr->deleteLaterRequested;
(gdb) bt
#0  ignition::gui::Plugin::DeleteLaterRequested (this=0x7250b0) at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/ign_gui/src/Plugin.cc:161
#1  0x00007ffff7b884e6 in ignition::gui::addPluginsToWindow () at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/ign_gui/src/Iface.cc:589
#2  0x00007ffff7b8e35b in ignition::gui::MainWindow::OnAddPlugin (this=<optimized out>, _plugin=...) at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/ign_gui/src/MainWindow.cc:269
#3  0x00007ffff7bba5a4 in ignition::gui::MainWindow::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>)
    at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/build/ign_gui/include/ignition/gui/moc_MainWindow.cpp:88
#4  0x00007ffff6fc7d2a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007ffff6fd21a5 in QSignalMapper::mapped(QString const&) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#6  0x00007ffff6fd36f2 in QSignalMapper::map(QObject*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007ffff6fc7d2a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007ffff760b412 in QAction::triggered(bool) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9  0x00007ffff760d898 in QAction::activate(QAction::ActionEvent) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#10 0x00007ffff778fe52 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff77960ec in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12 0x00007ffff779a060 in QMenu::mouseReleaseEvent(QMouseEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff7657fc8 in QWidget::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff779aab3 in QMenu::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff761505c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff761ac19 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff6f9938b in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#18 0x00007ffff7619b32 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#19 0x00007ffff767291d in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#20 0x00007ffff7674b7b in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#21 0x00007ffff761505c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007ffff761a516 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#23 0x00007ffff6f9938b in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ffff53ad4e1 in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#25 0x00007ffff53af1a5 in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#26 0x00007ffff5392f08 in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#27 0x00007ffff7ead200 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#28 0x00007ffff4a98197 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#29 0x00007ffff4a983f0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#30 0x00007ffff4a9849c in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#31 0x00007ffff6fef7cf in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#32 0x00007ffff6f96b4a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#33 0x00007ffff6f9ebec in QCoreApplication::exec() () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#34 0x00007ffff7b86790 in ignition::gui::runMainWindow () at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/ign_gui/src/Iface.cc:659
#35 0x000000000040163e in main (argc=<optimized out>, argv=<optimized out>) at /home/andres/OSRF/DrakeFrontEnd/delphyne_ws/src/delphyne_gui/visualizer/visualizer.cc:102

• Our class is defined as:

class RenderWidget : public ignition::gui::Plugin {
  Q_OBJECT

 public:
  /// \brief Default constructor.
  explicit RenderWidget(QWidget* parent = 0);
[...]
}

and

RenderWidget::RenderWidget(QWidget* parent)
    : Plugin(), initializedScene(false), engine(nullptr) {
[...]
}

And of course, let me know if you need more info. Any pointers / hints will be greatly appreciated.

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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Interesting problem, some ideas:

The result is that all the plugins shipped with ign_gui call the constructor, but those defined outside (i.e. in an app that use ign_gui as a library) don't call it.

Have you tried the example plugins? They are essentially "external plugins".

While this->dataPtr is not a nullptr, it seems to have garbage

Yeah I suppose that would happen if the constructor is not called

explicit RenderWidget(QWidget* parent = 0);

Can't imagine how that could change anything, but have you tried removing the explicit?

[osrf-migration]

Original comment by Andrés Fortier (Bitbucket: andres_fortier).

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Thanks for the quick reply @chapulina ! After nuking my entire workspace things started to work again (I've tried already nuking the build dir with no luck). I really have no idea what is/was going on, but it seems to be fixed for now. Thanks again and sorry for stealing time from you.

[osrf-migration]

Original comment by Andrés Fortier (Bitbucket: andres_fortier).

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• changed state from "new" to "invalid"

The issue didn't have to do with ign_gui.

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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Glad to hear you figured it out!