The documentation for KJob::exec states that

"Note that the event loop started by this method does not process user input events, which means your user interface will effectivly be blocked. Other events like paint or network events are still being processed. The advantage of not processing user input events is that the chance of accidental reentrancy is greatly reduced. Still you should avoid calling this function."

Yet it is possible for UI events to "come through" e.g. by clicking on a systray icon as can be seen from this extract of the crash backtrace from https://bugs.kde.org/show_bug.cgi?id=340624 :


#6  0x00007f9f95149ae4 in MailCommon::FolderTreeWidget::folderTreeView (this=<optimized out>, this=<optimized out>) at ../../mailcommon/folder/foldertreewidget.cpp:261
#7  0x00007f9f955043df in folderTreeView (this=<optimized out>, this=<optimized out>) at ../../kmail/kmmainwidget.h:146
#8  KMKernel::treeviewModelSelection (this=<optimized out>) at ../../kmail/kmkernel.cpp:1939
#9  0x00007f9f954efa8e in KMail::KMSystemTray::slotContextMenuAboutToShow (this=0x1675610) at ../../kmail/kmsystemtray.cpp:307
#10 0x00007f9ffb8b795a in QMetaObject::activate (sender=0x178f060, m=<optimized out>, local_signal_index=<optimized out>, argv=0x0) at kernel/qobject.cpp:3567
#11 0x00007f9ffb8aab17 in QMetaMethod::invoke (this=this@entry=0x7fff49f44340, object=object@entry=0x178f060, connectionType=Qt::DirectConnection, connectionType@entry=Qt::AutoConnection, returnValue=..., val0=..., val1=..., val2=..., val3=..., val4=..., val5=..., val6=..., val7=..., val8=..., val9=...) at kernel/qmetaobject.cpp:1664
...
#27 0x00007f9ffb8a5d07 in QCoreApplication::sendPostedEvents (receiver=0x7f9ffb8d3060 <postEventSourceDispatch(GSource*, GSourceFunc, gpointer)>, receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1470
#28 0x00007f9ffb8d3073 in sendPostedEvents () at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:236
#29 postEventSourceDispatch (s=0x8ecde0) at kernel/qeventdispatcher_glib.cpp:287
#30 0x00007f9ff4adce04 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#31 0x00007f9ff4add048 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007f9ff4add0ec in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007f9ffb8d270f in QEventDispatcherGlib::processEvents (this=0x8b9bb0, flags=...) at kernel/qeventdispatcher_glib.cpp:434
#34 0x00007f9ffc350d86 in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#35 0x00007f9ffb8a05af in QEventLoop::processEvents (this=this@entry=0x7fff49f45260, flags=...) at kernel/qeventloop.cpp:149
#36 0x00007f9ffb8a08ed in QEventLoop::exec (this=0x7fff49f45260, flags=...) at kernel/qeventloop.cpp:204
#37 0x00007f9ffbd38af3 in KJob::exec (this=0x11f0110) at ../../kdecore/jobs/kjob.cpp:210
#38 0x00007f9f9550090b in KMKernel::cleanup (this=0x11a8f20) at ../../kmail/kmkernel.cpp:1311


Reproducible: Always

Steps to Reproduce:
1. start Kontact, wait until it gets blocked on a "Retrieving Folder Contents" without any progress indicator showing activity (preferably start with --nofork from a terminal). Configure it to show the mail indicator systray icon.
2. Quit Kontact through the menu action
3. Observe that the systray icon is still there (and kontact still runs); right-click the systray icon

Actual Results:  
crash because the user input event is not ignored

Expected Results:  
in this context: user event ignored