[Bug 1290514] [NEW] LibreOffice KDE4 backend crashes in Qt4 recursive paint events

[Jan-Marek Glogowski]

Public bug reported:

Distribution: Kubuntu 12.04
Package: qt4-x11 4:4.8.1-0ubuntu4.6 
Program: LibreOffice 4.3 (current master build).

This bugs origin is a crash when using KDE4 native file picker dialogs
from within LibreOffice [1].

There are several similar ones in all distributions.

In the backtrace you can see, that LO calls the Qt event dispatcher,
which processes a draw event, which - at some point - waits for the
clipboard using the Qt / LO event loop, which processes another (now
nested) draw event -> crash.

This happens, because LO sets
  qApp->clipboard()->setProperty( "useEventLoopWhenWaiting", true );
because the X11 event processing in clipboardWaitForEvents would interfere with the LO X11 handling / locking.

QX11Data::clipboardWaitForEvents (gui/kernel/qclipboard_x11.cpp:520)
actually installs the correct clipboard event filter in the qapp, which
should just handle Clipboard events.

But actually the Qt internal glib event loop doesn't honor the
QEventLoop::ExcludeSocketNotifiers (see
src/corelib/kernel/qeventdispatcher_glib.cpp).

Originally I opened [2], because I didn't want to deep dive into the Qt
codebase. Since I didn't get any usable replies and all my implemented
workarounds in LibreOffice didn't work I tried to understand the
backtrace from [2] and realized the missing
QEventLoop::ExcludeSocketNotifiers handling in the Qt glib code. I
opened [3] and pushed a patch for Qt review as [4].

[1] https://bugs.freedesktop.org/show_bug.cgi?id=69002
[2] https://bugreports.qt-project.org/browse/QTBUG-34614
[3] https://bugreports.qt-project.org/browse/QTBUG-37380
[4] https://codereview.qt-project.org/#change,80528

** Affects: qt4-x11 (Ubuntu)
     Importance: Undecided
         Status: Triaged

** Affects: qt4-x11 (Ubuntu Precise)
     Importance: Undecided
         Status: New

** Affects: qt4-x11 (Ubuntu Saucy)
     Importance: Undecided
         Status: New

** Affects: qt4-x11 (Ubuntu Trusty)
     Importance: Undecided
         Status: Triaged


** Tags: kubuntu

** Attachment added: "LibreOffice backtrace of the crash."
   https://bugs.launchpad.net/bugs/1290514/+attachment/4017137/+files/bt-repaint-error.txt

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https://bugs.launchpad.net/bugs/1290514

Title:
  LibreOffice KDE4 backend crashes in Qt4 recursive paint events

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