File 0256-fix-recursive-backingstore-sync-crash.diff of Package libqt4

qt-bugs@ issue : N227209
Trolltech task ID : none yet number : 174065
applied: yes
author: Szymon Tomasz Stefanek <>

This patch fixes a crash deep inside the qt painting engine.

The toplevel shared painter is instantiated by the topmost window
which "owns" the backingstore buffer. The topmost window then recursively
asks the children to paint themselves with the shared painter.
With certain widget hierarchies it turns out that the topmost window
may be asked to paint itself deep inside the recursive painting stack:
a sort of "hierarchy-looping recursion".
The window will do the job and then happily destroy the shared
painter leaving the outer stack frames with a dangling pointer.

This patch stops the "looping recursion" when it's triggered
with a shared painter already active. The bug doesn't seem to
be present in qt 4.5 snapshots, but in the meantime we need this fix.

Index: src/gui/painting/qbackingstore.cpp
--- src/gui/painting/qbackingstore.cpp	(revision 879741)
+++ src/gui/painting/qbackingstore.cpp	(working copy)
@@ -987,8 +987,12 @@
-        if (tlw->updatesEnabled()) {
+        // With certain widget hierarchies we may end up being called recursively
+        // on the same toplevel. This is likely to explode once the painter is released
+        // in the code below (since there is no reference counting). Avoid it.
+        bool alreadyPainting = tlwExtra->sharedPainter && tlwExtra->sharedPainter->isActive();
+        if (tlw->updatesEnabled() && !alreadyPainting) {
             // hw: XXX the toClean region is not correct if !dirtyWidgets.isEmpty()
             // Pre render config
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