MacOS X: using cursor.setBlockFormat in QSyntaxHighlighter::highlightBlock deadlocks

The attached program deadlocks in QFragmentMapData<QTextBlockData>::next(unsigned int). The problem is the use of cursor.setBlockFormat in the QSyntaxHighlighter::highlightBlock method.

To consistently reproduce the bug, set MallocPreScribble=1 and MalloScribble=1 in the environment. Without these two environment variables, the behaviour is less consistent. Sometimes the app crashes, sometimes it just hangs.

Steps to reproduce the bug:

compile with:

g++ -framework QtGui -framework QtCore -F/usr/local/qt/lib/ -I/usr/local/qt/include/QtGui crash.cc -o crash

run with:

MallocPreScribble=1 MallocScribble=1 ./crash

The same code works flawlessly with Qt 4.6 and earlier.