Found an impressive threading bugp
I used to lock QReadWriteLock twice for reading when creating an iterator. Once for the method itself with a QReadLocker and once for the iterator. Sometimes that would block.

The problem is that QReadWriteLock favors write locks over read locks, i.e. if a write lock is pending no read lock can be aquired, even a recursive one like the one I did would not get through.

Now for example listStatement would lock, then create the iterator, and then try to lock again. But if between the first and second lock a write operation started, the second read lock would block and wait for the write which in turn waits for the first read lock. -> dead lock.

Wow, it took me hours and hours to find that bug. But it really shows how tricky threaded programming is. :)