QMutex isn't the right tool for this:
- If tryLock succeeds, the code didn't unlock the mutex again, leading
to the runtime warning "QMutex: destroying locked mutex"
- But if we add "else data.mutex.unlock()" then we risk that the other thread also
calls unlock(), on a now-unlocked mutex, which is undefined behaviour.
QWaitCondition is the right tool for waiting for something to happen,
allowing us to unlock the mutex correctly in all cases.