Threads::number_of_threads() is a plain _int_ but accessed sometimes like it was an atomic that never can go back to zero. Some use-cases are certainly suspicious and we must investigate if they can handle the raciness of this method. If you have Threads_lock this method behaves as intended, except it can go back to zero before VM has finished exiting. Which makes a check against zero also mean we are exiting.

Most of the use cases wants to know if we have 'booted' and the other should probably use a (SMR) ThreadsList. So I suggest removing this method and create a new method for 'have VM booted'.