Used a weakref.WeakSet just to stay on the safe side against memory leaks
Used a weakref.WeakSet just to stay on the safe side against memory leaks


Former-commit-id: 601382823eeb3f62e36e722b49e43fa658e5591a
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine