Occaisonally crashes on exit

[daniyall]

Stack trace

Traceback (most recent call last):
  File "/usr/lib/python3.5/multiprocessing/process.py", line 252, in _bootstrap
    util._exit_function()
  File "/usr/lib/python3.5/multiprocessing/util.py", line 288, in _exit_function
    _run_finalizers(0)
  File "/usr/lib/python3.5/multiprocessing/util.py", line 248, in _run_finalizers
    items = [x for x in list(_finalizer_registry.items()) if f(x)]
RuntimeError: dictionary changed size during iteration