Remove threading patches and don't remove multiprocessing module #796
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This was referenced Dec 18, 2020
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Related to #144
We currently patch included packages to replace
threadingbydummy_threadingimports. Alsomultiprocessingmodule is removed (and so fails to import), requiring us to patch any package that uses it.In this PR,
threadingbydummy_threading. Usingdummy_threadingwill lead to deadlocks when thread communication is expected, while objects likethreading.RLlock,threading.Lockare actually working fine. Whenthreading.Thread(..).start()is called it will produce a "can't start new thread" exception which I think is a reasonable message in the context of pyodide.If the use of
dummy_threadingis desired it can be monkeypatched globally at run time cf Handling unsupported CPython stdlib modules #144 (comment)multiprocessing.cpu_count()returns 16 (maybe that is hard-coded in emscripten?). Starting a new Process will raise aResource temporarily unavailableexception, while callingos.fork()which I think is also reasonable.Overall the goal is to noticeably reduce the amount of patches we have to maintain, and also support more packages from PyPi with micropip without requiring patches.
I added tests to checks the above mentioned behavior. This also adds a few test improvements with a more systematic use of
pytest.raiseswhen needed.