Well, it's hard for me to tell from this stack trace what's happening here. I don't recognize PyObject_VectorcallMethod. Is this a new thing? Last time I wro

Well, it's hard for me to tell from this stack trace what's happening here. I don't recognize PyObject_VectorcallMethod. Is this a new thing? Last time I wrote any C code for Python I don't recall seeing / using this. Anyways, another common pitfall in multiprocessing is attempting to serialize multithreading / multiprocessing primitives s.a. locks, variables or mutexes. My memory may fail me, but, I think, it may result in deadlock too. I think, multiprocessing code tries to guard against it, but there are some weird rules for when it's OK for serialized objects to have those primitives (I think, initialization in __init__ is fine, but not so much otherwise or something like that), but the check isn't very good / just a heuristic... But, really, I don't remember this part well.