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Subclasses of JSONEncoder that use check_circular=False can segfault json:
>>> import json
>>> class EndlessJSONEncoder(json.JSONEncoder):
... def default(self, o):
... return [o]
... 
>>> EndlessJSONEncoder(check_circular=False).encode(5j)
Segmentation fault
The attached patch fixes it raising a "RuntimeError: maximum recursion depth exceeded".
There might be other ways to get a segfault, because there are other recursive calls involving more functions (i.e. a calls b, and b calls a) that are not covered by the patch.