Persistent.__setstate__ C SILENTLY fails on non-builtin dict

[agroszer]

Key interning introduced in https://github.com/zopefoundation/persistent/commit/7bd85c189ae17be514f4989bfb8f79f4d8013943 makes __setstate__ SILENTLY fail when a non-builtin dict is passed as state.

See https://github.com/agroszer/persistent/commit/1c1cebaf79914420a9de61e55fb72a75981965b2 how to provoke that.

• is there any constraint that state must be a builtin dict ?
• the C and python implementation differs, because the python one does not fail
• a major issue is that it silently fails, just not setting attributes and returning

[stephan-hof]

I see a few possible options here:

• Check if this optimization is needed at all. The dict object got some improvements in newer python versions. For example: https://www.python.org/dev/peps/pep-0412/ However this applies only to python3 and not python2, so I think it's not a real option.
• Use of PyMapping_Items and PyObject_SetItem instead of PyDict_Next. If you need the speed of PyDict_Next a check via PyDict_CheckExact could be done. If it returns False use the generic methods as a fallback.
• raise an exception if the dict is not a builtin dict.
• Don't do the optimization if PyDict_CheckExact returns False. Which is the most promising one, since its easy to implement. Yes you loose the optimization if a non builtin dict comes in, but I guess this does not happen very often.