bpo-1635741 - try to port pickle to multi-stage init

[koubaa]

Right now this patch causes a segfault:

test_unpickle_module_race (test.test_pickle.PyUnpicklerTests) ... Segmentation fault

It's really a clever test. Anyways hopefully the cause is apparent to reviewers. Otherwise I can try to debug it on my own some more

https://bugs.python.org/issue1635741

[koubaa]

Found the problem. PyState_FindModule is used heavily (via _Pickle_GetGlobalState) and this is only supported for single phase init. I could replace some of those with PyType_GetModuleState but some types use Py_TPFLAGS_BASETYPE which means it may not be corrent in one for subclasses.

@vstinner Does PEP 573 (or a subsequent PEP) offer any alternative to that?

[vstinner]

@vstinner Does PEP 573 (or a subsequent PEP) offer any alternative to that?

Not yet. Until PEP 573 is completed, I suggest to leave extensions which require this feature unmodified. So I suggest to close this PR for now, for example.

cc @corona10 @shihai1991 @encukou

[shihai1991]

ps: you can attention petr's thread if you have free time: https://mail.python.org/archives/list/capi-sig@python.org/thread/T3P2QNLNLBRFHWSKYSTPMVEIL2EEKFJU/