Fix dill._dill submodule being saved as GLOBAL "dill._shims" "_dill"

[leogama]

I'm tracing some problems with my "portable" mode prototype and hit a bug with references to the _dill submodule saved as global:

>>> import dill
>>> dill.dumps(dill._dill, 0)
b'cdill._shims\n_dill\np0\n.'

The _dill submodule is saved as an attribute of the dill._shims submodule, which is valid, but it should be just dill._dill.

The _dill submodule is special-cased to be saved as global:

dill/dill/_dill.py

Lines 1807 to 1810 in 8e5e450

	elif PY3 and obj.__name__ == "dill._dill":
	log.info("M2: %s" % obj)
	pickler.save_global(obj, name="_dill")
	log.info("# M2")

But pickle.whichmodule misidentifies it as pertaining to dill._shims, because this entry is set earlier in sys.modules and it picks the first match.

---

The change fixes things for this especial case, but there is potential for new bugs related to other submodules:

>>> import dill, glob, importlib, os, pickle, pprint
>>> os.chdir(dill.__path__[0])
>>> modules = [mod.rpartition('.')[0] for mod in glob.glob('*py') if not mod.startswith('__')]
>>> modules = {mod: importlib.import_module('dill.' + mod) for mod in modules}
>>> pprint.pprint({name: pickle.whichmodule(mod, name) for name, mod in modules.items()})
{'_dill': 'dill._shims',
 '_objects': 'dill',
 '_shims': 'dill._dill',
 'detect': 'dill',
 'objtypes': 'dill',
 'pointers': 'dill',
 'settings': '__main__',
 'source': 'dill',
 'temp': 'dill'}

Note how _shims is attributed to dill._dill and settings is attributed to __main__(???). But currently they are not saved as globals.

[anivegesana]

That behavior is bizarre. I guess in CPython, they assumed that every global would have a __module__ and they didn't allow modules to be pickled, so it worked out fine for them. Just not for us.

[mmckerns]

LGTM

[mmckerns]

Nice find. I can't wait to see what unintended consequences this has...