Fixed crashed in _dill._is_builtin_module when a module's __file__ is None

[miguelinux314]

Summary

The _is_builtin_module function of dill/_dill.py has the following check in line 1587

if not hasattr(module, "__file__"): return True

If the check is not passed, then os.path.realpath(module.__file__) is called later in that function (line 1591).

This causes dill to crash when a module has __file__ set to None. This PR fixes that check so that when module.__file__ is None, the module is recognized as not builtin.

Notes:

• Even though https://docs.python.org/3/reference/import.html#file__ specifies that when __file__ is set it is a string, it will actually be none when importing a module without an __init__.py file in it.
• This PR only changes the behavior of dill to prevent TypeError crashes. Code that did not crash before is unaffected by this PR.
• No issue created prior to this PR)

[mmckerns]

@miguelinux314: Thanks for catching this. In theory, __file__ could be set to anything, right? So, to mitigate a TypeError, would it be better to either type-check (i.e. with type) or to catch a TypeError? If None is the most common case, then you could check that as you have now first...

[miguelinux314]

Thank you @mmckerns for the quick reply.

Yes, the __file__ attribute of any module (built-in or not) can be manually set to anything.

On the other hand, under certain conditions (see https://github.com/python/cpython/blob/423459be2f0b6d007e5f235f39d80044cb099faf/Lib/importlib/_bootstrap.py#L667, caused by https://bugs.python.org/issue32305) importlib does set it to None without complaining or manually fidgeting with the __file__ attribute.

Based on this, I don't think one can get the __file__ attribute to differ both from a valid path string and None without actively trying to break the code. That's why I checked only for None and left other cases to crash. However, if you feel the dill library would be sturdier considering the manual fiddling cases, my recommendation would be a try-catch of lines 1620 to 1629 with TypeError. I'd be happy with either solution.

Thanks again for you time and help

[mmckerns]

I'll take it as is. thanks