bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions

[habnabit]

http://bugs.python.org/issue29403

Cython will, in the right circumstances, offer a MethodType instance
where im_func is a builtin function. Any instance of MethodType is
automatically assumed to be a python-defined function (more
specifically, a function that has an inspectable signature), but
_set_signature was still conservative in its assumptions. As a result
_set_signature would return early with None instead of a mock since the
im_func had no inspectable signature. This causes problems deeper inside
mock, as _set_signature is assumed to always return a mock, and
nothing checked its return value.

In similar corner cases, autospec will simply not check the spec of the
function, so _set_signature is amended to now return early with the
original, not-wrapped mock object.

[the-knights-who-say-ni]

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[habnabit]

I signed the CLA when I opened the bug on bpo last week, but my account doesn't say received still: http://bugs.python.org/user25445

[ShalbafZadeh]

@habnabit
have you added you github username to your bpo account?
i can't see it
https://bugs.python.org/user6636

[habnabit]

@ShalbafZadeh that was an old account; I made a new one last week from a newer openid provider. http://bugs.python.org/user25445 is the new one, which I'm logged into now.

[ShalbafZadeh]

@habnabit seems odd

[habnabit]

Rebased and pushed, including a test fix. The third-party mock used six, and the stdlib does not include six.

[habnabit]

Rebased again!

[codecov]

Codecov Report

Merging #3 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   82.37%   82.37%   +<.01%     
==========================================
  Files        1427     1427              
  Lines      350948   350959      +11     
==========================================
+ Hits       289088   289104      +16     
+ Misses      61860    61855       -5

---

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[berkerpeksag]

I haven't had a time to dive into the whole life cycle of _set_signature() yet, but from a quick glance it looks good to me.

[berkerpeksag]

Please add an entry to Misc/NEWS. It should be in the "Library" section.

[habnabit]

Okay, rebased and added a NEWS entry.

[Yhg1s]

This change looks good to me: returning None does not make sense in the one place where _set_signature is called, and I don't think there's a better solution here. If the code wants to try harder and figure out what signature to use, that would be a change to _get_signature, not _set_signature, and this change would still make sense.

@voidspace, do you want to take a look, or can we merge?

[buhtz]

Can we merge this?