ENH some steps to make cloudpickle dynamic function/classes more deterministic

[tomMoral]

• add some utilities to create pickle diff to check for determinist pickle.
• add some tests to check if the same dynamic class, constructed from scratch or loaded from a cloudpickle stream, leads to the same pickle file:
  • Check for attribute order issues.
  • Check for some string interning issues.

One case seems unsolvable: when updating the class state, all attrname for a class are interned. This is why we are also interning the dynamic func names, to be consistent between their name and the method names (all interned).
But if some values in the pickle stream (anywhere) have the same value as one attrname, then these two strings which are the same in the original pickle file become different in the unpickled objects. Typically, pickling the following original dynamic class results in memorization of join between the method name and the argument value, while pickling the unpickled class leads to no memorization: arg_1 = "join" value is not interned. In contrast, the join method name is.

class A:
    '''Class with potential string interning issues.'''
    arg_1 = "join"

    def join(self, arg_1):
        pass

Not sure if there is a solution for this one.
Note that this test can be run locally with pytest -vv --runxfail -k determinist_subworker_str_interning

Related issues: #510, #453.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (25aef95) 96.08% compared to head (e18678f) 96.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
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+ Coverage   96.08%   96.12%   +0.04%     
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  Files           3        3              
  Lines         562      568       +6     
  Branches      116      123       +7     
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+ Hits          540      546       +6     
  Misses         11       11              
  Partials       11       11              

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[ogrisel]

Some early feedback.

[ogrisel]

Here is a pass of feedback. I included some comments on my analysis of the xfailed test about string literals in class attributes. Interning is probably not a safe solution in this case...

[tomMoral]

I found a better solution to string interning: prevent memoization for problematic strings that might be interned or not.
This is the reverse operation compared to the original proposal that tried to intern everything but this allows to only act on the problematic memoization.
Now all tests pass, except the one about the class hash that we will tackle in a follow-up PR.

For the use of difflib, I am not really convinced because it is useful to see the diff with the full payload to be able to debug (otherwise if is hard to tell from where BINGET 11 comes. This is possible with pytest -vv and the original solution, while difflib is not really telling the full story. So I propose to revert the difflib commit.

[ogrisel]

More feedback on the new strategy.

[ogrisel]

Can you please add a changelog entry?

There seems to be a leftover call to sys.intern (see below).

Also, I would rather avoid duplicating variations of the same inline comment all over the place and instead make them all point to the inline comment of _class_setstate to limit redundancy and make it more maintainable.

[ogrisel]

LGTM with the missing changelog entry.

[ogrisel]

Thanks @tomMoral!