fix[tool]: update VarAccess pickle implementation

[charles-cooper]

What I did

fix a bug where unpickling annotated_vyper_module would lead to a crash:

AttributeError: 'VarAccess' object has no attribute 'variable'

a repro can be found at charles-cooper@2a5facf.

this is a blocker for tooling, for instance titanoboa relies on pickle/unpickle to cache CompilerData objects:
https:/vyperlang/titanoboa/blob/86df8936654db20686410488738d7abaf165a4c9/boa/util/disk_cache.py#L65-L66

the underlying issue is that pickle.loads() calls obj.__hash__() for objects that are keys in a hashed data structure - namely, dicts, sets and frozensets. this causes a crash when there is a cycle in the object graph, because the object is not fully instantiated at the time that __hash__() is called. this is a cpython bug, reported at python/cpython#124937.

@serhiy-storchaka suggested the approach taken in this PR, which breaks the loop before pickling: python/cpython#124937 (comment).

How I did it

How to verify it

Commit message

fix a bug where unpickling `annotated_vyper_module` would lead to
a crash:

```
AttributeError: 'VarAccess' object has no attribute 'variable'
```

this is a blocker for tooling, for instance, titanoboa
relies on pickle/unpickle to cache `CompilerData` objects:
https:/vyperlang/titanoboa/blob/86df8936654db2068641/boa/util/disk_cache.py#L65-L66

the underlying issue is that `pickle.loads()` calls `obj.__hash__()`
for objects that are keys in a hashed data structure - namely, dicts,
sets and frozensets. this causes a crash when there is a cycle in
the object graph, because the object is not fully instantiated at the
time that `__hash__()` is called. this is a cpython issue, reported at
python/cpython#124937.

@serhiy-storchaka suggested the approach taken in this PR, which breaks
the loop before pickling:
https:/python/cpython/issues/124937#issuecomment-2392227290

note that the implementation of `__reduce__()` in this PR is safe, since
there is no cycle in the hash function itself, since the recursion
breaks in `VarInfo.__hash__()`. in other words, there is no possibility
of `VarAccess.__hash__()` changing mid-way through reconstructing the
object.

Description for the changelog

Cute Animal Picture

[cyberthirst]

can we add an explainer comment on why's this necessary?

[charles-cooper]

can we add an explainer comment on why's this necessary?

will get to it

[serhiy-storchaka]

You could also use the following trick if all fields can be passed as keyword arguments to constructor in subclasses:

Suggested change

	return self.__class__._produce, (dict_obj,)
	return partial(self.__class__, **dict_obj), ()

[charles-cooper]

interesting, is there a performance difference?

[serhiy-storchaka]

It may be slower. The advantage is that you do not need to add private method _produce().

[charles-cooper]

copying in some offline discussion with @cyberthirst about why the hash is safe even though the data structure is cyclic:

There is no cycle in the hash function, it breaks at VarInfo.

vyper/vyper/semantics/analysis/base.py

Line 182 in c02d2d8

return hash(id(self))

So the contents of the _expr_info._reads/_writes can't affect the value of VarAccess hash.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.07%. Comparing base (c02d2d8) to head (15efb2e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4270      +/-   ##
==========================================
- Coverage   91.30%   90.07%   -1.23%     
==========================================
  Files         110      110              
  Lines       15764    15770       +6     
  Branches     3461     3463       +2     
==========================================
- Hits        14393    14205     -188     
- Misses        935     1095     +160     
- Partials      436      470      +34     

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