Avoid reference cycling by the garbage collector during response reading

[hauntsaninja]

Pull Request check-list

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Description of change

This won't meaningfully affect most users, so I won't feel bad if this is closed :-)

It's often important for performance to control when garbage collection runs and to reduce the need to run garbage collection in the first place.

During AbstractionConnection.on_connect, it's common to raise and catch ResponseError, since e.g. CLIENT SETINFO is very new. However, because response is in a local, it creates ref cycles that keep all locals in all calling stack frames alive.

The use of a local to store the exception is unfortunate, since exceptions hold references to their tracebacks, which hold references to the relevant frames, which holds a reference to the local exception. See https://peps.python.org/pep-0344/#open-issue-garbage-collection and https://peps.python.org/pep-3110/#rationale

This breaks the cycle by deleting the local when we raise, so frames are destroyed by the normal reference counting mechanism.

Fix suggested by JelleZijlstra

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% ⚠️

Comparison is base (e3de026) 91.29% compared to head (f1df163) 91.27%.
Report is 5 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2932      +/-   ##
==========================================
- Coverage   91.29%   91.27%   -0.03%     
==========================================
  Files         126      126              
  Lines       32391    32393       +2     
==========================================
- Hits        29571    29566       -5     
- Misses       2820     2827       +7     

Files Changed	Coverage Δ
redis/connection.py	81.17% <100.00%> (+0.05%)	⬆️

... and 5 files with indirect coverage changes

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