gh-3108: Avoid materializing f_locals for KI protection

[graingert]

Fixes #3108
Fixes #2670

In this pull request, we avoid materializing f_locals by using weak references to code objects instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62377%. Comparing base (13d4bad) to head (5d76de2).
Report is 250 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main       #3110         +/-   ##
===================================================
+ Coverage   99.58714%   99.62377%   +0.03662%     
===================================================
  Files            121         122          +1     
  Lines          18166       18340        +174     
  Branches        3268        3281         +13     
===================================================
+ Hits           18091       18271        +180     
+ Misses            52          47          -5     
+ Partials          23          22          -1     

Files with missing lines	Coverage Δ
src/trio/_core/_ki.py	100.00000% <100.00000%> (ø)
src/trio/_core/_run.py	99.02344% <100.00000%> (-0.00381%)	⬇️
src/trio/_core/_run_context.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_ki.py	99.77169% <100.00000%> (+1.93218%)	⬆️
src/trio/_core/_tests/test_run.py	100.00000% <100.00000%> (ø)
src/trio/_tools/gen_exports.py	95.90164% <100.00000%> (ø)

[A5rocks]

Plenty of nitpicks or me being confused; please don't assume I know what I'm doing with these comments. If the question doesn't make sense, it's cause I don't understand and not some weird encoded question.

[graingert]

It looks like this will fix #1752

and maybe in conjunction with #3112 it will fix this #2454

[A5rocks]

A bunch of really minor comments. This could merge as is without any changes IMO.

[A5rocks]

Nevermind, I was thinking about whether a deprecation is possible but it isn't because the problem is functions having ki protection only sometimes.

[graingert]

pre-commit.ci autofix

[A5rocks]

Just to be explicit, since all my comments were fixed and all my questions answered.

[CoolCat467]

As far as I can tell, this looks pretty great