gh-119333: Add c-api to have contextvar enter/exit callbacks

[fried]

This PR implements gh-119333, allowing for setting a callback function on PyContext_Enter and PyContext_Exit from the c-api

Co-authored-by: @itamaro

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📚 Documentation preview 📚: https://cpython-previews--119335.org.readthedocs.build/

[1st1]

I'm OK with this change. The motivation for adding it seems reasonable. I'll let @ericsnowcurrently review it properly, but I skimmed through the code and I like the approach.

[erlend-aasland]

Some minor remarks after a quick look: please align the naming with PEP-7 and use Py (not PY) prefixes for the macros.

[fried]

@ericsnowcurrently Am I doing this wrong, Having an issue with Check if generated files are up to date I followed the example for the other watcher checks and they use the same style globals. Also I can't run the check locally it on my mac it doesn't seem to like clang.

[ericsnowcurrently]

Am I doing this wrong, Having an issue with Check if generated files are up to date I followed the example for the other watcher checks and they use the same style globals.

Try copying similar entries in Tools/c-analyzer/cpython/ignored.tsv and then editing them to match the new variables.

Also I can't run the check locally it on my mac it doesn't seem to like clang.

Yeah, sorry, this is definitely a gap.

[kumaraditya303]

FWIW you can get interp from tstate which is already known, it would have avoided one more tls read which is slow on msvc.

[gvanrossum]

Sorry. We’ll fix it tomorrow. Any other concerns?

[kumaraditya303]

I don't understand this comment, what is meant by clue in? Also what is change in active python context?

[kumaraditya303]

Remove the =, it adds no useful info and is confusing.