gh-151524: Avoid using instrumentation callback result after Py_DECREF

[lpyu001]

• Issue: instrumentation.c: Use-after-Py_DECREF comparison in call_one_instrument #151524

[chris-eibl]

The change lgtm, but I'd create a news entry, like almost all of the fixes for the umbrella issue #146102 did so far.

cc @pablogsal

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[lpyu001]

The change lgtm, but I'd create a news entry, like almost all of the fixes for the umbrella issue #146102 did so far.

cc @pablogsal

I’ve submitted the news entry.thanks

[sobolevn]

Question: was it ever tested? Or was just the RC of res higher than 1, so no crash happened?

[sobolevn]

This would be a user-facing news entry. Users care about crashes (which could happen here), not about RC :)
Let's rephrase it.

[ZeroIntensity]

This isn't UB. _PyInstrumentation_DISABLE is an immortal object; Py_DECREF operations on it are a no-op.

>>> import sys
>>> sys._is_immortal(sys.monitoring.DISABLE)
True

That said, I do agree that it's misleading. Let's either remove the Py_DECREF call entirely and/or add an assertion that it's immortal.

[ZeroIntensity]

This is technically just a refactor, so it's not user-facing. Let's remove this.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.