gh-115618: Remove improper Py_XDECREFs in property methods

[serhiy-storchaka]

• Issue: Incorrect Py_XDECREFs in property #115618

[serhiy-storchaka]

It has no effect in main, but in 3.11 the test crashes without the fix.

[eltoder]

@serhiy-storchaka Currently your test passes on 3.12 and main without the fix, because None is immortal. AFAICT it is not possible to write a test to catch refcount bugs for immortal objects. What do you think if we change the debug versions of Py_INCREF/Py_DECREF to update the stats even for immortal objects? Then your test will catch this bug even on 3.12+.

[Eclips4]

@serhiy-storchaka Currently your test passes on 3.12 and main without the fix, because None is immortal. AFAICT it is not possible to write a test to catch refcount bugs for immortal objects. What do you think if we change the debug versions of Py_INCREF/Py_DECREF to update the stats even for immortal objects? Then your test will catch this bug even on 3.12+.

Personally I do not think that this is worth the effort, and this isn't will make test catch this bug (because this test goes along with the fix), it's just make bug reproducible.

Actually there's a test (Lib/test/test_capi/test_immortal) which relies on current behaviour.

[eltoder]

@Eclips4 no, that test doesn't do anything with the total refcount, it only checks that decrefing an immortal object doesn't change its refcount. So it will be fine. We can improve the test by checking incref as well, which will also make it balanced. The test also hard-codes the boundaries of the smallint cache instead of using the macros, which can also be improved.

[serhiy-storchaka]

What do you think if we change the debug versions of Py_INCREF/Py_DECREF to update the stats even for immortal objects?

I afraid that it is too late, and many tests will show unbalanced refcounts.

[miss-islington-app]

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @serhiy-storchaka, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 090dd21ab9379d6a2a6923d6cbab697355fb7165 3.12

[miss-islington-app]

Sorry, @serhiy-storchaka, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 090dd21ab9379d6a2a6923d6cbab697355fb7165 3.11

[bedevere-app]

GH-115620 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-115621 is a backport of this pull request to the 3.11 branch.