gh-123619: Fix PyUnstable_Object_EnableDeferredRefcount()

[vstinner]

Return 0 if the object is not tracked by the GC.

• Issue: Instance method performance issue with free threading (3.13rc1) #123619

[vstinner]

cc @ZeroIntensity @colesbury

[vstinner]

I propose backporting the fix to 3.14 and 3.15 branches. Enabling deferred refcount on an object which is not tracked by the GC can lead to nasty bugs.

[vstinner]

Tests / Sanitizers / TSan (free-threading) (pull_request): Failing after 23m

test_ssl failed.

So far, I failed to reproduce the issue locally. I built Python with ./configure --config-cache --with-thread-sanitizer --with-pydebug --disable-gil CC=clang LD=clang on Ubuntu 24.04. TSAN_OPTIONS='handle_segv=0' ./python -u -m test --tsan -v test_ssl 2>&1|tee log does not fail, but logs a warning when running test_sni_callback_race().

[vstinner]

Tests / Sanitizers / TSan (free-threading) (pull_request): Failing after 23m
test_ssl failed.

I created #151268 to log the output when a TSAN test fails.

[ZeroIntensity]

test_ssl being thread-unsafe is a known issue. This looks right.

[vstinner]

I created #151277 for the TSan+test_ssl issue.

[miss-islington-app]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-151326 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-151327 is a backport of this pull request to the 3.14 branch.