gh-148450: abc.register needs to update type_version when tp_flags is changed.

[cocolato]

• Issue: abc.register changes tp_flags without updating type_version #148450

[nascheme]

This looks correct to me. Setting flags means that caches based on the type version should be invalidated. You can't call type_modified_unlocked() with the world stopped, since it is re-entrant. So you need the tricky process of acquiring type lock, ensuring it stays locked on stop-the-world.

Minor improvement suggestion: I would adjust the comment to explain that ideally the setting of flags and clearing (invalidation) of version would be done in one step. However, because the version invalidation must be done with the world not stopped, we need to do these two steps.

Regarding the 3.13 backport: the code is different in that version, there is no stop-the-world done. With 3.13, there are data races on on tp_flags (since they are read without a lock and without atomics). For the backport, you would just not worry about that and call type_modified_unlocked() while holding the type lock.