gh-132942: Fix races in type lookup cache

[nascheme]

Two races related to the type lookup cache, when used in the free-threaded build. This caused test_opcache to sometimes fail (as well as other hard to reproduce failures).

The first problem is that find_name_in_mro() can block on some mutex and then release critical sections. If that happens, the type version used for the cache entry can be wrong (too new). Assigning the version before doing the find fixes this issue. If it does race, you will add an entry that uses an out-of-date version.

The second problem was much harder to track down. There is a hard to trigger race in update_cache(), writing to cache, and _PyType_LookupStackRefAndVersion(), reading from cache. We use a sequence lock to avoid races. However, if the reader reads the old entry value and the new entry version, it will try to execute _Py_TryXGetStackRef() on a stale cache entry value. If that value has been deallocated, PyStackRef_XCLOSE() will crash. This could happen before because the version was written first and then new value second.

The fix is simply to write the entry value first and the version after. That way, the reader always sees a value at least as new as the version.

Possible scenarios for the reader of the cache entry, as it is being written to concurrently:

entry version	entry value	outcome
old	old	Okay, type version will not match
old	new	Okay, incref/decref works, seq check fails
new	old	Bad, incref/decref on old value might crash
new	new	Okay, incref/decref works, seq check fails

• Issue: test_opcache fails randomly (failure or crash) #132942

[nascheme]

Here is a script that triggers the crash. It can take a while, especially if running under "rr".

crash_mro_lookup.py.txt

[colesbury]

LGTM

[miss-islington-app]

Thanks @nascheme for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @nascheme, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 31d1342de9489f95384dbc748130c2ae6f092e84 3.13