Use `dict.setdefault()` in functools caching for better thread-safety

Feature or enhancement

Proposal:

Currently, functools.cache family of functions uses _PyDict_SetItem_KnownHash. But I am wondering if it shouldn't use SetDefault style access at basically no-cost?

That way, when working with many threads, while multiple evaluations are still possible, all threads are guaranteed to end up with the same object.
There may be great reasons why that isn't wanted, but on first sight it seems like a simple and practically free way to remove a pit-fall for threaded code.
(Of course building a custom version that does this is easy, but functools.cache is great and used quite a lot.)

Admittedly, an optional once= would also be a very cool feature (or maybe as a dedicated pattern).

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-154069

Start with the issue's description of functools.cache, _PyDict_SetItem_KnownHash, and SetDefault, then review linked PR gh-154069. Determine the intended thread-safety and object-identity semantics before changing anything; done would require an accepted design and corresponding implementation and tests.