gh-159136: Avoid stop-the-world when setting function name attributes

[nascheme]

Setting __name__, __qualname__, __doc__ or __module__ on a function stopped the world in the free-threaded build since gh-154826 and gh-154851, making functools.wraps() orders of magnitude slower with multiple threads.

Use the per-object critical section and _PyObject_XSetRefDelayed() instead, as gen_set_name() already does. Use acquire loads in the readers and hold strong references across error reporting, tracing, and eval-frame recording that may execute Python or detach the thread. The __code__, __defaults__, __kwdefaults__ and __closure__ setters still stop the world to clear func_version.

Document synchronization requirements for direct field access and the lifetime of borrowed function attributes.

Benchmark results (release free-threaded build, each thread creates its own function, 5000 iterations, time in seconds):

workload	build	1t	2t	4t	8t
set __name__	main	0.002	4.95	4.86	5.99
set __name__	this PR	0.001	0.001	0.001	0.002
set __name__ + __doc__	main	0.004	7.70	10.02	11.47
set __name__ + __doc__	this PR	0.002	0.002	0.001	0.002
functools.wraps	main	0.014	18.53	22.58	28.79
functools.wraps	this PR	0.010	0.010	0.010	0.014

Note: this PR was created with assistance from LLM agents (Claude Fable and GPT 6.1 Sol).

• Issue: Free-threaded build: setting a function's __name__/__doc__ stops the world; functools.wraps from multiple threads is orders of magnitude slower since 3.14.7 #159136

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #35106100 | 📁 Comparing 7837be5 against main (0768eed)

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6 files changed · ± 6 modified

± Modified

• builtins/stdtypes.html
• c-api/function.html
• c-api/reflection.html
• library/multiprocessing.html
• reference/expressions.html
• whatsnew/changelog.html