gh-142495 fix __missing__ data race condition

[fatelei]

The original defdict_missing function in

cpython/Modules/_collectionsmodule.c:2216-2239 had a race

condition where:

1. When missing is called, the factory function is invoked
2. Between the factory call and PyObject_SetItem(), another thread could insert a value for the same key
3. PyObject_SetItem() would overwrite the existing value, violating atomic

fix

1. Atomic Behavior: Uses PyDict_SetDefaultRef() which provides atomic "get or insert default" behavior
2. No Overwrites: Guarantees that existing values are never replaced
3. Thread Safety: The function handles concurrent access correctly
4. Factory Called Only When Needed: The factory is still only called when a key is initially missing
5. Preserves Semantics: The existing behavior and API remain unchanged

• Issue: defaultdict.__missing__ has data races #142495

[picnixz]

Note that someone actually asked on the issue if they could work on it. Please, communicate with them. cc @LindaSummer

In addition, read our policy about https://devguide.python.org/getting-started/generative-ai/ (the PR summary looks like AI-generated so I want to be sure that the PR hasn't been so).

[fatelei]

Note that someone actually asked on the issue if they could work on it. Please, communicate with them. cc @LindaSummer

In addition, read our policy about https://devguide.python.org/getting-started/generative-ai/ (the PR summary looks like AI-generated so I want to be sure that the PR hasn't been so).

the test code is generate by llm, the source code is not

[picnixz]

Tests shouldn't be written by LLM as they are exactly here to exercise whether the change is correct. In addition, it takes too much to review AI-generated test code so I'm going to close this, sorry.