ctypes thread safety auditing (and fixing)

[ZeroIntensity]

Feature or enhancement

This is a tracking issue for all thread safety problems related to ctypes. I'll be working on this, but others can feel free to give me some help.

First of all, we need to find where the thread safety problems are.

Auditing

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1. Audit _ctypes.c
2. Audit _ctypes_test.c (and probably the generated file too)
3. Audit callbacks.c
4. Audit callproc.c
5. Audit cfield.c
6. Audit malloc_closure.c
7. Audit stgdict.c

I'll be tracking the issues that get found when auditing here. The plan is to just create a new issue and link to it for each new problem instead of flooding this issue with PRs.

Generally, the workflow for fixes should follow most of the rules from #116738, but I suspect we'll need recursive mutexes for quite a few things related to callbacks, because it's difficult to tell what might be re-entrant, and we can't use critical sections for arbitrary function pointers.

Known Issues

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1. Crash when modifying DLL function pointers conconcurrently #127946
   3.13 3.14 extension-modules topic-ctypes topic-free-threading type-crash +6
   
2. ctypes pointer writes are not thread safe #128182
   3.13 3.14 extension-modules topic-ctypes topic-free-threading type-bug +6

cc @encukou, as the ctypes genius, and @colesbury as the free-threading mastermind.