Missing critical sections for `PyDict_Next` calls in `_functoolsmodule.c`

[bkap123]

Bug report

Bug description:

After some discussion on PR #145362, I noticed that in Modules/_functoolsmodule.c, all use cases of PyDict_Next do not use a critical section, which is necessary in for the free-threaded build. The docs say that for PyDict_Next:

The function is not thread-safe in the free-threaded build without external synchronization. You can use Py_BEGIN_CRITICAL_SECTION to lock the dictionary while iterating over it

I have not checked if other files are missing a critical section, but it is definitely possible. I will draft a PR shortly for this issue in the coming days, but if anyone else finds other occurrences of PyDict_Next that are not thread-safe, please let me know.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-145446: Add critical section in functools module for PyDict_Next #145487

[aisk]

A critical section is not free, it will introduce some performance regressions. If the code is only accessed by a single thread, the additional performance overhead is unnecessary. Therefore, not all APIs are thread-safe in Python under the nogil build.

I’m not sure whether we should make PyDict_Next thread-safe, but I think we should evaluate the performance impact before continuing the work.

[aisk]

There is the original issue which intentionally removed the critical section: #120858, according to it, I think we should not make the PyDict_Next thread-safe.

I will tag this issue as pending and will close it if no additional information is provided.

[aisk]

Sorry, I re-read your issue, you mean make _functoolsmodule thread-safe when calling PyDict_Next by adding a critical section? The title is a little bit misleading 😂 Updated the title, and please feel free to modify it if I'm wrong.