Make queue.SimpleQueue thread-safe in `--disable-gil` builds

[mpage]

Feature or enhancement

Proposal:

Instances of queue.SimpleQueue have mutable state that is not safe to mutate without the GIL. The previous implementation (in 3.12) was colesbury/nogil-3.12@7e60a01aee.

@colesbury - You mentioned in chat that porting the 3.12 approach wouldn't be straightforward. Can you say more?

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-113884: Make queue.SimpleQueue thread-safe in --disable-gil builds #114161
• gh-113884: Refactor queue.SimpleQueue to use a ring buffer to store items #114259

[colesbury]

@mpage, after looking it over again, I think the nogil-3.12 approach is mostly the right direction with a few modifications:

1. Replace the manual _PyMutex_lock()/unlock() calls with critical sections and remove the _PyMutex mutex field. Since, the code already uses Argument Clinic (AC), this is mostly just a matter of adding the @critical_section to the AC input. This will avoid the extra lock operations in the default build, which can rely on the GIL.
2. The parking lot API changed slightly; it's not a huge difference. For example, the unpark operation now takes a callback

This means we will go from a SimpleQueue implementation built around list to run that more explicitly manages a buffer. I think that's worthwhile here; otherwise I think handling the synchronization across both the list operations and the containing SimpleQueue will be too messy.

[erlend-aasland]

Thank you, @mpage and @colesbury 🙏