[PR #11464/945837cc backport][3.12] fix thread safety of http writer

[patchback]

This is a backport of PR #11464 as merged into master (945837c).

What do these changes do?

This makes the http writer cython extension thread safe. Previously it used a global static buffer which would be shared across threads, now the buffer is stack allocated which avoids this and makes it thread safe.

This also marks the cython extensions with freethreading_compatible marker.

Are there changes in behavior for the user?

No change.

Is it a substantial burden for the maintainers to support this?

No

Related issue number

#8796

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES/ folder

  • name it <issue_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

  • if you don't have an issue number, change it to the pull request
    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    • .feature: A new behavior, public APIs. That sort of stuff.
    • .deprecation: A declaration of future API removals and breaking
      changes in behavior.
    • .breaking: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    • .doc: Notable updates to the documentation structure or build
      process.
    • .packaging: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    • .contrib: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

  • Make sure to use full sentences with correct case and punctuation,
    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

[codspeed-hq]

CodSpeed Performance Report

Merging #11468 will not alter performance

_{Comparing patchback/backports/3.12/945837cc2d04341cd54df57f17444b8f9a349eb4/pr-11464 (c1f5f4a) with 3.12 (04b2ef6)}

Summary

✅ 59 untouched benchmarks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.22%. Comparing base (04b2ef6) to head (c1f5f4a).
⚠️ Report is 1 commits behind head on 3.12.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             3.12   #11468      +/-   ##
==========================================
- Coverage   98.23%   98.22%   -0.01%     
==========================================
  Files         130      130              
  Lines       43653    43653              
  Branches     2374     2374              
==========================================
- Hits        42882    42879       -3     
- Misses        595      598       +3     
  Partials      176      176              

Flag	Coverage Δ
CI-GHA	98.12% <ø> (-0.01%)	⬇️
OS-Linux	97.87% <ø> (-0.01%)	⬇️
OS-Windows	95.68% <ø> (+<0.01%)	⬆️
OS-macOS	97.22% <ø> (+0.01%)	⬆️
Py-3.10.11	96.86% <ø> (ø)
Py-3.10.18	97.26% <ø> (ø)
Py-3.11.13	97.45% <ø> (+<0.01%)	⬆️
Py-3.11.9	97.06% <ø> (+<0.01%)	⬆️
Py-3.12.10	97.14% <ø> (ø)
Py-3.12.11	97.55% <ø> (-0.01%)	⬇️
Py-3.13.3	97.78% <ø> (-0.01%)	⬇️
Py-3.9.13	96.76% <ø> (+<0.01%)	⬆️
Py-3.9.23	97.16% <ø> (ø)
Py-pypy7.3.16	90.47% <ø> (+4.90%)	⬆️
VM-macos	97.22% <ø> (+0.01%)	⬆️
VM-ubuntu	97.87% <ø> (-0.01%)	⬇️
VM-windows	95.68% <ø> (+<0.01%)	⬆️

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