What do these changes do?

Add a new USE_SYSTEM_DEPS environment variable that can be used to enable building against the system install of llhttp library rather than the vendored one.

This is implemented using the pkgconfig package to query llhttp install via pkg-config. The package is only used when USE_SYSTEM_DEPS is enabled, however due to limitations of pyproject.toml it is listed unconditionally as a build dependency. That said, flake8 claims that the dependency is not listed and I don't really know how to solve that:

setup.py:36:5: I900 'pkgconfig' not listed as a requirement

I have also updated aiohttp/_cparser.pxd to reference the llhttp.h header via its name rather than full path. I have poisoned the system header to confirm that the correct header is used both when building against the system library and the vendored library.

Are there changes in behavior for the user?

• no changes for users installing from wheels
• users installing from source will see an additional pkgconfig build requirement
• users installing from source who enable USE_SYSTEM_DEPS will be using the system llhttp library

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

I don't think so. The code uses pkg-config, so it should be pretty forward-secure to potentially nonstandard llhttp installs. On top of that, presumably the feature will be primarily used by downstreams / expert users, who would rather be willing to submit fixes, should any issues arise (though that implies the burden of reviewing them).

Related issue number

Discussion #10759.

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
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• Add a new news fragment into the CHANGES/ folder

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

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    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
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    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

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