[ptweezy]

For the "Check latest documentation looks fine" step in #267.

On https://python-isal.readthedocs.io/en/latest/, the API documentation for isal_zlib, igzip and igzip_threaded is empty. Read the Docs builds on Ubuntu 22.04 (build.os in .readthedocs.yml), which installs libisal 2.30.0. The log of the latest build shows:

isal_zlib.cpython-314-x86_64-linux-gnu.so: undefined symbol: isal_zlib_header_init
WARNING: autodoc: failed to import module 'isal_zlib' from module 'isal'

isal_zlib_header_init was added in ISA-L 2.31. It is called by the preset dictionary fix from #257, so this started after 1.8.0. Anyone who links dynamically against ISA-L 2.30, for example on Debian bookworm or Ubuntu 22.04, gets the same ImportError. The 1.9.0 tag would have the same empty API pages.

This PR:

1. Removes the isal_zlib_header_init call. The function only sets the four fields of struct isal_zlib_header to zero, and the next four lines set all of them. ISA-L 2.30 already has the struct and isal_write_zlib_header.
2. Runs the docs and twine_check jobs on ubuntu-22.04, the same release as build.os. The docs job runs sphinx-build -W, so it fails when a module cannot be imported on Read the Docs. On ubuntu-latest it got libisal 2.31, which is why CI did not catch this.

Results with each build linked dynamically against the distribution's libisal, in containers:

Build	libisal	Import	sphinx-build -W	Tests
develop, Debian bookworm, CPython 3.14.8	2.30.0	undefined symbol	fails
This PR, Debian bookworm, CPython 3.14.8	2.30.0	works	passes	9304 passed, 216 failed
1.8.0 with its own tests, Debian bookworm, CPython 3.14.8	2.30.0	works	passes	9107 passed, 216 failed
This PR, Ubuntu 24.04, CPython 3.12.3	2.31.0	works	passes	9520 passed

The 216 failures are the same on 1.8.0 and on this PR. They are all in test_compat.py::test_compress_compressobj with wbits 9 to 14. ISA-L 2.30 always writes a window size of 32 KiB (CINFO 7) in the zlib header, and CPython's zlib.decompress rejects that when wbits is smaller. So this PR does not make ISA-L 2.30 work fully. It restores the 1.8.0 behavior, so the modules import and the documentation builds.

If you would rather require ISA-L 2.31, setting build.os to ubuntu-24.04 (libisal 2.31.0) also fixes Read the Docs, but dynamic builds against 2.30 would still fail at import.

There is no CHANGELOG entry, because no release has this problem.

Checklist

• Pull request details were added to CHANGELOG.rst (not needed: no release has this problem)
• Documentation was updated (if needed)