Provide a clear option to depend on zlib-ng

[hmaarrfk]

This is more of a nit than anything, but the problem arose in conda-forge where we still haven't gone all-in on zlib-ng.

As such, we ship BOTH zlib, and zlib-ng, which are designed to co-exist.

I thus wrote a patch for pillow to be able to directly target zlib-ng.

I figured I would upstream it and maybe this can help other distributions with this predicament.

xref:

• Pillow 12.0.0 with zlib-ng  conda-forge/pillow-feedstock#173
• Question: Can this be used as a drop-in replacement for the regular zlib? conda-forge/zlib-ng-feedstock#10
• zlib-ng strategy -- drop in for zlib vs side by side installation conda-forge/conda-forge.github.io#2638

I'm happy to address any issues you all have with this but the idea is that:

if zlib-ng is found, then choose that, and just use the zlib-ng headers directly instead of zlib.h.

---

TODO:

• Add an explicit option to link to prioritize zlib.h or zlib-ng.h -- @mgorny

[aclark4life]

I figured I would upstream it and maybe this can help other distributions with this predicament.

Are we the upstream and what is the predicament? 😄

[hmaarrfk]

Are we the upstream

I think!? right?

the predicament

Today: i want to take advantage of zlib-ng with Pillow, but:

1. Either I migrate the whole OS/distribution to zlib-ng
2. I stick to zlib

I want to have both:

1. Keep all the old packages on zlib
2. Use zlib-ng with pillow.

[hmaarrfk]

now if only i understood the windows failures ^_^

[radarhere]

Today: i want to take advantage of zlib-ng with Pillow, but:

1. Either I migrate the whole OS/distribution to zlib-ng
2. I stick to zlib

So, just spelling this out, you're saying that if both zlib and zlib-ng are installed, then Pillow may pick zlib over zlib-ng? Probably just based on the order of the paths that it searches?

Have you considered adjusting the search paths? https://pillow.readthedocs.io/en/stable/installation/building-from-source.html#installing

If they are installed in a non-standard location, you may need to configure setuptools to use those locations by editing setup.py or pyproject.toml, or by adding environment variables on the command line:

CFLAGS="-I/usr/pkg/include" python3 -m pip install --upgrade Pillow --no-binary :all:

[hmaarrfk]

So, just spelling this out

If you build out zlib and zlib-ng so that they can be co-installed, you have to disable zlib-ng's "compatibility" mode.

Once you've disabled compatibility mode, we are "forced" to use <zlib.h> from zlib.

Have you considered adjusting the search paths?

Yeah this is generally what we do. we set enough flags that it uses the isolated environment.

[mgorny]

This introduces an automagic dependency on zlib-ng when both zlib-ng and zlib are installed. As such, Gentoo (as well as anyone who doesn't build in an entirely isolated environment) would end up having to patch this out in order to use plain zlib. I'm not opposed to having zlib-ng as an option, but then it should be an option, one that can be explicitly disabled.

[hmaarrfk]

thanks for bringing this up. it is true. it is "priority for zlib-ng".

I'm happy to add an explicit "switch" between the two.

I'll add that to the todo list if the general strategy is agreeable.

[aclark4life]

Still working on the general strategy being agreeable. That is usually determined by interest and the number of folks looking for a fix for this issue. At present, we depend on zlib and zlib-ng satisfies that dependency via compat mode. How many folks are looking for co-existence ?

[hmaarrfk]

That’s a good question.

I’m. Happy to let this PR sit