Building Python 2 packages with Meson might fail

[jtojnar]

Currently, python2.pkgs.pycairo, which is transitively depended on by GIMP, is failing on staging-next due to Meson crashing in configure step:

Traceback (most recent call last):
  File "/nix/store/xp66zc4w184hwipy6zljah1mzk7dprqg-meson-0.61.2/bin/meson", line 6, in <module>
    from mesonbuild.mesonmain import main
  File "/nix/store/xp66zc4w184hwipy6zljah1mzk7dprqg-meson-0.61.2/lib/python3.9/site-packages/mesonbuild/mesonmain.py", line 27, in <module>
    from . import mesonlib
  File "/nix/store/xp66zc4w184hwipy6zljah1mzk7dprqg-meson-0.61.2/lib/python3.9/site-packages/mesonbuild/mesonlib/__init__.py", line 21, in <module>
    from .universal import *
  File "/nix/store/xp66zc4w184hwipy6zljah1mzk7dprqg-meson-0.61.2/lib/python3.9/site-packages/mesonbuild/mesonlib/universal.py", line 28, in <module>
    import typing as T
  File "/nix/store/sgk7zd1sqax5gh9avijwp3k7h8451b1g-python2.7-typing-3.10.0.0/lib/python2.7/site-packages/typing.py", line 782, in <module>
    AnyStr = TypeVar('AnyStr', bytes, unicode)
NameError: name 'unicode' is not defined

https://hydra.nixos.org/build/171372612/nixlog/31

It appears that Meson (a Python 3 app) is picking up python2.pkgs.typing from a transitive dependency in checkInputs (likely introduced in 58f6365), and we were just lucky that we did not encounter this sooner (thanks to Meson being largely self-contained).

cc @jonringer @FRidh

[jtojnar]

Not really sure how to fix it, other than by wrapping Meson with --set PYTHONPATH "". That might break third party meson modules, thought I do not think we have any packaged.

[yu-re-ka]

Or maybe keep an old version of meson around that is itself using python 2?

[jtojnar]

I do not think there was a Python 2 version. And even if there were, it would be so ancient that it could not be used to build any ~current project.

[FRidh]

This was bound to happen sometime. It's surprising really we did not see this any sooner with meson though. It's this issue #118452.

Workaround in 4f30fc6.