Note, this doesn't explicitly do GioUnix, but as that comes with Gio (but is platform-specific), I'm hoping that's enough to fix it.

What about

    try:
        gi.require_version("GioUnix", "2.0")
    except ValueError:
        # GioUnix is not available on this platform, which is fine
        pass

I tried it in #30535

Sorry I was going down the issue list in order and merged #30535 before I got to this one.

That is okay; this has turned into more of a cleanup on the tests a bit than fully fixing the original issue.

Upstream should have made a new release (or two) with the correct exceptions to make the previous change unnecessary, so I've reverted #30535, fixing #30537.

Is the test failure real? It's suspicious that a gtk test segfaults after this change.

_________ test_backend_getattr[matplotlib.backends.backend_gtk4cairo] __________
[gw0] linux -- Python 3.12.11 /opt/hostedtoolcache/Python/3.12.11/arm64/bin/python

module_name = 'matplotlib.backends.backend_gtk4cairo'

[...]

E               subprocess.CalledProcessError: Command '['/opt/hostedtoolcache/Python/3.12.11/arm64/bin/python', '-c', "import importlib.util;_spec = importlib.util.spec_from_file_location('matplotlib.tests.test_getattr', '/home/runner/work/matplotlib/matplotlib/lib/matplotlib/tests/test_getattr.py');_module = importlib.util.module_from_spec(_spec);_spec.loader.exec_module(_module);_module._test_module_getattr()", 'matplotlib.backends.backend_gtk4cairo']' died with <Signals.SIGSEGV: 11>.

/opt/hostedtoolcache/Python/3.12.11/arm64/lib/python3.12/subprocess.py:571: CalledProcessError
------------------------------ Captured log call -------------------------------
ERROR    matplotlib.testing:__init__.py:112 Subprocess error:
Fatal Python error: Segmentation fault

It does seem related, yes. Previously, GTK4 would never fully be hit by this test because GTK3 was already imported. How it manages to segfault though, I'm unsure just yet.

Is the test failure real? It's suspicious that a gtk test segfaults after this change.

It looks like the update to Ubuntu 24.04 fixed it. And since the ubuntu-22.04-arm image seems to have accidentally switched to 24.04 as well, I won't spend any time trying to figure it out.