[Bug]: test_other_signal_before_sigint fails with Qt on macOS CI

[QuLogic]

Bug summary

We've previously not been installing Qt wrappers such as PyQt6 or PySide. #27723 adds those back, but we have discovered that test_other_signal_before_sigint fails on CI. The subprocess appears to segfault somewhere in the Qt event loop.

I had tried to debug it with c1afbac, but then it stopped failing. Reverting that change then failed again, though one time it passed. So this seems to be fairly flaky.

I went through all CI runs, and found that this is not particularly limited to any parametrization:

      4 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtcairo-QT_API=PyQt6-BACKEND_DEPS=PyQt6,cairocffi] _
      3 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtcairo-QT_API=PySide6-BACKEND_DEPS=PySide6,cairocffi] _
      3 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtcairo-QT_API=PyQt5-BACKEND_DEPS=PyQt5,cairocffi] _
      3 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtagg-QT_API=PyQt6-BACKEND_DEPS=PyQt6] _
      2 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtcairo-QT_API=PySide2-BACKEND_DEPS=PySide2,cairocffi] _
      2 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtagg-QT_API=PySide6-BACKEND_DEPS=PySide6] _
      1 _ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtagg-QT_API=PySide2-BACKEND_DEPS=PySide2] _

Unfortunately, I don't think @tacaswell was able to reproduce the crash, and @ksunden was only able to reproduce once, but only as part of a full test suite, and never again later in a smaller run.

So as part of #27723, I've marked these as xfail, but if we can figure out a way to reproduce, we should be able to fix this.

Code for reproduction

pytest -k test_other_signal_before_sigint lib/matplotlib/tests/test_backends_interactive.py

Actual outcome

_ test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtagg-QT_API=PySide6-BACKEND_DEPS=PySide6] _
[gw1] darwin -- Python 3.12.2 /Library/Frameworks/Python.framework/Versions/3.12/bin/python

env = {'BACKEND_DEPS': 'PySide6', 'MPLBACKEND': 'qtagg', 'QT_API': 'PySide6'}
target = 'show', kwargs = {'block': True}
request = <FixtureRequest for <Function test_other_signal_before_sigint[show-kwargs0-MPLBACKEND=qtagg-QT_API=PySide6-BACKEND_DEPS=PySide6]>>

    @pytest.mark.skipif(sys.platform == 'win32',
                        reason='No other signal available to send on Windows')
    @pytest.mark.parametrize("env", _get_testable_interactive_backends())
    @pytest.mark.parametrize("target, kwargs", [
        ('show', {'block': True}),
        ('pause', {'interval': 10})
    ])
    def test_other_signal_before_sigint(env, target, kwargs, request):
        backend = env.get("MPLBACKEND")
        if not backend.startswith(("qt", "macosx")):
            pytest.skip("SIGINT currently only tested on qt and macosx")
        if backend == "macosx":
            request.node.add_marker(pytest.mark.xfail(reason="macosx backend is buggy"))
        proc = _WaitForStringPopen(
            [sys.executable, "-c",
             inspect.getsource(_test_other_signal_before_sigint_impl) +
             "\n_test_other_signal_before_sigint_impl("
                f"{backend!r}, {target!r}, {kwargs!r})"])
        try:
            proc.wait_for('DRAW')
            os.kill(proc.pid, signal.SIGUSR1)
            proc.wait_for('SIGUSR1')
            os.kill(proc.pid, signal.SIGINT)
            stdout, _ = proc.communicate(timeout=_test_timeout)
        except Exception:
            proc.kill()
            stdout, _ = proc.communicate()
            raise
        print(stdout)
>       assert 'SUCCESS' in stdout
E       AssertionError

/Users/runner/work/matplotlib/matplotlib/lib/matplotlib/tests/test_backends_interactive.py:790: AssertionError
----------------------------- Captured stdout call -----------------------------

----------------------------- Captured stderr call -----------------------------
Fatal Python error: Segmentation fault

Current thread 0x00000001d88a9c40 (most recent call first):
  File "/Users/runner/work/matplotlib/matplotlib/lib/matplotlib/backends/qt_compat.py", line 159 in _exec
  File "/Users/runner/work/matplotlib/matplotlib/lib/matplotlib/backends/backend_qt.py", line 633 in start_main_loop
  File "/Users/runner/work/matplotlib/matplotlib/lib/matplotlib/backend_bases.py", line 3536 in show
  File "/Users/runner/work/matplotlib/matplotlib/lib/matplotlib/pyplot.py", line 590 in show
  File "<string>", line 21 in _test_other_signal_before_sigint_impl
  File "<string>", line 25 in <module>

Extension modules: numpy.core._multiarray_umath, numpy.core._multiarray_tests, numpy.linalg._umath_linalg, numpy.fft._pocketfft_internal, numpy.random._common, numpy.random.bit_generator, numpy.random._bounded_integers, numpy.random._mt19937, numpy.random.mtrand, numpy.random._philox, numpy.random._pcg64, numpy.random._sfc64, numpy.random._generator, PIL._imaging, kiwisolver._cext, PyQt6.QtCore, PyQt6.QtGui, PyQt6.QtWidgets (total: 18)

Expected outcome

Tests pass

Additional information

No response

Operating system

macOS

Matplotlib Version

main

Matplotlib Backend

QtAgg

Python version

3.10 or 3.12

Jupyter version

No response

Installation

git checkout