TST: Check cross Qt import results from parent process

We are only interested in whether we emitted the warning, and don't care
if the test segfaults or otherwise crashes. Thus it is better to check
the output from the parent process, instead of `pytest.warns` in the
subprocess and checking the return code.

Author: QuLogic

lib/matplotlib/tests/test_backends_interactive.py

@@ -424,9 +424,9 @@ def test_qt_missing():
 
 
 def _impl_test_cross_Qt_imports():
-    import sys
     import importlib
-    import pytest
+    import sys
+    import warnings
 
     _, host_binding, mpl_binding = sys.argv
     # import the mpl binding.  This will force us to use that binding
@@ -436,11 +436,12 @@ def _impl_test_cross_Qt_imports():
     host_qwidgets = importlib.import_module(f'{host_binding}.QtWidgets')
 
     host_app = host_qwidgets.QApplication(["mpl testing"])
-    with pytest.warns(UserWarning, match="Mixing Qt major"):
-        matplotlib.backends.backend_qt._create_qApp()
+    warnings.filterwarnings("error", message=r".*Mixing Qt major.*",
+                            category=UserWarning)
+    matplotlib.backends.backend_qt._create_qApp()
 
 
-def test_cross_Qt_imports():
+def qt5_and_qt6_pairs():
     qt5_bindings = [
         dep for dep in ['PyQt5', 'PySide2']
         if importlib.util.find_spec(dep) is not None
@@ -450,25 +451,29 @@ def test_cross_Qt_imports():
         if importlib.util.find_spec(dep) is not None
     ]
     if len(qt5_bindings) == 0 or len(qt6_bindings) == 0:
-        pytest.skip('need both QT6 and QT5 bindings')
+        yield pytest.param(None, None,
+                           marks=[pytest.mark.skip('need both QT6 and QT5 bindings')])
+        return
 
     for qt5 in qt5_bindings:
         for qt6 in qt6_bindings:
             for pair in ([qt5, qt6], [qt6, qt5]):
-                try:
-                    _run_helper(_impl_test_cross_Qt_imports,
-                                *pair,
-                                timeout=_test_timeout)
-                except subprocess.CalledProcessError as ex:
-                    # if segfault, carry on.  We do try to warn the user they
-                    # are doing something that we do not expect to work
-                    if ex.returncode == -signal.SIGSEGV:
-                        continue
-                    # We got the abort signal which is likely because the Qt5 /
-                    # Qt6 cross import is unhappy, carry on.
-                    elif ex.returncode == -signal.SIGABRT:
-                        continue
-                    raise
+                yield pair
+
+
+@pytest.mark.parametrize('host, mpl', [*qt5_and_qt6_pairs()])
+def test_cross_Qt_imports(host, mpl):
+    try:
+        proc = _run_helper(_impl_test_cross_Qt_imports, host, mpl,
+                           timeout=_test_timeout)
+    except subprocess.CalledProcessError as ex:
+        # We do try to warn the user they are doing something that we do not
+        # expect to work, so we're going to ignore if the subprocess crashes or
+        # is killed, and just check that the warning is printed.
+        stderr = ex.stderr
+    else:
+        stderr = proc.stderr
+    assert "Mixing Qt major versions may not work as expected." in stderr
 
 
 @pytest.mark.skipif('TF_BUILD' in os.environ,