Test failures with "RuntimeError: dictionary changed size during iteration"

[mrbean-bremen]

This came up in this comment by @schuylermartin45:

I'm seeing some odd test failures in our conda-forge build for CRM that are not present elsewhere. Looks like some of the pyfakefs fixtures are not being initialized properly: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=1274733&view=logs&j=7b6f2c87-f3a7-5133-8d84-7c03a75d9dfc&t=9eb77fd2-8ddd-5444-8fc0-71cb28dcb736&l=5034

Here's a small snippet from that log:

        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       AttributeError: 'NoneType' object has no attribute 'add_real_directory'

tests/fetcher/test_http_artifact_fetcher.py:100: AttributeError
___________________ test_scan[dummy_py_project_01-expected0] ___________________
[gw0] linux -- Python 3.11.13 $PREFIX/bin/python

project_name = 'dummy_py_project_01'
expected = {ProjectDependency(data=MatchSpec("python"), type=<DependencySection.RUN: 3>), ProjectDependency(data=MatchSpec("conda... type=<DependencySection.RUN: 3>), ProjectDependency(data=MatchSpec("requests"), type=<DependencySection.RUN: 3>), ...}
request = <FixtureRequest for <Function test_scan[dummy_py_project_01-expected0]>>

    @pytest.mark.parametrize(
        "project_name,expected",
        [
            (
                "dummy_py_project_01",
                {
                    ProjectDependency(MatchSpec("conda_recipe_manager"), DependencySection.RUN),
                    ProjectDependency(MatchSpec("matplotlib"), DependencySection.RUN),  # Two imports on one line
                    ProjectDependency(MatchSpec("networkx"), DependencySection.RUN),  # Two imports on one line
                    ProjectDependency(MatchSpec("python"), DependencySection.HOST),
                    ProjectDependency(MatchSpec("python"), DependencySection.RUN),
                    ProjectDependency(MatchSpec("pyyaml"), DependencySection.TESTS),
                    ProjectDependency(MatchSpec("requests"), DependencySection.RUN),  # Found in source and test code.
                },
            ),
        ],
    )
    def test_scan(project_name: str, expected: set[ProjectDependency], request: pytest.FixtureRequest) -> None:
        """
        Tests scanning for Python dependencies with a mocked-out Python project.
    
        :param project_name: Name of the dummy Python project directory to use
        :param expected: Expected value
        """
        fs = cast(FakeFilesystem, request.getfixturevalue("fs"))
        project_path: Final[Path] = get_test_path() / "software_projects" / project_name
>       fs.add_real_directory(project_path)
        ^^^^^^^^^^^^^^^^^^^^^
E       AttributeError: 'NoneType' object has no attribute 'add_real_directory'

While examining the log file from the test, I found the following part:

==================================== ERRORS ====================================
_ ERROR at setup of test_bump_recipe_cli[pytest-pep8.yaml-None-bump_recipe/pytest-pep8_build_num_2.yaml] _
[gw0] linux -- Python 3.11.13 $PREFIX/bin/python

request = <SubRequest 'fs' for <Function test_bump_recipe_cli[pytest-pep8.yaml-None-bump_recipe/pytest-pep8_build_num_2.yaml]>>

    @pytest.fixture
    def fs(request):
        """Fake filesystem."""
        if hasattr(request, "param"):
            # pass optional parameters via @pytest.mark.parametrize
            patcher = Patcher(*request.param)
        else:
            patcher = Patcher()
>       patcher.setUp()

../_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_place/lib/python3.11/site-packages/pyfakefs/pytest_plugin.py:38: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_place/lib/python3.11/site-packages/pyfakefs/fake_filesystem_unittest.py:1033: in setUp
    self._find_modules()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pyfakefs.fake_filesystem_unittest.Patcher object at 0x7ff8021acf90>

    def _find_modules(self) -> None:
        """Find and cache all modules that import file system modules.
        Later, `setUp()` will stub these with the fake file system
        modules.
        """
        module_names = list(self._fake_module_classes.keys()) + [PATH_MODULE]
>       for name, module in list(sys.modules.items()):
                            ^^^^^^^^^^^^^^^^^^^^^^^^^
E       RuntimeError: dictionary changed size during iteration

I'm not sure if this is the root of the problem, but it looks like the modules change while iterating over them - probably some module behaving unexpectedly, but this should be avoided, maybe by making a copy of the modules before iterating over them (have to check the performance impact).

A related problem (#338) had been reported (and fixed) quite some time ago, though I have never seen this specific problem until now.