def METHOD_NAME(base_uri, download_arg, list_return_val):
    """
    Verifies that all asynchronous artifact downloads are joined before `download_artifacts()`
    returns a result to the caller
    """
    def list_artifacts(path):
        fullpath = posixpath.join(base_uri, path)
        if fullpath.endswith(_MODEL_DIR) or fullpath.endswith(_MODEL_DIR + "/"):
            return [FileInfo(item, False, _DUMMY_FILE_SIZE) for item in list_return_val]
        elif fullpath.endswith(_PARENT_MODEL_DIR) or fullpath.endswith(_PARENT_MODEL_DIR + "/"):
            return [FileInfo(posixpath.join(path, _MODEL_DIR), True, _EMPTY_FILE_SIZE)]
        else:
            return []
    with mock.patch.object(SlowArtifactRepositoryImpl, "list_artifacts") as list_artifacts_mock:
        list_artifacts_mock.side_effect = list_artifacts
        repo = SlowArtifactRepositoryImpl(base_uri)
        repo.download_artifacts(download_arg)