Preserve legacy API options in worker init

tests/integration/test_checker.py

@@ -289,9 +289,14 @@ def test_acquire_when_multiprocessing_pool_can_initialize():
     This simulates the behaviour on most common platforms.
     """
     with mock.patch("multiprocessing.Pool") as pool:
-        result = checker._try_initialize_processpool(2, [])
+        options = mock.Mock()
+        result = checker._try_initialize_processpool(2, [], options)
 
-    pool.assert_called_once_with(2, checker._mp_init, initargs=([],))
+    pool.assert_called_once_with(
+        2,
+        checker._mp_init,
+        initargs=([], options),
+    )
     assert result is pool.return_value
 
 
@@ -308,9 +313,14 @@ def test_acquire_when_multiprocessing_pool_can_not_initialize():
     https://github.com/python/cpython/blob/4e02981de0952f54bf87967f8e10d169d6946b40/Lib/multiprocessing/synchronize.py#L30-L33
     """
     with mock.patch("multiprocessing.Pool", side_effect=ImportError) as pool:
-        result = checker._try_initialize_processpool(2, [])
+        options = mock.Mock()
+        result = checker._try_initialize_processpool(2, [], options)
 
-    pool.assert_called_once_with(2, checker._mp_init, initargs=([],))
+    pool.assert_called_once_with(
+        2,
+        checker._mp_init,
+        initargs=([], options),
+    )
     assert result is None
 
 

tests/unit/test_checker_manager.py

@@ -73,6 +73,23 @@ def test_jobs_count_limited_to_file_count():
     assert manager.jobs == 2
 
 
+def test_mp_init_preserves_supplied_options():
+    parsed_plugins = mock.Mock(checkers="parsed-checkers")
+    parsed_options = mock.Mock()
+    options_override = mock.Mock()
+
+    with (
+        mock.patch.object(checker, "_mp", None),
+        mock.patch.object(
+            checker,
+            "parse_args",
+            return_value=(parsed_plugins, parsed_options),
+        ),
+    ):
+        checker._mp_init([], options_override)
+        assert checker._mp == ("parsed-checkers", options_override)
+
+
 def test_make_checkers():
     """Verify that we create a list of FileChecker instances."""
     style_guide = style_guide_mock()