Preserve legacy API options in parallel checks

src/flake8/api/legacy.py

@@ -162,7 +162,9 @@ class StyleGuide:
         # Stop cringing... I know it's gross.
         self._application.make_guide()
         self._application.file_checker_manager = None
-        self._application.make_file_checker_manager([])
+        self._application.make_file_checker_manager(
+            [], use_initialized_options=True,
+        )
 
     def input_file(
         self,
@@ -212,5 +214,5 @@ def get_style_guide(**kwargs: Any) -> StyleGuide:
             LOG.error('Could not update option "%s"', key)
     application.make_formatter()
     application.make_guide()
-    application.make_file_checker_manager([])
+    application.make_file_checker_manager([], use_initialized_options=True)
     return StyleGuide(application)

src/flake8/checker.py

@@ -7,6 +7,7 @@ import errno
 import logging
 import multiprocessing.pool
 import operator
+import pickle
 import signal
 import tokenize
 from collections.abc import Generator
@@ -61,16 +62,24 @@ def _mp_prefork(
         _mp = None
 
 
-def _mp_init(argv: Sequence[str]) -> None:
+def _mp_init(
+    argv: Sequence[str],
+    plugins: Checkers | None = None,
+    options: argparse.Namespace | None = None,
+) -> None:
     global _mp
 
     # Ensure correct signaling of ^C using multiprocessing.Pool.
     signal.signal(signal.SIGINT, signal.SIG_IGN)
 
+    if plugins is not None and options is not None:
+        _mp = plugins, options
+        return
+
     # for `fork` this'll already be set
     if _mp is None:
-        plugins, options = parse_args(argv)
-        _mp = plugins.checkers, options
+        parsed_plugins, parsed_options = parse_args(argv)
+        _mp = parsed_plugins.checkers, parsed_options
 
 
 def _mp_run(filename: str) -> tuple[str, Results, dict[str, int]]:
@@ -105,6 +114,7 @@ class Manager:
         style_guide: StyleGuideManager,
         plugins: Checkers,
         argv: Sequence[str],
+        use_initialized_options: bool = False,
     ) -> None:
         """Initialize our Manager instance."""
         self.style_guide = style_guide
@@ -119,6 +129,7 @@ class Manager:
         }
         self.exclude = (*self.options.exclude, *self.options.extend_exclude)
         self.argv = argv
+        self.use_initialized_options = use_initialized_options
         self.results: list[tuple[str, Results, dict[str, int]]] = []
 
     def _process_statistics(self) -> None:
@@ -191,8 +202,12 @@ class Manager:
 
     def run_parallel(self) -> None:
         """Run the checkers in parallel."""
+        plugins = self.plugins if self.use_initialized_options else None
+        options = self.options if self.use_initialized_options else None
         with _mp_prefork(self.plugins, self.options):
-            pool = _try_initialize_processpool(self.jobs, self.argv)
+            pool = _try_initialize_processpool(
+                self.jobs, self.argv, plugins=plugins, options=options,
+            )
 
         if pool is None:
             self.run_serial()
@@ -547,13 +562,21 @@ class FileChecker:
 def _try_initialize_processpool(
     job_count: int,
     argv: Sequence[str],
+    *,
+    plugins: Checkers | None = None,
+    options: argparse.Namespace | None = None,
 ) -> multiprocessing.pool.Pool | None:
     """Return a new process pool instance if we are able to create one."""
     try:
-        return multiprocessing.Pool(job_count, _mp_init, initargs=(argv,))
+        return multiprocessing.Pool(
+            job_count, _mp_init, initargs=(argv, plugins, options),
+        )
     except OSError as err:
         if err.errno not in SERIAL_RETRY_ERRNOS:
             raise
+    except (AttributeError, pickle.PicklingError, TypeError):
+        if plugins is None and options is None:
+            raise
     except ImportError:
         pass
 

src/flake8/main/application.py

@@ -79,7 +79,9 @@ class Application:
             self.options, self.formatter,
         )
 
-    def make_file_checker_manager(self, argv: Sequence[str]) -> None:
+    def make_file_checker_manager(
+        self, argv: Sequence[str], use_initialized_options: bool = False,
+    ) -> None:
         """Initialize our FileChecker Manager."""
         assert self.guide is not None
         assert self.plugins is not None
@@ -87,6 +89,7 @@ class Application:
             style_guide=self.guide,
             plugins=self.plugins.checkers,
             argv=argv,
+            use_initialized_options=use_initialized_options,
         )
 
     def run_checks(self) -> None:

tests/integration/test_api_legacy.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 from flake8.api import legacy
+from flake8.main.options import JobsArgument
 
 
 def test_legacy_api(tmpdir):
@@ -13,3 +14,17 @@ def test_legacy_api(tmpdir):
         style_guide = legacy.get_style_guide()
         report = style_guide.check_files([t_py.strpath])
         assert report.total_errors == 1
+
+
+def test_legacy_api_uses_initialized_options_for_parallel_checks(tmpdir):
+    with tmpdir.as_cwd():
+        a_py = tmpdir.join("a.py")
+        b_py = tmpdir.join("b.py")
+        a_py.write('x = "' + "a" * 80 + '"\n')
+        b_py.write('y = "' + "b" * 80 + '"\n')
+
+        style_guide = legacy.get_style_guide(color="never", max_line_length=88)
+        style_guide.options.jobs = JobsArgument("2")
+        report = style_guide.check_files([a_py.strpath, b_py.strpath])
+
+        assert report.total_errors == 0

tests/integration/test_checker.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 import importlib.metadata
+import pickle
 from unittest import mock
 
 import pytest
@@ -291,7 +292,24 @@ def test_acquire_when_multiprocessing_pool_can_initialize():
     with mock.patch("multiprocessing.Pool") as pool:
         result = checker._try_initialize_processpool(2, [])
 
-    pool.assert_called_once_with(2, checker._mp_init, initargs=([],))
+    pool.assert_called_once_with(
+        2, checker._mp_init, initargs=([], None, None),
+    )
+    assert result is pool.return_value
+
+
+def test_acquire_when_multiprocessing_pool_uses_initialized_options():
+    plugins = mock.Mock()
+    options = mock.Mock()
+
+    with mock.patch("multiprocessing.Pool") as pool:
+        result = checker._try_initialize_processpool(
+            2, [], plugins=plugins, options=options,
+        )
+
+    pool.assert_called_once_with(
+        2, checker._mp_init, initargs=([], plugins, options),
+    )
     assert result is pool.return_value
 
 
@@ -310,7 +328,21 @@ def test_acquire_when_multiprocessing_pool_can_not_initialize():
     with mock.patch("multiprocessing.Pool", side_effect=ImportError) as pool:
         result = checker._try_initialize_processpool(2, [])
 
-    pool.assert_called_once_with(2, checker._mp_init, initargs=([],))
+    pool.assert_called_once_with(
+        2, checker._mp_init, initargs=([], None, None),
+    )
+    assert result is None
+
+
+def test_acquire_falls_back_when_initialized_options_are_unpickleable():
+    with mock.patch(
+        "multiprocessing.Pool", side_effect=pickle.PicklingError,
+    ) as pool:
+        result = checker._try_initialize_processpool(
+            2, [], plugins=mock.Mock(), options=mock.Mock(),
+        )
+
+    pool.assert_called_once()
     assert result is None