Enforce usage of fork method on macOS

src/flake8/checker.py

@@ -5,6 +5,7 @@ import errno
 import itertools
 import logging
 import multiprocessing.pool
+import platform
 import signal
 import tokenize
 from typing import Any
@@ -101,7 +102,11 @@ class Manager:
         # - the user provided some awful input
 
         # class state is only preserved when using the `fork` strategy.
-        if multiprocessing.get_start_method() != "fork":
+        # since Python 3.8, macOS has the method `spawn` as the default value
+        if (
+            platform.system() != "Darwin"
+            and multiprocessing.get_start_method() != "fork"
+        ):
             LOG.warning(
                 "The multiprocessing module is not available. "
                 "Ignoring --jobs arguments."
@@ -595,7 +600,8 @@ def _try_initialize_processpool(
 ) -> Optional[multiprocessing.pool.Pool]:
     """Return a new process pool instance if we are able to create one."""
     try:
-        return multiprocessing.Pool(job_count, _pool_init)
+        context = multiprocessing.get_context("fork")
+        return context.Pool(job_count, _pool_init)
     except OSError as err:
         if err.errno not in SERIAL_RETRY_ERRNOS:
             raise

tests/integration/test_checker.py

@@ -292,11 +292,11 @@ def test_acquire_when_multiprocessing_pool_can_initialize():
 
     This simulates the behaviour on most common platforms.
     """
-    with mock.patch("multiprocessing.Pool") as pool:
+    with mock.patch("multiprocessing.get_context") as context:
         result = checker._try_initialize_processpool(2)
 
-    pool.assert_called_once_with(2, checker._pool_init)
-    assert result is pool.return_value
+    context.return_value.Pool.assert_called_once_with(2, checker._pool_init)
+    assert result is context.return_value.Pool.return_value
 
 
 def test_acquire_when_multiprocessing_pool_can_not_initialize():
@@ -311,10 +311,11 @@ 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:
+    with mock.patch("multiprocessing.get_context") as context:
+        context.return_value.Pool.side_effect = ImportError
         result = checker._try_initialize_processpool(2)
 
-    pool.assert_called_once_with(2, checker._pool_init)
+    context.return_value.Pool.assert_called_once_with(2, checker._pool_init)
     assert result is None
 
 

tests/unit/test_checker_manager.py

@@ -1,6 +1,7 @@
 """Tests for the Manager object for FileCheckers."""
 import errno
 import multiprocessing
+import platform
 from unittest import mock
 
 import pytest
@@ -51,14 +52,28 @@ def test_oserrors_are_reraised(_):
     assert serial.call_count == 0
 
 
+@mock.patch.object(platform, "system")
 @mock.patch.object(multiprocessing, "get_start_method", return_value="spawn")
-def test_multiprocessing_is_disabled(_):
+def test_multiprocessing_is_disabled(_, mock_system):
     """Verify not being able to import multiprocessing forces jobs to 0."""
     style_guide = style_guide_mock()
     manager = checker.Manager(style_guide, finder.Checkers([], [], []))
     assert manager.jobs == 0
 
 
+@mock.patch.object(platform, "system", return_value="Darwin")
+@mock.patch.object(multiprocessing, "get_start_method", return_value="spawn")
+def test_multiprocessing_is_enabled_for_macos(_, mock_system):
+    """Verify jobs are returned on macOS.
+
+    Since Python 3.8, `spawn` is the default value on macOS, which is
+    not currently supported by flake8.
+    """
+    style_guide = style_guide_mock()
+    manager = checker.Manager(style_guide, finder.Checkers([], [], []))
+    assert manager.jobs > 0
+
+
 def test_multiprocessing_cpu_count_not_implemented():
     """Verify that jobs is 0 if cpu_count is unavailable."""
     style_guide = style_guide_mock()