Fix warn mode to work when warnings are configured as errors

Remove catch_warnings() workaround from production code and instead
fix the root cause: tests were allowing sys.warnoptions to leak into
the forkserver subprocess via _args_from_interpreter_flags().

When CI runs with -W error, the forkserver subprocess was inheriting
this flag and converting our ImportWarning calls into exceptions,
causing it to crash.

Solution: Save and clear sys.warnoptions in test setUp, restore in
tearDown. This gives the forkserver subprocess a clean warning state
where warnings.warn() works as intended.

Also remove unnecessary set_forkserver_preload([]) call from tearDown.

Author: gpshead

Lib/multiprocessing/forkserver.py

@@ -244,13 +244,11 @@ def _handle_preload(preload, main_path=None, sys_path=None, on_error='ignore'):
                     raise
                 case 'warn':
                     import warnings
-                    with warnings.catch_warnings():
-                        warnings.simplefilter('always', ImportWarning)
-                        warnings.warn(
-                            f"Failed to preload __main__ from {main_path!r}: {e}",
-                            ImportWarning,
-                            stacklevel=2
-                        )
+                    warnings.warn(
+                        f"Failed to preload __main__ from {main_path!r}: {e}",
+                        ImportWarning,
+                        stacklevel=2
+                    )
                 case 'ignore':
                     pass
         finally:
@@ -265,13 +263,11 @@ def _handle_preload(preload, main_path=None, sys_path=None, on_error='ignore'):
                     raise
                 case 'warn':
                     import warnings
-                    with warnings.catch_warnings():
-                        warnings.simplefilter('always', ImportWarning)
-                        warnings.warn(
-                            f"Failed to preload module {modname!r}: {e}",
-                            ImportWarning,
-                            stacklevel=2
-                        )
+                    warnings.warn(
+                        f"Failed to preload module {modname!r}: {e}",
+                        ImportWarning,
+                        stacklevel=2
+                    )
                 case 'ignore':
                     pass
 

Lib/test/test_multiprocessing_forkserver/test_preload.py

@@ -1,6 +1,7 @@
 """Tests for forkserver preload functionality."""
 
 import multiprocessing
+import sys
 import tempfile
 import unittest
 from multiprocessing import forkserver
@@ -10,12 +11,14 @@ class TestForkserverPreload(unittest.TestCase):
     """Tests for forkserver preload functionality."""
 
     def setUp(self):
+        self._saved_warnoptions = sys.warnoptions.copy()
+        sys.warnoptions.clear()
         self.ctx = multiprocessing.get_context('forkserver')
         forkserver._forkserver._stop()
 
     def tearDown(self):
+        sys.warnoptions[:] = self._saved_warnoptions
         forkserver._forkserver._stop()
-        self.ctx.set_forkserver_preload([])
 
     @staticmethod
     def _send_value(conn, value):