gh-135335: flush stdout/stderr in forkserver after preloading modules

Lib/multiprocessing/forkserver.py

@@ -222,6 +222,10 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
             except ImportError:
                 pass
 
+        # gh-135335: flush stdout/stderr in case any of the preloaded modules
+        # wrote to them, otherwise children might inherit buffered data
+        util._flush_std_streams()
+
     util._close_stdin()
 
     sig_r, sig_w = os.pipe()

Lib/test/_test_multiprocessing.py

@@ -6801,6 +6801,35 @@ def test_child_sys_path(self):
         self.assertEqual(child_sys_path[1:], sys.path[1:])
         self.assertIsNone(import_error, msg=f"child could not import {self._mod_name}")
 
+    def test_std_streams_flushed_after_preload(self):
+        # gh-135335: Check fork server flushes standard streams after
+        # preloading modules
+        if multiprocessing.get_start_method() != "forkserver":
+            self.skipTest("forkserver specific test")
+
+        # Create a test module in the temporary directory on the child's path
+        # TODO: This can all be simplified once gh-126631 is fixed and we can
+        #       use __main__ instead of a module.
+        dirname = os.path.join(self._temp_dir, 'preloaded_module')
+        init_name = os.path.join(dirname, '__init__.py')
+        os.mkdir(dirname)
+        with open(init_name, "w") as f:
+            cmd = '''if 1:
+                import sys
+                print('stderr', end='', file=sys.stderr)
+                print('stdout', end='', file=sys.stdout)
+            '''
+            f.write(cmd)
+
+        name = os.path.join(os.path.dirname(__file__), 'mp_preload_flush.py')
+        env = {'PYTHONPATH': self._temp_dir}
+        _, out, err = test.support.script_helper.assert_python_ok(name, **env)
+
+        # Check stderr first, as it is more likely to be useful to see in the
+        # event of a failure.
+        self.assertEqual(err.decode().rstrip(), 'stderr')
+        self.assertEqual(out.decode().rstrip(), 'stdout')
+
 
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):

Lib/test/mp_preload_flush.py

@@ -0,0 +1,15 @@
+import multiprocessing
+import sys
+
+modname = 'preloaded_module'
+if __name__ == '__main__':
+    if modname in sys.modules:
+        raise AssertionError(f'{modname!r} is not in sys.modules')
+    multiprocessing.set_start_method('forkserver')
+    multiprocessing.set_forkserver_preload([modname])
+    for _ in range(2):
+        p = multiprocessing.Process()
+        p.start()
+        p.join()
+elif modname not in sys.modules:
+    raise AssertionError(f'{modname!r} is not in sys.modules')

Misc/NEWS.d/next/Library/2025-06-10-21-42-04.gh-issue-135335.WnUqb_.rst

@@ -0,0 +1,2 @@
+:mod:`multiprocessing`: Flush ``stdout`` and ``stderr`` after preloading
+modules in the ``forkserver``.