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

If a preloaded module writes to stdout or stderr, and the stream is buffered,
child processes will inherit the buffered data after forking. Attempt to
prevent this by flushing the streams after preload.

Author: duaneg

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 +6801,29 @@ 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.
+        os.mkdir(os.path.join(self._temp_dir, 'a'))
+        with open(os.path.join(self._temp_dir, 'a', '__init__.py'), "w") as f:
+            f.write('''if 1:
+                    import sys
+                    print('stdout', file=sys.stdout)
+                    print('stderr', file=sys.stderr)''')
+
+        name = os.path.join(os.path.dirname(__file__), 'mp_preload_flush.py')
+        env = {'PYTHONPATH': ":".join(sys.path)}
+        rc, out, err = test.support.script_helper.assert_python_ok(name, **env)
+        self.assertEqual(rc, 0)
+        self.assertEqual(out.decode().rstrip(), 'stdout')
+        self.assertEqual(err.decode().rstrip(), 'stderr')
+
 
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):

Lib/test/mp_preload_flush.py

@@ -0,0 +1,7 @@
+import multiprocessing
+if __name__ == '__main__':
+    multiprocessing.set_forkserver_preload(['a'])
+    for _ in range(2):
+        p = multiprocessing.Process()
+        p.start()
+        p.join()

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

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