[3.13] gh-126631: gh-137996: fix pre-loading of __main__ (GH-135295)

Lib/multiprocessing/forkserver.py

@@ -127,12 +127,13 @@ def ensure_running(self):
             cmd = ('from multiprocessing.forkserver import main; ' +
                    'main(%d, %d, %r, **%r)')
 
+            main_kws = {}
             if self._preload_modules:
-                desired_keys = {'main_path', 'sys_path'}
                 data = spawn.get_preparation_data('ignore')
-                data = {x: y for x, y in data.items() if x in desired_keys}
-            else:
-                data = {}
+                if 'sys_path' in data:
+                    main_kws['sys_path'] = data['sys_path']
+                if 'init_main_from_path' in data:
+                    main_kws['main_path'] = data['init_main_from_path']
 
             with socket.socket(socket.AF_UNIX) as listener:
                 address = connection.arbitrary_address('AF_UNIX')
@@ -147,7 +148,7 @@ def ensure_running(self):
                 try:
                     fds_to_pass = [listener.fileno(), alive_r]
                     cmd %= (listener.fileno(), alive_r, self._preload_modules,
-                            data)
+                            main_kws)
                     exe = spawn.get_executable()
                     args = [exe] + util._args_from_interpreter_flags()
                     args += ['-c', cmd]

Lib/test/_test_multiprocessing.py

@@ -6545,6 +6545,18 @@ def child():
         self.assertEqual(q.get_nowait(), "done")
         close_queue(q)
 
+    def test_preload_main(self):
+        # gh-126631: Check that __main__ can be pre-loaded
+        if multiprocessing.get_start_method() != "forkserver":
+            self.skipTest("forkserver specific test")
+
+        name = os.path.join(os.path.dirname(__file__), 'mp_preload_main.py')
+        _, out, err = test.support.script_helper.assert_python_ok(name)
+        self.assertEqual(err, b'')
+
+        # The trailing empty string comes from split() on output ending with \n
+        out = out.decode().split("\n")
+        self.assertEqual(out, ['__main__', '__mp_main__', 'f', 'f', ''])
 
 #
 # Mixins

Lib/test/mp_preload_main.py

@@ -0,0 +1,14 @@
+import multiprocessing
+
+print(f"{__name__}")
+
+def f():
+    print("f")
+
+if __name__ == "__main__":
+    ctx = multiprocessing.get_context("forkserver")
+    ctx.set_forkserver_preload(['__main__'])
+    for _ in range(2):
+        p = ctx.Process(target=f)
+        p.start()
+        p.join()

Misc/NEWS.d/next/Library/2025-06-10-21-00-48.gh-issue-126631.eITVJd.rst

@@ -0,0 +1,2 @@
+Fix :mod:`multiprocessing` ``forkserver`` bug which prevented ``__main__``
+from being preloaded.