Add regression test & blurb

Author: duaneg

Lib/test/_test_multiprocessing.py

@@ -6866,6 +6866,19 @@ 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')
+        rc, out, err = test.support.script_helper.assert_python_ok(name)
+        self.assertFalse(err, msg=err.decode())
+        self.assertEqual(rc, 0)
+
+        # TODO: Where is the extra empty line coming from?
+        out = out.decode().split("\n")
+        self.assertEqual(out, ['__main__', '__mp_main__', 'f', 'f', ''])
 
 #
 # Mixins

Lib/test/mp_preload_main.py

@@ -0,0 +1,20 @@
+import multiprocessing
+import sys
+
+print(f"{__name__}")
+
+# TODO: This is necessary as the fork server doesn't flush output after
+#       preloading modules, and hence buffered output is inherited by child
+#       processes. See gh-135335 (this should be removed once that is fixed).
+sys.stdout.flush()
+
+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.