Issue #28779: multiprocessing.set_forkserver_preload() would crash the forkserver process if a preloaded module instantiated some multiprocessing objects such as locks.

Lib/test/mp_preload.py

@@ -0,0 +1,18 @@
+import multiprocessing
+
+multiprocessing.Lock()
+
+
+def f():
+    print("ok")
+
+
+if __name__ == "__main__":
+    ctx = multiprocessing.get_context("forkserver")
+    modname = "test.mp_preload"
+    # Make sure it's importable
+    __import__(modname)
+    ctx.set_forkserver_preload([modname])
+    proc = ctx.Process(target=f)
+    proc.start()
+    proc.join()