gh-89240: Enable multiprocessing on Windows to use large process pools (GH-107873)

We add _winapi.BatchedWaitForMultipleObjects to wait for larger numbers of handles.
This is an internal module, hence undocumented, and should be used with caution.
Check the docstring for info before using BatchedWaitForMultipleObjects.

Lib/test/_test_multiprocessing.py

@@ -6113,6 +6113,24 @@ class MiscTestCase(unittest.TestCase):
         self.assertEqual(rc, 0)
         self.assertFalse(err, msg=err.decode('utf-8'))
 
+    def test_large_pool(self):
+        #
+        # gh-89240: Check that large pools are always okay
+        #
+        testfn = os_helper.TESTFN
+        self.addCleanup(os_helper.unlink, testfn)
+        with open(testfn, 'w', encoding='utf-8') as f:
+            f.write(textwrap.dedent('''\
+                import multiprocessing
+                def f(x): return x*x
+                if __name__ == '__main__':
+                    with multiprocessing.Pool(200) as p:
+                        print(sum(p.map(f, range(1000))))
+            '''))
+        rc, out, err = script_helper.assert_python_ok(testfn)
+        self.assertEqual("332833500", out.decode('utf-8').strip())
+        self.assertFalse(err, msg=err.decode('utf-8'))
+
 
 #
 # Mixins