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_winapi.py

@@ -0,0 +1,94 @@
+# Test the Windows-only _winapi module
+
+import random
+import threading
+import time
+import unittest
+from test.support import import_helper
+
+_winapi = import_helper.import_module('_winapi', required_on=['win'])
+
+MAXIMUM_WAIT_OBJECTS = 64
+MAXIMUM_BATCHED_WAIT_OBJECTS = (MAXIMUM_WAIT_OBJECTS - 1) ** 2
+
+class WinAPIBatchedWaitForMultipleObjectsTests(unittest.TestCase):
+    def _events_waitall_test(self, n):
+        evts = [_winapi.CreateEventW(0, False, False, None) for _ in range(n)]
+
+        with self.assertRaises(TimeoutError):
+            _winapi.BatchedWaitForMultipleObjects(evts, True, 100)
+
+        # Ensure no errors raised when all are triggered
+        for e in evts:
+            _winapi.SetEvent(e)
+        try:
+            _winapi.BatchedWaitForMultipleObjects(evts, True, 100)
+        except TimeoutError:
+            self.fail("expected wait to complete immediately")
+
+        # Choose 8 events to set, distributed throughout the list, to make sure
+        # we don't always have them in the first chunk
+        chosen = [i * (len(evts) // 8) for i in range(8)]
+
+        # Replace events with invalid handles to make sure we fail
+        for i in chosen:
+            old_evt = evts[i]
+            evts[i] = -1
+            with self.assertRaises(OSError):
+                _winapi.BatchedWaitForMultipleObjects(evts, True, 100)
+            evts[i] = old_evt
+
+
+    def _events_waitany_test(self, n):
+        evts = [_winapi.CreateEventW(0, False, False, None) for _ in range(n)]
+
+        with self.assertRaises(TimeoutError):
+            _winapi.BatchedWaitForMultipleObjects(evts, False, 100)
+
+        # Choose 8 events to set, distributed throughout the list, to make sure
+        # we don't always have them in the first chunk
+        chosen = [i * (len(evts) // 8) for i in range(8)]
+
+        # Trigger one by one. They are auto-reset events, so will only trigger once
+        for i in chosen:
+            with self.subTest(f"trigger event {i} of {len(evts)}"):
+                _winapi.SetEvent(evts[i])
+                triggered = _winapi.BatchedWaitForMultipleObjects(evts, False, 10000)
+                self.assertSetEqual(set(triggered), {i})
+
+        # Trigger all at once. This may require multiple calls
+        for i in chosen:
+            _winapi.SetEvent(evts[i])
+        triggered = set()
+        while len(triggered) < len(chosen):
+            triggered.update(_winapi.BatchedWaitForMultipleObjects(evts, False, 10000))
+        self.assertSetEqual(triggered, set(chosen))
+
+        # Replace events with invalid handles to make sure we fail
+        for i in chosen:
+            with self.subTest(f"corrupt event {i} of {len(evts)}"):
+                old_evt = evts[i]
+                evts[i] = -1
+                with self.assertRaises(OSError):
+                    _winapi.BatchedWaitForMultipleObjects(evts, False, 100)
+                evts[i] = old_evt
+
+
+    def test_few_events_waitall(self):
+        self._events_waitall_test(16)
+
+    def test_many_events_waitall(self):
+        self._events_waitall_test(256)
+
+    def test_max_events_waitall(self):
+        self._events_waitall_test(MAXIMUM_BATCHED_WAIT_OBJECTS)
+
+
+    def test_few_events_waitany(self):
+        self._events_waitany_test(16)
+
+    def test_many_events_waitany(self):
+        self._events_waitany_test(256)
+
+    def test_max_events_waitany(self):
+        self._events_waitany_test(MAXIMUM_BATCHED_WAIT_OBJECTS)