pythongh-89240: Enable multiprocessing on Windows to use large process pools (pythonGH-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.

Author: zooba

Include/internal/pycore_global_objects_fini_generated.h

@@ -372,6 +372,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(defaultaction)
         STRUCT_FOR_ID(delete)
         STRUCT_FOR_ID(depth)
+        STRUCT_FOR_ID(desired_access)
         STRUCT_FOR_ID(detect_types)
         STRUCT_FOR_ID(deterministic)
         STRUCT_FOR_ID(device)
@@ -462,6 +463,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(groups)
         STRUCT_FOR_ID(h)
         STRUCT_FOR_ID(handle)
+        STRUCT_FOR_ID(handle_seq)
         STRUCT_FOR_ID(hash_name)
         STRUCT_FOR_ID(header)
         STRUCT_FOR_ID(headers)
@@ -479,9 +481,12 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(indexgroup)
         STRUCT_FOR_ID(inf)
         STRUCT_FOR_ID(infer_variance)
+        STRUCT_FOR_ID(inherit_handle)
         STRUCT_FOR_ID(inheritable)
         STRUCT_FOR_ID(initial)
         STRUCT_FOR_ID(initial_bytes)
+        STRUCT_FOR_ID(initial_owner)
+        STRUCT_FOR_ID(initial_state)
         STRUCT_FOR_ID(initial_value)
         STRUCT_FOR_ID(initval)
         STRUCT_FOR_ID(inner_size)
@@ -537,6 +542,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(locals)
         STRUCT_FOR_ID(logoption)
         STRUCT_FOR_ID(loop)
+        STRUCT_FOR_ID(manual_reset)
         STRUCT_FOR_ID(mapping)
         STRUCT_FOR_ID(match)
         STRUCT_FOR_ID(max_length)
@@ -553,6 +559,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(metadata)
         STRUCT_FOR_ID(method)
         STRUCT_FOR_ID(microsecond)
+        STRUCT_FOR_ID(milliseconds)
         STRUCT_FOR_ID(minute)
         STRUCT_FOR_ID(mod)
         STRUCT_FOR_ID(mode)
@@ -562,6 +569,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(month)
         STRUCT_FOR_ID(mro)
         STRUCT_FOR_ID(msg)
+        STRUCT_FOR_ID(mutex)
         STRUCT_FOR_ID(mycmp)
         STRUCT_FOR_ID(n)
         STRUCT_FOR_ID(n_arg)
@@ -665,6 +673,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(sched_priority)
         STRUCT_FOR_ID(scheduler)
         STRUCT_FOR_ID(second)
+        STRUCT_FOR_ID(security_attributes)
         STRUCT_FOR_ID(seek)
         STRUCT_FOR_ID(seekable)
         STRUCT_FOR_ID(selectors)
@@ -752,6 +761,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(values)
         STRUCT_FOR_ID(version)
         STRUCT_FOR_ID(volume)
+        STRUCT_FOR_ID(wait_all)
         STRUCT_FOR_ID(warnings)
         STRUCT_FOR_ID(warnoptions)
         STRUCT_FOR_ID(wbits)

Include/internal/pycore_global_strings.h

@@ -1011,8 +1011,20 @@ def _exhaustive_wait(handles, timeout):
         # returning the first signalled might create starvation issues.)
         L = list(handles)
         ready = []
+        # Windows limits WaitForMultipleObjects at 64 handles, and we use a
+        # few for synchronisation, so we switch to batched waits at 60.
+        if len(L) > 60:
+            try:
+                res = _winapi.BatchedWaitForMultipleObjects(L, False, timeout)
+            except TimeoutError:
+                return []
+            ready.extend(L[i] for i in res)
+            if res:
+                L = [h for i, h in enumerate(L) if i > res[0] & i not in res]
+            timeout = 0
         while L:
-            res = _winapi.WaitForMultipleObjects(L, False, timeout)
+            short_L = L[:60] if len(L) > 60 else L
+            res = _winapi.WaitForMultipleObjects(short_L, False, timeout)
             if res == WAIT_TIMEOUT:
                 break
             elif WAIT_OBJECT_0 <= res < WAIT_OBJECT_0 + len(L):

Include/internal/pycore_runtime_init_generated.h

@@ -6113,6 +6113,24 @@ def test_spawn_sys_executable_none_allows_import(self):
         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