bpo-26903: Limit ProcessPoolExecutor to 61 workers on Windows (GH-13132)

Co-Authored-By: brianquinlan <brian@sweetapp.com>

Lib/concurrent/futures/process.py

@@ -57,6 +57,7 @@ import threading
 import weakref
 from functools import partial
 import itertools
+import sys
 import traceback
 
 # Workers are created as daemon threads and processes. This is done to allow the
@@ -109,6 +110,12 @@ def _python_exit():
 EXTRA_QUEUED_CALLS = 1
 
 
+# On Windows, WaitForMultipleObjects is used to wait for processes to finish.
+# It can wait on, at most, 63 objects. There is an overhead of two objects:
+# - the result queue reader
+# - the thread wakeup reader
+_MAX_WINDOWS_WORKERS = 63 - 2
+
 # Hack to embed stringification of remote traceback in local traceback
 
 class _RemoteTraceback(Exception):
@@ -505,9 +512,16 @@ class ProcessPoolExecutor(_base.Executor):
 
         if max_workers is None:
             self._max_workers = os.cpu_count() or 1
+            if sys.platform == 'win32':
+                self._max_workers = min(_MAX_WINDOWS_WORKERS,
+                                        self._max_workers)
         else:
             if max_workers <= 0:
                 raise ValueError("max_workers must be greater than 0")
+            elif (sys.platform == 'win32' and
+                max_workers > _MAX_WINDOWS_WORKERS):
+                raise ValueError(
+                    f"max_workers must be <= {_MAX_WINDOWS_WORKERS}")
 
             self._max_workers = max_workers