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

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

Author: brianquinlan

Doc/library/concurrent.futures.rst

@@ -216,6 +216,10 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
  given, it will default to the number of processors on the machine.
  If *max_workers* is lower or equal to ``0``, then a :exc:`ValueError`
  will be raised.
+ On Windows, *max_workers* must be equal or lower than ``61``. If it is not
+ then :exc:`ValueError` will be raised. If *max_workers* is ``None``, then
+ the default chosen will be at most ``61``, even if more processors are
+ available.
  *mp_context* can be a multiprocessing context or None. It will be used to
  launch the workers. If *mp_context* is ``None`` or not given, the default
  multiprocessing context is used.

Lib/concurrent/futures/process.py

@@ -57,6 +57,7 @@
 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 @@ def __init__(self, max_workers=None, mp_context=None,
 
  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
 

Lib/test/test_concurrent_futures.py

@@ -755,6 +755,13 @@ def test_default_workers(self):
 
 
 class ProcessPoolExecutorTest(ExecutorTest):
+
+ @unittest.skipUnless(sys.platform=='win32', 'Windows-only process limit')
+ def test_max_workers_too_large(self):
+ with self.assertRaisesRegex(ValueError,
+ "max_workers must be <= 61"):
+ futures.ProcessPoolExecutor(max_workers=62)
+
  def test_killed_child(self):
  # When a child process is abruptly terminated, the whole pool gets
  # "broken".

Misc/NEWS.d/next/Library/2019-05-06-19-17-04.bpo-26903.4payXb.rst

@@ -0,0 +1 @@
+Limit `max_workers` in `ProcessPoolExecutor` to 61 to work around a WaitForMultipleObjects limitation.