GH-94597: deprecate SafeChildWatcher, FastChildWatcher and MultiLoopChildWatcher child watchers

Doc/whatsnew/3.12.rst

@@ -102,6 +102,24 @@ New Modules
 Improved Modules
 ================
 
+asyncio
+-------
+
+* On Linux, :mod:`asyncio` uses :class:`~asyncio.PidfdChildWatcher` by default
+ if :func:`os.pidfd_open` is available and functional instead of
+ :class:`~asyncio.ThreadedChildWatcher`.
+ (Contributed by Kumar Aditya in :gh:`98024`.)
+
+* The child watcher classes :class:`~asyncio.MultiLoopChildWatcher`,
+ :class:`~asyncio.FastChildWatcher` and
+ :class:`~asyncio.SafeChildWatcher` are deprecated and
+ will be removed in Python 3.14. It is recommended to not manually
+ configure a child watcher as the event loop now uses the best available
+ child watcher for each platform (:class:`~asyncio.PidfdChildWatcher`
+ if supported and :class:`~asyncio.ThreadedChildWatcher` otherwise).
+ (Contributed by Kumar Aditya in :gh:`94597`.)
+
+
 pathlib
 -------
 

Lib/asyncio/unix_events.py

@@ -1022,6 +1022,13 @@ class SafeChildWatcher(BaseChildWatcher):
  big number of children (O(n) each time SIGCHLD is raised)
  """
 
+ def __init__(self):
+ super().__init__()
+ warnings._deprecated("SafeChildWatcher",
+ "{name!r} is deprecated as of Python 3.12 and will be "
+ "removed in Python {remove}.",
+ remove=(3, 14))
+
  def close(self):
  self._callbacks.clear()
  super().close()
@@ -1100,6 +1107,10 @@ def __init__(self):
  self._lock = threading.Lock()
  self._zombies = {}
  self._forks = 0
+ warnings._deprecated("FastChildWatcher",
+ "{name!r} is deprecated as of Python 3.12 and will be "
+ "removed in Python {remove}.",
+ remove=(3, 14))
 
  def close(self):
  self._callbacks.clear()
@@ -1212,6 +1223,10 @@ class MultiLoopChildWatcher(AbstractChildWatcher):
  def __init__(self):
  self._callbacks = {}
  self._saved_sighandler = None
+ warnings._deprecated("MultiLoopChildWatcher",
+ "{name!r} is deprecated as of Python 3.12 and will be "
+ "removed in Python {remove}.",
+ remove=(3, 14))
 
  def is_active(self):
  return self._saved_sighandler is not None

Lib/test/test_asyncio/test_events.py

@@ -22,7 +22,7 @@
 import unittest
 from unittest import mock
 import weakref
-
+import warnings
 if sys.platform not in ('win32', 'vxworks'):
  import tty
 
@@ -2055,7 +2055,9 @@ def test_remove_fds_after_closing(self):
  class UnixEventLoopTestsMixin(EventLoopTestsMixin):
  def setUp(self):
  super().setUp()
- watcher = asyncio.SafeChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter('ignore', DeprecationWarning)
+ watcher = asyncio.SafeChildWatcher()
  watcher.attach_loop(self.loop)
  asyncio.set_child_watcher(watcher)
 
@@ -2652,7 +2654,9 @@ def setUp(self):
  asyncio.set_event_loop(self.loop)
 
  if sys.platform != 'win32':
- watcher = asyncio.SafeChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter('ignore', DeprecationWarning)
+ watcher = asyncio.SafeChildWatcher()
  watcher.attach_loop(self.loop)
  asyncio.set_child_watcher(watcher)
 

Lib/test/test_asyncio/test_streams.py

@@ -9,6 +9,7 @@
 import threading
 import unittest
 from unittest import mock
+import warnings
 from test.support import socket_helper
 try:
  import ssl
@@ -791,8 +792,9 @@ def test_read_all_from_pipe_reader(self):
  protocol = asyncio.StreamReaderProtocol(reader, loop=self.loop)
  transport, _ = self.loop.run_until_complete(
  self.loop.connect_read_pipe(lambda: protocol, pipe))
-
- watcher = asyncio.SafeChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter('ignore', DeprecationWarning)
+ watcher = asyncio.SafeChildWatcher()
  watcher.attach_loop(self.loop)
  try:
  asyncio.set_child_watcher(watcher)

Lib/test/test_asyncio/test_subprocess.py

@@ -4,7 +4,6 @@
 import sys
 import unittest
 import warnings
-import functools
 from unittest import mock
 
 import asyncio
@@ -31,19 +30,6 @@
  'sys.stdout.buffer.write(data)'))]
 
 
-@functools.cache
-def _has_pidfd_support():
- if not hasattr(os, 'pidfd_open'):
- return False
-
- try:
- os.close(os.pidfd_open(os.getpid()))
- except OSError:
- return False
-
- return True
-
-
 def tearDownModule():
  asyncio.set_event_loop_policy(None)
 
@@ -688,7 +674,7 @@ def setUp(self):
  self.loop = policy.new_event_loop()
  self.set_event_loop(self.loop)
 
- watcher = self.Watcher()
+ watcher = self._get_watcher()
  watcher.attach_loop(self.loop)
  policy.set_child_watcher(watcher)
 
@@ -703,32 +689,38 @@ def tearDown(self):
  class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
  test_utils.TestCase):
 
- Watcher = unix_events.ThreadedChildWatcher
-
- @unittest.skip("bpo-38323: MultiLoopChildWatcher has a race condition \
- and these tests can hang the test suite")
- class SubprocessMultiLoopWatcherTests(SubprocessWatcherMixin,
- test_utils.TestCase):
-
- Watcher = unix_events.MultiLoopChildWatcher
+ def _get_watcher(self):
+ return unix_events.ThreadedChildWatcher()
 
  class SubprocessSafeWatcherTests(SubprocessWatcherMixin,
  test_utils.TestCase):
 
- Watcher = unix_events.SafeChildWatcher
+ def _get_watcher(self):
+ with self.assertWarns(DeprecationWarning):
+ return unix_events.SafeChildWatcher()
+
+ class MultiLoopChildWatcherTests(test_utils.TestCase):
+
+ def test_warns(self):
+ with self.assertWarns(DeprecationWarning):
+ unix_events.MultiLoopChildWatcher()
 
  class SubprocessFastWatcherTests(SubprocessWatcherMixin,
  test_utils.TestCase):
 
- Watcher = unix_events.FastChildWatcher
+ def _get_watcher(self):
+ with self.assertWarns(DeprecationWarning):
+ return unix_events.FastChildWatcher()
 
  @unittest.skipUnless(
- _has_pidfd_support(),
+ unix_events.can_use_pidfd(),
  "operating system does not support pidfds",
  )
  class SubprocessPidfdWatcherTests(SubprocessWatcherMixin,
  test_utils.TestCase):
- Watcher = unix_events.PidfdChildWatcher
+
+ def _get_watcher(self):
+ return unix_events.PidfdChildWatcher()
 
 
  class GenericWatcherTests(test_utils.TestCase):
@@ -758,7 +750,7 @@ async def execute():
 
 
  @unittest.skipUnless(
- _has_pidfd_support(),
+ unix_events.can_use_pidfd(),
  "operating system does not support pidfds",
  )
  def test_create_subprocess_with_pidfd(self):

Lib/test/test_asyncio/test_unix_events.py

@@ -12,6 +12,7 @@
 import threading
 import unittest
 from unittest import mock
+import warnings
 from test.support import os_helper
 from test.support import socket_helper
 
@@ -1686,12 +1687,16 @@ def test_close(self, m_waitpid):
 
 class SafeChildWatcherTests (ChildWatcherTestsMixin, test_utils.TestCase):
  def create_watcher(self):
- return asyncio.SafeChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter("ignore", DeprecationWarning)
+ return asyncio.SafeChildWatcher()
 
 
 class FastChildWatcherTests (ChildWatcherTestsMixin, test_utils.TestCase):
  def create_watcher(self):
- return asyncio.FastChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter("ignore", DeprecationWarning)
+ return asyncio.FastChildWatcher()
 
 
 class PolicyTests(unittest.TestCase):
@@ -1724,7 +1729,9 @@ def test_get_default_child_watcher(self):
 
  def test_get_child_watcher_after_set(self):
  policy = self.create_policy()
- watcher = asyncio.FastChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter("ignore", DeprecationWarning)
+ watcher = asyncio.FastChildWatcher()
 
  policy.set_child_watcher(watcher)
  self.assertIs(policy._watcher, watcher)
@@ -1745,7 +1752,9 @@ def f():
  policy.get_event_loop().close()
 
  policy = self.create_policy()
- policy.set_child_watcher(asyncio.SafeChildWatcher())
+ with warnings.catch_warnings():
+ warnings.simplefilter("ignore", DeprecationWarning)
+ policy.set_child_watcher(asyncio.SafeChildWatcher())
 
  th = threading.Thread(target=f)
  th.start()
@@ -1757,7 +1766,9 @@ def test_child_watcher_replace_mainloop_existing(self):
 
  # Explicitly setup SafeChildWatcher,
  # default ThreadedChildWatcher has no _loop property
- watcher = asyncio.SafeChildWatcher()
+ with warnings.catch_warnings():
+ warnings.simplefilter("ignore", DeprecationWarning)
+ watcher = asyncio.SafeChildWatcher()
  policy.set_child_watcher(watcher)
  watcher.attach_loop(loop)
 

Misc/NEWS.d/next/Library/2022-10-08-06-59-46.gh-issue-94597.TsS0oT.rst

@@ -0,0 +1 @@
+The child watcher classes :class:`~asyncio.MultiLoopChildWatcher`, :class:`~asyncio.FastChildWatcher` and :class:`~asyncio.SafeChildWatcher` are deprecated and will be removed in Python 3.14. Patch by Kumar Aditya.