GH-94597: deprecate `SafeChildWatcher`, `FastChildWatcher` and `MultiLoopChildWatcher` child watchers (#98089)

Doc/whatsnew/3.12.rst

@@ -109,6 +109,24 @@ 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 @@ class FastChildWatcher(BaseChildWatcher):
         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 errno
 import unittest
 from unittest import mock
 import weakref
-
+import warnings
 if sys.platform not in ('win32', 'vxworks'):
     import tty
 
@@ -2055,7 +2055,9 @@ else:
     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 @@ class GetEventLoopTestsMixin:
         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 sys
 import threading
 import unittest
 from unittest import mock
+import warnings
 from test.support import socket_helper
 try:
     import ssl
@@ -791,8 +792,9 @@ os.close(fd)
         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 signal
 import sys
 import unittest
 import warnings
-import functools
 from unittest import mock
 
 import asyncio
@@ -31,19 +30,6 @@ PROGRAM_CAT = [
               '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 @@ if sys.platform != 'win32':
             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 @@ if sys.platform != 'win32':
     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 @@ if sys.platform != 'win32':
 
 
         @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 sys
 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 @@ class ChildWatcherTestsMixin:
 
 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 @@ class PolicyTests(unittest.TestCase):
 
     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 @@ class PolicyTests(unittest.TestCase):
             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 @@ class PolicyTests(unittest.TestCase):
 
         # 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.