gh-82504: deprecate the MultiLoopChildWatcher

Doc/library/asyncio-policy.rst

@@ -232,6 +232,7 @@ implementation used by the asyncio event loop:
    :py:data:`SIGCHLD` is received).
 
    .. versionadded:: 3.8
+   .. deprecated:: 3.12
 
 .. class:: SafeChildWatcher
 

Lib/asyncio/unix_events.py

@@ -1229,6 +1229,7 @@ class MultiLoopChildWatcher(AbstractChildWatcher):
     # but retrieves the current loop by get_running_loop()
 
     def __init__(self):
+        warnings._deprecated("asyncio.MultiLoopChildWatcher", remove=(3, 14))
         self._callbacks = {}
         self._saved_sighandler = None
 

Lib/test/test_asyncio/test_subprocess.py

@@ -668,13 +668,6 @@ class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
 
         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
-
     class SubprocessSafeWatcherTests(SubprocessWatcherMixin,
                                      test_utils.TestCase):
 
@@ -702,6 +695,12 @@ class SubprocessPidfdWatcherTests(SubprocessWatcherMixin,
                                       test_utils.TestCase):
         Watcher = unix_events.PidfdChildWatcher
 
+
+    class MultiLoopChildWatcherTests(unittest.TestCase):
+        def test_deprecated(self):
+            with self.assertWarns(DeprecationWarning):
+                asyncio.MultiLoopChildWatcher()
+
 else:
     # Windows
     class SubprocessProactorTests(SubprocessMixin, test_utils.TestCase):

Misc/NEWS.d/next/Library/2022-07-07-09-04-49.gh-issue-82504.6xtYAz.rst

@@ -0,0 +1 @@
+Deprecate :class:`asyncio.MultiLoopChildWatcher` to be removed in Python 3.14.