Issue #28368: Refuse monitoring processes if the child watcher has no loop attached.

Patch by Vincent Michel.

Lib/test/test_asyncio/test_subprocess.py

@@ -433,6 +433,13 @@ class SubprocessMixin:
         # the transport was not notified yet
         self.assertFalse(killed)
 
+        # Unlike SafeChildWatcher, FastChildWatcher does not pop the
+        # callbacks if waitpid() is called elsewhere. Let's clear them
+        # manually to avoid a warning when the watcher is detached.
+        if sys.platform != 'win32' and \
+           isinstance(self, SubprocessFastWatcherTests):
+            asyncio.get_child_watcher()._callbacks.clear()
+
     def test_popen_error(self):
         # Issue #24763: check that the subprocess transport is closed
         # when BaseSubprocessTransport fails

Lib/test/test_asyncio/test_unix_events.py

@@ -11,6 +11,7 @@ import sys
 import tempfile
 import threading
 import unittest
+import warnings
 from unittest import mock
 
 if sys.platform == 'win32':
@@ -1391,7 +1392,9 @@ class ChildWatcherTestsMixin:
         with mock.patch.object(
                 old_loop, "remove_signal_handler") as m_remove_signal_handler:
 
-            self.watcher.attach_loop(None)
+            with self.assertWarnsRegex(
+                    RuntimeWarning, 'A loop is being detached'):
+                self.watcher.attach_loop(None)
 
             m_remove_signal_handler.assert_called_once_with(
                 signal.SIGCHLD)
@@ -1463,6 +1466,15 @@ class ChildWatcherTestsMixin:
                 if isinstance(self.watcher, asyncio.FastChildWatcher):
                     self.assertFalse(self.watcher._zombies)
 
+    @waitpid_mocks
+    def test_add_child_handler_with_no_loop_attached(self, m):
+        callback = mock.Mock()
+        with self.create_watcher() as watcher:
+            with self.assertRaisesRegex(
+                    RuntimeError,
+                    'the child watcher does not have a loop attached'):
+                watcher.add_child_handler(100, callback)
+
 
 class SafeChildWatcherTests (ChildWatcherTestsMixin, test_utils.TestCase):
     def create_watcher(self):