gh-94182: run the PidfdChildWatcher on the running loop

Lib/asyncio/unix_events.py

@@ -912,46 +912,29 @@ class PidfdChildWatcher(AbstractChildWatcher):
     recent (5.3+) kernels.
     """
 
-    def __init__(self):
-        self._loop = None
-        self._callbacks = {}
-
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, exc_traceback):
         pass
 
     def is_active(self):
-        return self._loop is not None and self._loop.is_running()
+        return True
 
     def close(self):
-        self.attach_loop(None)
+        pass
 
     def attach_loop(self, loop):
-        if self._loop is not None and loop is None and self._callbacks:
-            warnings.warn(
-                'A loop is being detached '
-                'from a child watcher with pending handlers',
-                RuntimeWarning)
-        for pidfd, _, _ in self._callbacks.values():
-            self._loop._remove_reader(pidfd)
-            os.close(pidfd)
-        self._callbacks.clear()
-        self._loop = loop
+        pass
 
     def add_child_handler(self, pid, callback, *args):
-        existing = self._callbacks.get(pid)
-        if existing is not None:
-            self._callbacks[pid] = existing[0], callback, args
-        else:
-            pidfd = os.pidfd_open(pid)
-            self._loop._add_reader(pidfd, self._do_wait, pid)
-            self._callbacks[pid] = pidfd, callback, args
+        loop = events.get_running_loop()
+        pidfd = os.pidfd_open(pid)
+        loop._add_reader(pidfd, self._do_wait, pid, pidfd, callback, args)
 
-    def _do_wait(self, pid):
-        pidfd, callback, args = self._callbacks.pop(pid)
-        self._loop._remove_reader(pidfd)
+    def _do_wait(self, pid, pidfd, callback, args):
+        loop = events.get_running_loop()
+        loop._remove_reader(pidfd)
         try:
             _, status = os.waitpid(pid, 0)
         except ChildProcessError:
@@ -969,12 +952,9 @@ def _do_wait(self, pid):
         callback(pid, returncode, *args)
 
     def remove_child_handler(self, pid):
-        try:
-            pidfd, _, _ = self._callbacks.pop(pid)
-        except KeyError:
-            return False
-        self._loop._remove_reader(pidfd)
-        os.close(pidfd)
+        # asyncio never calls remove_child_handler() !!!
+        # The method is no-op but is implemented because
+        # abstract base classes require it.
         return True
 
 

Lib/test/test_asyncio/test_subprocess.py

@@ -4,6 +4,7 @@
 import sys
 import unittest
 import warnings
+import functools
 from unittest import mock
 
 import asyncio
@@ -30,6 +31,19 @@
               '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)
 
@@ -708,17 +722,8 @@ class SubprocessFastWatcherTests(SubprocessWatcherMixin,
 
         Watcher = unix_events.FastChildWatcher
 
-    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
-
     @unittest.skipUnless(
-        has_pidfd_support(),
+        _has_pidfd_support(),
         "operating system does not support pidfds",
     )
     class SubprocessPidfdWatcherTests(SubprocessWatcherMixin,
@@ -751,6 +756,35 @@ async def execute():
                 mock.call.__exit__(RuntimeError, mock.ANY, mock.ANY),
             ])
 
+
+        @unittest.skipUnless(
+            _has_pidfd_support(),
+            "operating system does not support pidfds",
+        )
+        def test_create_subprocess_with_pidfd(self):
+            async def in_thread():
+                proc = await asyncio.create_subprocess_exec(
+                    *PROGRAM_CAT,
+                    stdin=subprocess.PIPE,
+                    stdout=subprocess.PIPE,
+                )
+                stdout, stderr = await proc.communicate(b"some data")
+                return proc.returncode, stdout
+
+            async def main():
+                # asyncio.Runner did not call asyncio.set_event_loop()
+                with self.assertRaises(RuntimeError):
+                    asyncio.get_event_loop_policy().get_event_loop()
+                return await asyncio.to_thread(asyncio.run, in_thread())
+
+            asyncio.set_child_watcher(asyncio.PidfdChildWatcher())
+            try:
+                with asyncio.Runner(loop_factory=asyncio.new_event_loop) as runner:
+                    returncode, stdout = runner.run(main())
+                self.assertEqual(returncode, 0)
+                self.assertEqual(stdout, b'some data')
+            finally:
+                asyncio.set_child_watcher(None)
 else:
     # Windows
     class SubprocessProactorTests(SubprocessMixin, test_utils.TestCase):

Misc/NEWS.d/next/Library/2022-06-24-08-49-47.gh-issue-94182.Wknau0.rst

@@ -0,0 +1 @@
+run the :class:`asyncio.PidfdChildWatcher` on the running loop, this allows event loops to run subprocesses when there is no default event loop running on the main thread