gh-94182: run the PidfdChildWatcher on the running loop

Lib/asyncio/unix_events.py

@@ -911,46 +911,29 @@ 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:
@@ -968,12 +951,9 @@ 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

@@ -712,12 +712,12 @@ def setUp(self):
             self.set_event_loop(self.loop)
 
 
-class GenericWatcherTests:
+class GenericWatcherTests(test_utils.TestCase):
 
     def test_create_subprocess_fails_with_inactive_watcher(self):
 
         async def execute():
-            watcher = mock.create_authspec(asyncio.AbstractChildWatcher)
+            watcher = mock.create_autopec(asyncio.AbstractChildWatcher)
             watcher.is_active.return_value = False
             asyncio.set_child_watcher(watcher)
 
@@ -727,9 +727,42 @@ async def execute():
 
             watcher.add_child_handler.assert_not_called()
 
-        self.assertIsNone(self.loop.run_until_complete(execute()))
+        self.assertIsNone(asyncio.run(execute()))
+
+
+    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(),
+        "operating system does not support pidfds",
+    )
+    def test_create_subprocess_with_pidfd(self):
+        async def in_thread():
+            proc = await asyncio.create_subprocess_exec(
+                *args,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+            )
+            stdout, stderr = await proc.communicate(b"some data")
+            return proc.returncode, stdout
 
+        async def main():
+            return await asyncio.to_thread(asyncio.run, in_thread())
 
+        asyncio.set_child_watcher(asyncio.PidfdChildWatcher())
+        try:
+            returncode, stdout = asyncio.run(main())
+            self.assertEqual(returncode, 0)
+            self.assertEqual(stdout, b'some data')
+        finally:
+            asyncio.set_child_watcher(None)
 
 
 if __name__ == '__main__':