GH-94182: Run the PidfdChildWatcher on the running loop (#94184)

There is no reason for this watcher to be attached to any particular loop.
This should make it safe to use regardless of the lifetime of the event loop running in the main thread
(relative to other loops).

Co-authored-by: Yury Selivanov <yury@edgedb.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

1 files changed, 12 insertions, 32 deletions

diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 96e6d73a759..0f67b4d469f 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -912,10 +912,6 @@ class PidfdChildWatcher(AbstractChildWatcher):
     recent (5.3+) kernels.
     """
 
-    def __init__(self):
-        self._loop = None
-        self._callbacks = {}
-
     def __enter__(self):
         return self
 
@@ -923,35 +919,22 @@ class PidfdChildWatcher(AbstractChildWatcher):
         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 @@ class PidfdChildWatcher(AbstractChildWatcher):
         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