gh-151179: Fix pidfd leak in asyncio _PidfdChildWatcher

Author: deadlovelll

Lib/asyncio/unix_events.py

@@ -876,19 +876,20 @@ 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:
-            # The child process is already reaped
-            # (may happen if waitpid() is called elsewhere).
-            returncode = 255
-            logger.warning(
-                "child process pid %d exit status already read: "
-                " will report returncode 255",
-                pid)
-        else:
-            returncode = waitstatus_to_exitcode(status)
-
-        os.close(pidfd)
+            try:
+                _, status = os.waitpid(pid, 0)
+            except ChildProcessError:
+                # The child process is already reaped
+                # (may happen if waitpid() is called elsewhere).
+                returncode = 255
+                logger.warning(
+                    "child process pid %d exit status already read: "
+                    " will report returncode 255",
+                    pid)
+            else:
+                returncode = waitstatus_to_exitcode(status)
+        finally:
+            os.close(pidfd)
         callback(pid, returncode, *args)
 
 class _ThreadedChildWatcher:

Lib/test/test_asyncio/test_unix_events.py

@@ -1333,5 +1333,44 @@ async def child_main():
 
         self.assertEqual(result.value, 0)
 
+
+@unittest.skipUnless(
+    unix_events.can_use_pidfd(),
+    "operating system does not support pidfd",
+)
+class PidfdChildWatcherTests(test_utils.TestCase):
+
+    def setUp(self):
+        super().setUp()
+        self.loop = asyncio.new_event_loop()
+        self.set_event_loop(self.loop)
+
+    def test_pidfd_closed_when_waitpid_raises(self):
+        # _do_wait() must close the pidfd even when waitpid()
+        # fails with something other than ChildProcessError, otherwise the
+        # pidfd is leaked
+        watcher = unix_events._PidfdChildWatcher()
+        pid = os.posix_spawn(sys.executable, [sys.executable, '-c', ''],
+                             os.environ)
+        pidfd = os.pidfd_open(pid)
+        try:
+            async def coro():
+                with mock.patch.object(os, 'waitpid',
+                                       side_effect=OSError('unexpected')):
+                    with self.assertRaises(OSError):
+                        watcher._do_wait(pid, pidfd, lambda *a: None, ())
+
+            self.loop.run_until_complete(coro())
+
+            with self.assertRaises(OSError):
+                os.fstat(pidfd)
+        finally:
+            try:
+                os.close(pidfd)
+            except OSError:
+                pass
+            os.waitpid(pid, 0)
+
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2026-06-09-19-43-24.gh-issue-151179.hl_6Z0.rst

@@ -0,0 +1,3 @@
+Fix a pidfd leak in :class:`_PidfdChildWatcher` on Linux: the watcher no
+longer leaks the process file descriptor when ``waitpid()`` fails with an
+error other than :exc:`ChildProcessError`.