gh-153962: Re-raise unexpected OSError in subprocess Popen._internal_poll

On POSIX, Popen._internal_poll() caught every OSError from os.waitpid()
and, unless it was ECHILD or the call came from __del__ (a non-None
_deadstate), silently discarded it, leaving returncode as None and
masking a real failure.

Add an else branch that re-raises such unexpected errors. The re-raise is
only reachable when _deadstate is None; __del__ and _cleanup() always
pass a non-None _deadstate, so __del__ still never raises. ECHILD
handling is unchanged.

Author: fedonman

Lib/subprocess.py

@@ -2191,6 +2191,13 @@ def _internal_poll(self, _deadstate=None, _del_safe=_del_safe):
                         # can't get the status.
                         # http://bugs.python.org/issue15756
                         self.returncode = 0
+                    else:
+                        # An unexpected error (e.g. EINVAL) must not be
+                        # silently swallowed, leaving returncode as None:
+                        # surface it to the caller.  This branch is never
+                        # reached from __del__, which always passes a
+                        # non-None _deadstate.
+                        raise
                 finally:
                     self._waitpid_lock.release()
             return self.returncode

Lib/test/test_subprocess.py

@@ -3419,6 +3419,25 @@ def test_leak_fast_process_del_killed(self):
         else:
             self.assertNotIn(ident, [id(o) for o in subprocess._active])
 
+    def test_internal_poll_reraises_unexpected_oserror(self):
+        # An unexpected OSError from waitpid() (i.e. not ECHILD, and not
+        # during the __del__/_deadstate path) must not be silently swallowed
+        # by poll(); doing so would leave returncode as None and mask a real
+        # failure.
+        p = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(3)"])
+        self.addCleanup(p.wait)
+        self.addCleanup(p.kill)
+
+        def unexpected_waitpid(pid, flags):
+            raise OSError(errno.EINVAL, "simulated unexpected error")
+
+        with mock.patch.object(subprocess._del_safe, "waitpid",
+                               unexpected_waitpid):
+            with self.assertRaises(OSError) as cm:
+                p.poll()
+        self.assertEqual(cm.exception.errno, errno.EINVAL)
+        self.assertIsNone(p.returncode)
+
     def test_close_fds_after_preexec(self):
         fd_status = support.findfile("fd_status.py", subdir="subprocessdata")
 

Misc/NEWS.d/next/Library/2026-07-18-16-35-33.gh-issue-153962.Kp7xQ2.rst

@@ -0,0 +1,4 @@
+On POSIX, :meth:`subprocess.Popen.poll` now re-raises an unexpected
+:exc:`OSError` from the underlying :func:`os.waitpid` call instead of
+silently ignoring it and leaving :attr:`!returncode` set to ``None``. The
+special handling of :const:`~errno.ECHILD` is unchanged.