gh-96863: Fix subprocess.Popen.wait to raise ProcessLookupError for external waits

Lib/subprocess.py

@@ -2020,11 +2020,17 @@ def _try_wait(self, wait_flags):
             try:
                 (pid, sts) = os.waitpid(self.pid, wait_flags)
             except ChildProcessError:
-                # This happens if SIGCLD is set to be ignored or waiting
+                # This happens if SIGCHLD is set to be ignored or waiting
                 # for child processes has otherwise been disabled for our
-                # process.  This child is dead, we can't get the status.
-                pid = self.pid
-                sts = 0
+                # process. This child is dead, we can't get the status.
+                if signal.getsignal(signal.SIGCHLD) == signal.SIG_IGN:
+                    # SIGCHLD was ignored; for compatibility return a zero
+                    # status for this child.
+                    pid = self.pid
+                    sts = 0
+                    return (pid, sts)
+
+                raise ProcessLookupError(f'Process {self.pid} is already waited on externally')
             return (pid, sts)
 
 

Lib/test/test_asyncio/test_subprocess.py

@@ -607,7 +607,8 @@ async def kill_running():
 
             # kill the process (but asyncio is not notified immediately)
             proc.kill()
-            proc.wait()
+            with self.assertRaises(ProcessLookupError):
+                proc.wait()
 
             proc.kill = mock.Mock()
             proc_returncode = proc.poll()

Lib/test/test_subprocess.py

@@ -3361,6 +3361,25 @@ def test_stopped(self):
 
         self.assertEqual(returncode, -3)
 
+    def test_wait_after_external_wait(self):
+        """Test behavior when process is already waited on externally; issue96863."""
+        # Create a process with non-zero exit code
+        proc = subprocess.Popen([sys.executable, '-c', 'import sys; sys.exit(1)'])
+
+        # Wait for the process externally using os.wait()
+        pid, status = os.wait()
+        self.assertEqual(pid, proc.pid)
+        self.assertEqual(os.WEXITSTATUS(status), 1)
+
+        # Now calling proc.wait() should raise ProcessLookupError
+        # instead of returning 0 (the old buggy behavior)
+        with self.assertRaises(ProcessLookupError) as cm:
+            proc.wait()
+
+        # Verify the exception contains proper information
+        exc = cm.exception
+        self.assertIn('%d is already waited on externally' % proc.pid, str(exc))
+
     def test_send_signal_race(self):
         # bpo-38630: send_signal() must poll the process exit status to reduce
         # the risk of sending the signal to the wrong process.

Misc/NEWS.d/next/Library/2025-08-15-14-06-33.gh-issue-96863.rUk3aJ.rst

@@ -0,0 +1,3 @@
+Fix :meth:`subprocess.Popen.wait` returning zero always when process already
+waited on externally. Now raises :exc:`ProcessLookupError` instead of losing
+exit status. Preserves ``SIGCHLD=SIG_IGN`` compatibility.