gh-158287: Use watchdog timer in test_stopped_child_does_not_block_event_loop

Author: JohnnyWilson16

Lib/test/test_asyncio/test_subprocess.py

@@ -3,6 +3,7 @@
 import signal
 import sys
 import textwrap
+import threading
 import unittest
 import warnings
 from unittest import mock
@@ -1169,25 +1170,55 @@ def test_stopped_child_does_not_block_event_loop(self):
             # stopped children (si_code == CLD_STOPPED). The child watcher
             # must not mistake this for process termination and attempt to reap
             # the child with waitpid(), which blocks the event loop thread.
-            async def run():
-                async with asyncio.timeout(support.SHORT_TIMEOUT):
+            proc = None
+            timed_out = False
+
+            def watchdog():
+                nonlocal timed_out
+                timed_out = True
+                # If the event loop gets stuck in waitpid() because of this bug,
+                # killing the child process unblocks waitpid() so the test fails
+                # instead of hanging indefinitely.
+                if proc is not None:
+                    if hasattr(signal, 'SIGCONT'):
+                        try:
+                            os.kill(proc.pid, signal.SIGCONT)
+                        except ProcessLookupError:
+                            pass
+                    try:
+                        os.kill(proc.pid, signal.SIGKILL)
+                    except ProcessLookupError:
+                        pass
+
+            timer = threading.Timer(support.SHORT_TIMEOUT, watchdog)
+            timer.start()
+            try:
+                async def run():
+                    nonlocal proc
                     proc = await asyncio.create_subprocess_exec(*PROGRAM_BLOCKED)
                     try:
                         proc.send_signal(signal.SIGSTOP)
                         # Verify event loop remains responsive while child is stopped
                         for _ in range(3):
                             await asyncio.sleep(0.05)
+                        self.assertFalse(timed_out, "event loop was blocked by stopped child")
                         self.assertIsNone(proc.returncode)
                     finally:
                         if hasattr(signal, 'SIGCONT'):
                             try:
                                 proc.send_signal(signal.SIGCONT)
                             except ProcessLookupError:
                                 pass
-                        proc.kill()
+                        try:
+                            proc.kill()
+                        except ProcessLookupError:
+                            pass
                         await proc.wait()
 
-            self.loop.run_until_complete(run())
+                self.loop.run_until_complete(run())
+            finally:
+                timer.cancel()
+                timer.join()
 
     @unittest.skipUnless(
         unix_events.can_use_pidfd(),