bpo-33613, test_semaphore_tracker_sigint: fix race condition (#7850)

Fail `test_semaphore_tracker_sigint` if no warnings are expected and one is received.

Fix race condition when the child receives SIGINT before it can register signal handlers for it.

The race condition occurs when the parent calls
`_semaphore_tracker.ensure_running()` (which in turn spawns the
semaphore_tracker using `_posixsubprocess.fork_exec`), the child
registers the signal handlers and the parent tries to kill the child.
What seem to happen is that in some slow systems, the parent sends the
signal to kill the child before the child protects against the signal.

1 files changed, 21 insertions, 6 deletions

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 749cf8c4fd..a5509ce913 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -20,6 +20,7 @@ import logging
 import struct
 import operator
 import weakref
+import warnings
 import test.support
 import test.support.script_helper
 from test import support
@@ -4517,17 +4518,19 @@ class TestSemaphoreTracker(unittest.TestCase):
         # bpo-31310: if the semaphore tracker process has died, it should
         # be restarted implicitly.
         from multiprocessing.semaphore_tracker import _semaphore_tracker
-        _semaphore_tracker.ensure_running()
         pid = _semaphore_tracker._pid
+        if pid is not None:
+            os.kill(pid, signal.SIGKILL)
+            os.waitpid(pid, 0)
+        with warnings.catch_warnings(record=True) as all_warn:
+            _semaphore_tracker.ensure_running()
+        pid = _semaphore_tracker._pid
+
         os.kill(pid, signum)
         time.sleep(1.0)  # give it time to die
 
         ctx = multiprocessing.get_context("spawn")
-        with contextlib.ExitStack() as stack:
-            if should_die:
-                stack.enter_context(self.assertWarnsRegex(
-                    UserWarning,
-                    "semaphore_tracker: process died"))
+        with warnings.catch_warnings(record=True) as all_warn:
             sem = ctx.Semaphore()
             sem.acquire()
             sem.release()
@@ -4537,11 +4540,23 @@ class TestSemaphoreTracker(unittest.TestCase):
             del sem
             gc.collect()
             self.assertIsNone(wr())
+            if should_die:
+                self.assertEqual(len(all_warn), 1)
+                the_warn = all_warn[0]
+                issubclass(the_warn.category, UserWarning)
+                self.assertTrue("semaphore_tracker: process died"
+                                in str(the_warn.message))
+            else:
+                self.assertEqual(len(all_warn), 0)
 
     def test_semaphore_tracker_sigint(self):
         # Catchable signal (ignored by semaphore tracker)
         self.check_semaphore_tracker_death(signal.SIGINT, False)
 
+    def test_semaphore_tracker_sigterm(self):
+        # Catchable signal (ignored by semaphore tracker)
+        self.check_semaphore_tracker_death(signal.SIGTERM, False)
+
     def test_semaphore_tracker_sigkill(self):
         # Uncatchable signal.
         self.check_semaphore_tracker_death(signal.SIGKILL, True)