bpo-43406: Fix test_signal.test_stress_modifying_handlers() (GH-24815) (GH-24817)

Fix a race condition of test_stress_modifying_handlers() of
test_signal: only raise signals while we are in the
catch_unraisable_exception() context manager.
Moreover, don't check if we received at least one
signal if at least one signal got ignored.
(cherry picked from commit 1fa17e8)

Co-authored-by: Victor Stinner <[email protected]>

Co-authored-by: Victor Stinner <[email protected]>

Author: miss-islington

Lib/test/test_signal.py

@@ -1272,11 +1272,16 @@ def cycle_handlers():
 
         old_handler = signal.signal(signum, custom_handler)
         self.addCleanup(signal.signal, signum, old_handler)
+
         t = threading.Thread(target=set_interrupts)
-        t.start()
         try:
+            ignored = False
             with support.catch_unraisable_exception() as cm:
+                t.start()
                 cycle_handlers()
+                do_stop = True
+                t.join()
+
                 if cm.unraisable is not None:
                     # An unraisable exception may be printed out when
                     # a signal is ignored due to the aforementioned
@@ -1285,8 +1290,13 @@ def cycle_handlers():
                     self.assertIn(
                         f"Signal {signum} ignored due to race condition",
                         str(cm.unraisable.exc_value))
-            # Sanity check that some signals were received, but not all
-            self.assertGreater(num_received_signals, 0)
+                    ignored = True
+
+            # bpo-43406: Even if it is unlikely, it's technically possible that
+            # all signals were ignored because of race conditions.
+            if not ignored:
+                # Sanity check that some signals were received, but not all
+                self.assertGreater(num_received_signals, 0)
             self.assertLess(num_received_signals, num_sent_signals)
         finally:
             do_stop = True