gh-116622: Switch test_stress_delivery_simultaneous from SIGUSR1 to SIGUSR2 (#123981)

Use SIGUSR1 instead of SIGUSR2 to improve reliability of signal stress test on Android.

Android/testbed/app/src/main/python/main.py

@@ -5,9 +5,25 @@ import signal
 import sys
 
 # Some tests use SIGUSR1, but that's blocked by default in an Android app in
-# order to make it available to `sigwait` in the "Signal Catcher" thread. That
-# thread's functionality is only relevant to the JVM ("forcing GC (no HPROF) and
-# profile save"), so disabling it should not weaken the tests.
+# order to make it available to `sigwait` in the Signal Catcher thread.
+# (https://cs.android.com/android/platform/superproject/+/android14-qpr3-release:art/runtime/signal_catcher.cc).
+# That thread's functionality is only useful for debugging the JVM, so disabling
+# it should not weaken the tests.
+#
+# There's no safe way of stopping the thread completely (#123982), but simply
+# unblocking SIGUSR1 is enough to fix most tests.
+#
+# However, in tests that generate multiple different signals in quick
+# succession, it's possible for SIGUSR1 to arrive while the main thread is busy
+# running the C-level handler for a different signal. In that case, the SIGUSR1
+# may be sent to the Signal Catcher thread instead, which will generate a log
+# message containing the text "reacting to signal".
+#
+# Such tests may need to be changed in one of the following ways:
+#   * Use a signal other than SIGUSR1 (e.g. test_stress_delivery_simultaneous in
+#     test_signal.py).
+#   * Send the signal to a specific thread rather than the whole process (e.g.
+#     test_signals in test_threadsignals.py.
 signal.pthread_sigmask(signal.SIG_UNBLOCK, [signal.SIGUSR1])
 
 sys.argv[1:] = shlex.split(os.environ["PYTHON_ARGS"])

Lib/test/test_signal.py

@@ -1325,15 +1325,18 @@ class StressTest(unittest.TestCase):
         def handler(signum, frame):
             sigs.append(signum)
 
-        self.setsig(signal.SIGUSR1, handler)
+        # On Android, SIGUSR1 is unreliable when used in close proximity to
+        # another signal – see Android/testbed/app/src/main/python/main.py.
+        # So we use a different signal.
+        self.setsig(signal.SIGUSR2, handler)
         self.setsig(signal.SIGALRM, handler)  # for ITIMER_REAL
 
         expected_sigs = 0
         while expected_sigs < N:
             # Hopefully the SIGALRM will be received somewhere during
-            # initial processing of SIGUSR1.
+            # initial processing of SIGUSR2.
             signal.setitimer(signal.ITIMER_REAL, 1e-6 + random.random() * 1e-5)
-            os.kill(os.getpid(), signal.SIGUSR1)
+            os.kill(os.getpid(), signal.SIGUSR2)
 
             expected_sigs += 2
             # Wait for handlers to run to avoid signal coalescing