bpo-43406: Fix possible race condition where ``PyErr_CheckSignals`` tries to execute a non-Python signal handler (GH-24756)
We can receive signals (at the C level, in `trip_signal()` in signalmodule.c) while `signal.signal` is being called to modify the corresponding handler. Later when `PyErr_CheckSignals()` is called to handle the given signal, the handler may be a non-callable object and would raise a cryptic asynchronous exception.
diff --git a/Lib/test/test_signal.py b/Lib/test/test_signal.py
index 6a43fe7..c9de4a4 100644
--- a/Lib/test/test_signal.py
+++ b/Lib/test/test_signal.py
@@ -6,6 +6,7 @@
import statistics
import subprocess
import sys
+import threading
import time
import unittest
from test import support
@@ -1251,6 +1252,55 @@ def handler(signum, frame):
# Python handler
self.assertEqual(len(sigs), N, "Some signals were lost")
+ @unittest.skipUnless(hasattr(signal, "SIGUSR1"),
+ "test needs SIGUSR1")
+ def test_stress_modifying_handlers(self):
+ # bpo-43406: race condition between trip_signal() and signal.signal
+ signum = signal.SIGUSR1
+ num_sent_signals = 0
+ num_received_signals = 0
+ do_stop = False
+
+ def custom_handler(signum, frame):
+ nonlocal num_received_signals
+ num_received_signals += 1
+
+ def set_interrupts():
+ nonlocal num_sent_signals
+ while not do_stop:
+ signal.raise_signal(signum)
+ num_sent_signals += 1
+
+ def cycle_handlers():
+ while num_sent_signals < 100:
+ for i in range(20000):
+ # Cycle between a Python-defined and a non-Python handler
+ for handler in [custom_handler, signal.SIG_IGN]:
+ signal.signal(signum, handler)
+
+ old_handler = signal.signal(signum, custom_handler)
+ self.addCleanup(signal.signal, signum, old_handler)
+ t = threading.Thread(target=set_interrupts)
+ t.start()
+ try:
+ with support.catch_unraisable_exception() as cm:
+ cycle_handlers()
+ if cm.unraisable is not None:
+ # An unraisable exception may be printed out when
+ # a signal is ignored due to the aforementioned
+ # race condition, check it.
+ self.assertIsInstance(cm.unraisable.exc_value, OSError)
+ 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)
+ self.assertLess(num_received_signals, num_sent_signals)
+ finally:
+ do_stop = True
+ t.join()
+
+
class RaiseSignalTest(unittest.TestCase):
def test_sigint(self):