bpo-37933: do not segfault canceling never started faulthandler dump

Lib/test/test_faulthandler.py

@@ -817,6 +817,17 @@ def test_disable_windows_exc_handler(self):
         self.assertEqual(output, [])
         self.assertEqual(exitcode, 0xC0000005)
 
+    def test_cancel_later_without_dump_traceback_later(self):
+        # bpo-37933: Calling cancel_dump_traceback_later()
+        # without dump_traceback_later() must not segfault.
+        code = dedent("""
+            import faulthandler
+            faulthandler.cancel_dump_traceback_later()
+        """)
+        output, exitcode = self.get_output(code)
+        self.assertEqual(output, [])
+        self.assertEqual(exitcode, 0)
+
 
 if __name__ == "__main__":
     unittest.main()

Modules/faulthandler.c

@@ -631,6 +631,11 @@ faulthandler_thread(void *unused)
 static void
 cancel_dump_traceback_later(void)
 {
+    /* If not scheduled, nothing to cancel */
+    if (!thread.cancel_event) {
+        return;
+    }
+
     /* Notify cancellation */
     PyThread_release_lock(thread.cancel_event);