def test_wakerOverflow(self):
        """
        Try to make an overflow on the reactor waker using callFromThread.
        """
        def cb(ign):
            self.failure = None
            waiter = threading.Event()
            def threadedFunction():
                # Hopefully a hundred thousand queued calls is enough to
                # trigger the error condition
                for i in range(100000):
                    try:
                        reactor.callFromThread(lambda: None)
                    except:
                        self.failure = failure.Failure()
                        break
                waiter.set()
            reactor.callInThread(threadedFunction)
            waiter.wait(120)
            if not waiter.isSet():
                self.fail("Timed out waiting for event")
            if self.failure is not None:
                return defer.fail(self.failure)
        return self._waitForThread().addCallback(cb)

        Try to make an overflow on the reactor waker using callFromThread.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_threads.py