File lock backend doesn't lock

[n6151h]

The backend.lock.file module has a bug in it that results in subsequent instances of CronJob classes being allowed to run even though ALLOW_PARALLELL_RUNS=False.

Take a look at line 20 (or 21 in 0.6.0, which is what pypi is providing):

    def lock(self):
        lock_name = self.get_lock_name()
        breakpoint()
        try:
            self.__lock_fd = open(lock_name, 'w+b', 1)
            locks.lock(self.__lock_fd, locks.LOCK_EX | locks.LOCK_NB)  <<< PROBLEM HERE
        except IOError:
            return False
        return True

In locks.lock(...) a BlockingIOError exception is thrown when a second (or more) instance tries to start, so it returns False, as it should. However, this return is effectively ignored (see <<< PROBLEM HERE) and the try/except block falls through to return True, which allows the instance to start regardless of the exclusion lock.

The problem line should simply return the result of locks.lock.