When refreshing the lock, should check whether the lock is held.

[brodyxchen]

Hello, I am looking a HA solution for celery beat and found this project, Thanks for you project, It is useful to me.

But When I learn the code, I found one possible problem about how to refreshing/extending the lock.

There could check lock before extended lock, so should use a lua script do check lock and extend lock, this ensures atomicity.

Finally, we also need to check the result of the refresh/extend lock, it is possible that the refresh/extend failed, meaning missing lock or network error.

---

Like this: (from redbeat)
https://github.com/sibson/redbeat/blob/5d1d5c154d2e080d3135afee235321abf5d2da5b/redbeat/schedulers.py#L37

    local token = redis.call('get', KEYS[1])
    if not token or token ~= ARGV[1] then
        return 0
    end
    local expiration = redis.call('pttl', KEYS[1])
    if not expiration then
        expiration = 0
    end
    if expiration < 0 then
        return 0
    end
    redis.call('pexpire', KEYS[1], ARGV[2])
    return 1

or Like this: （from golang redlock lib）
https://github.com/go-redsync/redsync/blob/311e82d385dcf0b228cb1b4974b690f8bb2dc127/mutex.go#L254

	if redis.call("GET", KEYS[1]) == ARGV[1] then
		return redis.call("PEXPIRE", KEYS[1], ARGV[2])
	else
		return 0
	end