request help: limit-count-redis time_window 1 may cause reject to some request under count

[jinyuxiaoqiang]

Issue description

While using plugin 'limit-count',i encountered a problem:
situation：count:3000,time_window:1,rejected_code:503,policy:"redis",key:$remote_addr。
reproduce: when requsting a route with above limit-count config a bit frequent about 500qps,apisix will return rejected_code.
Adding log to limit-count.lua _M.access() and limit-count-redis.lua. I find an issue:
local ret, err = red:ttl(key) => ret = 0
local b_key, err = red:get(key) =>b_key = null
remaining, err = red:incrby(key, -1) => remaining = -1
local a_key, err = red:get(key) => a_key = -1
then leads to return nil, "rejected".
so ret = 0 means key will expire soon,when executing the code after get ttl, the key is very likely deleted.
please help to confirm the issue,thanks

Environment

• apisix version (cmd: apisix version):2.2
• OS (cmd: uname -a): Linux 820301ecfc7b 5.4.0-48-generic doc: add doc for limit count.  #52-Ubuntu SMP Thu Sep 10 10:58:49 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): nginx version: openresty/1.19.3.1
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API): etcdctl version: 3.4.13. API version: 3.4
• apisix-dashboard version, if have:2.3.0
• the plugin runner version, if the issue is about a plugin runner (cmd: depended on the kind of runner):
• luarocks version, if the issue is about installation (cmd: luarocks --version):
  3.5.0

[tzssangglass]

maybe solved by:#3991