why doesn't resty.redis work with the ngx.timer? - nginx

Question

I've asked here but thought I'd post on SO as well:
given this code:
local redis = require('resty.redis')
local client = redis:new()
client:connect(host,port)
ngx.thread.spawn(function()
ngx.say(ngx.time(),' ',#client:keys('*'))
end)
ngx.timer.at(2,function()
ngx.say(ngx.time(),' ',#client:keys('*'))
end)
I get this error:
---urhcdhw2pqoz---
1611628086 5
2021/01/26 10:28:08 [error] 4902#24159: *4 lua entry thread aborted: runtime error: ...local/Cellar/openresty/1.19.3.1_1/lualib/resty/redis.lua:349: bad request
stack traceback:
coroutine 0:
[C]: in function 'send'
...local/Cellar/openresty/1.19.3.1_1/lualib/resty/redis.lua:349: in function 'keys'
./src/main.lua:20: in function <./src/main.lua:19>, context: ngx.timer
so it seems that threads work with redis but timers don't. Why is that?

Answer 1

There are two errors in your code.
It is not possible to pass the cosocket object between Lua handlers (emphasis added by me):
The cosocket object created by this API function has exactly the same lifetime as the Lua handler creating it. So never pass the cosocket object to any other Lua handler (including ngx.timer callback functions) and never share the cosocket object between different Nginx requests.
https://github.com/openresty/lua-nginx-module#ngxsockettcp
In your case, the reference to the cosocket object is stored in the client table (client._sock).
ngx.print/ngx.say are not available in the ngx.timer.* context.
https://github.com/openresty/lua-nginx-module#ngxsay (check the context: section).
You can use ngx.log instead (it writes to nginx log, set error_log stderr debug; in nginx.conf to print logs to stderr).
The following code works as expected:
ngx.timer.at(2, function()
local client = redis:new()
client:connect('127.0.0.1' ,6379)
ngx.log(ngx.DEBUG, #client:keys('*'))
end)