is_socket_object.lua

local tcp_sock_mt = getmetatable(ngx.socket.tcp())
local udp_sock_mt = getmetatable(ngx.socket.udp())

local is_socket_object = function(obj)
    local obj_mt = getmetatable(obj)
    return obj_mt == tcp_sock_mt or obj_mt == udp_sock_mt
end

--------

local redis = require('resty.redis')
local client = redis:new()
ngx.say(is_socket_object(client))   -- false
ngx.say(is_socket_object('string'))   -- false
ngx.say(is_socket_object(client._sock))   -- true

https://stackoverflow.com/questions/65895230/why-doesnt-resty-redis-work-with-the-ngx-timer/65906836