perf(ip-restriction) cache globals and use table.size()

Also add checks to make sure #782 does not happen (even if I could not
reproduce).

kong/plugins/ip-restriction/access.lua

@@ -1,24 +1,18 @@
 local iputils = require "resty.iputils"
 local responses = require "kong.tools.responses"
-local utils = require "kong.tools.utils"
 
 local _M = {}
 
 function _M.execute(conf)
   local block = false
+  local remote_addr = ngx.var.remote_addr
 
-  if utils.table_size(conf.blacklist) > 0 then
-    if iputils.ip_in_cidrs(ngx.var.remote_addr, conf._blacklist_cache) then
-      block = true
-    end
+  if conf._blacklist_cache and #conf._blacklist_cache > 0 then
+    block = iputils.ip_in_cidrs(remote_addr, conf._blacklist_cache)
   end
 
-  if utils.table_size(conf.whitelist) > 0 then
-    if iputils.ip_in_cidrs(ngx.var.remote_addr, conf._whitelist_cache) then
-      block = false
-    else
-      block = true
-    end
+  if conf._whitelist_cache and #conf._whitelist_cache > 0 then
+    block = not iputils.ip_in_cidrs(remote_addr, conf._whitelist_cache)
   end
 
   if block then

kong/plugins/ip-restriction/init_worker.lua

@@ -3,7 +3,10 @@ local iputils = require "resty.iputils"
 local _M = {}
 
 function _M.execute()
-  iputils.enable_lrucache()
+  local ok, err = iputils.enable_lrucache()
+  if not ok then
+    ngx.log(ngx.ERR, "[ip-restriction] Could not enable lrucache: ", err)
+  end
 end
 
 return _M