[resty.resolver] do not allocate dns socket unless used

allocating dns resolver will cause opening a socket

flamegraphs: https://gist.github.com/mikz/22c650100409fb172dc543596c49a309

apicast/src/resty/resolver/dns.lua

@@ -1,8 +1,7 @@
 local resty_resolver = require 'resty.dns.resolver'
 
 local setmetatable = setmetatable
-local ipairs = ipairs
-local concat = table.concat
+local pairs = pairs
 
 local _M = {
   _VERSION = '0.1'
@@ -14,23 +13,34 @@ function _M.new(_, options)
   local opts = options or {}
   local nameservers = opts.nameservers or {}
 
-  for _,nameserver in ipairs(nameservers) do
-    resolvers[nameserver] = resty_resolver:new({ nameservers = { nameserver }})
-  end
-
   return setmetatable({
-    resolvers = resolvers
+    resolvers = resolvers,
+    nameservers = nameservers
   }, mt)
 end
 
+function _M:init_resolvers()
+  local resolvers = self.resolvers
+  local nameservers = self.nameservers
+
+  for i=1,#nameservers do
+    resolvers[nameservers[i]] = resty_resolver:new({ nameservers = { nameservers[i] }})
+  end
+  return resolvers
+end
+
 function _M.query(self, qname, opts)
   local resolvers = self.resolvers
   local answers, err
 
+  if #resolvers == 0 then
+    resolvers = self:init_resolvers()
+  end
+
   for nameserver, resolver in pairs(resolvers) do
     answers, err = resolver:query(qname, opts)
 
-    ngx.log(ngx.DEBUG, 'resolver query: ', qname, ' nameserver: ', concat(nameserver,':'))
+    ngx.log(ngx.DEBUG, 'resolver query: ', qname, ' nameserver: ', nameserver[1],':', nameserver[2])
 
     if answers and not answers.errcode and not err then
       break