refactor(dns): remove unnecessary DNS client initialization

changelog/unreleased/kong/fix-dns-initialization.yml

@@ -0,0 +1,3 @@
+message: Removed unnecessary dns initialization
+type: performance
+scope: Core

kong/globalpatches.lua

@@ -521,6 +521,7 @@ return function(options)
 
     local client = package.loaded["kong.resty.dns.client"]
     if not client then
+      -- dns initialization here is essential for busted tests.
       client = require("kong.tools.dns")()
     end
 

kong/init.lua

@@ -69,7 +69,6 @@ _G.kong = kong_global.new() -- no versioned PDK for plugins for now
 
 
 local DB = require "kong.db"
-local dns = require "kong.tools.dns"
 local meta = require "kong.meta"
 local lapis = require "lapis"
 local runloop = require "kong.runloop.handler"
@@ -645,6 +644,10 @@ function Kong.init()
   local conf_path = pl_path.join(ngx.config.prefix(), ".kong_env")
   local config = assert(conf_loader(conf_path, nil, { from_kong_env = true }))
 
+  -- The dns client has been initialized in conf_loader, so we set it directly.
+  -- Other modules should use 'kong.dns' to avoid reinitialization.
+  kong.dns = assert(package.loaded["kong.resty.dns.client"])
+
   reset_kong_shm(config)
 
   -- special math.randomseed from kong.globalpatches not taking any argument.
@@ -683,7 +686,6 @@ function Kong.init()
   assert(db:connect())
 
   kong.db = db
-  kong.dns = dns(config)
 
   if config.proxy_ssl_enabled or config.stream_ssl_enabled then
     certificate.init()

kong/runloop/balancer/targets.lua

@@ -48,7 +48,7 @@ local resolve_timer_running
 local queryDns
 
 function targets_M.init()
-  dns_client = require("kong.tools.dns")(kong.configuration)    -- configure DNS client
+  dns_client = assert(package.loaded["kong.resty.dns.client"])
   if renewal_heap:size() > 0 then
     renewal_heap = require("binaryheap").minUnique()
     renewal_weak_cache = setmetatable({}, { __mode = "v" })    

kong/timing/hooks/dns.lua

@@ -1,13 +1,6 @@
 local _M = {}
-
 local timing
 
-local client = package.loaded["kong.resty.dns.client"]
-if not client then
-  client = require("kong.tools.dns")()
-end
-
-
 local function before_toip(qname, _port, _dnsCacheOnly, _try_list)
   timing.enter_context("dns")
   timing.enter_context(qname)
@@ -30,6 +23,7 @@ function _M.register_hooks(timing_module)
     Here is the signature of the `toip()` function:
     function toip(self, qname, port, dnsCacheOnly, try_list)
   --]]
+  local client = assert(package.loaded["kong.resty.dns.client"])
   req_dyn_hook.hook_function("timing", client, "toip", 4, {
     befores = { before_toip },
     afters = { after_toip },