optimize: implemented delay_encode for json component, encoding is

only done when string output is really needed.

lua/apisix/core/json.lua

@@ -4,6 +4,12 @@ local type = type
 local pairs = pairs
 
 
+local _M = {
+    version = 0.1,
+    decode = require("cjson.safe").decode,
+}
+
+
 local function serialise_obj(data)
     if type(data) == "function" or type(data) == "userdata"
        or type(data) == "table" then
@@ -32,16 +38,31 @@ local function tab_clone_with_serialise(data)
     return t
 end
 
-local _M = {
-    version = 0.1,
-    encode = function(data, force)
-        if force then
-            data = tab_clone_with_serialise(data)
-        end
 
-        return json_encode(data)
-    end,
-    decode = require("cjson.safe").decode,
-}
+local function encode(data, force)
+    if force then
+        data = tab_clone_with_serialise(data)
+    end
+
+    return json_encode(data)
+end
+_M.encode = encode
+
+
+local delay_tab = setmetatable({data = "", force = false}, {
+    __tostring = function(self)
+        return encode(self.data, self.force)
+    end
+})
+
+
+-- this is a non-thread safe implementation
+-- it works well with log, eg: log.info(..., json.delay_encode({...}))
+function _M.delay_encode(data, force)
+    delay_tab.data = data
+    delay_tab.force = force
+    return delay_tab
+end
+
 
 return _M

lua/apisix/heartbeat.lua

@@ -57,7 +57,7 @@ local function report()
     if not res then
         log.error("failed to report heartbeat information: ", err)
     else
-        log.info("succed to report body: ", json.encode(res, true))
+        log.info("succed to report body: ", json.delay_encode(res, true))
     end
 end