fix(pdk): fix a bug that pdk.log.serialize() does not properly handle…

… JSON nulls (#13376)

* fix(pdk): fix a bug that pdk.log.serialize() will throw an error
when json entity set via pdk.log.set_serialize_value contains cjson.null
as value.

* handle cdata null

---------

Co-authored-by: Zachary Hu <6426329+outsinre@users.noreply.github.com>

changelog/unreleased/kong/pdk-log-error.yml

@@ -0,0 +1,3 @@
+message: Fixed an issue that pdk.log.serialize() will throw an error when JSON entity set by serialize_value contains json.null
+type: bugfix
+scope: PDK

kong/pdk/log.lua

@@ -17,6 +17,7 @@ local inspect = require("inspect")
 local phase_checker = require("kong.pdk.private.phases")
 local constants = require("kong.constants")
 local clear_tab = require("table.clear")
+local ngx_null = ngx.null
 
 
 local request_id_get = require("kong.observability.tracing.request_id").get
@@ -640,7 +641,12 @@ do
 
   local function is_valid_value(v, visited)
     local t = type(v)
-    if v == nil or t == "number" or t == "string" or t == "boolean" then
+
+    -- cdata is not supported by cjson.encode
+    if type(v) == 'cdata' then
+        return false
+
+    elseif v == nil or v == ngx_null or t == "number" or t == "string" or t == "boolean" then
       return true
     end
 

spec/01-unit/10-log_serializer_spec.lua

@@ -231,6 +231,20 @@ describe("kong.log.serialize", function()
         assert.not_equal(tostring(ngx.ctx.service),
                          tostring(res.service))
       end)
+
+      it("handle 'json.null' and 'cdata null'", function()
+        kong.log.set_serialize_value("response.body", ngx.null)
+        local pok, value = pcall(kong.log.serialize, {})
+        assert.is_true(pok)
+        assert.is_true(type(value) == "table")
+
+        local ffi = require "ffi"
+        local n = ffi.new("void*")
+        kong.log.set_serialize_value("response.body", n)
+        local pok, value = pcall(kong.log.serialize, {})
+        assert.is_false(pok)
+        assert.is_true(type(value) == "string")
+      end)
     end)
   end)