fix(response-transformer): fix the bug that response-transformer (#9463)

Fix the error that the response from upstream is a plain string with Header Content-Type: application/json, the response transformer throw exception when transforming.

FTI-4352

CHANGELOG.md

@@ -215,6 +215,12 @@
   [#9626](https://github.com/Kong/kong/pull/9626)
 
 
+#### Plugins
+
+- **Response-Transformer**: Fix the bug that Response-Transformer plugin 
+  breaks when receiving an unexcepted boy.
+  [#9463](https://github.com/Kong/kong/pull/9463)
+
 ## [3.0.0]
 
 > Released 2022/09/12

kong/plugins/response-transformer/body_transformer.lua

@@ -1,4 +1,5 @@
 local cjson = require("cjson.safe").new()
+local cjson_decode = cjson.decode
 
 
 local insert = table.insert
@@ -8,6 +9,8 @@ local sub = string.sub
 local gsub = string.gsub
 local match = string.match
 local lower = string.lower
+local tonumber = tonumber
+local pcall = pcall
 
 
 cjson.decode_array_with_array_mt(true)
@@ -39,9 +42,12 @@ local function cast_value(value, value_type)
 end
 
 
-local function read_json_body(body)
+local function parse_json(body)
   if body then
-    return cjson.decode(body)
+    local ok, res = pcall(cjson_decode, body)
+    if ok then
+      return res
+    end
   end
 end
 
@@ -90,9 +96,9 @@ end
 
 
 function _M.transform_json_body(conf, buffered_data)
-  local json_body = read_json_body(buffered_data)
+  local json_body = parse_json(buffered_data)
   if json_body == nil then
-    return
+    return nil, "failed parsing json body"
   end
 
   -- remove key:value to body

kong/plugins/response-transformer/handler.lua

@@ -20,16 +20,21 @@ end
 
 
 function ResponseTransformerHandler:body_filter(conf)
+
   if not is_body_transform_set(conf)
     or not is_json_body(kong.response.get_header("Content-Type"))
   then
     return
   end
 
   local body = kong.response.get_raw_body()
-  if body then
-    return kong.response.set_raw_body(body_transformer.transform_json_body(conf, body))
+
+  local json_body, err = body_transformer.transform_json_body(conf, body)
+  if err then
+    kong.log.warn("body transform failed: " .. err)
+    return
   end
+  return kong.response.set_raw_body(json_body)
 end
 
 

spec/03-plugins/15-response-transformer/02-body_transformer_spec.lua

@@ -322,4 +322,68 @@ describe("Plugin: response-transformer", function()
       assert.are.same(body, result)
     end)
   end)
+
+  describe("handle unexpected body type", function()
+    -- Related to issue https://github.com/Kong/kong/issues/9461
+
+    local old_kong, handler
+
+    lazy_setup(function()
+      old_kong = _G.kong
+      _G.kong = {
+        response = {
+          get_header = function(header)
+            if header == "Content-Type" then
+              return "application/json"
+            end
+          end,
+          get_raw_body = function()
+            return "not a json value"
+          end,
+          set_raw_body = function() end
+        },
+        log = {
+          warn = function() end
+        }
+      }
+
+      -- force module reload to use mock `_G.kong`
+      package.loaded["kong.plugins.response-transformer.handler"] = nil
+      handler = require("kong.plugins.response-transformer.handler")
+    end)
+
+    lazy_teardown(function()
+      _G.kong = old_kong
+    end)
+
+    it("gracefully fails transforming invalid json body", function()
+      local conf  = {
+        remove    = {
+          headers = {},
+          json    = { "foo" }
+        },
+        add       = {
+          headers = {},
+          json    = {},
+        },
+        append    = {
+          headers = {},
+          json    = {},
+        },
+        replace   = {
+          headers = {},
+          json    = {},
+        },
+      }
+
+      local spy_response_get_header   = spy.on(kong.response, "get_header")
+      local spy_response_get_raw_body = spy.on(kong.response, "get_raw_body")
+      local spy_response_set_raw_body = spy.on(kong.response, "set_raw_body")
+
+      assert.is_nil(handler:body_filter(conf))
+      assert.spy(spy_response_get_header).was_called_with("Content-Type")
+      assert.spy(spy_response_get_raw_body).was_called()
+      assert.spy(spy_response_set_raw_body).was_not_called()
+    end)
+  end)
 end)