fix(response-transformer): ensure body transformation is successful

kong/plugins/response-transformer/body_transformer.lua

@@ -92,7 +92,7 @@ end
 function _M.transform_json_body(conf, buffered_data)
   local json_body = read_json_body(buffered_data)
   if json_body == nil then
-    return
+    return nil, "failed reading json body"
   end
 
   -- remove key:value to body

kong/plugins/response-transformer/handler.lua

@@ -28,7 +28,12 @@ function ResponseTransformerHandler:body_filter(conf)
 
   local body = kong.response.get_raw_body()
   if body then
-    return kong.response.set_raw_body(body_transformer.transform_json_body(conf, body))
+    local transf_body, err = body_transformer.transform_json_body(conf, body)
+    if err then
+      kong.log.warn("body transformation failed: ", err)
+      return
+    end
+    return kong.response.set_raw_body(transf_body)
   end
 end
 

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

@@ -322,4 +322,69 @@ 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)