fix(utils) add_error list behaviour

Adding more than twice the same key will keep adding values to a list,
instead of creating nested tables.

Also handle object error values.

Fix Kong#509

kong/tools/utils.lua

@@ -82,6 +82,7 @@ function _M.deep_copy(orig)
   return copy
 end
 
+local err_list_mt = {}
 
 --- Add an error message to a key/value table.
 -- If the key already exists, a sub table is created with the original and the new value.
@@ -93,10 +94,11 @@ function _M.add_error(errors, k, v)
   if not errors then errors = {} end
 
   if errors and errors[k] then
-    local list = {}
-    table.insert(list, errors[k])
-    table.insert(list, v)
-    errors[k] = list
+    if getmetatable(errors[k]) ~= err_list_mt then
+      errors[k] = setmetatable({errors[k]}, err_list_mt)
+    end
+
+    table.insert(errors[k], v)
   else
     errors[k] = v
   end

spec/unit/tools/utils_spec.lua

@@ -47,24 +47,53 @@ describe("Utils", function()
     end)
 
     describe("#add_error()", function()
+      local add_error = utils.add_error
 
       it("should create a table if given `errors` is nil", function()
-        assert.are.same({ hello = "world" }, utils.add_error(nil, "hello", "world"))
+        assert.same({hello = "world"}, add_error(nil, "hello", "world"))
       end)
 
-      it("should add a key/value when given `errors` already exists", function()
-        local errors = { hello = "world" }
-        assert.are.same({
+      it("should add a key/value when the key does not exists", function()
+        local errors = {hello = "world"}
+        assert.same({
           hello = "world",
           foo = "bar"
-        }, utils.add_error(errors, "foo", "bar"))
+        }, add_error(errors, "foo", "bar"))
       end)
 
-      it("should create a list if the same key is given twice", function()
-        local errors = { hello = "world" }
-        assert.are.same({
-          hello = {"world", "universe"}
-        }, utils.add_error(errors, "hello", "universe"))
+      it("should transform previous values to a list if the same key is given again", function()
+        local e
+
+        e = add_error(e, "key1", "value1")
+        e = add_error(e, "key2", "value2")
+        assert.same({key1 = "value1", key2 = "value2"}, e)
+
+        e = add_error(e, "key1", "value3")
+        e = add_error(e, "key1", "value4")
+        assert.same({key1 = {"value1", "value3", "value4"}, key2 = "value2"}, e)
+
+        e = add_error(e, "key1", "value5")
+        e = add_error(e, "key1", "value6")
+        e = add_error(e, "key2", "value7")
+        assert.same({key1 = {"value1", "value3", "value4", "value5", "value6"}, key2 = {"value2", "value7"}}, e)
+      end)
+
+      it("should also list tables pushed as errors", function()
+        local e
+
+        e = add_error(e, "key1", "value1")
+        e = add_error(e, "key2", "value2")
+        e = add_error(e, "key1", "value3")
+        e = add_error(e, "key1", "value4")
+
+        e = add_error(e, "keyO", {message = "some error"})
+        e = add_error(e, "keyO", {message = "another"})
+
+        assert.same({
+          key1 = {"value1", "value3", "value4"},
+          key2 = "value2",
+          keyO = {{message = "some error"}, {message = "another"}}
+        }, e)
       end)
 
     end)