fix(db): ensure that flattened_errors is encoded as a JSON array

I wonder how many more times we'll have to fix this sorta thing...

changelog/unreleased/kong/fix-error-flattening-json.yml

@@ -0,0 +1,3 @@
+message: "Fixed an issue where `POST /config?flatten_errors=1` could return a JSON object instead of an empty array."
+type: bugfix
+scope: Core

kong/db/errors.lua

@@ -3,6 +3,7 @@ local pl_keys = require("pl.tablex").keys
 local nkeys = require("table.nkeys")
 local table_isarray = require("table.isarray")
 local uuid = require("kong.tools.uuid")
+local json = require("kong.tools.cjson")
 
 
 local type         = type
@@ -21,6 +22,7 @@ local concat       = table.concat
 local sort         = table.sort
 local insert       = table.insert
 local remove       = table.remove
+local new_array    = json.new_array
 
 
 local sorted_keys = function(tbl)
@@ -720,7 +722,7 @@ do
   ---@param ns?        string
   ---@param flattened? table
   local function categorize_errors(errs, ns, flattened)
-    flattened = flattened or {}
+    flattened = flattened or new_array()
 
     for field, err in drain(errs) do
       local errtype = type(err)
@@ -1020,7 +1022,7 @@ do
   ---@param  input table
   ---@return table
   function flatten_errors(input, err_t)
-    local flattened = {}
+    local flattened = new_array()
 
     for entity_type, section_errors in drain(err_t) do
       if type(section_errors) ~= "table" then

kong/tools/cjson.lua

@@ -1,6 +1,9 @@
 local cjson = require "cjson.safe".new()
 local CJSON_MAX_PRECISION = require "kong.constants".CJSON_MAX_PRECISION
+local new_tab = require("table.new")
 
+local setmetatable = setmetatable
+local array_mt = cjson.array_mt
 
 cjson.decode_array_with_array_mt(true)
 cjson.encode_sparse_array(nil, nil, 2^15)
@@ -14,7 +17,19 @@ _M.encode = cjson.encode
 _M.decode_with_array_mt = cjson.decode
 
 
-_M.array_mt = cjson.array_mt
-
+_M.array_mt = array_mt
+
+--- Creates a new table with the cjson array metatable.
+---
+--- This ensures that the table will be encoded as a JSON array, even if it
+--- is empty.
+---
+---@param size? integer
+---@return table
+function _M.new_array(size)
+  local t = size and new_tab(size, 0) or {}
+  setmetatable(t, array_mt)
+  return t
+end
 
 return _M