Redis Lua Differetiating empty array and object - json

Question

I encountered this bug in cjson lua when I was using a script in redis 3.2 to set a particular value in a json object.
Currently, the lua in redis does not differentiate between an empty json array or an empty json object. Which causes serious problems when serialising json objects that have arrays within them.
eval "local json_str = '{\"items\":[],\"properties\":{}}' return cjson.encode(cjson.decode(json_str))" 0
Result:
"{\"items\":{},\"properties\":{}}"
I found this solution https://github.com/mpx/lua-cjson/issues/11 but I wasn't able to implement in a redis script.
This is an unsuccessful attempt :
eval
"function cjson.mark_as_array(t)
local mt = getmetatable(t) or {}
mt.__is_cjson_array = true
return setmetatable(t, mt)
end
function cjson.is_marked_as_array(t)
local mt = getmetatable(t)
return mt and mt.__is_cjson_array end
local json_str = '{\"items\":[],\"properties\":{}}'
return cjson.encode(cjson.decode(json_str))"
0
Any help or pointer appreciated.

Answer 1

There are two plans.
Modify the lua-cjson source code and compile redis, click here for details.
Fix by code:
local now = redis.call("time")
-- local timestamp = tonumber(now[1]) * 1000 + math.floor(now[2]/1000)
math.randomseed(now[2])
local emptyFlag = "empty_" .. now[1] .. "_" .. now[2] .. "_" .. math.random(10000)
local emptyArrays = {}
local function emptyArray()
if cjson.as_array then
-- cjson fixed: https://github.com/xiyuan-fengyu/redis-lua-cjson-empty-table-fix
local arr = {}
setmetatable(arr, cjson.as_array)
return arr
else
-- plan 2
local arr = {}
table.insert(emptyArrays, arr)
return arr
end
end
local function toJsonStr(obj)
if #emptyArrays > 0 then
-- plan 2
for i, item in ipairs(emptyArrays) do
if #item == 0 then
-- empty array, insert a special mark
table.insert(item, 1, emptyFlag)
end
end
local jsonStr = cjson.encode(obj)
-- replace empty array
jsonStr = (string.gsub(jsonStr, '%["' .. emptyFlag .. '"]', "[]"))
for i, item in ipairs(emptyArrays) do
if item[1] == emptyFlag then
table.remove(item, 1)
end
end
return jsonStr
else
return cjson.encode(obj)
end
end
-- example
local arr = emptyArray()
local str = toJsonStr(arr)
print(str) -- "[]"