How to check for nil/null in Redis' Lua cjson?

Question

I have a lua script with code block as below:

local call_data     = cjson.decode(ARGV[1])
local other_data    = cjson.decode(ARGV[2])
local data          = {}
local next          = next
local populate_data = function(source)
  if next(source) == nil then
    return
  end

  for property,value in pairs(source) do
    redis.call('HSET', KEYS[2], property, value)
  end
end
populate_data(call_data)
populate_data(other_data)

When I try to run the script with the following command KEYS and ARGV as:-

redis-cli --eval dialed.lua "inflight_stats:18" "calls:AC443d7a8111a96ba8074f54a71f0521ce:CA1ec49703eee1959471c71506f43bb42e:dialed" , "{\"from\":\"+18035224181\",\"to\":\"+919943413333\",\"sid\":\"CA1ec49703eee1959471c71506f43bb42e\",\"status\":\"queued\",\"direction\":\"outbound-api\",\"date_created\":null,\"account_sid\":\"AC443d8a8111a96ba8074f54a71f0521ce\"}" "{\"phone\":\"919943413333\",\"campaign_id\":18,\"caller_session_sid\":\"CA828b163153bf5cc301ef5285e38925f9\"}" 0

Error :-

(error) ERR Error running script (call to f_08dcc69ee8baa0200e0cf552948ab4bc338c9978): @user_script:11: @user_script: 11: Lua redis() command arguments must be strings or integers

Υou can use: ./redis-cli --ldb --eval /tmp/script.lua mykey somekey , arg1 arg2 — georgeliatsos, Oct 19 '18 at 09:10
@gliatsos please check the above description for once. I am already doing it with the same for testing — Jaswinder, Oct 19 '18 at 10:32
Running this script returns (nil). As you're not calling `populate_data`, I can't see how you get to line 11. — Itamar Haber, Oct 19 '18 at 12:20
@ItamarHaber yes I haven't mentioned calling reference to pupulate_data due to some private concerns but yes it has been called. I think it is something related to lightuserdata for 'value' getting 'light-userdata@(nil)' but not sure how to identify this 'value' equals nil — Jaswinder, Oct 19 '18 at 12:28
@ItamarHaber you have a great experience with Lua scripts and I really appreciate if you could help me out of this. — Jaswinder, Oct 19 '18 at 12:52

Itamar Haber · Accepted Answer · 2018-10-21T10:19:01.967

TL;DR for values returned by cjson.decode(), use cjson.null to compare to JSON's null value.

Explanation: Lua uses nil in tables to mark deleted entries. If JSONinc nulls were converted to Lunatic nils, the decoded objects would be corrupt. Therefore, the cjson lib uses a lightweight userdata type to represent null/nil.

Your 'call_data' has a 'date_created' field that is null - that causes the error.

The funny thing is that Redis, like Lua, will not store a nil/null value, so you'll have to either ignore null values or use a special value in Redis to flag them.

Assuming you'll be ignoring them, here's one way around it:

local call_data     = cjson.decode(ARGV[1])
local other_data    = cjson.decode(ARGV[2])
local data          = {}
local next          = next
local null          = cjson.null
local populate_data = function(source)
  if next(source) == nil then
    return
  end

  for property,value in pairs(source) do
    if value ~= null then
      redis.call('HSET', KEYS[2], property, value)
    end
  end
end
populate_data(call_data)
populate_data(other_data)

Also, a small optimization would be to batch the updates, like so:

  local payload = {}
  for property,value in pairs(source) do
    if value ~= null then
      table.insert(payload, property)
      table.insert(payload, value)
    end
  end
  redis.call('HSET', KEYS[2], unpack(payload))

P.S. if you want, look at ReJSON that I wrote - it is designed to help with what it appears that you're trying to do.

How to check for nil/null in Redis' Lua cjson?

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