-- Insert one cache entry only when the data key is absent: the bytes go
-- under KEYS[1], their fingerprint under KEYS[2], and ARGV[3] carries the
-- TTL in seconds ('' for none). Keeping the check, both writes and both
-- expirations in one execution is what makes add() insert-only across
-- processes: a background drain finishing late cannot land between the
-- check and the write and overwrite a newer fill.
if redis.call('EXISTS', KEYS[1]) ~= 0 then
  return 0
end
redis.call('SET', KEYS[1], ARGV[1])
redis.call('SET', KEYS[2], ARGV[2])
if ARGV[3] ~= '' then
  redis.call('EXPIRE', KEYS[1], ARGV[3])
  redis.call('EXPIRE', KEYS[2], ARGV[3])
end
return 1