Redis LUA script for atomically setting a key's expiration if its value is unchanged

redis-expireeq.rb

# EXPIREEQ key value_assertion seconds
# Set a timeout on key if the current value equals value_assertion.
# EXPIREEQ is short for "EXPIRE if EQual"

# The lua script:
expireeq = <<-EOF
  local key = KEYS[1]
  local val_assert = ARGV[1]
  local ex = ARGV[2]
  local val = redis.call("GET", key)
  if val == val_assert then
    return redis.call("EXPIRE", key, ex)
  end
  return 0
EOF

# usage with redis-rb:
# redis.eval expireeq, keys: [key], argv: [value_assert, seconds]

# Use case:
# A worker grabs a lock for processing a partition of a queue:
worker_name = "andrew"

if redis.set("queue-partition-1", worker_name, ex: 5.minutes, nx: true)
  # we got the exclusive lock
  loop do
    # before getting more jobs, make sure we still have the lock
    abort("lost our lock!") unless redis.eval(expireeq, keys: ["queue-partition-1"], argv: [worker_name, 5.minutes])
    
    jobs = queue.get_jobs
    # process jobs that might take a long time…
  end
else
  # we didn't get the lock :(
end