require 'securerandom' module Gitlab # This class implements an 'exclusive lease'. We call it a 'lease' # because it has a set expiry time. We call it 'exclusive' because only # one caller may obtain a lease for a given key at a time. The # implementation is intended to work across GitLab processes and across # servers. It is a cheap alternative to using SQL queries and updates: # you do not need to change the SQL schema to start using # ExclusiveLease. # class ExclusiveLease LUA_CANCEL_SCRIPT = <<~EOS.freeze local key, uuid = KEYS[1], ARGV[1] if redis.call("get", key) == uuid then redis.call("del", key) end EOS LUA_RENEW_SCRIPT = <<~EOS.freeze local key, uuid, ttl = KEYS[1], ARGV[1], ARGV[2] if redis.call("get", key) == uuid then redis.call("expire", key, ttl) return uuid end EOS def self.get_uuid(key) Gitlab::Redis::SharedState.with do |redis| redis.get(redis_shared_state_key(key)) || false end end def self.cancel(key, uuid) Gitlab::Redis::SharedState.with do |redis| redis.eval(LUA_CANCEL_SCRIPT, keys: [redis_shared_state_key(key)], argv: [uuid]) end end def self.redis_shared_state_key(key) "gitlab:exclusive_lease:#{key}" end def initialize(key, uuid: nil, timeout:) @redis_shared_state_key = self.class.redis_shared_state_key(key) @timeout = timeout @uuid = uuid || SecureRandom.uuid end # Try to obtain the lease. Return lease UUID on success, # false if the lease is already taken. def try_obtain # Performing a single SET is atomic Gitlab::Redis::SharedState.with do |redis| redis.set(@redis_shared_state_key, @uuid, nx: true, ex: @timeout) && @uuid end end # Try to renew an existing lease. Return lease UUID on success, # false if the lease is taken by a different UUID or inexistent. def renew Gitlab::Redis::SharedState.with do |redis| result = redis.eval(LUA_RENEW_SCRIPT, keys: [@redis_shared_state_key], argv: [@uuid, @timeout]) result == @uuid end end # Returns true if the key for this lease is set. def exists? Gitlab::Redis::SharedState.with do |redis| redis.exists(@redis_shared_state_key) end end # Returns the TTL of the Redis key. # # This method will return `nil` if no TTL could be obtained. def ttl Gitlab::Redis::SharedState.with do |redis| ttl = redis.ttl(@redis_shared_state_key) ttl if ttl.positive? end end end end