69 lines
2.1 KiB
Ruby
69 lines
2.1 KiB
Ruby
require 'securerandom'
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module Gitlab
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# This class implements an 'exclusive lease'. We call it a 'lease'
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# because it has a set expiry time. We call it 'exclusive' because only
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# one caller may obtain a lease for a given key at a time. The
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# implementation is intended to work across GitLab processes and across
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# servers. It is a cheap alternative to using SQL queries and updates:
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# you do not need to change the SQL schema to start using
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# ExclusiveLease.
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#
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class ExclusiveLease
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LUA_CANCEL_SCRIPT = <<~EOS.freeze
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local key, uuid = KEYS[1], ARGV[1]
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if redis.call("get", key) == uuid then
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redis.call("del", key)
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end
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EOS
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LUA_RENEW_SCRIPT = <<~EOS.freeze
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local key, uuid, ttl = KEYS[1], ARGV[1], ARGV[2]
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if redis.call("get", key) == uuid then
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redis.call("expire", key, ttl)
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return uuid
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end
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EOS
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def self.cancel(key, uuid)
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Gitlab::Redis::SharedState.with do |redis|
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redis.eval(LUA_CANCEL_SCRIPT, keys: [redis_shared_state_key(key)], argv: [uuid])
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end
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end
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def self.redis_shared_state_key(key)
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"gitlab:exclusive_lease:#{key}"
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end
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def initialize(key, timeout:)
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@redis_shared_state_key = self.class.redis_shared_state_key(key)
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@timeout = timeout
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@uuid = SecureRandom.uuid
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end
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# Try to obtain the lease. Return lease UUID on success,
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# false if the lease is already taken.
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def try_obtain
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# Performing a single SET is atomic
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Gitlab::Redis::SharedState.with do |redis|
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redis.set(@redis_shared_state_key, @uuid, nx: true, ex: @timeout) && @uuid
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end
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end
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# Try to renew an existing lease. Return lease UUID on success,
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# false if the lease is taken by a different UUID or inexistent.
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def renew
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Gitlab::Redis::SharedState.with do |redis|
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result = redis.eval(LUA_RENEW_SCRIPT, keys: [@redis_shared_state_key], argv: [@uuid, @timeout])
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result == @uuid
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end
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end
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# Returns true if the key for this lease is set.
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def exists?
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Gitlab::Redis::SharedState.with do |redis|
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redis.exists(@redis_shared_state_key)
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end
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end
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end
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end
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