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618fb5b34d
When originally introduced, `attempts` was only used in the read-only check
context.
With the introduction of the exponential backoff in cda370db, `attempts` was
also used to count loop iterations, but was left inside the if block instead of
being incremented every loop, meaning the exponential backoff was only
happening when the site was recently readonly.
Co-authored-by: jbrw <jamie.wilson@discourse.org>
108 lines
2.7 KiB
Ruby
108 lines
2.7 KiB
Ruby
# frozen_string_literal: true
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# Cross-process locking using Redis.
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# Expiration happens when the current time is greater than the expire time
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class DistributedMutex
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DEFAULT_VALIDITY = 60
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CHECK_READONLY_ATTEMPTS = 5
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LOCK_SCRIPT = DiscourseRedis::EvalHelper.new <<~LUA
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local now = redis.call("time")[1]
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local expire_time = now + ARGV[1]
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local current_expire_time = redis.call("get", KEYS[1])
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if current_expire_time and tonumber(now) <= tonumber(current_expire_time) then
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return nil
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else
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local result = redis.call("setex", KEYS[1], ARGV[1] + 1, tostring(expire_time))
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return expire_time
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end
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LUA
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UNLOCK_SCRIPT = DiscourseRedis::EvalHelper.new <<~LUA
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local current_expire_time = redis.call("get", KEYS[1])
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if current_expire_time == ARGV[1] then
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local result = redis.call("del", KEYS[1])
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return result ~= nil
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else
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return false
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end
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LUA
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def self.synchronize(key, redis: nil, validity: DEFAULT_VALIDITY, &blk)
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self.new(
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key,
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redis: redis,
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validity: validity
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).synchronize(&blk)
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end
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def initialize(key, redis: nil, validity: DEFAULT_VALIDITY)
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@key = key
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@using_global_redis = true if !redis
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@redis = redis || Discourse.redis
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@mutex = Mutex.new
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@validity = validity
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end
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# NOTE wrapped in mutex to maintain its semantics
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def synchronize
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result = nil
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@mutex.synchronize do
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expire_time = get_lock
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begin
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result = yield
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ensure
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current_time = redis.time[0]
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if current_time > expire_time
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warn("held for too long, expected max: #{@validity} secs, took an extra #{current_time - expire_time} secs")
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end
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unlocked = UNLOCK_SCRIPT.eval(redis, [prefixed_key], [expire_time.to_s])
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if !unlocked && current_time <= expire_time
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warn("the redis key appears to have been tampered with before expiration")
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end
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end
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end
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result
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end
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private
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attr_reader :key
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attr_reader :redis
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attr_reader :validity
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def get_lock
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attempts = 0
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while true
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expire_time = LOCK_SCRIPT.eval(redis, [prefixed_key], [validity])
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return expire_time if expire_time
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# Exponential backoff, max duration 1s
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interval = attempts < 10 ? (0.001 * 2**attempts) : 1
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sleep interval
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attempts += 1
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# in readonly we will never be able to get a lock
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if @using_global_redis && Discourse.recently_readonly? && attempts > CHECK_READONLY_ATTEMPTS
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raise Discourse::ReadOnly
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end
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end
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end
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def prefixed_key
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@prefixed_key ||= redis.respond_to?(:namespace_key) ? redis.namespace_key(key) : key
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end
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def warn(msg)
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Rails.logger.warn("DistributedMutex(#{key.inspect}): #{msg}")
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end
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end
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