discourse/lib/distributed_memoizer.rb

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class DistributedMemoizer
# never wait for longer that 1 second for a cross process lock
MAX_WAIT = 2
LOCK = Mutex.new
# memoize a key across processes and machines
def self.memoize(key, duration = 60 * 60 * 24, redis = nil)
redis ||= $redis
redis_key = self.redis_key(key)
unless result = redis.get(redis_key)
redis_lock_key = self.redis_lock_key(key)
start = Time.new
got_lock = false
begin
while Time.new < start + MAX_WAIT && !got_lock
LOCK.synchronize do
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got_lock = get_lock(redis, redis_lock_key)
end
sleep 0.001
end
unless result = redis.get(redis_key)
result = yield
redis.setex(redis_key, duration, result)
end
ensure
# NOTE: delete regardless so next one in does not need to wait MAX_WAIT again
redis.del(redis_lock_key)
end
end
result
end
def self.redis_lock_key(key)
"memoize_lock_" << key
end
def self.redis_key(key)
"memoize_" << key
end
# Used for testing
def self.flush!
$redis.scan_each(match: "memoize_*").each { |key| $redis.del(key) }
end
protected
def self.get_lock(redis, redis_lock_key)
redis.watch(redis_lock_key)
current = redis.get(redis_lock_key)
return false if current
unique = SecureRandom.hex
result = redis.multi do
redis.setex(redis_lock_key, MAX_WAIT, unique)
end
redis.unwatch
result == ["OK"]
end
end