discourse/lib/rate_limiter.rb

require_dependency 'rate_limiter/limit_exceeded'
require_dependency 'rate_limiter/on_create_record'

# A redis backed rate limiter.
class RateLimiter

  # We don't observe rate limits in test mode
  def self.disabled?
    Rails.env.test?
  end

  def initialize(user, key, max, secs)
    @user = user
    @key = "l-rate-limit:#{@user.id}:#{key}"
    @max = max
    @secs = secs
  end

  def clear!
    $redis.del(@key)
  end

  def can_perform?
    rate_unlimited? || is_under_limit?
  end

  def performed!
    return if rate_unlimited?

    if is_under_limit?
      # simple ring buffer.
      $redis.lpush(@key, Time.now.to_i)
      $redis.ltrim(@key, 0, @max - 1)

      # let's ensure we expire this key at some point, otherwise we have leaks
      $redis.expire(@key, @secs * 2)
    else
      raise LimitExceeded.new(seconds_to_wait)
    end
  end

  def rollback!
    return if RateLimiter.disabled?
    $redis.lpop(@key)
  end

  private

  def seconds_to_wait
    @secs - age_of_oldest
  end

  def age_of_oldest
    # age of oldest event in buffer, in seconds
    Time.now.to_i - $redis.lrange(@key, -1, -1).first.to_i
  end

  def is_under_limit?
      # number of events in buffer less than max allowed? OR
      ($redis.llen(@key) < @max) ||
      # age bigger than silding window size?
      (age_of_oldest > @secs)
  end

  def rate_unlimited?
    !!(RateLimiter.disabled? || @user.staff?)
  end
end