thread_safe.rb

class Database
  def initialize
    @mutex = Mutex.new
    @value = 0
  end

  def read
    io_blocking { @value }
  end

  def write(value)
    io_blocking { @value = value }
  end

  def synchronize
    @mutex.synchronize { yield }
  end

  private

  def io_blocking
    sleep(0.1)
    yield
  end
end

module Current
  def self.sign_in(user_id)
    Thread.current[:user_id] = user_id
  end

  def self.user_id
    Thread.current[:user_id]
  end
end

database = Database.new

threads = 10.times.map do |i|
  Thread.start do
    Current.sign_in(i)

    database.synchronize do
      current_value = database.read
      new_value = current_value + Current.user_id

      database.write(new_value)
    end
  end
end

# 全てのthreadの完了を待つ
threads.each(&:join)

# 必ず45になる
puts database.read