consumer.rb

def on_request
  key = nil
  Concurrent.atomically do
    key = RsaBox.take
  end
  
  ...
end

producer.rb

RsaBox = Concurrent::MVar.new

producer = Thread.new do
  loop do
    key = expensive_computation
    Concurrent.atomically do
      RsaBox.put key
    end
  end
end


...
producer.join