ex.rb

Pool = TVar.new([])

t = Thread.new do
  loop do
    key = compute_rsa_key

    Concurrent.atomically do
      if Pool.value.length > 10
        raise Transaction::AbortError
      end

      Pool.value.push key
    end
  end
end

t.join



def on_request
  rsa = nil

  Concurrent.atomically do
    rsa = Pool.value.pop
  end
end