为什么tensorflow tf.fifoqueue在下面的代码中很早就关闭了?

我正在尝试实现一个具有 enqueue 在后台运行 dequeue 在主线程中运行。

目标是在循环中运行优化器,该优化器依赖于存储在缓冲区中的值,并且仅随优化过程中的每个步骤而更改。下面是一个简单的例子来说明:

VarType = tf.int32

data0 = np.array([1.0])

init = tf.placeholder(VarType, [1])
q = tf.FIFOQueue(capacity=1, shapes=[1], dtypes=VarType)
nq_init = q.enqueue(init)
# I use a Variable intermediary because I will want to access the
# data multiple times, but I do not want the next data point in the
# queue until I initialize the variable again.
data_ = tf.Variable(q.dequeue(), trainable=False, collections=[])

# Notice that data_ is accessed twice, but should be the same
# in a single sess.run
# so "data_ = q.dequeue()" would not be correct
# plus there needs to be access to initial data
data1 = data_ + 1
data2 = data_ * data1
qr = tf.train.QueueRunner(q, [q.enqueue(data2)] * 1)
tf.train.add_queue_runner(qr)

with tf.Session() as sess:
    sess.run(tf.global_variables_initializer())

    coord = tf.train.Coordinator()
    threads = tf.train.start_queue_runners(coord=coord)

    sess.run(nq_init, feed_dict={init:data0})
    # this first initialization works fine
    sess.run(data_.initializer)
    for n in range(10):
        print(sess.run(data2))
        # this second initialization errors out: 
        sess.run(data_.initializer)

    coord.request_stop()
    coord.join(threads)

print('Done')

这段代码出错,错误如下:

outofrangeerror(有关回溯,请参阅上面的内容):fifo queue''u0'fifo'u queue'已关闭,并且元素不足(请求的1,当前大小0)

为什么,这是怎么解决的?