使用flask服务器时，rabbitMQ的pika崩溃

Question

So we have a single thread flask server running where we receive requests from a python app client. In this flask server we use rabbitMQ with pika library to distribute messages to other clients. What is happening is that in the get function the program is crashing with the error:

pika.exceptions.ConnectionClosed: (505, 'UNEXPECTED_FRAME - expected content header for class 60, got non content header frame instead')

I've searched a lot of topics about this in stack overflow and others but they all address problems with multi threading which is not the case. Flask should only serve with one thread unless it is called in app.run(threaded=yes).

The program normally crashes when multiple messages are sent in a short interval (eg 5 per second) and it's also important to note that messages are being received every second with a request to this function:

@app.route('/api/users/getMessages', methods=['POST'])  
def get_Messages():  
    data = json.loads(request.data)
    token = data['token']

    payload = jwt.decode(token, 'SECRET', algorithms=['HS256'])
    istid = payload['istid']
    print('istid: '+istid)

    messages = []

    queue = channel.queue_declare(queue=istid)
    for i in range(queue.method.message_count):
        method_frame, header_frame, body = channel.basic_get(queue=istid, no_ack=True)
        if method_frame:
            #print(method_frame, header_frame, body)
            messages.append(body)
        else:
            print('No message returned')

    res = {'messages':messages, 'error':0}
    return jsonify(res)

In this code it crashes normally in the line:

queue = channel.queue_declare(queue=istid)

But we also tried to change the code to use a while instead of a for where it ends when the body is None and it crashes in the line:
method_frame, header_frame, body = channel.basic_get(queue=istid, no_ack=True) in that case. Also important, the crashes are random and it can work a few times and then randomly crashes after a get request while messages are being sent. If anyone knows anything related to this we would appreciate any help.

Another note, we thought about using basic_consume with callback instead of basic_get but we didn't find a way in which this would work since we have to send the messages back and have several user making requests to this same function.

EDIT #1: In the rabbitMQ docs rabbitmq if you search for the function "def basic_get" you will notice there are some TODO comments and also a reference to this

Due to implementation details, this cannot be called a second time until the callback is executed.

So I suspected that this could be what was happening but even if it is I don't know how could it be solved.

Answer 1

For anyone interested in the solution, as it is in the other comments, the program was not thread safe since flask as of version 1.0 uses threaded = True as default.
The solution is either:
1) running flask with app.run(threaded = False)
2) Making the program thread safe by implementing locks whenever accessing the channel /connection with pika.