Unable to receive more than 20 MQTT messages using Mosquitto/Paho for Python
Question: I'm using the Mosquitto (now Paho) python MQTT client to connect to a HiveMQ
broker. The code is very basic, as taken from their documentation here -
<https://pypi.python.org/pypi/paho-mqtt>

    
    
    #SUBSCRIBER
    import paho.mqtt.client as mqtt
    
    # The callback for when the client receives a CONNACK response from the server.
    def on_connect(client, userdata, rc):
        client.subscribe("GB/node0/", 2) 
    
    # The callback for when a PUBLISH message is received from the server.
    def on_message(client, userdata, msg):
        print "Topic: ", msg.topic+'\nMessage: '+str(msg.payload)
    
    client = mqtt.Client()
    client.on_connect = on_connect
    client.on_message = on_message
    
    client.connect("192.168.15.4", 1883, 60)
    
    # Blocking call that processes network traffic, dispatches callbacks and
    # handles reconnecting.
    client.loop_forever()
    

As you notice in client.subscribe(), the QoS is 2. This is the only
modification to the official documentation's code.

The publishing client's code is-

    
    
    #PUBLISHER
    import paho.mqtt.client as mqtt    
    mqttc = mqtt.Client("python_pub")
    mqttc.connect("192.168.15.4", 1883, 60)   
    mqttc.publish("GB/node0/", "Hello, World baby!",2) 
    

Here also, the QoS is 2.

This modification of QoS results in only 20 messages being received by the
subscriber. Upon further probing, I realized the problem is probably due to
`max_inflight_messages_set()`, which is an option function which sets the
maximum number of messages with QoS>0 that can be part way through their
network flow at once. The default is 20.

However, changing it to any other value does not work. Also, why does the
client think these messages are still inflight when they've been received? How
do I resolve the problem? How do I ensure that the client understands these
messages are not "inflight" and have been delivered?


Answer: Try calling `mqttc.loop(2,10)` after the `mqttc.publish()` in the publisher so
the publisher can handle the QOS2 acknowledgement from the broker that it has
received the publish.

The 2 second timeout and the 10 packets is probably more than is needed but it
should work

