umqtt: keepalive feature doesn’t function correctly

Hi!

Here’s some example code to reproduce the issue:

import ssl
import machine
import network
from umqtt.simple import MQTTClient

client_id = machine.unique_id().hex()
mqtt_server = 'XXX.s1.eu.hivemq.cloud'
mqtt_user = 'XXX'
mqtt_password = 'XXX'
mqtt_topic_sub = b'test/topic'

wlan = network.WLAN(network.STA_IF)
wlan.active(True)
wlan.connect('XXX','XXX')

ssl_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)

def mqtt_connect():
    client = MQTTClient(client_id, mqtt_server, 8883, mqtt_user, mqtt_password, 15, ssl_context)
    client.connect()
    print('Connected to %s MQTT Broker'%(mqtt_server))
    return client

def reconnect():
    print('Failed to connect to the MQTT Broker. Reconnecting...')
    machine.reset()
    
def callback(topic, msg):
    print(topic, msg)

try:
    client = mqtt_connect()
    client.set_callback(callback)
    client.subscribe(mqtt_topic_sub)
except OSError as e:
    reconnect()
while True:
    client.wait_msg()

The library allows setting the keepalive to any value (default is 0, which disables it). However, when you set a keepalive value, the library does not automatically send pings to the server as expected. This causes the server to disconnect the client after 1.5 times the specified keepalive duration.

Are there any possible solutions to this?

I tested this with Mosquitto and HiveMQ serverless.

Start at the umqtt.simple MQTTClient implementation used by the example and trace how the keepalive argument is handled while wait_msg() blocks. Reproduce against Mosquitto or HiveMQ with keepalive=15; done means the client sends the required pings and remains connected beyond the broker timeout, with regression coverage for the behavior.