Fix: retained message delayed on TLS connection

[wpickard]

Hi, my name is William Pickard and I work for Toro Robotics Colorado.

We noticed that on subscription to topics with a retained message over TLS, the retained message for that topic is received after a considerable delay (seems to be until after the first keepalive).

A review of the paho-mqtt project turned up and older PR addressing this issue:

See: eclipse-paho/paho.mqtt.python#131

This PR ports the above fix from paho-mqtt to the aiomqtt client.

Minimal steps to reproduce:

import aiomqtt
import asyncio
from async_timeout import timeout
import ssl

HOSTNAME = 'test.mosquitto.org'
TOPIC = '/tests/aiomqtt/test_delayed_retain_ssl'

# The TLS cert on test.mosquitto.org seems broken
CONTEXT = ssl.create_default_context()
CONTEXT.check_hostname = False
CONTEXT.verify_mode = ssl.VerifyMode.CERT_NONE

async def test_retained_sub():
    async with aiomqtt.Client(HOSTNAME, 8883, tls_context=CONTEXT) as client:
        await client.publish(TOPIC, payload='hello', retain=True)
        print('Published retained message')
        await client.subscribe(TOPIC)
        
        # This isn't a perfet timing test. It should be almost immediate.
        # The timeout can be removed to observed the total delay.
        async with timeout(1):
            async for message in client.messages:
                print(f'Received {message.payload.decode()}')
                break
            
asyncio.run(test_retained_sub())

[tsturzl]

Experiencing this issue also, this fix seems to resolve the issue.