umqtt.simple's check_msg triggers OSError -1 with TLS servers

Please consider this code:

import utime
from umqtt.simple import MQTTClient

mqtt = MQTTClient("test_mqtt_client_id", "test.mosquitto.org",
                  port=8883, keepalive=30, ssl=True)

def on_topic_updated(topic, msg):
    print((topic, msg))

mqtt.set_callback(on_topic_updated)
mqtt.connect()
mqtt.subscribe(b"Topic/For/Sylvain")
while True:
    print("Checking msg...")
    mqtt.check_msg()
    utime.sleep(1)

It systematically gives the same output (and I can reproduce it with another broker e.g. AWS IoT)

$ /usr/local/Cellar/micropython/1.11/bin/micropython  main.py
Checking msg...
Checking msg...
Traceback (most recent call last):
  File "main.py", line 15, in <module>
  File "/Users/sylvain/.micropython/lib/umqtt/simple.py", line 204, in check_msg
  File "/Users/sylvain/.micropython/lib/umqtt/simple.py", line 173, in wait_msg
OSError: -1

What's strange is that if I use port 1883 and ssl=False (i.e. no encryption), the same code works.

I reproduced this issue on my Mac (MicroPython 1.11) and on my ESP8266 board (MicroPython 1.12).

Looping over mqtt.wait_msg() works like a charm, so I suspect switching back and forth a TLS socket from blocking to non-blocking generates such error.

Read umqtt/simple.py around check_msg() and wait_msg(), then reproduce the example with a TLS broker and compare it with the working wait_msg() loop. Done means check_msg() can be used repeatedly with TLS without raising OSError -1, while the existing wait_msg() behavior remains intact.