umqtt.robust: Fix check_msg blocking after reconnect.

After `reconnect()`, MQTTClient.socket is blocking by default, and
check_msg() can block.  This commit aims to fix that behaviour by
reimplementing `check_msg()` for umqtt.robust and setting the socket to
non-blocking.

Fixes issue #192.

micropython/umqtt.robust/example_check_msg_robust.py

@@ -0,0 +1,14 @@
+import umqtt.robust
+import time
+
+# Instantiate an MQTTClient with a keepalive time of 5 seconds (to help us test
+# what happens to check_msg() with a broken connection)
+m = umqtt.robust.MQTTClient(host="localhost", debug=True, keepalive=5)
+
+m.connect()
+
+# Wait for the broker to consider us dead
+time.sleep(6)
+
+# This should initiate a reconnect() and return immediately
+m.check_msg()

micropython/umqtt.robust/manifest.py

@@ -1,5 +1,5 @@
 metadata(
-    description='Lightweight MQTT client for MicroPython ("robust" version).', version="1.0.1"
+    description='Lightweight MQTT client for MicroPython ("robust" version).', version="1.0.2"
 )
 
 # Originally written by Paul Sokolovsky.

micropython/umqtt.robust/umqtt/robust.py

@@ -42,3 +42,13 @@ class MQTTClient(simple.MQTTClient):
             except OSError as e:
                 self.log(False, e)
             self.reconnect()
+
+    def check_msg(self, attempts=2):
+        while attempts:
+            self.sock.setblocking(False)
+            try:
+                return super().wait_msg()
+            except OSError as e:
+                self.log(False, e)
+            self.reconnect()
+            attempts -= 1