Catch response write errors in the MQTT callback.

If the client that forwarded the request is no longer available (either
because an exception or a timeout was raised) then its I/O buffer and
event loop may be closed.

In this case, the response callback should handle and report the
exception, and still set the event, so that any other threads waiting
for the response can move on.

platypush/plugins/mqtt/__init__.py

@@ -479,8 +479,9 @@ class MqttPlugin(RunnablePlugin):
         client.stop()
         del client
 
-    @staticmethod
-    def _response_callback(reply_topic: str, event: threading.Event, buffer: IO[bytes]):
+    def _response_callback(
+        self, reply_topic: str, event: threading.Event, buffer: IO[bytes]
+    ):
         """
         A response callback that writes the response to an IOBuffer and stops
         the client loop.
@@ -490,9 +491,15 @@ class MqttPlugin(RunnablePlugin):
             if msg.topic != reply_topic:
                 return
 
-            buffer.write(msg.payload)
-            client.loop_stop()
-            event.set()
+            try:
+                buffer.write(msg.payload)
+                client.loop_stop()
+            except Exception as e:
+                self.logger.warning(
+                    'Could not write the response back to the MQTT client: %s', e
+                )
+            finally:
+                event.set()
 
         return on_message