PUBACK messages are now sent only after on_message has returned.

ChangeLog.txt

@@ -12,6 +12,8 @@ v1.6.0 - 2021-xx-xx
 - The `rc` parameter in the `on_disconnect` callback now has meaningful values
   in the case of an error. Closes #441.
 - Callbacks can now be applied to client instances using decorators.
+- PUBACK messages are now sent to the broker only after the on_message
+  callback has returned.
 
 
 v1.5.1 - 2020-09-22

src/paho/mqtt/client.py

@@ -3313,9 +3313,8 @@ class Client(object):
             self._handle_on_message(message)
             return MQTT_ERR_SUCCESS
         elif message.qos == 1:
-            rc = self._send_puback(message.mid)
             self._handle_on_message(message)
-            return rc
+            return self._send_puback(message.mid)
         elif message.qos == 2:
             rc = self._send_pubrec(message.mid)
             message.state = mqtt_ms_wait_for_pubrel