Do not assign _multi_output_topics when the topic is from a loader (#997)

Author: dachengx

strax/processors/post_office.py

@@ -132,8 +132,10 @@ def register_producer(
             else:
                 # Multi-output producer, recurse
                 for sub_topic in topic:
-                    self._multi_output_topics[sub_topic] = topic
+                    # if sub_topic is already registered as loader,
+                    # we can not handle it as _multi_output_topics
                     if sub_topic not in registered:
+                        self._multi_output_topics[sub_topic] = topic
                         self.register_producer(iterator, sub_topic)
                 return
         assert isinstance(topic, str)
@@ -264,7 +266,10 @@ def _fetch_new(self, topic):
                 # This is what our caller wants
                 desired_sub_msg = sub_msg
             log.debug(f"Got submessage {sub_msg} for sub topic {sub_msg_topic}")
-            self._ack_msg_produced(sub_msg, sub_msg_topic)
+            # sub_msg_topic not in self._multi_output_topics means
+            # it already has a loader (as producer)
+            if sub_msg_topic in self._multi_output_topics:
+                self._ack_msg_produced(sub_msg, sub_msg_topic)
         return desired_sub_msg
 
     def _ack_msg_produced(self, msg, topic):