Harden reply-topic creation against a hardcoded replicas=0

faust/agents/replies.py

@@ -189,7 +189,7 @@ def _reply_topic(self, topic: str) -> TopicT:
         return self.app.topic(
             topic,
             partitions=1,
-            replicas=0,
+            replicas=self.app.conf.topic_replication_factor,
             deleting=True,
             retention=self.app.conf.reply_expires,
             value_type=ReqRepResponse,

tests/unit/agents/test_replies.py

@@ -254,7 +254,12 @@ def test_reply_topic(self, *, c, app):
         topic = c._reply_topic("foo")
         assert topic.get_topic_name() == "foo"
         assert topic.partitions == 1
-        assert topic.replicas == 0
+        # Must not hardcode 0: most brokers reject a replication factor of
+        # 0 outright (InvalidReplicationFactorError, see
+        # faust-streaming/faust#76). Use the app's configured default
+        # instead, consistent with every other topic faust creates.
+        assert topic.replicas == app.conf.topic_replication_factor
+        assert topic.replicas > 0
         assert topic.deleting
         assert topic.retention == app.conf.reply_expires
         assert topic.value_type is ReqRepResponse