Don't call subscribe when there's no topics to subscribe to.

This is important if one wants to use Consumer.assign instead of
a subscriptions.

Author: HuwCampbell

src/Kafka/Consumer.hs

@@ -125,22 +125,44 @@ newConsumer props (Subscription ts tp) = liftIO $ do
   _   <- setDefaultTopicConf kc tp'
   rdk <- newRdKafkaT RdKafkaConsumer kc'
   case rdk of
-    Left err   -> return . Left $ KafkaError err
+    Left err ->
+      return $
+        Left (KafkaError err)
+
     Right rdk' -> do
+      let
+        kafka =
+          KafkaConsumer (Kafka rdk') kc
+
       when (cpCallbackPollMode props == CallbackPollModeAsync) $ do
-        msgq <- rdKafkaQueueNew rdk'
-        writeIORef qref (Just msgq)
-      let kafka = KafkaConsumer (Kafka rdk') kc
+        messageQueue <- rdKafkaQueueNew rdk'
+        writeIORef qref (Just messageQueue)
+
       redErr <- redirectCallbacksPoll kafka
       case redErr of
-        Just err -> closeConsumer kafka >> return (Left err)
-        Nothing  -> do
-          forM_ (cpLogLevel cp) (setConsumerLogLevel kafka)
-          sub <- subscribe kafka ts
-          case sub of
-            Nothing  -> (when (cpCallbackPollMode props == CallbackPollModeAsync) $
-              runConsumerLoop kafka (Just $ Timeout 100)) >> return (Right kafka)
-            Just err -> closeConsumer kafka >> return (Left err)
+        Just err -> do
+          _ <- closeConsumer kafka
+          return (Left err)
+
+        Nothing -> do
+          forM_ (cpLogLevel cp) $
+            setConsumerLogLevel kafka
+
+          subscribeError <-
+            if Set.null ts then
+              pure Nothing
+            else
+              subscribe kafka ts
+
+          case subscribeError of
+            Nothing -> do
+              when (cpCallbackPollMode props == CallbackPollModeAsync) $
+                runConsumerLoop kafka (Just $ Timeout 100)
+              return (Right kafka)
+
+            Just err -> do
+              _ <- closeConsumer kafka
+              return (Left err)
 
 -- | Polls a single message
 pollMessage :: MonadIO m