queue: improve is_duplicate duplication logic

Check if the message is a duplicate only when the backing queue
`is_duplicate` returns false.

Otherwise, we might cache headers belonging to duplicate messages
generated internally by the broker.

This would, at least, pollute the cache hit statistics.

Signed-off-by: Matteo Cafasso <[email protected]>

Author: noxdafox

lib/rabbit_message_deduplication_queue.ex

@@ -372,11 +372,9 @@ defmodule RabbitMQ.MessageDeduplicationPlugin.Queue do
   end
 
   def is_duplicate(message, state = dqstate(queue: queue, queue_state: qs)) do
-    duplicate = duplicate?(queue, message)
-
     case passthrough2(state, do: is_duplicate(message, qs)) do
       {true, state} -> {true, state}
-      {false, state} -> {duplicate, state}
+      {false, state} -> {duplicate?(queue, message), state}
     end
   end