Handle generic data structures in priority queue info callback

The priority queue implementation makes assumptions over the
data format returned by the underlying backing queue
`info` callback.

This causes priority queues to crash if used together with other
modules implementing the `rabbit_backing_queue` behaviour and
returning additional data with the `info` callback.

As we cannot predict the format of the data the backing queue
`info` callback could return, we just replace it with the undefined
keyword.

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

Author: noxdafox

src/rabbit_priority_queue.erl

@@ -666,8 +666,8 @@ cse(_, lazy)                -> lazy;
 cse(lazy, _)                -> lazy;
 %% numerical stats
 cse(A, B) when is_number(A) -> A + B;
-cse({delta, _, _, _, _}, _)    -> {delta, todo, todo, todo, todo};
-cse(A, B)                   -> exit({A, B}).
+cse({delta, _, _, _, _}, _) -> {delta, todo, todo, todo, todo};
+cse(_, _)                   -> undefined.
 
 %% When asked about 'head_message_timestamp' fro this priority queue, we
 %% walk all the backing queues, starting by the highest priority. Once a