queue_topology should not return undefined for members

it's easier to deal with the result if members is an empty
list than when it is either a list or `undefined`.

Author: mkuratczyk

deps/rabbit/src/rabbit_amqp_management.erl

@@ -440,12 +440,13 @@ encode_queue(Q, NumMsgs, NumConsumers) ->
                {{utf8, <<"type">>}, {utf8, rabbit_queue_type:to_binary(QType)}},
                {{utf8, <<"arguments">>}, QArgs}
               ],
-    KVList1 = if is_list(Replicas) ->
-                     [{{utf8, <<"replicas">>},
-                       {array, utf8, [{utf8, atom_to_binary(R)} || R <- Replicas]}
-                      } | KVList0];
-                 Replicas =:= undefined ->
-                     KVList0
+    KVList1 = case length(Replicas) > 0 of
+                  true ->
+                      [{{utf8, <<"replicas">>},
+                        {array, utf8, [{utf8, atom_to_binary(R)} || R <- Replicas]}
+                       } | KVList0];
+                  false ->
+                      KVList0
               end,
     KVList = case Leader of
                  undefined ->
@@ -459,7 +460,7 @@ encode_queue(Q, NumMsgs, NumConsumers) ->
 
 %% The returned Replicas contain both online and offline replicas.
 -spec queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 queue_topology(Q) ->
     Type = amqqueue:get_type(Q),
     Type:queue_topology(Q).

deps/rabbit/src/rabbit_classic_queue.erl

@@ -678,7 +678,7 @@ send_queue_event(Pid, QName, Event) ->
     gen_server:cast(Pid, {queue_event, QName, Event}).
 
 -spec queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 queue_topology(Q) ->
     Pid = amqqueue:get_pid(Q),
     Node = node(Pid),

deps/rabbit/src/rabbit_queue_type.erl

@@ -275,7 +275,7 @@
     ok.
 
 -callback queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 
 -spec discover(binary() | atom()) -> queue_type().
 discover(<<"undefined">>) ->

deps/rabbit/src/rabbit_quorum_queue.erl

@@ -2226,12 +2226,12 @@ maybe_log_leader_health_check_result(Result) ->
     rabbit_log:warning("Leader health check result (unhealthy leaders detected): ~tp", [Qs]).
 
 -spec queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 queue_topology(Q) ->
     [{leader, Leader0},
-             {members, Members}] = rabbit_queue_type:info(Q, [leader, members]),
-            Leader = case Leader0 of
-                         '' -> undefined;
-                         _ -> Leader0
-                     end,
-            {Leader, Members}.
+     {members, Members}] = rabbit_queue_type:info(Q, [leader, members]),
+    Leader = case Leader0 of
+                 '' -> undefined;
+                 _ -> Leader0
+             end,
+    {Leader, Members}.

deps/rabbit/src/rabbit_stream_queue.erl

@@ -1397,7 +1397,7 @@ delivery_count_add(Count, N) ->
     serial_number:add(Count, N).
 
 -spec queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 queue_topology(Q) ->
     #{name := StreamId} = amqqueue:get_type_state(Q),
     case rabbit_stream_coordinator:members(StreamId) of
@@ -1407,6 +1407,6 @@ queue_topology(Q) ->
                          (Node, {_Pid, replica}, {Writer, Replicas}) ->
                               {Writer, [Node | Replicas]}
                       end, {undefined, []}, Members);
-                {error, _} ->
-            {undefined, undefined}
+        {error, _} ->
+            {undefined, []}
     end.

deps/rabbitmq_mqtt/src/rabbit_mqtt_qos0_queue.erl

@@ -305,7 +305,7 @@ state_info(A1) ->
     ?UNSUPPORTED([A1]).
 
 -spec queue_topology(amqqueue:amqqueue()) ->
-    {Leader :: undefined | node(), Replicas :: undefined | [node(),...]}.
+    {Leader :: undefined | node(), Replicas :: [node(),...]}.
 queue_topology(Q) ->
     Pid = amqqueue:get_pid(Q),
     Node = node(Pid),