Fix quorum queue queue federation bugs

Quorum queues would due to a misunderstanding whilst porting
queue decorators from classic queues calculate the wrong maximum
consumer priority leading to strange behaviour when using queue
federation.

Also notify decorators on startup. In this case we had to do this
when a member becomes leader which isn't always at startup so
will have to rely on the mirroried supervisor to ensure we don't
start multiple federation links.

deps/rabbit/src/rabbit_fifo.erl

@@ -861,8 +861,10 @@ state_enter0(leader, #?MODULE{consumers = Cons,
     Mons = [{monitor, process, P} || P <- Pids],
     Nots = [{send_msg, P, leader_change, ra_event} || P <- Pids],
     NodeMons = lists:usort([{monitor, node, node(P)} || P <- Pids]),
-    FHReservation = [{mod_call, rabbit_quorum_queue, file_handle_leader_reservation, [Resource]}],
-    Effects = TimerEffs ++ Mons ++ Nots ++ NodeMons ++ FHReservation,
+    FHReservation = [{mod_call, rabbit_quorum_queue,
+                      file_handle_leader_reservation, [Resource]}],
+    NotifyDecs = notify_decorators_startup(Resource),
+    Effects = TimerEffs ++ Mons ++ Nots ++ NodeMons ++ FHReservation ++ [NotifyDecs],
     case BLH of
         undefined ->
             Effects;
@@ -1299,9 +1301,8 @@ query_notify_decorators_info(#?MODULE{consumers = Consumers} = State) ->
     MaxActivePriority = maps:fold(
                           fun(_, #consumer{credit = C,
                                            status = up,
-                                           cfg = #consumer_cfg{priority = P0}},
+                                           cfg = #consumer_cfg{priority = P}},
                               MaxP) when C > 0 ->
-                                  P = -P0,
                                   case MaxP of
                                       empty -> P;
                                       MaxP when MaxP > P -> MaxP;
@@ -2477,6 +2478,10 @@ notify_decorators_effect(QName, MaxActivePriority, IsEmpty) ->
     {mod_call, rabbit_quorum_queue, spawn_notify_decorators,
      [QName, consumer_state_changed, [MaxActivePriority, IsEmpty]]}.
 
+notify_decorators_startup(QName) ->
+    {mod_call, rabbit_quorum_queue, spawn_notify_decorators,
+     [QName, startup, []]}.
+
 convert(To, To, State) ->
     State;
 convert(0, To, State) ->