Do not return unacked messages in basic.cancel by quorum queues

It's not part of the spec. Messages should stay unacked as the consumer
can still ack them, it simply won't receive new messages.
Consumer state should be kept if unacked messages are present, but not listed.

Author: dcorbacho

src/rabbit_fifo.erl

@@ -184,7 +184,7 @@
          %% command: `{consumer_credit, ReceiverDeliveryCount, Credit}'
          credit_mode = simple_prefetch :: credit_mode(), % part of snapshot data
          lifetime = once :: once | auto,
-         suspected_down = false :: boolean()
+         suspected_down = false :: 'cancel' | boolean()
         }).
 
 -type consumer() :: #consumer{}.
@@ -426,10 +426,7 @@ apply(Meta, #checkout{spec = {dequeue, Settlement},
             end
     end;
 apply(Meta, #checkout{spec = cancel, consumer_id = ConsumerId}, State0) ->
-    {State, Effects} = cancel_consumer(ConsumerId, State0, []),
-    % TODO: here we should really demonitor the pid but _only_ if it has no
-    % other consumers or enqueuers. leaving a monitor in place isn't harmful
-    % however
+    {State, Effects} = cancel_consumer(ConsumerId, State0, [], consumer_cancel),
     checkout(Meta, State, Effects);
 apply(Meta, #checkout{spec = Spec, meta = ConsumerMeta,
                       consumer_id = {_, Pid} = ConsumerId},
@@ -469,7 +466,8 @@ apply(_, {down, ConsumerPid, noconnection},
     {Cons, State, Effects1} = maps:fold(
                                   fun({_, P} = K,
                                       #consumer{checked_out = Checked0} = C,
-                                      {Co, St0, Eff}) when node(P) =:= Node ->
+                                      {Co, St0, Eff}) when (node(P) =:= Node) and
+                                                           (C#consumer.suspected_down =/= cancel)->
                                           St = return_all(St0, Checked0),
                                           Credit = increase_credit(C, maps:size(Checked0)),
                                           Eff1 = ConsumerUpdateActiveFun(St, K, C, false, suspected_down, Eff),
@@ -514,7 +512,7 @@ apply(Meta, {down, Pid, _Info}, #state{consumers = Cons0,
     DownConsumers = maps:keys(
                       maps:filter(fun({_, P}, _) -> P =:= Pid end, Cons0)),
     {State, Effects} = lists:foldl(fun(ConsumerId, {S, E}) ->
-                                           cancel_consumer(ConsumerId, S, E)
+                                           cancel_consumer(ConsumerId, S, E, down)
                                    end, {State2, Effects1}, DownConsumers),
     checkout(Meta, State, Effects);
 apply(Meta, {nodeup, Node}, #state{consumers = Cons0,
@@ -538,7 +536,7 @@ apply(Meta, {nodeup, Node}, #state{consumers = Cons0,
     ConsumerUpdateActiveFun = consumer_active_flag_update_function(State0),
     {Cons1, SQ, Effects} =
         maps:fold(fun({_, P} = ConsumerId, C, {CAcc, SQAcc, EAcc})
-                        when node(P) =:= Node ->
+                        when (node(P) =:= Node) and (C#consumer.suspected_down =/= cancel) ->
                           EAcc1 = ConsumerUpdateActiveFun(State0, ConsumerId, C, true, up, EAcc),
                           update_or_remove_sub(
                             ConsumerId, C#consumer{suspected_down = false},
@@ -606,7 +604,8 @@ maybe_mark_suspect_waiting_consumers(Node,
              _ ->
                  {ConsumerId, Consumer}
          end
-     end || {{_, P} = ConsumerId, Consumer} <- WaitingConsumers].
+     end || {{_, P} = ConsumerId, Consumer} <- WaitingConsumers,
+            Consumer#consumer.suspected_down =/= cancel].
 
 -spec state_enter(ra_server:ra_state(), state()) -> ra_machine:effects().
 state_enter(leader, #state{consumers = Cons,
@@ -758,18 +757,24 @@ query_consumers(#state{consumers = Consumers,
                                           end
                                       end
                               end,
-    FromConsumers = maps:map(fun ({Tag, Pid}, #consumer{meta = Meta} = Consumer) ->
-                                    {Active, ActivityStatus} = ActiveActivityStatusFun({Tag, Pid}, Consumer),
-                                    {Pid, Tag,
-                                      maps:get(ack, Meta, undefined),
-                                      maps:get(prefetch, Meta, undefined),
-                                      Active,
-                                      ActivityStatus,
-                                      maps:get(args, Meta, []),
-                                      maps:get(username, Meta, undefined)}
-                             end, Consumers),
+    FromConsumers = maps:fold(fun (_, #consumer{suspected_down = cancel}, Acc) ->
+                                      Acc;
+                                   ({Tag, Pid}, #consumer{meta = Meta} = Consumer, Acc) ->
+                                      {Active, ActivityStatus} = ActiveActivityStatusFun({Tag, Pid}, Consumer),
+                                      maps:put({Tag, Pid},
+                                               {Pid, Tag,
+                                                maps:get(ack, Meta, undefined),
+                                                maps:get(prefetch, Meta, undefined),
+                                                Active,
+                                                ActivityStatus,
+                                                maps:get(args, Meta, []),
+                                                maps:get(username, Meta, undefined)},
+                                               Acc)
+                              end, #{}, Consumers),
     FromWaitingConsumers =
-        lists:foldl(fun({{Tag, Pid}, #consumer{meta = Meta} = Consumer}, Acc) ->
+        lists:foldl(fun ({_, #consumer{suspected_down = cancel}}, Acc) ->
+                                      Acc;
+                        ({{Tag, Pid}, #consumer{meta = Meta} = Consumer}, Acc) ->
                             {Active, ActivityStatus} = ActiveActivityStatusFun({Tag, Pid}, Consumer),
                             maps:put({Tag, Pid},
                                      {Pid, Tag,
@@ -854,42 +859,42 @@ num_checked_out(#state{consumers = Cons}) ->
                 end, 0, maps:values(Cons)).
 
 cancel_consumer(ConsumerId,
-                #state{consumer_strategy = default} = State, Effects) ->
+                #state{consumer_strategy = default} = State, Effects, Reason) ->
     %% general case, single active consumer off
-    cancel_consumer0(ConsumerId, State, Effects);
+    cancel_consumer0(ConsumerId, State, Effects, Reason);
 cancel_consumer(ConsumerId,
                 #state{consumer_strategy = single_active,
                        waiting_consumers = []} = State,
-                Effects) ->
+                Effects, Reason) ->
     %% single active consumer on, no consumers are waiting
-    cancel_consumer0(ConsumerId, State, Effects);
+    cancel_consumer0(ConsumerId, State, Effects, Reason);
 cancel_consumer(ConsumerId,
                 #state{consumers = Cons0,
                        consumer_strategy = single_active,
                        waiting_consumers = WaitingConsumers0} = State0,
-               Effects0) ->
+               Effects0, Reason) ->
     %% single active consumer on, consumers are waiting
     case maps:take(ConsumerId, Cons0) of
-        {#consumer{checked_out = Checked0}, _} ->
+        {Consumer, Cons1} ->
             % The active consumer is to be removed
             % Cancel it
-            State1 = return_all(State0, Checked0),
-            Effects1 = cancel_consumer_effects(ConsumerId, State1, Effects0),
+            {State1, Effects1} = maybe_return_all(ConsumerId, Consumer, Cons1, State0, Effects0, Reason),
+            Effects2 = cancel_consumer_effects(ConsumerId, State1, Effects1),
             % Take another one from the waiting consumers and put it in consumers
             [{NewActiveConsumerId, NewActiveConsumer}
              | RemainingWaitingConsumers] = WaitingConsumers0,
-            #state{service_queue = ServiceQueue} = State0,
+            #state{service_queue = ServiceQueue} = State1,
             ServiceQueue1 = maybe_queue_consumer(NewActiveConsumerId,
                                                  NewActiveConsumer,
                                                  ServiceQueue),
-            State = State1#state{consumers = #{NewActiveConsumerId =>
-                                               NewActiveConsumer},
+            State = State1#state{consumers = maps:put(NewActiveConsumerId,
+                                                      NewActiveConsumer, State1#state.consumers),
                                  service_queue = ServiceQueue1,
                                  waiting_consumers = RemainingWaitingConsumers},
-            Effects2 = consumer_update_active_effects(State, NewActiveConsumerId,
+            Effects = consumer_update_active_effects(State, NewActiveConsumerId,
                                                       NewActiveConsumer, true,
-                                                      single_active, Effects1),
-            {State, Effects2};
+                                                      single_active, Effects2),
+            {State, Effects};
         error ->
             % The cancelled consumer is not the active one
             % Just remove it from idle_consumers
@@ -914,23 +919,39 @@ consumer_update_active_effects(#state{queue_resource = QName },
       [QName, ConsumerId, false, Ack, Prefetch, Active, ActivityStatus, Args]}
       | Effects].
 
-cancel_consumer0(ConsumerId,
-                #state{consumers = C0} = S0, Effects0) ->
+cancel_consumer0(ConsumerId, #state{consumers = C0} = S0, Effects0, Reason) ->
     case maps:take(ConsumerId, C0) of
-        {#consumer{checked_out = Checked0}, Cons} ->
-            S = return_all(S0, Checked0),
-            Effects = cancel_consumer_effects(ConsumerId, S, Effects0),
-            case maps:size(Cons) of
+        {Consumer, Cons1} ->
+            {S, Effects2} = maybe_return_all(ConsumerId, Consumer, Cons1, S0, Effects0, Reason),
+            Effects = cancel_consumer_effects(ConsumerId, S, Effects2),
+            case maps:size(S#state.consumers) of
                 0 ->
-                    {S#state{consumers = Cons}, [{aux, inactive} | Effects]};
+                    {S, [{aux, inactive} | Effects]};
                 _ ->
-                    {S#state{consumers = Cons}, Effects}
+                    {S, Effects}
             end;
         error ->
             %% already removed: do nothing
             {S0, Effects0}
     end.
 
+maybe_return_all(ConsumerId, #consumer{checked_out = Checked0} = Consumer, Cons1,
+                 #state{consumers = C0,
+                        service_queue = SQ0} = S0, Effects0, Reason) ->
+    case Reason of
+        consumer_cancel ->
+            {Cons, SQ, Effects1} =
+                update_or_remove_sub(ConsumerId,
+                                     Consumer#consumer{lifetime = once,
+                                                       credit = 0,
+                                                       suspected_down = cancel},
+                                     C0, SQ0, Effects0),
+            {S0#state{consumers = Cons, service_queue = SQ}, Effects1};
+        down ->
+            S1 = return_all(S0, Checked0),
+            {S1#state{consumers = Cons1}, Effects0}
+    end.
+
 apply_enqueue(#{index := RaftIdx} = Meta, From, Seq, RawMsg, State0) ->
     Bytes = message_size(RawMsg),
     case maybe_enqueue(RaftIdx, From, Seq, RawMsg, [], State0) of
@@ -1303,6 +1324,8 @@ checkout_one(#state{service_queue = SQ0,
                             %% can happen when draining
                             %% recurse without consumer on queue
                             checkout_one(InitState#state{service_queue = SQ1});
+                        {ok, #consumer{suspected_down = cancel}} ->
+                            checkout_one(InitState#state{service_queue = SQ1});
                         {ok, #consumer{suspected_down = true}} ->
                             checkout_one(InitState#state{service_queue = SQ1});
                         {ok, #consumer{checked_out = Checked0,