rabbit_fifo bugfixes

Bugfixes around noconnect consumer handling as well as refactoring to
make these kinds of errors less likely.

[#164691411]

3 files changed, 185 insertions, 112 deletions

diff --git a/src/rabbit_fifo.erl b/src/rabbit_fifo.erl
index e551834490..c09c8a4f22 100644
--- a/src/rabbit_fifo.erl
+++ b/src/rabbit_fifo.erl
@@ -184,11 +184,9 @@ apply(Meta, #discard{msg_ids = MsgIds, consumer_id = ConsumerId},
 apply(Meta, #return{msg_ids = MsgIds, consumer_id = ConsumerId},
       #?MODULE{consumers = Cons0} = State) ->
     case Cons0 of
-        #{ConsumerId := Con0 = #consumer{checked_out = Checked0}} ->
-            Checked = maps:without(MsgIds, Checked0),
+        #{ConsumerId := #consumer{checked_out = Checked0}} ->
             Returned = maps:with(MsgIds, Checked0),
-            MsgNumMsgs = maps:values(Returned),
-            return(Meta, ConsumerId, MsgNumMsgs, Con0, Checked, [], State);
+            return(Meta, ConsumerId, Returned, [], State);
         _ ->
             {State, ok}
     end;
@@ -318,17 +316,23 @@ apply(_, {down, Pid, noconnection},
     Node = node(Pid),
     %% if the pid refers to the active consumer, mark it as suspected and return
     %% it to the waiting queue
-    {State1, Effects0} = case maps:to_list(Cons0) of
-                 [{{_, P} = Cid, C}] when node(P) =:= Node ->
-                     %% the consumer should be returned to waiting
-                     %%
-                     Effs = consumer_update_active_effects(
-                             State0, Cid, C, false, suspected_down, []),
-                     {State0#?MODULE{consumers = #{},
-                                    waiting_consumers = Waiting0 ++ [{Cid, C}]},
-                      Effs};
-                 _ -> {State0, []}
-             end,
+    {State1, Effects0} =
+        case maps:to_list(Cons0) of
+            [{{_, P} = Cid, C0}] when node(P) =:= Node ->
+                %% the consumer should be returned to waiting
+                %% and checked out messages should be returned
+                Effs = consumer_update_active_effects(
+                         State0, Cid, C0, false, suspected_down, []),
+                Checked = C0#consumer.checked_out,
+                Credit = increase_credit(C0, maps:size(Checked)),
+                {St, Effs1} = return_all(State0, Effs,
+                                         Cid, C0#consumer{credit = Credit}),
+                #{Cid := C} = St#?MODULE.consumers,
+                {St#?MODULE{consumers = #{},
+                            waiting_consumers = Waiting0 ++ [{Cid, C}]},
+                 Effs1};
+            _ -> {State0, []}
+        end,
     WaitingConsumers = update_waiting_consumer_status(Node, State1,
                                                       suspected_down),
 
@@ -357,21 +361,20 @@ apply(_, {down, Pid, noconnection},
     Node = node(Pid),
     ConsumerUpdateActiveFun = consumer_active_flag_update_function(State0),
 
-    {Cons, State, Effects1} =
-        maps:fold(fun({_, P} = K, #consumer{checked_out = Checked0,
-                                            status = up} = C,
-                      {Co, St0, Eff}) when node(P) =:= Node ->
-                          {St, Eff0} = return_all(St0, Checked0, Eff, K, C),
-                          Credit = increase_credit(C, maps:size(Checked0)),
-                          Eff1 = ConsumerUpdateActiveFun(St, K, C, false,
+    {State, Effects1} =
+    maps:fold(fun({_, P} = Cid, #consumer{checked_out = Checked0,
+                                          status = up} = C0,
+                      {St0, Eff}) when node(P) =:= Node ->
+                          Credit = increase_credit(C0, map_size(Checked0)),
+                          C = C0#consumer{status = suspected_down,
+                                          credit = Credit},
+                          {St, Eff0} = return_all(St0, Eff, Cid, C),
+                          Eff1 = ConsumerUpdateActiveFun(St, Cid, C, false,
                                                          suspected_down, Eff0),
-                          {maps:put(K, C#consumer{status = suspected_down,
-                                                  credit = Credit,
-                                                  checked_out = #{}}, Co),
-                           St, Eff1};
-                     (K, C, {Co, St, Eff}) ->
-                          {maps:put(K, C, Co), St, Eff}
-                  end, {#{}, State0, []}, Cons0),
+                          {St, Eff1};
+                     (_, _, {St, Eff}) ->
+                          {St, Eff}
+                  end, {State0, []}, Cons0),
     Enqs = maps:map(fun(P, E) when node(P) =:= Node ->
                             E#enqueuer{status = suspected_down};
                        (_, E) -> E
@@ -380,14 +383,14 @@ apply(_, {down, Pid, noconnection},
     % Monitor the node so that we can "unsuspect" these processes when the node
     % comes back, then re-issue all monitors and discover the final fate of
     % these processes
-    Effects2 = case maps:size(Cons) of
+    Effects2 = case maps:size(State#?MODULE.consumers) of
                    0 ->
                        [{aux, inactive}, {monitor, node, Node}];
                    _ ->
                        [{monitor, node, Node}]
                end ++ Effects1,
     %% TODO: should we run a checkout here?
-    {State#?MODULE{consumers = Cons, enqueuers = Enqs}, ok, Effects2};
+    {State#?MODULE{enqueuers = Enqs}, ok, Effects2};
 apply(Meta, {down, Pid, _Info}, #?MODULE{consumers = Cons0,
                                          enqueuers = Enqs0} = State0) ->
     % Remove any enqueuer for the same pid and enqueue any pending messages
@@ -795,9 +798,9 @@ consumer_update_active_effects(#?MODULE{cfg = #cfg{resource = QName}},
       | Effects].
 
 cancel_consumer0(ConsumerId, #?MODULE{consumers = C0} = S0, Effects0, Reason) ->
-    case maps:take(ConsumerId, C0) of
-        {Consumer, Cons1} ->
-            {S, Effects2} = maybe_return_all(ConsumerId, Consumer, Cons1, S0,
+    case C0 of
+        #{ConsumerId := Consumer} ->
+            {S, Effects2} = maybe_return_all(ConsumerId, Consumer, S0,
                                              Effects0, Reason),
             %% The effects are emitted before the consumer is actually removed
             %% if the consumer has unacked messages. This is a bit weird but
@@ -810,7 +813,7 @@ cancel_consumer0(ConsumerId, #?MODULE{consumers = C0} = S0, Effects0, Reason) ->
                 _ ->
                     {S, Effects}
             end;
-        error ->
+        _ ->
             %% already removed: do nothing
             {S0, Effects0}
     end.
@@ -847,9 +850,9 @@ activate_next_consumer(#?MODULE{consumers = Cons,
 
 
 
-maybe_return_all(ConsumerId, #consumer{checked_out = Checked0} = Consumer,
-                 Cons1, #?MODULE{consumers = C0,
-                                 service_queue = SQ0} = S0,
+maybe_return_all(ConsumerId, Consumer,
+                 #?MODULE{consumers = C0,
+                          service_queue = SQ0} = S0,
                  Effects0, Reason) ->
     case Reason of
         consumer_cancel ->
@@ -859,11 +862,12 @@ maybe_return_all(ConsumerId, #consumer{checked_out = Checked0} = Consumer,
                                                        credit = 0,
                                                        status = cancelled},
                                      C0, SQ0, Effects0),
-            {S0#?MODULE{consumers = Cons, service_queue = SQ}, Effects1};
+            {S0#?MODULE{consumers = Cons,
+                        service_queue = SQ}, Effects1};
         down ->
-            {S1, Effects1} = return_all(S0, Checked0, Effects0, ConsumerId,
-                                        Consumer),
-            {S1#?MODULE{consumers = Cons1}, Effects1}
+            {S1, Effects1} = return_all(S0, Effects0, ConsumerId, Consumer),
+            {S1#?MODULE{consumers = maps:remove(ConsumerId, S1#?MODULE.consumers)},
+             Effects1}
     end.
 
 apply_enqueue(#{index := RaftIdx} = Meta, From, Seq, RawMsg, State0) ->
@@ -980,38 +984,43 @@ maybe_enqueue(RaftIdx, From, MsgSeqNo, RawMsg, Effects0,
 snd(T) ->
     element(2, T).
 
-return(Meta, ConsumerId, MsgNumMsgs, Con0, Checked,
-       Effects0, #?MODULE{consumers = Cons0, service_queue = SQ0} = State0) ->
-    Con = Con0#consumer{checked_out = Checked,
-                        credit = increase_credit(Con0, length(MsgNumMsgs))},
-    {Cons, SQ, Effects1} = update_or_remove_sub(ConsumerId, Con, Cons0,
-                                               SQ0, Effects0),
-    {State1, Effects2} = lists:foldl(
-                           fun({'$prefix_msg', _} = Msg, {S0, E0}) ->
-                                   return_one(0, Msg, S0, E0,
-                                              ConsumerId, Con);
-                              ({MsgNum, Msg}, {S0, E0}) ->
-                                   return_one(MsgNum, Msg, S0, E0,
-                                              ConsumerId, Con)
-                           end, {State0, Effects1}, MsgNumMsgs),
-    checkout(Meta, State1#?MODULE{consumers = Cons,
-                                  service_queue = SQ},
-             Effects2).
+return(Meta, ConsumerId, Returned,
+       Effects0, #?MODULE{service_queue = SQ0} = State0) ->
+    {State1, Effects1} = maps:fold(
+                          fun(MsgId, {'$prefix_msg', _} = Msg, {S0, E0}) ->
+                                  return_one(MsgId, 0, Msg, S0, E0, ConsumerId);
+                             (MsgId, {MsgNum, Msg}, {S0, E0}) ->
+                                  return_one(MsgId, MsgNum, Msg, S0, E0,
+                                             ConsumerId)
+                          end, {State0, Effects0}, Returned),
+    #{ConsumerId := Con0} = Cons0 = State1#?MODULE.consumers,
+    Con = Con0#consumer{credit = increase_credit(Con0, map_size(Returned))},
+    {Cons, SQ, Effects2} = update_or_remove_sub(ConsumerId, Con, Cons0,
+                                                SQ0, Effects1),
+    State = State1#?MODULE{consumers = Cons,
+                           service_queue = SQ},
+    checkout(Meta, State, Effects2).
 
 % used to processes messages that are finished
-complete(ConsumerId, MsgRaftIdxs, NumDiscarded,
-         Con0, Checked, Effects0,
+complete(ConsumerId, Discarded,
+         #consumer{checked_out = Checked} = Con0, Effects0,
          #?MODULE{consumers = Cons0, service_queue = SQ0,
                   ra_indexes = Indexes0} = State0) ->
+    MsgRaftIdxs = [RIdx || {_, {RIdx, _}} <- maps:values(Discarded)],
     %% credit_mode = simple_prefetch should automatically top-up credit
     %% as messages are simple_prefetch or otherwise returned
-    Con = Con0#consumer{checked_out = Checked,
-                        credit = increase_credit(Con0, NumDiscarded)},
+    Con = Con0#consumer{checked_out = maps:without(maps:keys(Discarded), Checked),
+                        credit = increase_credit(Con0, maps:size(Discarded))},
     {Cons, SQ, Effects} = update_or_remove_sub(ConsumerId, Con, Cons0,
                                                SQ0, Effects0),
     Indexes = lists:foldl(fun rabbit_fifo_index:delete/2, Indexes0,
                           MsgRaftIdxs),
-    {State0#?MODULE{consumers = Cons,
+    State1 = lists:foldl(fun({_, {_, {_, RawMsg}}}, Acc) ->
+                                 add_bytes_settle(RawMsg, Acc);
+                            ({'$prefix_msg', _} = M, Acc) ->
+                                 add_bytes_settle(M, Acc)
+                         end, State0, maps:values(Discarded)),
+    {State1#?MODULE{consumers = Cons,
                     ra_indexes = Indexes,
                     service_queue = SQ}, Effects}.
 
@@ -1030,19 +1039,9 @@ increase_credit(#consumer{credit = Current}, Credit) ->
 complete_and_checkout(#{index := IncomingRaftIdx} = Meta, MsgIds, ConsumerId,
                       #consumer{checked_out = Checked0} = Con0,
                       Effects0, State0) ->
-    Checked = maps:without(MsgIds, Checked0),
     Discarded = maps:with(MsgIds, Checked0),
-    MsgRaftIdxs = [RIdx || {_, {RIdx, _}} <- maps:values(Discarded)],
-    State1 = lists:foldl(fun({_, {_, {_, RawMsg}}}, Acc) ->
-                                 add_bytes_settle(RawMsg, Acc);
-                            ({'$prefix_msg', _} = M, Acc) ->
-                                 add_bytes_settle(M, Acc)
-                         end, State0, maps:values(Discarded)),
-    %% need to pass the length of discarded as $prefix_msgs would be filtered
-    %% by the above list comprehension
-    {State2, Effects1} = complete(ConsumerId, MsgRaftIdxs,
-                                  maps:size(Discarded),
-                                  Con0, Checked, Effects0, State1),
+    {State2, Effects1} = complete(ConsumerId, Discarded, Con0,
+                                  Effects0, State0),
     {State, ok, Effects} = checkout(Meta, State2, Effects1),
     % settle metrics are incremented separately
     update_smallest_raft_index(IncomingRaftIdx, State, Effects).
@@ -1102,69 +1101,70 @@ find_next_cursor(Smallest, Cursors0, Potential) ->
             {Potential, Cursors0}
     end.
 
-return_one(0, {'$prefix_msg', Header0},
+return_one(MsgId, 0, {'$prefix_msg', Header0},
            #?MODULE{returns = Returns,
+                    consumers = Consumers,
                     cfg = #cfg{delivery_limit = DeliveryLimit}} = State0,
-           Effects0, ConsumerId, Con) ->
-    Header = maps:update_with(delivery_count,
-                              fun (C) -> C+1 end,
+           Effects0, ConsumerId) ->
+    #consumer{checked_out = Checked} = Con0 = maps:get(ConsumerId, Consumers),
+    Header = maps:update_with(delivery_count, fun (C) -> C+1 end,
                               1, Header0),
     Msg = {'$prefix_msg', Header},
     case maps:get(delivery_count, Header) of
         DeliveryCount when DeliveryCount > DeliveryLimit ->
-            Checked = Con#consumer.checked_out,
-            {State1, Effects} = complete(ConsumerId, [], 1, Con, Checked,
-                                          Effects0, State0),
-            {add_bytes_settle(Msg, State1), Effects};
+            complete(ConsumerId, #{MsgId => Msg}, Con0, Effects0, State0);
         _ ->
             %% this should not affect the release cursor in any way
-            {add_bytes_return(Msg,
-                              State0#?MODULE{returns = lqueue:in(Msg, Returns)}),
+            Con = Con0#consumer{checked_out = maps:remove(MsgId, Checked)},
+            {add_bytes_return(
+               Msg,
+               State0#?MODULE{consumers = Consumers#{ConsumerId => Con},
+                              returns = lqueue:in(Msg, Returns)}),
              Effects0}
     end;
-return_one(MsgNum, {RaftId, {Header0, RawMsg}},
+return_one(MsgId, MsgNum, {RaftId, {Header0, RawMsg}},
            #?MODULE{returns = Returns,
+                    consumers = Consumers,
                     cfg = #cfg{delivery_limit = DeliveryLimit}} = State0,
-           Effects0, ConsumerId, Con) ->
-    Header = maps:update_with(delivery_count,
-                              fun (C) -> C+1 end,
+           Effects0, ConsumerId) ->
+    #consumer{checked_out = Checked} = Con0 = maps:get(ConsumerId, Consumers),
+    Header = maps:update_with(delivery_count, fun (C) -> C+1 end,
                               1, Header0),
     Msg = {RaftId, {Header, RawMsg}},
     case maps:get(delivery_count, Header) of
         DeliveryCount when DeliveryCount > DeliveryLimit ->
             DlMsg = {MsgNum, Msg},
-            Effects = dead_letter_effects(delivery_limit,
-                                          #{none => DlMsg},
+            Effects = dead_letter_effects(delivery_limit, #{none => DlMsg},
                                           State0, Effects0),
-            Checked = Con#consumer.checked_out,
-            {State1, Effects1} = complete(ConsumerId, [RaftId], 1, Con, Checked,
-                                          Effects, State0),
-            {add_bytes_settle(RawMsg, State1), Effects1};
+            complete(ConsumerId, #{MsgId => DlMsg}, Con0, Effects, State0);
         _ ->
+            Con = Con0#consumer{checked_out = maps:remove(MsgId, Checked)},
             %% this should not affect the release cursor in any way
-            {add_bytes_return(RawMsg,
-                              State0#?MODULE{returns =
-                                             lqueue:in({MsgNum, Msg}, Returns)}),
+            {add_bytes_return(
+               RawMsg,
+               State0#?MODULE{consumers = Consumers#{ConsumerId => Con},
+                              returns = lqueue:in({MsgNum, Msg}, Returns)}),
              Effects0}
     end.
 
-return_all(State0, Checked0, Effects0, ConsumerId, Consumer) ->
+return_all(#?MODULE{consumers = Cons} = State0, Effects0, ConsumerId,
+           #consumer{checked_out = Checked0} = Con) ->
     %% need to sort the list so that we return messages in the order
     %% they were checked out
     Checked = lists:sort(maps:to_list(Checked0)),
-    lists:foldl(fun ({_, {'$prefix_msg', _} = Msg}, {S, E}) ->
-                        return_one(0, Msg, S, E, ConsumerId, Consumer);
-                    ({_, {MsgNum, Msg}}, {S, E}) ->
-                        return_one(MsgNum, Msg, S, E, ConsumerId, Consumer)
-                end, {State0, Effects0}, Checked).
+    State = State0#?MODULE{consumers = Cons#{ConsumerId => Con}},
+    lists:foldl(fun ({MsgId, {'$prefix_msg', _} = Msg}, {S, E}) ->
+                        return_one(MsgId, 0, Msg, S, E, ConsumerId);
+                    ({MsgId, {MsgNum, Msg}}, {S, E}) ->
+                        return_one(MsgId, MsgNum, Msg, S, E, ConsumerId)
+                end, {State, Effects0}, Checked).
 
 %% checkout new messages to consumers
 %% reverses the effects list
 checkout(#{index := Index}, State0, Effects0) ->
     {State1, _Result, Effects1} = checkout0(checkout_one(State0),
                                             Effects0, #{}),
-    case evaluate_limit(State0#?MODULE.ra_indexes, false,
-                        State1, Effects1) of
+    case evaluate_limit(false, State1, Effects1) of
         {State, true, Effects} ->
             update_smallest_raft_index(Index, State, Effects);
         {State, false, Effects} ->
@@ -1187,17 +1187,16 @@ checkout0({Activity, State0}, Effects0, Acc) ->
                end,
     {State0, ok, lists:reverse(Effects1)}.
 
-evaluate_limit(_OldIndexes, Result,
+evaluate_limit(Result,
                #?MODULE{cfg = #cfg{max_length = undefined,
                                    max_bytes = undefined}} = State,
                Effects) ->
     {State, Result, Effects};
-evaluate_limit(OldIndexes, Result,
-               State0, Effects0) ->
+evaluate_limit(Result, State0, Effects0) ->
     case is_over_limit(State0) of
         true ->
             {State, Effects} = drop_head(State0, Effects0),
-            evaluate_limit(OldIndexes, true, State, Effects);
+            evaluate_limit(true, State, Effects);
         false ->
             {State0, Result, Effects0}
     end.
diff --git a/test/rabbit_fifo_SUITE.erl b/test/rabbit_fifo_SUITE.erl
index cf14a68d9d..fccda9772a 100644
--- a/test/rabbit_fifo_SUITE.erl
+++ b/test/rabbit_fifo_SUITE.erl
@@ -283,6 +283,19 @@ duplicate_enqueue_test(_) ->
     ?assertNoEffect({send_msg, _, {delivery, _, [{_, {_, first}}]}, _}, Effects3),
     ok.
 
+return_test(_) ->
+    Cid = {<<"cid">>, self()},
+    Cid2 = {<<"cid2">>, self()},
+    {State0, _} = enq(1, 1, msg, test_init(test)),
+    {State1, _} = check_auto(Cid, 2, State0),
+    {State2, _} = check_auto(Cid2, 3, State1),
+    {State3, _, _} = apply(meta(4), rabbit_fifo:make_return(Cid, [0]), State2),
+    ?assertMatch(#{Cid := #consumer{checked_out = C}} when map_size(C) == 0,
+                                                           State3#rabbit_fifo.consumers),
+    ?assertMatch(#{Cid2 := #consumer{checked_out = C2}} when map_size(C2) == 1,
+                                                           State3#rabbit_fifo.consumers),
+    ok.
+
 return_non_existent_test(_) ->
     Cid = {<<"cid">>, self()},
     {State0, [_, _Inactive]} = enq(1, 1, second, test_init(test)),
@@ -556,7 +569,7 @@ purge_with_checkout_test(_) ->
     ?assertEqual(1, maps:size(Checked)),
     ok.
 
-down_returns_checked_out_in_order_test(_) ->
+down_noproc_returns_checked_out_in_order_test(_) ->
     S0 = test_init(?FUNCTION_NAME),
     %% enqueue 100
     S1 = lists:foldl(fun (Num, FS0) ->
@@ -572,6 +585,30 @@ down_returns_checked_out_in_order_test(_) ->
     {S, _, _} = apply(meta(102), {down, self(), noproc}, S2),
     Returns = lqueue:to_list(S#rabbit_fifo.returns),
     ?assertEqual(100, length(Returns)),
+    ?assertEqual(0, maps:size(S#rabbit_fifo.consumers)),
+    %% validate returns are in order
+    ?assertEqual(lists:sort(Returns), Returns),
+    ok.
+
+down_noconnection_returns_checked_out_test(_) ->
+    S0 = test_init(?FUNCTION_NAME),
+    NumMsgs = 20,
+    S1 = lists:foldl(fun (Num, FS0) ->
+                         {FS, _} = enq(Num, Num, Num, FS0),
+                         FS
+                     end, S0, lists:seq(1, NumMsgs)),
+    ?assertEqual(NumMsgs, maps:size(S1#rabbit_fifo.messages)),
+    Cid = {<<"cid">>, self()},
+    {S2, _} = check(Cid, 101, 1000, S1),
+    #consumer{checked_out = Checked} = maps:get(Cid, S2#rabbit_fifo.consumers),
+    ?assertEqual(NumMsgs, maps:size(Checked)),
+    %% simulate down
+    {S, _, _} = apply(meta(102), {down, self(), noconnection}, S2),
+    Returns = lqueue:to_list(S#rabbit_fifo.returns),
+    ?assertEqual(NumMsgs, length(Returns)),
+    ?assertMatch(#consumer{checked_out = Ch}
+                   when map_size(Ch) == 0,
+                        maps:get(Cid, S#rabbit_fifo.consumers)),
     %% validate returns are in order
     ?assertEqual(lists:sort(Returns), Returns),
     ok.
@@ -736,6 +773,41 @@ single_active_consumer_cancel_consumer_when_channel_is_down_test(_) ->
 
     ok.
 
+single_active_returns_messages_on_noconnection_test(_) ->
+    R = rabbit_misc:r("/", queue, atom_to_binary(?FUNCTION_NAME, utf8)),
+    State0 = init(#{name => ?FUNCTION_NAME,
+                    queue_resource => R,
+                    release_cursor_interval => 0,
+                    single_active_consumer_on => true}),
+    Meta = #{index => 1},
+    Nodes = [n1],
+    ConsumerIds = [{_, DownPid}] =
+        [begin
+             B = atom_to_binary(N, utf8),
+             {<<"ctag_", B/binary>>,
+              test_util:fake_pid(N)}
+         end || N <- Nodes],
+    % adding some consumers
+    State1 = lists:foldl(
+               fun(CId, Acc0) ->
+                       {Acc, _, _} =
+                           apply(Meta,
+                                 make_checkout(CId,
+                                               {once, 1, simple_prefetch}, #{}),
+                                 Acc0),
+                       Acc
+               end, State0, ConsumerIds),
+    {State2, _} = enq(4, 1, msg1, State1),
+    % simulate node goes down
+    {State3, _, _} = apply(meta(5), {down, DownPid, noconnection}, State2),
+    %% assert the consumer is up
+    ?assertMatch([_], lqueue:to_list(State3#rabbit_fifo.returns)),
+    ?assertMatch([{_, #consumer{checked_out = Checked}}]
+                 when map_size(Checked) == 0,
+                      State3#rabbit_fifo.waiting_consumers),
+
+    ok.
+
 single_active_consumer_replaces_consumer_when_down_noconnection_test(_) ->
     R = rabbit_misc:r("/", queue, atom_to_binary(?FUNCTION_NAME, utf8)),
     State0 = init(#{name => ?FUNCTION_NAME,
diff --git a/test/rabbit_fifo_prop_SUITE.erl b/test/rabbit_fifo_prop_SUITE.erl
index 76b8d7e715..0f5fb8bf58 100644
--- a/test/rabbit_fifo_prop_SUITE.erl
+++ b/test/rabbit_fifo_prop_SUITE.erl
@@ -285,7 +285,9 @@ scenario17(_Config) ->
                 make_checkout(C2, cancel),
                 make_enqueue(E,2,<<"two">>),
                 {nodeup,rabbit@fake_node1},
+                %% this has no effect as was returned
                 make_settle(C1, [0]),
+                %% this should settle "one"
                 make_settle(C1, [1])
                 ],
     run_snapshot_test(#{name => ?FUNCTION_NAME,