From ed793f20af50f0ef0f62aaa281a2dd7c3cbbcad3 Mon Sep 17 00:00:00 2001
From: Iliia Khaprov <iliia.khaprov@broadcom.com>
Date: Thu, 14 Aug 2025 15:03:46 +0200
Subject: [PATCH] Preserve order for missing deliveries (get_checked_out)

---
 deps/rabbit/src/rabbit_fifo.erl            |  2 +-
 deps/rabbit/test/rabbit_fifo_int_SUITE.erl | 46 +++++++++++++++++++++-
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/deps/rabbit/src/rabbit_fifo.erl b/deps/rabbit/src/rabbit_fifo.erl
index bbc27d57a1b5..e91a188ecd0b 100644
--- a/deps/rabbit/src/rabbit_fifo.erl
+++ b/deps/rabbit/src/rabbit_fifo.erl
@@ -1091,7 +1091,7 @@ handle_aux(_, _, {get_checked_out, ConsumerKey, MsgIds}, Aux0, RaAux0) ->
                                   {S, Acc}
                           end
                   end, {RaAux0, []}, maps:with(MsgIds, Checked)),
-            {reply, {ok,  IdMsgs}, Aux0, RaState};
+            {reply, {ok,  lists:reverse(IdMsgs)}, Aux0, RaState};
         _ ->
             {reply, {error, consumer_not_found}, Aux0, RaAux0}
     end;
diff --git a/deps/rabbit/test/rabbit_fifo_int_SUITE.erl b/deps/rabbit/test/rabbit_fifo_int_SUITE.erl
index 68811230ec0c..029970bedd18 100644
--- a/deps/rabbit/test/rabbit_fifo_int_SUITE.erl
+++ b/deps/rabbit/test/rabbit_fifo_int_SUITE.erl
@@ -45,7 +45,9 @@ all_tests() ->
      flow,
      test_queries,
      duplicate_delivery,
-     usage
+     usage,
+
+     missing_messages_order
     ].
 
 groups() ->
@@ -296,6 +298,48 @@ usage(Config) ->
     ?assert(Use > 0.0),
     ok.
 
+missing_messages_order(Config) ->
+    ClusterName = ?config(cluster_name, Config),
+    ServerId = ?config(node_id, Config),
+    UId = ?config(uid, Config),
+    Tag = UId,
+    ok = start_cluster(ClusterName, [ServerId]),
+    F1 = rabbit_fifo_client:init([ServerId]),
+    {empty, F3} = rabbit_fifo_client:dequeue(ClusterName, Tag, settled, F1),
+
+    {ok, F4, []} = rabbit_fifo_client:enqueue(ClusterName, msg1, F3),
+    {_, _, F5} = process_ra_events(receive_ra_events(1, 0), ClusterName, F4),
+
+    {ok, F6, []} = rabbit_fifo_client:enqueue(ClusterName, msg2, F5),
+    {_, _, F7} = process_ra_events(receive_ra_events(1, 0), ClusterName, F6),
+
+    {ok, F8, []} = rabbit_fifo_client:enqueue(ClusterName, msg3, F7),
+    {_, _, F9} = process_ra_events(receive_ra_events(1, 0), ClusterName, F8),
+
+    FC = rabbit_fifo_client:init([ServerId]),
+    {ok, _, FC3} = rabbit_fifo_client:checkout(Tag, {simple_prefetch, 10}, #{}, FC),
+
+    {ok, F10, []} = rabbit_fifo_client:enqueue(ClusterName, msg4, F9),
+    {_, E, F11}= process_ra_events(receive_ra_events(1, 0), ClusterName, F10),
+
+        receive
+            {ra_event, Qname, Evt1} = Msg ->
+                {ok, FC3b, Actions1} =
+                    rabbit_fifo_client:handle_ra_event(Qname, Qname, Evt1, FC3),
+                [{deliver, _, true,
+                 [{_,
+                   missing_messages_order,0,false,msg1},
+                  {_,
+                   missing_messages_order,1,false,msg2},
+                  {_,
+                   missing_messages_order,2,false,msg3},
+                  {_,
+                   missing_messages_order,3,false,msg4}]}] = Actions1
+        after ?TIMEOUT ->
+                flush(),
+                exit(await_delivery_timeout)
+        end.
+
 resends_lost_command(Config) ->
     ClusterName = ?config(cluster_name, Config),
     ServerId = ?config(node_id, Config),