fix(gateway): keep the replay buffer across a session transfer (#2276)

fluxer_gateway/src/session/session_lifecycle.erl

@@ -552,6 +552,10 @@ serialize_transfer_identity(State) ->
         debounce_reactions => maps:get(debounce_reactions, State, false)
     }.
 
+-spec transfer_buffer(eqwalizer:dynamic(limited_deque:deque() | [map()])) -> [term()].
+transfer_buffer(Buffer) when is_list(Buffer) -> Buffer;
+transfer_buffer(Buffer) -> limited_deque:to_list(Buffer).
+
 -spec serialize_transfer_runtime(session_state()) -> map().
 serialize_transfer_runtime(State) ->
     #{
@@ -559,7 +563,7 @@ serialize_transfer_runtime(State) ->
         relationships => maps:get(relationships, State, #{}),
         seq => maps:get(seq, State, 0),
         ack_seq => maps:get(ack_seq, State, 0),
-        buffer => maps:get(buffer, State, []),
+        buffer => transfer_buffer(maps:get(buffer, State, [])),
         collected_guild_states => maps:get(collected_guild_states, State, []),
         collected_sessions => maps:get(collected_sessions, State, []),
         collected_presences => maps:get(collected_presences, State, []),

fluxer_gateway/test/session_lifecycle_tests.erl

@@ -108,6 +108,34 @@ serialize_transfer_state_strips_socket_pid_test() ->
     TransferState = session_lifecycle:serialize_transfer_state(State),
     ?assertEqual(undefined, maps:get(socket_pid, TransferState)).
 
+serialize_transfer_state_preserves_deque_replay_buffer_test() ->
+    Deque = lists:foldl(
+        fun(Seq, Acc) ->
+            limited_deque:push(#{event => message_create, data => #{}, seq => Seq}, Acc)
+        end,
+        limited_deque:new(4096, 16777216),
+        [1000, 1001]
+    ),
+    State = #{
+        id => <<"session-deque-transfer">>,
+        user_id => 42,
+        user_data => #{},
+        version => 9,
+        token_hash => <<"token_hash">>,
+        auth_session_id_hash => <<"auth_hash">>,
+        properties => #{},
+        status => online,
+        afk => false,
+        mobile => false,
+        ready => undefined,
+        seq => 1001,
+        ack_seq => 999,
+        buffer => Deque
+    },
+    TransferState = session_lifecycle:serialize_transfer_state(State),
+    Restored = session_init:normalize_buffer(maps:get(buffer, TransferState)),
+    ?assertEqual([1000, 1001], [maps:get(seq, Event) || Event <- Restored]).
+
 heartbeat_ack_recalculates_buffer_bytes_test() ->
     Event1 = #{seq => 1, event => message_create, data => #{<<"content">> => <<"one">>}},
     Event2 = #{seq => 2, event => message_create, data => #{<<"content">> => <<"two">>}},