fix(dsrepl): Don't reverse elements of batches

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

@@ -672,22 +672,15 @@ apply(
         ?tag := ?BATCH,
         ?batch_messages := MessagesIn
     },
-    #{db_shard := DBShard, latest := Latest} = State
+    #{db_shard := DBShard, latest := Latest0} = State
 ) ->
     %% NOTE
     %% Unique timestamp tracking real time closely.
     %% With microsecond granularity it should be nearly impossible for it to run
     %% too far ahead than the real time clock.
-    {NLatest, Messages} = assign_timestamps(Latest, MessagesIn),
-    %% TODO
-    %% Batch is now reversed, but it should not make a lot of difference.
-    %% Even if it would be in order, it's still possible to write messages far away
-    %% in the past, i.e. when replica catches up with the leader. Storage layer
-    %% currently relies on wall clock time to decide if it's safe to iterate over
-    %% next epoch, this is likely wrong. Ideally it should rely on consensus clock
-    %% time instead.
+    {Latest, Messages} = assign_timestamps(Latest0, MessagesIn),
     Result = emqx_ds_storage_layer:store_batch(DBShard, Messages, #{}),
-    NState = State#{latest := NLatest},
+    NState = State#{latest := Latest},
     %% TODO: Need to measure effects of changing frequency of `release_cursor`.
     Effect = {release_cursor, RaftIdx, NState},
     {NState, Result, Effect};
@@ -730,7 +723,7 @@ assign_timestamps(Latest, [MessageIn | Rest], Acc) ->
             assign_timestamps(Latest + 1, Rest, [Message | Acc])
     end;
 assign_timestamps(Latest, [], Acc) ->
-    {Latest, Acc}.
+    {Latest, lists:reverse(Acc)}.
 
 assign_timestamp(TimestampUs, Message) ->
     {TimestampUs, Message}.