Fix mod_mam compatibility with RSM for other backends

src/mod_mam.erl

@@ -663,21 +663,23 @@ filter_by_rsm(_Msgs, #rsm_in{max = Max}) when Max =< 0 ->
     [];
 filter_by_rsm(Msgs, #rsm_in{max = Max, direction = Direction, id = ID}) ->
     NewMsgs = case Direction of
-                  aft ->
-                      lists:filter(
-                        fun(#archive_msg{id = I}) ->
-                                I > ID
-                        end, Msgs);
-                  before ->
-                      lists:foldl(
-                        fun(#archive_msg{id = I} = Msg, Acc) when I < ID ->
-                                [Msg|Acc];
-                           (_, Acc) ->
-                                Acc
-                        end, [], Msgs);
-                  _ ->
-                      Msgs
-              end,
+		  aft when ID /= <<"">> ->
+		      lists:filter(
+			fun(#archive_msg{id = I}) ->
+				I > ID
+			end, Msgs);
+		  before when ID /= <<"">> ->
+		      lists:foldl(
+			fun(#archive_msg{id = I} = Msg, Acc) when I < ID ->
+				[Msg|Acc];
+			   (_, Acc) ->
+				Acc
+			end, [], Msgs);
+		  before when ID == <<"">> ->
+		      lists:reverse(Msgs);
+		  _ ->
+		      Msgs
+	      end,
     filter_by_max(NewMsgs, Max).
 
 filter_by_max(Msgs, undefined) ->

test/ejabberd_SUITE.erl

@@ -1854,6 +1854,35 @@ mam_query_rsm(Config, NS) ->
 				    rsm = #rsm_set{count = 10,
 						   first = undefined,
 						   last = undefined}}]})
+    end,
+    %% Should receive 2 last messages
+    I5 = send(Config,
+	      #iq{type = Type,
+		  sub_els = [#mam_query{xmlns = NS,
+					rsm = #rsm_set{max = 2,
+						       before = none}}]}),
+    maybe_recv_iq_result(NS, I5),
+    lists:foreach(
+      fun(N) ->
+	      Text = #text{data = jlib:integer_to_binary(N)},
+	      ?recv1(#message{to = MyJID,
+			      sub_els =
+				  [#mam_result{
+				      xmlns = NS,
+				      sub_els =
+					  [#forwarded{
+					      delay = #delay{},
+					      sub_els =
+						  [#message{
+						      from = MyJID, to = Peer,
+						      body = [Text]}]}]}]})
+      end, lists:seq(4, 5)),
+    if NS == ?NS_MAM_TMP ->
+	    ?recv1(#iq{type = result, id = I5,
+		       sub_els = [#mam_query{xmlns = NS, rsm = #rsm_set{count = 5}}]});
+       true ->
+	    ?recv1(#message{
+		      sub_els = [#mam_fin{rsm = #rsm_set{count = 10}}]})
     end.
 
 client_state_master(Config) ->

tools/xmpp_codec.erl

@@ -4746,10 +4746,10 @@ encode_rsm_before(Cdata, _xmlns_attrs) ->
     _attrs = _xmlns_attrs,
     {xmlel, <<"before">>, _attrs, _els}.
 
-decode_rsm_before_cdata(__TopXMLNS, <<>>) -> undefined;
+decode_rsm_before_cdata(__TopXMLNS, <<>>) -> none;
 decode_rsm_before_cdata(__TopXMLNS, _val) -> _val.
 
-encode_rsm_before_cdata(undefined, _acc) -> _acc;
+encode_rsm_before_cdata(none, _acc) -> _acc;
 encode_rsm_before_cdata(_val, _acc) ->
     [{xmlcdata, _val} | _acc].
 

tools/xmpp_codec.hrl

@@ -280,7 +280,7 @@
                     units = [] :: [binary()]}).
 
 -record(rsm_set, {'after' :: binary(),
-                  before :: binary(),
+                  before :: 'none' | binary(),
                   count :: non_neg_integer(),
                   first :: #rsm_first{},
                   index :: non_neg_integer(),

tools/xmpp_codec.spec

@@ -2071,6 +2071,7 @@
 -xml(rsm_before,
      #elem{name = <<"before">>,
            xmlns = <<"http://jabber.org/protocol/rsm">>,
+	   cdata = #cdata{default = none},
            result = '$cdata'}).
 
 -xml(rsm_last,