mod_mam: Limit result set page size

If the client doesn't specify a maximum number of messages to retrieve
per page, set a limit of 50 messages.  If the client specifies a limit
larger than 250, cap the number to 250 messages.

These limits aren't enforced for MAM v0.2 requests though, as that
version of the XEP doesn't require clients to support RSM.  The newer
revisions say that "a server MAY place a reasonable limit on how many
stanzas may be pushed to a client in one request.  Whether or not the
client query included a <set/> element, the server MAY simply return its
limited results, modifying the <set/> element it returns appropriately."

src/mod_mam.erl

@@ -44,6 +44,9 @@
 -include("mod_muc_room.hrl").
 -include("ejabberd_commands.hrl").
 
+-define(DEF_PAGE_SIZE, 50).
+-define(MAX_PAGE_SIZE, 250).
+
 -define(BIN_GREATER_THAN(A, B),
 	((A > B andalso byte_size(A) == byte_size(B))
 	 orelse byte_size(A) > byte_size(B))).
@@ -469,8 +472,9 @@ process_iq(LServer, #jid{luser = LUser} = From, To, IQ, SubEl, Fs, MsgType) ->
 	{_Start, _End, _With, #rsm_in{index = Index}} when is_integer(Index) ->
 	    IQ#iq{type = error, sub_el = [SubEl, ?ERR_FEATURE_NOT_IMPLEMENTED]};
 	{Start, End, With, RSM} ->
+	    NS = xml:get_tag_attr_s(<<"xmlns">>, SubEl),
 	    select_and_send(LServer, From, To, Start, End,
-			    With, RSM, IQ, MsgType)
+			    With, limit_max(RSM, NS), IQ, MsgType)
     end.
 
 muc_process_iq(#iq{lang = Lang, sub_el = SubEl} = IQ,
@@ -910,12 +914,12 @@ select(LServer, #jid{luser = LUser} = JidRequestor,
 	   end,
     {Query, CountQuery} = make_sql_query(User, LServer,
 					 Start, End, With, RSM),
-    % XXX TODO from XEP-0313:
+    % TODO from XEP-0313 v0.2: "To conserve resources, a server MAY place a
-    % To conserve resources, a server MAY place a reasonable limit on
+    % reasonable limit on how many stanzas may be pushed to a client in one
-    % how many stanzas may be pushed to a client in one request. If a
+    % request. If a query returns a number of stanzas greater than this limit
-    % query returns a number of stanzas greater than this limit and
+    % and the client did not specify a limit using RSM then the server should
-    % the client did not specify a limit using RSM then the server
+    % return a policy-violation error to the client." We currently don't do this
-    % should return a policy-violation error to the client.
+    % for v0.2 requests, but we do limit #rsm_in.max for v0.3 and newer.
     case {ejabberd_odbc:sql_query(Host, Query),
 	  ejabberd_odbc:sql_query(Host, CountQuery)} of
 	{{selected, _, Res}, {selected, _, [[Count]]}} ->
@@ -1124,6 +1128,15 @@ filter_by_max(Msgs, Len) when is_integer(Len), Len >= 0 ->
 filter_by_max(_Msgs, _Junk) ->
     {[], true}.
 
+limit_max(RSM, ?NS_MAM_TMP) ->
+    RSM; % XEP-0313 v0.2 doesn't require clients to support RSM.
+limit_max(#rsm_in{max = Max} = RSM, _NS) when not is_integer(Max) ->
+    RSM#rsm_in{max = ?DEF_PAGE_SIZE};
+limit_max(#rsm_in{max = Max} = RSM, _NS) when Max > ?MAX_PAGE_SIZE ->
+    RSM#rsm_in{max = ?MAX_PAGE_SIZE};
+limit_max(RSM, _NS) ->
+    RSM.
+
 match_interval(Now, Start, End) ->
     (Now >= Start) and (Now =< End).