Don't send error stanza as reply to error stanza (EJAB-930)

src/ejabberd_router.erl

@@ -31,6 +31,7 @@
 
 %% API
 -export([route/3,
+	 route_error/4,
 	 register_route/1,
 	 register_route/2,
 	 register_routes/1,
@@ -83,6 +84,16 @@ route(From, To, Packet) ->
 	    ok
     end.
 
+%% Route the error packet only if the originating packet is not an error itself.
+%% RFC3920 9.3.1
+route_error(From, To, ErrPacket, OrigPacket) ->
+    case exmpp_stanza:is_stanza_error(OrigPacket) of
+	false ->
+	    route(From, To, ErrPacket);
+	true ->
+	    ok
+    end.
+
 register_route(Domain) ->
     register_route(Domain, undefined).
 

src/ejabberd_service.erl

@@ -347,7 +347,7 @@ handle_info({route, From, To, Packet}, StateName, StateData) ->
 	    send_element(StateData, El2);
 	deny ->
 	    Err = exmpp_stanza:reply_with_error(Packet, 'not-allowed'),
-	    ejabberd_router:route(To, From, Err)
+	    ejabberd_router:route_error(To, From, Err, Packet)
     end,
     {next_state, StateName, StateData}.
 

src/mod_muc/mod_muc.erl

@@ -338,7 +338,7 @@ do_route(Host, ServerHost, Access, HistorySize, RoomShaper,
         Err = exmpp_iq:error(Packet,exmpp_stanza:error(Packet#xmlel.ns,
                                                        'forbidden',
                                                        {Lang,ErrText})),
-	    ejabberd_router:route(To, From, Err)
+	    ejabberd_router:route_error(To, From, Err, Packet)
     end.