Do read piefxis file fully before starting processing included file

Previous method of processing files, could lead to mixing events generated
by parsers for multiple files, where we could get in inconsistant state.

After this change we gather all events generated by parser for single file,
before we start parsing any included file.

This fixes issue #3166

src/ejabberd_piefxis.erl

@@ -296,17 +296,26 @@ process(#state{xml_stream_state = XMLStreamState, fd = Fd} = State) ->
     end.
 
 process_els(State) ->
+    Els = gather_els(State, []),
+    process_els(State, lists:reverse(Els)).
+
+gather_els(State, List) ->
     receive
         {'$gen_event', El} ->
-            case process_el(El, State) of
-                {ok, NewState} ->
-                    process_els(NewState);
-                Err ->
-                    Err
-            end
+            gather_els(State, [El | List])
     after 0 ->
-            {ok, State}
-    end.
+        List
+end.
+
+process_els(State, [El | Tail]) ->
+    case process_el(El, State) of
+        {ok, NewState} ->
+            process_els(NewState, Tail);
+        Err ->
+            Err
+    end;
+process_els(State, []) ->
+    {ok, State}.
 
 process_el({xmlstreamstart, <<"server-data">>, Attrs}, State) ->
     case fxml:get_attr_s(<<"xmlns">>, Attrs) of