Fix race condition (EJABS-1677)

src/mod_ack.erl

@@ -233,7 +233,15 @@ handle_cast(_Msg, State) ->
 handle_info({timeout, _TRef, {ID, Pid}}, State) ->
     case lookup(Pid, ID, State#state.timers) of
 	{ok, _} ->
+            MRef = erlang:monitor(process, Pid),
             catch ejabberd_c2s:stop(Pid),
+            receive
+                {'DOWN', MRef, process, Pid, _Reason}->
+                    ok
+            after 5 ->
+                    catch exit(Pid, kill)
+            end,
+            erlang:demonitor(MRef, [flush]),
             handle_cast({del, Pid}, State);
 	error ->
 	    {noreply, State}