Adding upgrade procedure for tcpsyslog2

There is no direct update to be done between both components, but the
function could be used, for example, if logplex_tcpsyslog_drain decided
to use enter_loop to upgrade.

src/logplex_tcpsyslog_drain2.erl

@@ -257,6 +257,29 @@ handle_info(Info, StateName, State) ->
     ?MODULE:StateName(Info, State).
 
 %% @private
+code_change(v67, OldStateName, OldState, _Extra) ->
+    13 = tuple_size(OldState),
+    StateL = tuple_to_list(OldState),
+    State0 = list_to_tuple(StateL ++ [queue:new(),undefined]),
+    %% Prepare to have the new buffer checked
+    process_flag(trap_exit, true),
+    %% Unregister the channel and let the new buffer handle it
+    %% remain registered as a drain
+    Chan = {channel, State0#state.channel_id},
+    logplex_channel:unregister(Chan),
+    %% Convert messages to the new format and send them back
+    State = #state{buf=Pid} = start_drain_buffer(State0#state{buf=undefined}),
+    [logplex_drain_buffer:post(Pid, Msg)
+     || Msg <- logplex_msg_buffer:to_list(State0#state.buf)],
+    %% We'll have lost a few messages to updating -- they're probably in the
+    %% mailbox right now, but oh well.
+    StateName = case OldStateName of
+        disconnected -> disconnected;
+        sending -> connected;
+        ready_to_send -> connected
+    end,
+    {ok, StateName, State};
+
 code_change(_OldVsn, StateName, State, _Extra) ->
     {ok, StateName, State}.