This files is test codes for the code_change function of gen_X modules.

call_code_change.erl

-module(call_code_change).
-export([call_code_change/2, call_code_change/4]).

call_code_change(Name, OldVer) ->
  call_code_change(Name, Name, OldVer, {}).

call_code_change(Pid, Module, OldVer, Extra) when is_pid(Pid) ->
  sys:suspend(Pid),
  sys:change_code(Pid, Module, OldVer, Extra),
  sys:resume(Pid),
  ok;
call_code_change(Name, Module, OldVer, Extra) ->
  case whereis(Name) of
    Pid when is_pid(Pid) ->
      call_code_change(Pid, Module, OldVer, Extra);
    _Other ->
      not_started
  end.

code_change_test_ver1.erl

% version 1
-module(code_change_test).
-behaviour(gen_server).

-export([start/0, stop/0]).
-export([state/0]).

-export([
  init/1,
  handle_call/3, handle_cast/2, handle_info/2,
  terminate/2, code_change/3
]).

start() -> gen_server:start({local, ?MODULE}, ?MODULE, [], []).
stop()  -> gen_server:cast(?MODULE, stop).
state() -> gen_server:call(?MODULE, state).

init(_Args) ->
  {ok, ver1}.

handle_call(state, _From, State) ->
  io:fwrite("State=~p~n", [State]),
  {reply, State, State};
handle_call(_Message, _From, State) ->
  {reply, ok, State}.

handle_cast(stop, State) ->
  {stop, normal, State};
handle_cast(_Message, State) ->
  {noreply, State}.

handle_info(_Info, State) -> {noreply, State}.

terminate(normal, _State) -> ok;
terminate(_, _) -> ok.

code_change(_OldVsn, State, _Extra) ->
  {ok, State}.

code_change_test_ver2.erl

% version 2
-module(code_change_test).
-behaviour(gen_server).

-export([start/0, stop/0]).
-export([state/0]).

-export([
    init/1,
    handle_call/3, handle_cast/2, handle_info/2,
    terminate/2, code_change/3
]).

start() -> gen_server:start({local, ?MODULE}, ?MODULE, [], []).
stop()  -> gen_server:cast(?MODULE, stop).
state() -> gen_server:call(?MODULE, state).

init(_Args) ->
  {ok, {ver2, [foo, bar]}}.

handle_call(state, _From, {ver2, Args}=State) ->
  io:fwrite("State=~p~n", [Args]),
  {reply, Args, State};
handle_call(_Message, _From, State) ->
  {reply, ok, State}.

handle_cast(stop, State) ->
  {stop, normal, State};
handle_cast(_Message, State) ->
  {noreply, State}.

handle_info(_Info, State) -> {noreply, State}.

terminate(normal, _State) -> ok;
terminate(_, _) -> ok.

code_change(OldVsn, State, Extra) ->
  io:fwrite(
    lists:foldl(
      fun (Name, Format) -> Format ++ Name ++ "=~p~n" end,
      "code_change~n", ["OldVsn", "State", "Extra"]
    ),
    [OldVsn, State, Extra]
  ),
  {ok, {ver2, [foo, bar]}}.