[erlang-questions] Unused local function kills the process when the code updates

[Ivan Glushkov]

Thank you for the answer, Patric.
You mean there's no way to get the local function saved if it has smth
like "io:format" call in it?
It will always kill the process after the second code update, won't it?
Ivan.

On Wed, Jan 9, 2013 at 3:25 PM, Patrik Nyblom <pan@REDACTED> wrote:
> Hi,
>
> As you changed the code and reloaded it, your fun, which is held in the
> process belongs to old code. The function signature has changed when you
> changed and loaded the new version of the module. When you reload again, you
> need to purge the old code, which is referenced by your process and  the
> process is killed. The fact that the fun has not changed textually is only a
> guarantee if it does no local calls and has no environment. As soon as the
> code changes the fun's will get new signatures as there is a hash of the
> module included in the fun signature, there is no optimization to keep fun
> signatures for this case.
>
> If you want long living fun's just change the fun to a module fun:
>
> register(?MODULE, spawn_link(?MODULE, loop, [fun ?MODULE:logger/2])).
>
>
> Cheers,
> /Patrik
>
> On 01/09/2013 09:29 AM, Ivan Glushkov wrote:
>
> Hi there.
>
> I do know that using long-lived local function is not a good idea, but
> I'm a bit concerned about the following behavior.
>
> I have a small example:
> $ cat my.erl
> -module(my).
> %-export([foo/0]).
> -export([ start/0, loop/1 ]).
>
> start() -> register(?MODULE, spawn_link(?MODULE, loop, [fun logger/2])).
>
> %foo() -> apply(fun erlang:now/0, []).
>
> loop(Logger) ->
>     receive V ->
>             io:format("received ~p~n", [V]),
>             ?MODULE:loop(Logger)
>     after 100 -> ?MODULE:loop(Logger)
>     end.
>
> logger(Msg, Args) -> io:format(Msg, Args).
>
>
> You see, here I save 'fun logger/2' into a loop state, and I don't use
> it anymore.
> Then if I do the following steps:
> - compile and load the code
> - start loop
> - uncomment foo
> - compile and load the code twice
>
> it kills my process.
>
> I would understand such a behavior if I didn't use fully qualified
> name for the 'loop' function. But I do.
> Actually, the same thing is happened if I use a gen_server instead of
> loop, and save my variable in it's state.
>
> I've done some research and compiled to assembler both versions of the
> code (attached).
> The only valuable difference (beside adding 'foo' surely) is the labels:
>
> 1. where the anonymous function 'fun logger/2' is defined:
> -{function, '-start/0-fun-0-', 2, 14}.
> -  {label,13}.
> +{function, '-start/0-fun-0-', 2, 16}.
> +  {label,15}.
>      {line,[{location,"my.erl",5}]}.
>      {func_info,{atom,my},{atom,'-start/0-fun-0-'},2}.
> -  {label,14}.
> -    {call_only,2,{f,8}}.
> +  {label,16}.
> +    {call_only,2,{f,10}}.
>
> 2. where it is used to create a 'Logger' variable:
> -    {make_fun2,{f,14},0,0,0}.
> +    {make_fun2,{f,16},0,0,0}.
>
> 3. where the logger function itself is defined:
> -{function, logger, 2, 8}.
> -  {label,7}.
> +{function, logger, 2, 10}.
> +  {label,9}.
>      {line,[{location,"my.erl",16}]}.
>      {func_info,{atom,my},{atom,logger},2}.
> -  {label,8}.
> +  {label,10}.
>      {line,[{location,"my.erl",16}]}.
>      {call_ext_only,2,{extfunc,io,format,2}}.
>
> 4. and of course the total number of labels:
> -{labels, 15}.
> +{labels, 17}.
>
> I'm not an expert in the field, but it seems to me that this is not a
> great deal. Function signature hasn't been changed, I mean name and
> arity. Why does it break my code?
>
> What concerns me is that if for some reason I'm saving some opaque
> values (received from the user) in my gen_server state, I can't be
> sure that it won't break my code _even if it isn't used_.
> I'm almost sure it's a incorrect behavior.
>
> Best,
> Ivan.
>
>
>
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