release_handler

[Serge Aleynikov]

OTP team:

I've been studying the code of the sasl's release_handler, and came up 
with this question that perhaps someone qualified would be able to answer.

As far as I understand, the following functions do the following tasks:

application_controller:prep_config_change()
   - Fetch OLD applications' environment from
     application controller's internal ETS table.

application_controller:config_change(EnvBefore)
   - Notify running applications (via config_change/3 callback) about
     configuration changes derived from comparing EnvBefore with the
     content of the application controller's ETS table storing
     applications' environment.

application_controller:change_application_data(AppSpecs, Config)
   - Rereads applications' environment from config files stored on disk
     and stores it to the application controller's ETS table.

Now with this background let's look at how the release handler does the 
application upgrade:

==[begin]== sasl/src/release_handler.erl =====
...
eval_appup_script(App, ToVsn, ToDir, Script) ->
     EnvBefore = application_controller:prep_config_change(),
     AppSpecL = read_appspec(App, ToDir),
     Res = release_handler_1:eval_script(Script,
                     [], % [AppSpec]
                     [{App, ToVsn, ToDir}],
                     []), % [Opt]
     case Res of
     {ok, _Unpurged} ->
         application_controller:config_change(EnvBefore),
         application_controller:change_application_data(AppSpecL,[]);
     _Res ->
         ignore
     end,
     Res.
...
==[end]=======================================

Unless I am wrong, from this code it does *not* look like application's 
config_change/3 callback will ever be invoked by 
application_controller:config_change(EnvBefore) because EnvBefore has 
the same applications' environment information as the current content of 
the applications' controller's ETS table.  It seems to me that at least 
the config_change/1 and change_application_data/2 calls should be done 
in the opposite order, or better, do something like this:

...
{ok, _Unpurged} ->
     Files =
         case init:get_argument(config) of
         {ok, [ ConfFileNames ]} ->
             [begin
                 S = filename:basename(F,".config"),
                 filename:join(filename:dirname(F), S ++ ".config")
              end || F <- ConfFileNames],
         {ok, _} ->
             []
         end,
     application_controller:change_application_data(AppSpecL, Files);
     application_controller:config_change(EnvBefore);
_Res ->
     ignore
...

I'd appreciate some clarification on the subject.

Regards,

Serge