Don't make module developers write XML by hand when defining default configuration

Summary of a discussion at the Drupalcon Denver code sprint with ksenzee and beejeebus:

There should definitely be a drush command that provides this functionality, but it cannot be provided with drupal core, save a change in policy. There is a drush branch extant with replacements for vset and vget, work for drush should probably be done there.

One possibility for an interface is a simple key/value form with "add another" functionality (available under config -> development) that would let developers input their configuration settings, hit submit, and download an xml file in the proper format. There is some question whether this more properly belongs in

a) core
b) devel.module
c) someplace else.

This decision should probably be made by persons higher up the food chain :). (Or at least as a community-led decision, made after more discussion in this issue)

Is there a reason we can't just let module developers ship the default config via PHP code in the first place?

In addition to being simpler, I don't really understand how it would actually work to ask module developers to ship an XML file. When I look at how the variable system is used throughout core, for example, I find code like this (in menu_set_active_menu_names()):

    $active = variable_get('menu_default_active_menus', array_keys(menu_list_system_menus()));

Or this (in file_icon_path()):

    $icon_directory = variable_get('file_icon_directory', drupal_get_path('module', 'file') . '/icons');

If we assume those variables are going to be converted to the config system, then I don't see how the default values would be represented as static XML. In the latter case it might even be impossible; in the first case it's possible but poor developer experience (menu_list_system_menus() is just a static array, but it's a helper function that is used a number of other places so it would be unfortunate if its contents had to be separately duplicated as XML).

Tagging

Simple:

$file = new Drupal\Core\Config\FileStorage('mymodule.settings');
$file->write(array(
  'foo' => 'bar',
  'default' => 'setting',
));

The file is written into your site's actual config directory, but from there, you can simply copy it into your module's config directory.

If want to script that, then even that is possible:

copy($file->getFileName(), drupal_get_path('module', 'mymodule') . '/config');

Lastly, if you don't want to write, but only encode into the file format of the file storage (YAML):

$yaml = Drupal\Core\Config\FileStorage::encode(array(
  'foo' => 'bar',
  'default' => 'setting',
));

As you can see, everything is possible, very easily.

re: #2:

Separate issue. But anyway; in both cases, you simply fall back to the default programmatic result/value after checking for whether configuration exists:

$value = config('file.settings')->get('directory.icon');
$icon_directory = (!empty($value) ? $value : drupal_get_path('module', 'file') . '/icons');