More hosting_features checks in _drush_load hooks?

I think this is mostly a documentation task for core - unless I am missing something, most existing modules in core use some kind of toggle from the frontend already and don't enable themselves by mistake.

So yes, community site and api.php.

For 7.x-3.x, we should probably evaluate the existing toggles used in core, and consider switching to the hook_drush_load() method where appropriate. Since it'll mean fewer files loaded and hooks fired on each and every invocation of Drush, it should result in a smaller memory footprint and better performance.

agreed.

It looks like hooks in *.provision.inc files still get fired regardless of hook_drush_load(). After a quick grep through provision, I couldn't see where these are being auto-included. Is Drush just including *.inc in the root of Drush commands?

Moving these files into an includes/ directory and then explicitly including them into the main commandfile makes them respect the results of hook_drush_load().

this is based on the engine system.

example, for install:

function provision_drupal_drush_engine_drupal() {
  $engines = array();
  $engines['clear'] = array();
  $engines['cron_key'] = array();
  $engines['deploy'] = array();
  $engines['import'] = array();
  $engines['install'] = array();
  $engines['packages'] = array();
  $engines['verify'] = array();
  return $engines;
}
# [...]
function drush_provision_drupal_provision_install_backend() {
  drush_include_engine('drupal', 'install');
}

I am not sure how migrate is fired, but I assume there is something similar.

that seems like a good idea.

Since we need to #2102341: Differentiate between a feature being disabled vs absent, this'll end up having to look more like:

/*
 * Implements hook_drush_load().
 *
 * In a drush contrib check if the frontend part (hosting_hook variant) is enabled.
 */
function hook_drush_load() {
  $features = drush_get_option('hosting_features', array());
  return array_key_exists('hook', $features) // Front-end module is installed...
         && $features['hook'];               // ... and enabled.
}