Rebuild the registry before database updates are run

I think that drush_provision_tasks_extra_provision_deploy_pre_updatedb() should work, since it follows the naming convention used in all other extra tasks.

Anyway, it will not work when defined in the "Aegir Hosting Tasks Extra" module, because this hook is fired only in the deploy.provision.inc file, in drush_provision_drupal_post_provision_deploy(), which is not fired in "Aegir Hosting Tasks Extra" automatically, because here you run just updatedb. Also, you don't really want to run the full drush_provision_drupal_post_provision_deploy() here, hence you will never be able to trigger drush_command_invoke_all('provision_deploy_pre_updatedb');

Maybe this hook should be moved outside of drush_provision_drupal_post_provision_deploy() to be really re-usable.

In fact, I'm not sure if this extra level of abstraction is really needed, since it seems to just add more confusion, and you still need to duplicate some code anyway, to get these hooks fired.

Also, you can't really even access drush_provision_drupal_post_provision_deploy() from the contrib module, unless you will add this snippet within drush_provision_tasks_extra_provision_update():

  $list = drush_commandfile_list();
  $provision_dir = dirname($list['provision']);
  include_once($provision_dir . '/platform/deploy.provision.inc');

I don't think this makes much sense, though..

It is just too much (confusing) work, while all you really need is something like:

/**
 * Implements the provision-update command.
 */
function drush_provision_tasks_extra_provision_update() {
  drush_errors_on();
  // Run registry-rebuild without cache-clear, if available.
  if (function_exists('drush_registry_rebuild')) {
    if (drush_drupal_major_version() < 8) {
      provision_backend_invoke(d()->name, 'registry-rebuild --no-cache-clear');
      drush_log(dt('Registry rebuilt with --no-cache-clear in drush_provision_tasks_extra_provision_update'));
    }
  }
  $options = d()->options;
  if (isset($options['entity_updates']) && $options['entity_updates'] && drush_drupal_major_version(d()->root) >= 8) {
    provision_backend_invoke(d()->name, 'updatedb',  array('--entity-updates'));
    drush_log(dt('Drush updatedb task completed with --entity-updates'));
  }
  else {
    provision_backend_invoke(d()->name, 'updatedb');
    drush_log(dt('Drush updatedb task completed w/o --entity-updates'));
  }
  // Run registry-rebuild with cache-clear, if available.
  if (function_exists('drush_registry_rebuild')) {
    if (drush_drupal_major_version() < 8) {
      provision_backend_invoke(d()->name, 'registry-rebuild');
      drush_log(dt('Registry rebuilt without --no-cache-clear in drush_provision_tasks_extra_provision_update'));
    }
  }
}

It is my pure speculation, so I could be totally wrong, but maybe it helps?