Clients cannot create sites because of missing database options

Clients are unable to install sites due to the following error (tested on a single server):

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'db_server' cannot be null: INSERT INTO {hosting_site} (vid, nid, client, db_server, db_name, platform, profile, language, last_cron, cron_key, status, verified) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11); Array ( [:db_insert_placeholder_0] => 332 [:db_insert_placeholder_1] => 243 [:db_insert_placeholder_2] => 186 [:db_insert_placeholder_3] => [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => 137 [:db_insert_placeholder_6] => 61 [:db_insert_placeholder_7] => en [:db_insert_placeholder_8] => 0 [:db_insert_placeholder_9] => [:db_insert_placeholder_10] => 0 [:db_insert_placeholder_11] => 0 ) in hosting_site_insert() (line 267 of /var/aegir/hostmaster-7.x-3.7/profiles/hostmaster/modules/aegir/hosting/site/hosting_site.nodeapi.inc).

When looking at the 'Create Site' form, this makes sense as the 'Database server' option is not visible for clients, moreover contains an empty value. Because there is only a single database server in my setup. _hosting_side_field makes this a hidden field (why not a value, that would completely hide it from the front-end?).

  // snippet from hosting_site_form() in hosting_site.form.inc
  _hosting_site_field($form, $node, 'db_server', array(
    '#type' => 'radios',
    '#title' => t('Database server'),
    '#required' => TRUE,
    '#description' => t('The database server the site will use to host its content.'),
    '#options' => hosting_get_servers('db'),
    '#default_value' => isset($node->db_server) ? $node->db_server : HOSTING_DEFAULT_DB_SERVER,
  ), '_hosting_node_link');

Now the problem is here; in _hosting_site_field() the provided default value is ignored, but the first option (provided by hosting_get_servers('db')) is picked:

  // snippet from _hosting_site_field() in hosting_site.form.inc

  if (($element['#type'] == 'radios') && !count($element['#options'])) {
    $form[$item] = array(
      '#type' => 'hidden',
      '#value' => key($element['#options']),
    );
  }

In turn, hosting_get_servers() performs a query with a node_access tag added to it:

function hosting_get_servers($service) {
  $return = array();
  $query = db_select('node', 'n');
  $query->join('hosting_service', 's', 'n.vid = s.vid');
  $query
    ->fields('n', array('nid', 'title'))
    ->condition('s.available', '1')
    ->condition('s.service', $service)
    ->addTag('node_access');

  $result = $query->execute();
  
  ... 

So summarizing, because the client has not access to servers, hosting_get_servers() does not return any options, so _hosting_site_field() gets no value at all. (I tried granting view access to servers, but in the end, only override all node access permission made it show up).

I suppose _hosting_site_field() should honor the default value if provided (and set it as a value rather than a hidden):

  // snippet from _hosting_site_field() in hosting_site.form.inc

  if (($element['#type'] == 'radios') && !count($element['#options'])) {
    $form[$item] = array(
      '#type' => 'value',
      '#value' => !empty($element['#default_value']) ? $element['#default_value'] : key($element['#options']),
    );
  }