(#1387) ConfigSetting - Deprecation+guard for retrieve()/add()

These functions were traditionally used for accessing `config_backend`,
which was (mostly) dropped in v4.7 and became inert. However, the
functions are still referenced in a couple oddball cases.

Note that the references to `$domain->config_backend` are liable to raise
warnings if evaluated - since the field no longer exists on the DAO class.
Consequently, the extra `$hasBackend` guard should reduce warnings.

CRM/Core/BAO/ConfigSetting.php

@@ -41,6 +41,9 @@ class CRM_Core_BAO_ConfigSetting {
    *
    * @param array $params
    *   Associated array of civicrm variables.
+   * @deprecated
+   *   This method was historically used to access civicrm_domain.config_backend.
+   *   However, that has been fully replaced by the settings system since v4.7.
    */
   public static function add(&$params) {
     $domain = new CRM_Core_DAO_Domain();
@@ -69,6 +72,9 @@ class CRM_Core_BAO_ConfigSetting {
    * @param $defaults
    *
    * @return array
+   * @deprecated
+   *   This method was historically used to access civicrm_domain.config_backend.
+   *   However, that has been fully replaced by the settings system since v4.7.
    */
   public static function retrieve(&$defaults) {
     $domain = new CRM_Core_DAO_Domain();
@@ -80,7 +86,8 @@ class CRM_Core_BAO_ConfigSetting {
       $urlVar = 'task';
     }
 
-    if ($isUpgrade && CRM_Core_BAO_SchemaHandler::checkIfFieldExists('civicrm_domain', 'config_backend')) {
+    $hasBackend = CRM_Core_BAO_SchemaHandler::checkIfFieldExists('civicrm_domain', 'config_backend');
+    if ($isUpgrade && $hasBackend) {
       $domain->selectAdd('config_backend');
     }
     else {
@@ -89,7 +96,10 @@ class CRM_Core_BAO_ConfigSetting {
 
     $domain->id = CRM_Core_Config::domainID();
     $domain->find(TRUE);
-    if ($domain->config_backend) {
+    if ($hasBackend && $domain->config_backend) {
+      // This whole branch can probably be removed; the transitional loading
+      // is in SettingBag::loadValues(). Moreover, since 4.7.alpha1 dropped
+      // the column, anyone calling ::retrieve() has likely not gotten any data.
       $defaults = unserialize($domain->config_backend);
       if ($defaults === FALSE || !is_array($defaults)) {
         $defaults = [];