MDL-65856 session: UX review of session timeout

Add new setting 'sessiontimeoutwarning', gives logged in user ability to extend session when there is no activity.

lib/adminlib.php

@@ -3814,6 +3814,8 @@ class admin_setting_configduration extends admin_setting {
 
     /** @var int default duration unit */
     protected $defaultunit;
+    /** @var callable|null Validation function */
+    protected $validatefunction = null;
 
     /**
      * Constructor
@@ -3837,6 +3839,36 @@ class admin_setting_configduration extends admin_setting {
         parent::__construct($name, $visiblename, $description, $defaultsetting);
     }
 
+    /**
+     * Sets a validate function.
+     *
+     * The callback will be passed one parameter, the new setting value, and should return either
+     * an empty string '' if the value is OK, or an error message if not.
+     *
+     * @param callable|null $validatefunction Validate function or null to clear
+     * @since Moodle 3.10
+     */
+    public function set_validate_function(?callable $validatefunction = null) {
+        $this->validatefunction = $validatefunction;
+    }
+
+    /**
+     * Validate the setting. This uses the callback function if provided; subclasses could override
+     * to carry out validation directly in the class.
+     *
+     * @param int $data New value being set
+     * @return string Empty string if valid, or error message text
+     * @since Moodle 3.10
+     */
+    protected function validate_setting(int $data): string {
+        // If validation function is specified, call it now.
+        if ($this->validatefunction) {
+            return call_user_func($this->validatefunction, $data);
+        } else {
+            return '';
+        }
+    }
+
     /**
      * Returns selectable units.
      * @static
@@ -3922,6 +3954,12 @@ class admin_setting_configduration extends admin_setting {
             return get_string('errorsetting', 'admin');
         }
 
+        // Validate the new setting.
+        $error = $this->validate_setting($seconds);
+        if ($error) {
+            return $error;
+        }
+
         $result = $this->config_write($this->name, $seconds);
         return ($result ? '' : get_string('errorsetting', 'admin'));
     }