AlreadyFulfilledException added, for acceptable exceptions

lib/alreadyfulfilledexception.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * GNU social - a federating social network
+ * Copyright (C) 2014, Free Software Foundation, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+if (!defined('GNUSOCIAL')) { exit(1); }
+
+/**
+ * Parent class for an exception when trying to do something that was
+ * probably already done.
+ *
+ * This is a common case for example when remote sites are not up to
+ * date with our database. For example subscriptions, where a remote
+ * user may be unsubscribed from our user, but they request it anyway. 
+ *
+ * This exception is usually caught in a manner that lets the execution
+ * continue _as if_ the desired action did what it was supposed to do.
+ *
+ * @category Exception
+ * @package  GNUsocial
+ * @author   Mikael Nordfeldth <mmn@hethane.se>
+ * @license  http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPLv3
+ * @link     http://gnu.io/
+ */
+
+class AlreadyFulfilledException extends ServerException
+{
+    public function __construct($msg=null)
+    {
+        if ($msg === null) {
+            // TRANS: Exception text when attempting to perform something which seems already done.
+            $msg = _('Trying to do something that was already done.');
+        }
+
+        parent::__construct($msg, 409);
+    }
+}