disallow users to create calendars with reserved names

apps/dav/lib/CalDAV/CalendarHome.php

@@ -30,6 +30,7 @@
 use Sabre\CalDAV\Schedule\Outbox;
 use Sabre\CalDAV\Subscriptions\Subscription;
 use Sabre\DAV\Exception\NotFound;
+use Sabre\DAV\MkCol;
 
 class CalendarHome extends \Sabre\CalDAV\CalendarHome {
 
@@ -52,6 +53,19 @@ public function getCalDAVBackend() {
 		return $this->caldavBackend;
 	}
 
+	/**
+	 * @inheritdoc
+	 */
+	function createExtendedCollection($name, MkCol $mkCol) {
+		$reservedNames = [BirthdayService::BIRTHDAY_CALENDAR_URI];
+
+		if (in_array($name, $reservedNames)) {
+			throw new \Sabre\Dav\Exception\MethodNotAllowed('The resource you tried to create is a reserved name');
+		}
+
+		parent::createExtendedCollection($name, $mkCol);
+	}
+
 	/**
 	 * @inheritdoc
 	 */