fixes #2792 - only touch if writable

lib/files/storage/local.php

@@ -95,6 +95,9 @@ class Local extends \OC\Files\Storage\Common{
 		// sets the modification time of the file to the given value.
 		// If mtime is nil the current time is set.
 		// note that the access time of the file always changes to the current time.
+		if(!$this->isUpdatable($path)) {
+			return false;
+		}
 		if(!is_null($mtime)) {
 			$result=touch( $this->datadir.$path, $mtime );
 		}else{