(hunchentoot:session-db-lock acceptor &key whole-db-p (whole-db-p t))
|Function: A function which returns a lock that will be used to prevent concurrent access to sessions. The first argument will be the acceptor that handles the current request, the second argument is true if the whole (current) session database is modified. If it is NIL, only one existing session in the database is modified. This function can return NIL which means that sessions or session databases will be modified without a lock held (for example for single-threaded environments). The default is to always return a global lock (ignoring the ACCEPTOR argument) for Lisps that support threads and NIL otherwise.