Fixed #8311 -- Avoid an infinite loop with session key generation when using

        return {}

    def create(self):

        while True:

        # Because a cache can fail silently (e.g. memcache), we don't know if

        # we are failing to create a new session because of a key collision or

        # because the cache is missing. So we try for a (large) number of times

        # and then raise an exception. That's the risk you shoulder if using

        # cache backing.

        for i in xrange(10000):

            self.session_key = self._get_new_session_key()

            try:

                self.save(must_create=True)

                continue

            self.modified = True

            return

        raise RuntimeError("Unable to create a new session key.")

    def save(self, must_create=False):

        if must_create: