docs: def session_rollback(self, session):
          """Send session_rollback signal in sqlalchemy ``after_rollback``.
  
          This marks the failure of session so the session may enter commit
          phase.
          """
          # this may happen when there's nothing to rollback
          if not hasattr(session, 'meepo_unique_id'):
              self.logger.debug("skipped - session_rollback")
              return
  
          # del session meepo id after rollback
          self.logger.debug("%s - after_rollback" % session.meepo_unique_id)
          signal("session_rollback").send(session)
          self._session_del(session)