@gen.engine
  def _MaybeCheckVisitedUsers(self, callback):
    if len(self._visited_users) >= DatabaseChecker._MAX_VISITED_USERS:
      yield gen.Task(self._CheckVisitedUsers)
    callback()
If visited_users has grown too large, trigger CheckVisitedUsers, otherwise, simply return.
dataset/ETHPy150Open viewfinderco/viewfinder/backend/db/tools/dbchk.py/DatabaseChecker._MaybeCheckVisitedUsers