import threading


class LiveHub:
    def __init__(self) -> None:
        self._condition = threading.Condition()
        self.revision = 0

    def touch(self) -> None:
        with self._condition:
            self.revision += 1
            self._condition.notify_all()

    def wait(self, seen: int, timeout: float = 20) -> int:
        with self._condition:
            if self.revision == seen:
                self._condition.wait(timeout)
            return self.revision


live = LiveHub()