如何在python中使通过上下文管理器发布的资源失效？

Question

我的问题与Kazoo/Zookeeper有关，但更一般地说，它是关于分配一个资源，然后让该资源的调用者不应该暂停。来自Kazoo docs：

It’s highly recommended to add a state listener with add_listener() and watch for LOST and SUSPENDED state changes and re-act appropriately. In the event that a LOST state occurs, its certain that the lock and/or the lease has been lost.

我正在使用上下文管理器分配锁：

@contextmanager
def lock(self, path, identifier):
  lock = zk.Lock(path, identifier)
  lock.acquire()
  yield lock      <--- how to communicate connection loss from client?
  finally: 
    lock.release()

用法如下：

with lock('/some_op/') as lck:
  # do something with lock, but pause in case of connection loss? 

在连接丢失的情况下，如何分配锁但撤消访问？

Kazoo建议您如何实现connection state listener：

def my_listener(state):
    if state == KazooState.LOST:
        # Register somewhere that the session was lost
    elif state == KazooState.SUSPENDED:
        # Handle being disconnected from Zookeeper
    else:
        # Handle being connected/reconnected to Zookeeper

Answer 1

这可能不是最佳设计，但您可以提供对with语句主体的锁的引用：

@contextmanager
def acquire_lock(self, path, identifier):
  lock = zk.Lock(path, identifier)
  try: 
    retry = KazooRetry()
    retry(lock.acquire)
    yield lock
  finally: 
    lock.release()

with acquire_lock('/some_op/') as lock:
    ...
    if necessary:
        lock.release()
    ...

具体到您的问题（我不熟悉Kazoo），您可以在上下文管理器中定义my_listener，在这里它可以关闭锁。你知道吗

@contextmanager
def acquire_lock(self, path, identifier):
    lock = zk.Lock(path, identifier)
    def my_listener(state):
        if state == KazooState.LOST:
            lock.release()
        elif state == KazooState.SUSPENDED:
            ...

    try:
        retry = KazooRetry()
    # etc

您的上下文可能需要生成侦听器，或者注册它（或者您对侦听器所做的任何操作），但是无论您对它所做的任何操作，它都可以访问您在上下文管理器中创建的锁。你知道吗