If I have a ZooKeeper client connected to an ensemble then obviously I can register watches.
If the client is disconnected (for example by a failing ensemble member) then I get a disconnection event for all of my watches. If, on the other hand, my client is closed then I do not get a disconnection event. This asymmetry makes it really hard to clear up properly when using the asynchronous API, as there is no way to "fail" data reads/updates when the client is closed.
I believe that the correct behaviour should be for all watchers to receive a disconnection event when the client is closed. The watchers can then respond as appropriate, and can differentiate between a "server disconnect" and a "client disconnect" by checking the ZooKeeper#getState() method.
This would not be a breaking behaviour change as Watchers are already required to handle disconnection events.