CURATOR-710: Fix leaking watch in EnsembleTracker

[kezhuw]

CURATOR-667(#474) fixes asynchronous event path for getConfig to
"/zookeeper/config" by using CuratorFramework::usingNamespace(null) to
fetch data.

It causes watcher not registering to possible WatcherRemovalManager,
so leaking in WatcherRemoveCuratorFramework::removeWatchers.

[tisonkun]

It seems the watcherRemovalManager already set in the constructor by this.watcherRemovalManager = client.getWatcherRemovalManager();?

[tisonkun]

Or otherwise we can merge this setter into the constructor as a parameter.

[tisonkun]

Can you elaborate a bit when watcherRemovalManager can be different from client.getWatcherRemovalManager().

[eolivelli]

I agree with the question: we can save some memory by not having the field. The performance impact is negligible

[kezhuw]

1. Given a fresh new CuratorFrameworkImpl, say framework0.
2. Let's assume framework1 = framework0.newWatcherRemoveCuratorFramework(). framework1.getWatcherRemovalManager will be the WatcherRemovalManager.
3. Let's assume framework2 = frame1.usingNamespace(null). framework2 will loss WatcherRemovalManager from above.
4. framework1.removeWatchers will not drop watchers from new Watching(framework2, ...).

Step.2 is what exactly EnsembleTracker does.

curator/curator-framework/src/main/java/org/apache/curator/framework/imps/EnsembleTracker.java

Lines 87 to 90 in 6a27ff0

	EnsembleTracker(CuratorFramework client, EnsembleProvider ensembleProvider) {
	this.client = client.newWatcherRemoveCuratorFramework();
	this.ensembleProvider = ensembleProvider;
	}

Step.3 is what exactly GetConfigBuilderImpl(CURATOR-667(#474)) does currently.

curator/curator-framework/src/main/java/org/apache/curator/framework/imps/GetConfigBuilderImpl.java

Lines 41 to 45 in 6a27ff0

	public GetConfigBuilderImpl(CuratorFrameworkImpl client) {
	this.client = (CuratorFrameworkImpl) client.usingNamespace(null);
	backgrounding = new Backgrounding();
	watching = new Watching(this.client);
	}

Step.4 is where this bug emerges.

curator/curator-framework/src/main/java/org/apache/curator/framework/imps/EnsembleTracker.java

Lines 99 to 104 in 6a27ff0

	public void close() {
	if (state.compareAndSet(State.STARTED, State.CLOSED)) {
	client.removeWatchers();
	client.getConnectionStateListenable().removeListener(connectionStateListener);
	}
	}

I have pushed a fixup commit 6b78a3b with comments to doc this.