xyuanlu opened a new issue, #2509:
URL: https://github.com/apache/helix/issues/2509
### Describe the bug
`
public ChildrenSubscribeResult subscribeChildChanges(String path,
IZkChildListener listener, boolean skipWatchingNonExistNode) {
synchronized (_childListener) {
Set<IZkChildListener> listeners = _childListener.get(path);
if (listeners == null) {
listeners = new CopyOnWriteArraySet<>();
_childListener.put(path, listeners);
}
listeners.add(listener);
}
List<String> children = watchForChilds(path, skipWatchingNonExistNode);
if (children == null && skipWatchingNonExistNode) {
unsubscribeChildChanges(path, listener);
LOG.info("zkclient{}, watchForChilds failed to install no-existing
watch and add listener. Path: {}", _uid, path);
return new ChildrenSubscribeResult(children, false);
}
return new ChildrenSubscribeResult(children, true);
}
public void unsubscribeChildChanges(String path, IZkChildListener
childListener) {
synchronized (_childListener) {
final Set<IZkChildListener> listeners = _childListener.get(path);
if (listeners != null) {
listeners.remove(childListener);
}
}
}
`
Current on Helix master (May 24 2023, TOT on commit 07b1bb8), subscribe
child/data change does
1. Lock in memory map,
2. add new listener to map,
3. unlock map
4. Subscribe an one time listener to ZK
5. If subscribe failed, call unsubscribe child/data change (lock, remove,
unlock)
unsubscribe child/data change does
1.Lock in memory map
2. remove listener from map
3. unlock map
When multiple user calling subscribe and unsubscribe at the same time,
unsubscribe child/data change may happen between subscribe step 3 and 4. As a
result, the listener is not added to the map but the ZkClient is registered as
watcher in ZK. Leaving a untracked watcher.
### Expected behavior
When multiple user calling subscribe and unsubscribe at the same time, the
final result is undetermined. However, all registered ZK watcher should be
tracked.
### Additional context
Add any other context about the problem here.
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