Tomás Fernández Löbbe commented on SOLR-11535:

This may be related to the fact that we execute ZooKeeper watches in a 
multi-thread executor, so if multiple threads in parallel could call the 
{{StateWatcher.refreshAndWatch}} method concurrently (coming from two watches 
fired on the same znode). Note that we re-set the watch when we call 
{{fetchCollectionState}}. This is similar to what I saw while working on 
 Maybe the solution is to not call {{constructState}} when 
{{updateWatchedCollection}} returns {{false}}. Other option I think could be to 
DocCollection newState = fetchCollectionState(coll, this);
updateWatchedCollection(coll, newState);
inside the synchronize. Other option is to document that the watchers could be 
called more than once for the same data and make the watcher call handle it. 
Maybe [~shalinmangar] also has some thoughts.

> Weird behavior of CollectionStateWatcher
> ----------------------------------------
>                 Key: SOLR-11535
>                 URL: https://issues.apache.org/jira/browse/SOLR-11535
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>    Affects Versions: 7.2, master (8.0)
>            Reporter: Andrzej Bialecki 
>            Priority: Major
>         Attachments: test.log
> While working on SOLR-11320 I noticed a strange behavior in 
> {{ActiveReplicaWatcher}}, which is a subclass of {{CollectionStateWatcher}} - 
> it appears that its {{onStateChanged}} method can be called from multiple 
> threads with exactly the same {{DocCollection}} state, ie. unchanged between 
> the calls.
> This seems to run contrary to the javadoc, which implies that this method is 
> called only when the state actually changes, and it also doesn't mention 
> anything about the need for thread-safety in the method implementation.
> I attached the log, which has a lot of additional debugging - but the most 
> pertinent part being where a Watcher-s hashCode is printed together with the 
> {{DocCollection}} - notice that these overlapping calls both submit an 
> instance of {{DocCollection}} with the same zkVersion.
> [~dragonsinth], [~romseygeek] - could you please take a look at this? If this 
> behavior is expected then the javadoc should be updated to state clearly that 
> multiple calls can be made concurrently, with exactly the same state (which 
> is kind of a weak guarantee for a method called {{onStateChanged}} ;) ).

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