Gus Heck created SOLR-13491:
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Summary: SolrZkClient can register the same watch twice.
Key: SOLR-13491
URL: https://issues.apache.org/jira/browse/SOLR-13491
Project: Solr
Issue Type: Bug
Security Level: Public (Default Security Level. Issues are Public)
Components: SolrCloud
Affects Versions: 8.0
Reporter: Gus Heck
While working on SOLR-13420 and related tickets I noticed that we presently use
the wrapWatch() method to wrap all zookeeper watches. This allows the watch to
be processed asynchronously by an executor which is good, but it subtly changes
the way zookeeper behaves. Normally zookeeper keeps a Map<String,Set<Watch>>
that ensures that if the same watch is registered twice it's only notified
once. However, our wrapper breaks this because now, if the same watch is added
twice each one gets it's own wrapper with default Object.equals() and thus the
two wrapped versions of the Watch aren not .equals() and may co-exist in the
Set<Watch>
I do not know of any current cases for which this would cause a problem, but
changes I am contemplating for SOLR-13375 are likely to hit this case and at a
minimum create wasted re-notifications of the same event and excess watch load
in Zk.
Moving the anonymous class wrapping to an explicit inner class and writing
hashcode/equals carefully to act as a pass through such that any to wrappers
with the same watch object are .equals() should fix this.
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