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https://issues.apache.org/jira/browse/SOLR-13490?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16848230#comment-16848230
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ASF subversion and git services commented on SOLR-13490:
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Commit 94657636281d93dc9918e212d3bc3c07a4f9d3d3 in lucene-solr's branch
refs/heads/branch_8x from Chris M. Hostetter
[ https://gitbox.apache.org/repos/asf?p=lucene-solr.git;h=9465763 ]
Fix TestCloudSearcherWarming to work around SOLR-13490
Also clean up some crufty System.out/System.err pollution
(cherry picked from commit af4e1d324a389bac8cfcbffcbef20d4647957a61)
> waitForState/registerCollectionStateWatcher can see stale liveNodes data due
> to (Zk) Watcher race condition
> -----------------------------------------------------------------------------------------------------------
>
> Key: SOLR-13490
> URL: https://issues.apache.org/jira/browse/SOLR-13490
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Hoss Man
> Priority: Major
> Attachments: SOLR-13490.patch
>
>
> I was investigating some failures in
> {{TestCloudSearcherWarming.testRepFactor1LeaderStartup}} which lead me to the
> hunch that {{waitForState}} wasn't ensuring that the predicates registered
> would always be called if/when a node was shutdown.
> Digging into it a bit more, I found that the root cause seems to be the way
> the {{CollectionStateWatcher}} / {{CollectionStatePredicate}} APIs pass in
> *both* the {{DocCollection}}, and the "current" {{liveNodes}} - but are only
> _triggered_ by the {{StateWatcher}} on the {{state.json}} (which is used to
> rebuild the {{DocCollection}}) - when the {{CollectionStateWatcher}} /
> {{CollectionStatePredicate}} are called, they get the "fresh"
> {{DocCollection}} but they get the _cached_ {{ZkStateReader.liveNodes}}
> Meanwhile, the {{LiveNodeWatcher}} only calls {{refreshLiveNodes()}} only
> updates {{ZkStateReader.liveNodes}} and triggers any {{LiveNodesListener}} -
> it does *NOT* invoke any {{CollectionStateWatcher}} that may have replicas
> hosted on any of changed nodes.
> Since there is no garunteed order that Watchers will be triggered, this means
> there is a race condition where the following can happen...
> * client1 has a ZkStateReader with cached {{liveNodes=[N1, N2, N3]}}
> * client1 registers a {{CollectionStateWatcher}} "watcherZ" that cares if
> "replicaX" of collectionA is on a "down" node
> * client2 causes shutdown of node N1 which is hosting replicaX
> * client1's zkStateReader gets a WatchedEvent for state.json of collectionA
> ** DocCollection for collectionA is rebuilt
> ** watcherZ is fired w/cached {{liveNodes=[N1, N2, N3]}} and the new
> DocCollection
> *** watcherZ sees that replicaX is on N1, but thinks N1 is live
> *** watcherZ says "everything ok, not the event i was waiting for" and
> doesn't take any action
> * client1's zkStateReader gets a WatchedEvent for LIVE_NODES_ZKNODE
> ** zkStateReader.liveNodes is rebuilt
> ...at no point in this sequence (or after this) will watcherZ be notified
> fired with the updated liveNodes (unless/until another {{state.json}} change
> is made for collectionA.
> ----
> While this is definitely be problematic in _tests_ that deal with node
> lifecyle and use things like {{SolrCloudTestCase.waitForState(...,
> SolrCloudTestCase.clusterShape(...))}} to check for the expected
> shards/replicas, a cursory search of how/where
> {{ZkStateReader.waitForState(...)}} and
> {{ZkStateReader.registerCollectionStateWatcher(...)}} are used in solr-core
> suggests that could also lead to bad behavior in situations like reacting to
> shard leader loss, waiting for all leaders of SYSTEM_COLL to come online for
> upgrade, running PrepRecoveryOp, etc... (anywhere that liveNodes is used by
> the watcher/predicate)
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