Hoss Man created SOLR-13490:
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Summary: 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
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.
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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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