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https://issues.apache.org/jira/browse/KAFKA-2795?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14999181#comment-14999181
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Jason Gustafson commented on KAFKA-2795:
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Nice catch. Seems like we need to use the result of putIfNotExists. Maybe
something like this:
{code}
private def addGroup(groupId: String, group: GroupMetadata): GroupMetadata = {
val previous = groupsCache.putIfNotExists(groupId, group)
if (previous != null)
previous
else
group
}
{code}
> potential NPE in GroupMetadataManager
> -------------------------------------
>
> Key: KAFKA-2795
> URL: https://issues.apache.org/jira/browse/KAFKA-2795
> Project: Kafka
> Issue Type: Bug
> Reporter: Onur Karaman
> Assignee: Guozhang Wang
>
> I didn't run the code, but I took a look at GroupMetadataManager.addGroup and
> it looks like we can get a NullPointerException when a group is somehow
> removed between the groupsCache.putIfNotExists and groupsCache.get lines and
> someone tries to use the result of the addGroup. One way this can happen is
> by interleaving GroupMetadataManager.addGroup and
> GroupMetadataManager.removeGroupsForPartition.
> Here's the scenario:
> # thread-1 is in the middle of adding a group g which is in the offset topic
> partition p. thread-1 already hit the groupsCache.putIfNotExists line in
> GroupMetadataManager.addGroup
> # thread-2 is in the middle of migrating all groups for partition p. thread-2
> is in GroupMetadataManager.removeGroupsForPartition and called
> groupsCache.remove("g").
> # thread-1 now executes groupsCache.get("g"), which returns null since it's
> now gone.
> # thread-1 now goes back to the GroupCoordinator doJoinGroup with a null
> GroupMetadata and then tries to do a group synchronized {...}, resulting in
> an NPE.
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