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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-7162) Race condition on revisions head between compaction and scheduler could result in skipped commit
Date Mon, 05 Feb 2018 08:22:00 GMT

    [ https://issues.apache.org/jira/browse/OAK-7162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16352135#comment-16352135
] 

Michael Dürig commented on OAK-7162:
------------------------------------

[~dulceanu], I agree with your reasoning regarding removing the call to #{{refreshHead}}.
Should there be future cases where the head is changed from outside the scheduler we should
still be on the safe side as #{{refreshHead}} gets called every time the root node is retrieved.
The only difference is delaying distribution of the content changes from that concurrent modification
for a bit.

A few minor remarks wrt. the patch:
 * {{LockBasedSchedulerTest#temporaryFolder}} is not used.
 * Please add a message to the the assertion asserting the commit tasks do not return {{null}}.
The message should somehow capture that {{null}} means a commit got lost because of a race
condition.
 * There is the {{EmptyHook}} class that you could use in {{#createCommit}}.
 * Please add a few words of Javadoc to {{#testSimulatedRaceOnRevisions}} explaining what
the tests is guarding against. This issue's description might be a good start.

Otherwise the test looks really good, great stuff!

> Race condition on revisions head between compaction and scheduler could result in skipped
commit
> ------------------------------------------------------------------------------------------------
>
>                 Key: OAK-7162
>                 URL: https://issues.apache.org/jira/browse/OAK-7162
>             Project: Jackrabbit Oak
>          Issue Type: Bug
>          Components: segment-tar
>    Affects Versions: 1.8.0
>            Reporter: Andrei Dulceanu
>            Assignee: Andrei Dulceanu
>            Priority: Blocker
>              Labels: scalability
>             Fix For: 1.9.0, 1.10, 1.8.2
>
>         Attachments: OAK-7162-02.patch, OAK-7162-03.patch, OAK-7162-test.patch, OAK-7162.patch
>
>
> There is a race condition on {{TarRevisions#head}} between a running compaction trying
to set the new head [0] and the scheduler doing the same after executing a specific commit
[1]. If the compaction thread is first, then the head assignment in the scheduler will fail
and not be re-attempted. 
> IMO, the simple if statement should be changed to a while loop in which the head is refreshed
and the commit is re-applied against the new head, before attempting again to set a new head
in {{TarRevisions}}. This is somehow similar to what we previously had [2], but without the
unneeded optimistic/pessimistic strategies involving tokens.
> [0] https://github.com/apache/jackrabbit-oak/blob/trunk/oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java#L764
> [1] https://github.com/apache/jackrabbit-oak/blob/trunk/oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/scheduler/LockBasedScheduler.java#L253
> [2] https://github.com/apache/jackrabbit-oak/blob/1.6/oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentNodeStore.java#L686



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