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https://issues.apache.org/jira/browse/OAK-8063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16783356#comment-16783356
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Francesco Mari commented on OAK-8063:
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[~dulceanu], I like your patch very much since it greatly simplifies the
transfer logic. I propose a third version of the patch with a minor change just
to avoid repeating the same check in two different places and to consistently
log the root segment ID where the traversal starts.
> The cold standby client doesn't correctly handle backward references
> --------------------------------------------------------------------
>
> Key: OAK-8063
> URL: https://issues.apache.org/jira/browse/OAK-8063
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: segment-tar, tarmk-standby
> Affects Versions: 1.6.0
> Reporter: Andrei Dulceanu
> Assignee: Andrei Dulceanu
> Priority: Major
> Labels: cold-standby
> Fix For: 1.12, 1.11.0, 1.8.12, 1.10.2
>
> Attachments: OAK-8063-02.patch, OAK-8063-03.patch, OAK-8063.patch
>
>
> The logic from {{StandbyClientSyncExecution#copySegmentHierarchyFromPrimary}}
> has a flaw when it comes to "backward references". Suppose we have the
> following data segment graph to be transferred from primary: S1, which
> references \{S2, S3} and S3 which references S2. Then, the correct transfer
> order should be S2, S3 and S1.
> Going through the current logic employed by the method, here's what happens:
> {noformat}
> Step 0: batch={S1}
> Step 1: visited={S1}, data={S1}, batch={S2, S3}, queued={S2, S3}
> Step 2: visited={S1, S2}, data={S2, S1}, batch={S3}, queued={S2, S3}
> Step 3: visited={S1, S2, S3}, data={S3, S2, S1}, batch={}, queued={S2,
> S3}.{noformat}
> Therefore, at the end of the loop, the order of the segments to be
> transferred will be S3, S2, S1, which might trigger a
> {{SegmentNotFoundException}} when S3 is further processed, because S2 is
> missing on standby (see OAK-8006).
> /cc [~frm]
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