[OAK-7162] Race condition on revisions head between compaction and scheduler could result in skipped commit

• Type: Bug
• Status: Closed
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: 1.8.0
• Fix Version/s: 1.9.0, 1.10.0, 1.8.2
• Component/s: segment-tar
• Labels: scalability
• Epic Link: TarMK transaction rate
• Assignee: Andrei Dulceanu
• Reporter: Andrei Dulceanu
• Created: 16/Jan/18 12:17
• Updated: 04/Oct/19 17:51
• Resolved: 05/Feb/18 11:22

Description

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