keith-turner opened a new pull request, #4554:
URL: https://github.com/apache/accumulo/pull/4554
The following is an example of the problem this change fixes
1. Thread 1: A user compaction is currently running for a tablet
2. Thread 2: Tablet server receives a compaction request RPC from manager
and it checks to see if the compaction is needed for the same tablet. If finds
it is needed.
3. Thread 1: completes user compaction, so a compaction is no longer needed
for the tablet
4. Thread 2: Initiates a user compaction of the tablet because its check in
step 2 passed.
This change adds a second check in step 4 above that should prevent this
race condition because the check is done at a point when its known no
concurrent user compaction is running. The original check was left as a fail
fast check, but a comment was added explaining its not sufficient for
correctness.
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