Summary:
Prior to this PR the following sequence could happen:
1. `RunManualCompaction()` A schedules compaction to thread pool and waits
2. `RunManualCompaction()` B waits without scheduling anything due to conflict
3. `DisableManualCompaction()` bumps `manual_compaction_paused_` and wakes up both
4. `RunManualCompaction()` A (`scheduled && !unscheduled`) unschedules its compaction and marks itself done
5. `RunManualCompaction()` B (`!scheduled && !unscheduled`) schedules compaction to thread pool
6. `RunManualCompaction()` B (`scheduled && !unscheduled`) waits on its compaction
7. `RunManualCompaction()` B at some point wakes up and finishes, either by unscheduling or by compaction execution
8. `DisableManualCompaction()` returns as there are no more manual compactions running
Between 6. and 7. the wait can be long while the compaction sits in the thread pool queue. That wait is unnecessary. This PR changes the behavior from step 5. onward:
5'. `RunManualCompaction()` B (`!scheduled && !unscheduled`) marks itself done
6'. `DisableManualCompaction()` returns as there are no more manual compactions running
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12578
Reviewed By: cbi42
Differential Revision: D56528144
Pulled By: ajkr
fbshipit-source-id: 4da2467376d7d4ff435547aa74dd8f118db0c03b
d2ebdea566