The tombstone could be the only one available revision in database.
It happens when all historical revisions have been deleted in previous
compactions. Since tombstone revision is still in database, we should
restore it as valid key index. Otherwise, we lost that event.
Signed-off-by: Wei Fu <fuweid89@gmail.com>
Before this patch, the tombstone can be deleted if its revision is equal
compacted revision. It causes that the watch subscriber won't get this
DELETE event. Based on Compact API[1], we should keep tombstone revision
if it's not less than the compaction revision.
> CompactionRequest compacts the key-value store up to a given revision.
> All superseded keys with a revision less than the compaction revision
> will be removed.
[1]: https://etcd.io/docs/latest/dev-guide/api_reference_v3/
Signed-off-by: Wei Fu <fuweid89@gmail.com>
