Fixed an issue where the old leader's log could not be synced with the new one due to the absence of a previous LogEntry #54
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…e new one due to the absence of a previous LogEntry If an old leader's log contains a snapshot and only unreplicated entries, it can't be synchronized with a new leader's log. This happens because the new leader can't verify the previous LogEntry of the old leader since this entry is already snapshotted. As a fix, we should add a check if `snapshotIndex` is large or equal to `previousLogIndex`, and use snapshot term in this case.
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According to the Raft protocol, before a new
LogEntrycan be added to the follower log, we should check that the previousLogEntryexists and has the sametermnumber as the leader's entry.If an old leader's log contains a snapshot and only unreplicated entries, the new leader can't verify the previous
LogEntryon the old leader, since this entry is already snapshotted and not present in aRaftLog:LogEntry prevEntry = log.getLogEntry(request.getPreviousLogIndex());return that
prevEntryisnull, ifprevLogIndex<=snapshotIndex.As a result, an old leader's log can't be synchronised with a new leader's log.
As a fix, we should add a check if
snapshotIndexis large or equal topreviousLogIndex, and use the snapshot term in this case.