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raft: Clarify conditions for granting votes and prevotes. 

This includes one theoretical logic change: A node that knows the
leader of the current term will no longer grant votes, even if it has
not yet voted in this term. It also adds a `m.Type == MsgPreVote`
guard on the `m.Term > r.Term` check, which was previously thought to
be incorrect (see #8517) but was actually just unclear.

Closes #8517
Closes #8571
release-3.4
Ben Darnell 2018-01-23 15:05:11 -05:00
parent 495ed39fc8
commit 4e0291ff91
1 changed files with 8 additions and 3 deletions
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11
raft/raft.go
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@ -855,9 +855,14 @@ func (r *raft) Step(m pb.Message) error {
r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
return nil
}
// The m.Term > r.Term clause is for MsgPreVote. For MsgVote m.Term should
// always equal r.Term.
if (r.Vote == None || m.Term > r.Term || r.Vote == m.From) && r.raftLog.isUpToDate(m.Index, m.LogTerm) {
// We can vote if this is a repeat of a vote we've already cast...
canVote := r.Vote == m.From ||
// ...we haven't voted and we don't think there's a leader yet in this term...
(r.Vote == None && r.lead == None) ||
// ...or this is a PreVote for a future term...
(m.Type == pb.MsgPreVote && m.Term > r.Term)
// ...and we believe the candidate is up to date.
if canVote && r.raftLog.isUpToDate(m.Index, m.LogTerm) {
r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] cast %s for %x [logterm: %d, index: %d] at term %d",
r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
// When responding to Msg{Pre,}Vote messages we include the term