NRG: Campaign early unless already candidate

Signed-off-by: Maurice van Veen <[email protected]>

server/raft.go

@@ -4138,11 +4138,10 @@ func (n *raft) processVoteRequest(vr *voteRequest) error {
 		n.vote = vr.candidate
 		n.writeTermVote()
 		n.resetElectionTimeout()
-	} else {
-		if vr.term >= n.term && n.vote == noVote {
-			n.term = vr.term
-			n.resetElect(randCampaignTimeout())
-		}
+	} else if n.vote == noVote && n.State() != Candidate {
+		// We have a more up-to-date log, and haven't voted yet.
+		// Start campaigning earlier, but only if not candidate already, as that would short-circuit us.
+		n.resetElect(randCampaignTimeout())
 	}
 
 	// Term might have changed, make sure response has the most current

server/raft_test.go

@@ -526,6 +526,60 @@ func TestNRGUnsuccessfulVoteRequestDoesntResetElectionTimer(t *testing.T) {
 	require_True(t, followerEqual)
 }
 
+func TestNRGUnsuccessfulVoteRequestCampaignEarly(t *testing.T) {
+	n, cleanup := initSingleMemRaftNode(t)
+	defer cleanup()
+
+	nats0 := "S1Nunr6R" // "nats-0"
+	n.etlr = time.Time{}
+
+	// Simple case: we are follower and vote for a candidate.
+	require_NoError(t, n.processVoteRequest(&voteRequest{term: 1, lastTerm: 0, lastIndex: 0, candidate: nats0}))
+	require_Equal(t, n.term, 1)
+	require_Equal(t, n.vote, nats0)
+	require_NotEqual(t, n.etlr, time.Time{}) // Resets election timer as it voted.
+	n.etlr = time.Time{}
+
+	// We are follower and deny vote for outdated candidate.
+	n.pterm, n.pindex = 1, 100
+	require_NoError(t, n.processVoteRequest(&voteRequest{term: 2, lastTerm: 1, lastIndex: 2, candidate: nats0}))
+	require_Equal(t, n.term, 2)
+	require_Equal(t, n.vote, noVote)
+	require_NotEqual(t, n.etlr, time.Time{}) // Resets election timer as it starts campaigning.
+	n.etlr = time.Time{}
+
+	// Switch to candidate.
+	n.pterm, n.pindex = 2, 200
+	n.switchToCandidate()
+	require_Equal(t, n.term, 3)
+	require_Equal(t, n.State(), Candidate)
+	require_NotEqual(t, n.etlr, time.Time{}) // Resets election timer as part of switching state.
+	n.etlr = time.Time{}
+
+	// We are candidate and deny vote for outdated candidate. But they were on a more recent term, restart campaign.
+	require_NoError(t, n.processVoteRequest(&voteRequest{term: 4, lastTerm: 1, lastIndex: 2, candidate: nats0}))
+	require_Equal(t, n.term, 4)
+	require_Equal(t, n.vote, noVote)
+	require_NotEqual(t, n.etlr, time.Time{}) // Resets election timer as it restarts campaigning.
+	n.etlr = time.Time{}
+
+	// Switch to candidate.
+	n.pterm, n.pindex = 4, 400
+	n.switchToCandidate()
+	require_Equal(t, n.term, 5)
+	require_Equal(t, n.State(), Candidate)
+	require_NotEqual(t, n.etlr, time.Time{}) // Resets election timer as part of switching state.
+	n.etlr = time.Time{}
+
+	// We are candidate and deny vote for outdated candidate. Don't start campaigning early.
+	require_NoError(t, n.processVoteRequest(&voteRequest{term: 5, lastTerm: 1, lastIndex: 2, candidate: nats0}))
+	require_Equal(t, n.term, 5)
+	require_Equal(t, n.vote, noVote)
+	// Election timer must NOT be updated as that would mean another candidate that we don't vote
+	// for can short-circuit us by making us restart elections, denying us the ability to become leader.
+	require_Equal(t, n.etlr, time.Time{})
+}
+
 func TestNRGInvalidTAVDoesntPanic(t *testing.T) {
 	c := createJetStreamClusterExplicit(t, "R3S", 3)
 	defer c.shutdown()