NRG: Ignore AEs from older terms

Co-authored-by: Reuben Ninan <[email protected]>
Signed-off-by: Neil Twigg <[email protected]>

Author: neilalexander

server/raft.go

@@ -3271,6 +3271,10 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
 			n.debug("Term higher than ours and we are not a follower: %v, stepping down to %q", n.State(), ae.leader)
 			n.stepdown.push(ae.leader)
 		}
+	} else if ae.term < n.term && !catchingUp && isNew {
+		n.debug("Ignoring AppendEntry from a leader (%s) with term %d which is less than ours", ae.leader, ae.term)
+		n.Unlock()
+		return
 	}
 
 	if isNew && n.leader != ae.leader && n.State() == Follower {

server/raft_test.go

@@ -220,6 +220,53 @@ func TestNRGObserverMode(t *testing.T) {
 	}
 }
 
+func TestNRGAEFromOldLeader(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "R3S", 3)
+	defer c.shutdown()
+
+	nc, _ := jsClientConnect(t, c.leader(), nats.UserInfo("admin", "s3cr3t!"))
+	defer nc.Close()
+
+	rg := c.createMemRaftGroup("TEST", 3, newStateAdder)
+	rg.waitOnLeader()
+
+	// Listen out for catchup requests.
+	ch := make(chan *nats.Msg, 16)
+	_, err := nc.ChanSubscribe(fmt.Sprintf(raftCatchupReply, ">"), ch)
+	require_NoError(t, err)
+
+	// Start next term so that we can reuse term 1 in the next step.
+	leader := rg.leader().node().(*raft)
+	leader.StepDown()
+	time.Sleep(time.Millisecond * 100)
+	rg.waitOnLeader()
+	require_Equal(t, leader.Term(), 2)
+	leader = rg.leader().node().(*raft)
+
+	// Send an append entry with an outdated term. Beforehand, doing
+	// so would have caused a WAL reset and then would have triggered
+	// a Raft-level catchup.
+	ae := &appendEntry{
+		term:   1,
+		pindex: 0,
+		leader: leader.id,
+		reply:  nc.NewRespInbox(),
+	}
+	payload, err := ae.encode(nil)
+	require_NoError(t, err)
+	resp, err := nc.Request(leader.asubj, payload, time.Second)
+	require_NoError(t, err)
+
+	// Wait for the response, the server should have rejected it.
+	ar := leader.decodeAppendEntryResponse(resp.Data)
+	require_NotNil(t, ar)
+	require_Equal(t, ar.success, false)
+
+	// No catchup should happen at this point because no reset should
+	// have happened.
+	require_NoChanRead(t, ch, time.Second*2)
+}
+
 // TestNRGSimpleElection tests that a simple election succeeds. It is
 // simple because the group hasn't processed any entries and hasn't
 // suffered any interruptions of any kind, therefore there should be