Improvements on raft leader handoff. (#4142)

Signed-off-by: Derek Collison <derek@nats.io>

server/jetstream_cluster.go

@@ -2376,7 +2376,7 @@ func (js *jetStream) monitorStream(mset *stream, sa *streamAssignment, sendSnaps
 			ci := js.clusterInfo(rg)
 			mset.checkClusterInfo(ci)
 
-			newPeers, _, newPeerSet, oldPeerSet := genPeerInfo(rg.Peers, len(rg.Peers)-replicas)
+			newPeers, oldPeers, newPeerSet, oldPeerSet := genPeerInfo(rg.Peers, len(rg.Peers)-replicas)
 
 			// If we are part of the new peerset and we have been passed the baton.
 			// We will handle scale down.
@@ -2402,6 +2402,11 @@ func (js *jetStream) monitorStream(mset *stream, sa *streamAssignment, sendSnaps
 					continue
 				}
 
+				// We are good to go, can scale down here.
+				for _, p := range oldPeers {
+					n.ProposeRemovePeer(p)
+				}
+
 				csa := sa.copyGroup()
 				csa.Group.Peers = newPeers
 				csa.Group.Preferred = ourPeerId
@@ -2425,6 +2430,7 @@ func (js *jetStream) monitorStream(mset *stream, sa *streamAssignment, sendSnaps
 				// Check if we have a quorom.
 				if current >= neededCurrent {
 					s.Noticef("Transfer of stream leader for '%s > %s' to '%s'", accName, sa.Config.Name, newLeader)
+					n.UpdateKnownPeers(newPeers)
 					n.StepDown(newLeaderPeer)
 				}
 			}

server/raft.go

@@ -1364,7 +1364,15 @@ func (n *raft) StepDown(preferred ...string) error {
 	if maybeLeader != noLeader {
 		n.debug("Selected %q for new leader", maybeLeader)
 		prop.push(newEntry(EntryLeaderTransfer, []byte(maybeLeader)))
-		time.AfterFunc(250*time.Millisecond, func() { stepdown.push(noLeader) })
+		time.AfterFunc(250*time.Millisecond, func() {
+			n.RLock()
+			stillLeader := n.state == Leader
+			n.RUnlock()
+			// If we are still the leader force a stepdown.
+			if stillLeader {
+				stepdown.push(noLeader)
+			}
+		})
 	} else {
 		// Force us to stepdown here.
 		n.debug("Stepping down")
@@ -1402,6 +1410,7 @@ func (n *raft) campaign() error {
 func (n *raft) xferCampaign() error {
 	n.debug("Starting transfer campaign")
 	if n.state == Leader {
+		n.lxfer = false
 		return errAlreadyLeader
 	}
 	n.resetElect(10 * time.Millisecond)