[IMPROVED] Leadership transfer (#4145)

When doing leadership transfer stepdown as soon as we know we have sent
the EntryLeaderTransfer entry.

Delaying could allow something to be sent from the old leader which
would cause the new leader to bail on being a candidate even though it
would have gotten all the votes.

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

server/raft.go

@@ -1368,15 +1368,6 @@ 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() {
-			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")
@@ -2234,11 +2225,20 @@ func (n *raft) runAsLeader() {
 					continue
 				}
 				n.sendAppendEntry(entries)
+
+				// If this is us sending out a leadership transfer stepdown inline here.
+				if b.Type == EntryLeaderTransfer {
+					n.prop.recycle(&es)
+					n.debug("Stepping down due to leadership transfer")
+					n.switchToFollower(noLeader)
+					return
+				}
 				// We need to re-create `entries` because there is a reference
 				// to it in the node's pae map.
 				entries = nil
 			}
 			n.prop.recycle(&es)
+
 		case <-hb.C:
 			if n.notActive() {
 				n.sendHeartbeat()