fix leadership transfer on leave suggestions (#11387)

* add suggestions

* set isLeader to false when leadership transfer succeed

.changelog/11376.txt

@@ -1,3 +1,3 @@
 ```release-note:enhancement
-try to transfer leadership to another server when leaving.
+raft: Consul leaders will attempt to transfer leadership to another server as part of gracefully leaving the cluster.
 ```

agent/consul/server.go

@@ -988,6 +988,22 @@ func (s *Server) Shutdown() error {
 	return nil
 }
 
+func (s *Server) attemptLeadershipTransfer() (success bool) {
+	leadershipTransferVersion := version.Must(version.NewVersion(LeaderTransferMinVersion))
+
+	ok, _ := ServersInDCMeetMinimumVersion(s, s.config.Datacenter, leadershipTransferVersion)
+	if !ok {
+		return false
+	}
+
+	future := s.raft.LeadershipTransfer()
+	if err := future.Error(); err != nil {
+		s.logger.Error("failed to transfer leadership, removing the server", "error", err)
+		return false
+	}
+	return true
+}
+
 // Leave is used to prepare for a graceful shutdown of the server
 func (s *Server) Leave() error {
 	s.logger.Info("server starting leave")
@@ -1007,24 +1023,9 @@ func (s *Server) Leave() error {
 	// removed for some sane period of time.
 	isLeader := s.IsLeader()
 	if isLeader && numPeers > 1 {
-		leadershipTransferVersion := version.Must(version.NewVersion(LeaderTransferMinVersion))
-		removeServer := false
-		if ok, _ := ServersInDCMeetMinimumVersion(s, s.config.Datacenter, leadershipTransferVersion); !ok {
-			// Transfer leadership to another node then leave the cluster
-			future := s.raft.LeadershipTransfer()
-			if err := future.Error(); err != nil {
-				s.logger.Error("failed to transfer leadership, removing the server", "error", err)
-				// leadership transfer failed, fallback to removing the server from raft
-				removeServer = true
-			} else {
-				// we are not leader anymore, continue the flow to leave as follower
-				isLeader = false
-			}
+		if s.attemptLeadershipTransfer() {
+			isLeader = false
 		} else {
-			// Leadership transfer is not available in the current version, fallback to removing the server from raft
-			removeServer = true
-		}
-		if removeServer {
 			future := s.raft.RemoveServer(raft.ServerID(s.config.NodeID), 0, 0)
 			if err := future.Error(); err != nil {
 				s.logger.Error("failed to remove ourself as raft peer", "error", err)