Raft Leader Transfer

func (r *Raft) handleTransferLeader(m pb.Message) {
	if r.State != StateLeader {
		// forward the message to the current leader
		r.msgs = append(r.msgs, pb.Message{
			From:    r.id,
			To:      r.Lead,
			MsgType: pb.MessageType_MsgTransferLeader,
		})
		return
	}
 
	transferee := m.From
	if transferee == r.id {
		// log.Debugf("[TRANSFER][PEER %+v] transfer leader to itself", r.id)
		return
	}
 
	_, exists := r.Prs[transferee]
	if !exists {
		// Ignore if the peer is not part of the cluster
		// log.Debugf("[TRANSFER][PEER %+v] transfer leader to unknown peer %+v", r.id, transferee)
		return
	}
 
	if r.leadTransferee != None && r.leadTransferee != transferee && r.leaderTransfering {
		// log.Debugf("[TRANSFER][PEER %+v] interrupt previous leader transfer (%+v -> %+v), new transferee %+v", r.id, r.leadTransferee, transferee)
		r.leadTransferee = transferee
		r.leaderTransfering = false
	}
 
	r.leadTransferee = transferee
 
	// if transferee is up to date, transfer leader to transferee
	if r.Prs[transferee].Match == r.RaftLog.LastIndex() && r.commitUpdated[transferee] {
		lastIndex, lastTerm := r.getLastIndexAndTerm()
		log.Debugf("[LEADER TRANSFER][PEER %+v] transfer leader to %+v", r.id, transferee)
		r.msgs = append(r.msgs, pb.Message{
			MsgType: pb.MessageType_MsgTimeoutNow,
			To:      transferee,
			From:    r.id,
			Term:    r.Term,
			LogTerm: lastTerm,
			Index:   lastIndex,
			Commit:  r.RaftLog.committed,
		})
	} else {
		if !r.leaderTransfering {
			r.sendAppendForUpdateCommit(transferee)
			r.leaderTransfering = true
		}
		r.msgs = append(r.msgs, m)
	}
}