etcdserver: exclude learner from leader transfer

1. Maintenance API MoveLeader() returns ErrBadLeaderTransferee if
transferee does not exist or is raft learner.

2. etcdserver TransferLeadership() only choose voting member as
transferee.

etcdserver/api/membership/cluster.go

@@ -754,3 +754,25 @@ func (c *RaftCluster) IsLearner() bool {
 	}
 	return localMember.IsLearner
 }
+
+// IsMemberExist returns if the member with the given id exists in cluster.
+func (c *RaftCluster) IsMemberExist(id types.ID) bool {
+	c.Lock()
+	defer c.Unlock()
+	_, ok := c.members[id]
+	return ok
+}
+
+// VotingMemberIDs returns the ID of voting members in cluster.
+func (c *RaftCluster) VotingMemberIDs() []types.ID {
+	c.Lock()
+	defer c.Unlock()
+	var ids []types.ID
+	for _, m := range c.members {
+		if !m.IsLearner {
+			ids = append(ids, m.ID)
+		}
+	}
+	sort.Sort(types.IDSlice(ids))
+	return ids
+}

etcdserver/api/v3rpc/rpctypes/error.go

@@ -72,6 +72,7 @@ var (
 	ErrGRPCUnhealthy                  = status.New(codes.Unavailable, "etcdserver: unhealthy cluster").Err()
 	ErrGRPCCorrupt                    = status.New(codes.DataLoss, "etcdserver: corrupt cluster").Err()
 	ErrGPRCNotSupportedForLearner     = status.New(codes.FailedPrecondition, "etcdserver: rpc not supported for learner").Err()
+	ErrGRPCBadLeaderTransferee        = status.New(codes.FailedPrecondition, "etcdserver: bad leader transferee").Err()
 
 	errStringToError = map[string]error{
 		ErrorDesc(ErrGRPCEmptyKey):      ErrGRPCEmptyKey,
@@ -123,6 +124,7 @@ var (
 		ErrorDesc(ErrGRPCTimeoutDueToConnectionLost): ErrGRPCTimeoutDueToConnectionLost,
 		ErrorDesc(ErrGRPCUnhealthy):                  ErrGRPCUnhealthy,
 		ErrorDesc(ErrGRPCCorrupt):                    ErrGRPCCorrupt,
+		ErrorDesc(ErrGRPCBadLeaderTransferee):        ErrGRPCBadLeaderTransferee,
 	}
 )
 
@@ -176,6 +178,7 @@ var (
 	ErrTimeoutDueToConnectionLost = Error(ErrGRPCTimeoutDueToConnectionLost)
 	ErrUnhealthy                  = Error(ErrGRPCUnhealthy)
 	ErrCorrupt                    = Error(ErrGRPCCorrupt)
+	ErrBadLeaderTransferee        = Error(ErrGRPCBadLeaderTransferee)
 )
 
 // EtcdError defines gRPC server errors.

etcdserver/api/v3rpc/util.go

@@ -55,6 +55,7 @@ var toGRPCErrorMap = map[error]error{
 	etcdserver.ErrUnhealthy:                  rpctypes.ErrGRPCUnhealthy,
 	etcdserver.ErrKeyNotFound:                rpctypes.ErrGRPCKeyNotFound,
 	etcdserver.ErrCorrupt:                    rpctypes.ErrGRPCCorrupt,
+	etcdserver.ErrBadLeaderTransferee:        rpctypes.ErrGRPCBadLeaderTransferee,
 
 	lease.ErrLeaseNotFound:    rpctypes.ErrGRPCLeaseNotFound,
 	lease.ErrLeaseExists:      rpctypes.ErrGRPCLeaseExist,

etcdserver/errors.go

@@ -37,6 +37,7 @@ var (
 	ErrUnhealthy                  = errors.New("etcdserver: unhealthy cluster")
 	ErrKeyNotFound                = errors.New("etcdserver: key not found")
 	ErrCorrupt                    = errors.New("etcdserver: corrupt cluster")
+	ErrBadLeaderTransferee        = errors.New("etcdserver: bad leader transferee")
 )
 
 type DiscoveryError struct {

etcdserver/server.go

@@ -1377,16 +1377,16 @@ func (s *EtcdServer) triggerSnapshot(ep *etcdProgress) {
 	ep.snapi = ep.appliedi
 }
 
-func (s *EtcdServer) isMultiNode() bool {
-	return s.cluster != nil && len(s.cluster.MemberIDs()) > 1
-}
-
 func (s *EtcdServer) isLeader() bool {
 	return uint64(s.ID()) == s.Lead()
 }
 
 // MoveLeader transfers the leader to the given transferee.
 func (s *EtcdServer) MoveLeader(ctx context.Context, lead, transferee uint64) error {
+	if !s.cluster.IsMemberExist(types.ID(transferee)) || s.cluster.Member(types.ID(transferee)).IsLearner {
+		return ErrBadLeaderTransferee
+	}
+
 	now := time.Now()
 	interval := time.Duration(s.Cfg.TickMs) * time.Millisecond
 
@@ -1440,20 +1440,20 @@ func (s *EtcdServer) TransferLeadership() error {
 		return nil
 	}
 
-	if !s.isMultiNode() {
+	if s.cluster == nil || len(s.cluster.VotingMemberIDs()) <= 1 {
 		if lg := s.getLogger(); lg != nil {
 			lg.Info(
-				"skipped leadership transfer; it's a single-node cluster",
+				"skipped leadership transfer for single voting member cluster",
 				zap.String("local-member-id", s.ID().String()),
 				zap.String("current-leader-member-id", types.ID(s.Lead()).String()),
 			)
 		} else {
-			plog.Printf("skipped leadership transfer for single member cluster")
+			plog.Printf("skipped leadership transfer for single voting member cluster")
 		}
 		return nil
 	}
 
-	transferee, ok := longestConnected(s.r.transport, s.cluster.MemberIDs())
+	transferee, ok := longestConnected(s.r.transport, s.cluster.VotingMemberIDs())
 	if !ok {
 		return ErrUnhealthy
 	}