etcdserver: allow 1 learner in cluster

Hard-coded the maximum number of learners to 1.

clientv3/integration/cluster_test.go

@@ -259,3 +259,43 @@ func TestMemberPromoteForLearner(t *testing.T) {
 		t.Errorf("learner promoted, expect 0 learner, got %d", numberOfLearners)
 	}
 }
+
+// TestMaxLearnerInCluster verifies that the maximum number of learners allowed in a cluster is 1
+func TestMaxLearnerInCluster(t *testing.T) {
+	defer testutil.AfterTest(t)
+
+	// 1. start with a cluster with 3 voting member and 0 learner member
+	clus := integration.NewClusterV3(t, &integration.ClusterConfig{Size: 3})
+	defer clus.Terminate(t)
+
+	// 2. adding a learner member should succeed
+	resp1, err := clus.Client(0).MemberAddAsLearner(context.Background(), []string{"http://127.0.0.1:1234"})
+	if err != nil {
+		t.Fatalf("failed to add learner member %v", err)
+	}
+	numberOfLearners := 0
+	for _, m := range resp1.Members {
+		if m.IsLearner {
+			numberOfLearners++
+		}
+	}
+	if numberOfLearners != 1 {
+		t.Fatalf("Added 1 learner node to cluster, got %d", numberOfLearners)
+	}
+
+	// 3. cluster has 3 voting member and 1 learner, adding another learner should fail
+	_, err = clus.Client(0).MemberAddAsLearner(context.Background(), []string{"http://127.0.0.1:2345"})
+	if err == nil {
+		t.Fatalf("expect member add to fail, got no error")
+	}
+	expectedErrKeywords := "too many learner members in cluster"
+	if !strings.Contains(err.Error(), expectedErrKeywords) {
+		t.Fatalf("expecting error to contain %s, got %s", expectedErrKeywords, err.Error())
+	}
+
+	// 4. cluster has 3 voting member and 1 learner, adding a voting member should succeed
+	_, err = clus.Client(0).MemberAdd(context.Background(), []string{"http://127.0.0.1:3456"})
+	if err != nil {
+		t.Errorf("failed to add member %v", err)
+	}
+}

etcdserver/api/membership/cluster.go

@@ -40,6 +40,8 @@ import (
 	"go.uber.org/zap"
 )
 
+const maxLearners = 1
+
 // RaftCluster is a list of Members that belong to the same raft cluster
 type RaftCluster struct {
 	lg *zap.Logger
@@ -292,16 +294,15 @@ func (c *RaftCluster) ValidateConfigurationChange(cc raftpb.ConfChange) error {
 				plog.Panicf("unmarshal confChangeContext should never fail: %v", err)
 			}
 		}
-		// A ConfChangeAddNode to a existing learner node promotes it to a voting member.
-		if confChangeContext.IsPromote {
+
+		if confChangeContext.IsPromote { // promoting a learner member to voting member
 			if members[id] == nil {
 				return ErrIDNotFound
 			}
 			if !members[id].IsLearner {
 				return ErrMemberNotLearner
 			}
-		} else {
-			// add a learner or a follower case
+		} else { // adding a new member
 			if members[id] != nil {
 				return ErrIDExists
 			}
@@ -317,6 +318,18 @@ func (c *RaftCluster) ValidateConfigurationChange(cc raftpb.ConfChange) error {
 					return ErrPeerURLexists
 				}
 			}
+
+			if confChangeContext.Member.IsLearner { // the new member is a learner
+				numLearners := 0
+				for _, m := range members {
+					if m.IsLearner {
+						numLearners++
+					}
+				}
+				if numLearners+1 > maxLearners {
+					return ErrTooManyLearners
+				}
+			}
 		}
 	case raftpb.ConfChangeRemoveNode:
 		if members[id] == nil {

etcdserver/api/membership/errors.go

@@ -27,6 +27,7 @@ var (
 	ErrPeerURLexists    = errors.New("membership: peerURL exists")
 	ErrMemberNotLearner = errors.New("membership: can only promote a learner member")
 	ErrLearnerNotReady  = errors.New("membership: can only promote a learner member which is in sync with leader")
+	ErrTooManyLearners  = errors.New("membership: too many learner members in cluster")
 )
 
 func isKeyNotFound(err error) bool {

etcdserver/api/v3rpc/rpctypes/error.go

@@ -42,6 +42,7 @@ var (
 	ErrGRPCMemberNotFound         = status.New(codes.NotFound, "etcdserver: member not found").Err()
 	ErrGRPCMemberNotLearner       = status.New(codes.FailedPrecondition, "etcdserver: can only promote a learner member").Err()
 	ErrGRPCLearnerNotReady        = status.New(codes.FailedPrecondition, "etcdserver: can only promote a learner member which is in sync with leader").Err()
+	ErrGRPCTooManyLearners        = status.New(codes.FailedPrecondition, "etcdserver: too many learner members in cluster").Err()
 
 	ErrGRPCRequestTooLarge        = status.New(codes.InvalidArgument, "etcdserver: request is too large").Err()
 	ErrGRPCRequestTooManyRequests = status.New(codes.ResourceExhausted, "etcdserver: too many requests").Err()
@@ -97,6 +98,7 @@ var (
 		ErrorDesc(ErrGRPCMemberNotFound):         ErrGRPCMemberNotFound,
 		ErrorDesc(ErrGRPCMemberNotLearner):       ErrGRPCMemberNotLearner,
 		ErrorDesc(ErrGRPCLearnerNotReady):        ErrGRPCLearnerNotReady,
+		ErrorDesc(ErrGRPCTooManyLearners):        ErrGRPCTooManyLearners,
 
 		ErrorDesc(ErrGRPCRequestTooLarge):        ErrGRPCRequestTooLarge,
 		ErrorDesc(ErrGRPCRequestTooManyRequests): ErrGRPCRequestTooManyRequests,
@@ -154,6 +156,7 @@ var (
 	ErrMemberNotFound         = Error(ErrGRPCMemberNotFound)
 	ErrMemberNotLearner       = Error(ErrGRPCMemberNotLearner)
 	ErrMemberLearnerNotReady  = Error(ErrGRPCLearnerNotReady)
+	ErrTooManyLearners        = Error(ErrGRPCTooManyLearners)
 
 	ErrRequestTooLarge = Error(ErrGRPCRequestTooLarge)
 	ErrTooManyRequests = Error(ErrGRPCRequestTooManyRequests)

etcdserver/api/v3rpc/util.go

@@ -37,6 +37,7 @@ var toGRPCErrorMap = map[error]error{
 	membership.ErrPeerURLexists:           rpctypes.ErrGRPCPeerURLExist,
 	membership.ErrMemberNotLearner:        rpctypes.ErrGRPCMemberNotLearner,
 	membership.ErrLearnerNotReady:         rpctypes.ErrGRPCLearnerNotReady,
+	membership.ErrTooManyLearners:         rpctypes.ErrGRPCTooManyLearners,
 	etcdserver.ErrNotEnoughStartedMembers: rpctypes.ErrMemberNotEnoughStarted,
 
 	mvcc.ErrCompacted:             rpctypes.ErrGRPCCompacted,