Allow updateClusterVersion when downgrading from 3.5.

Signed-off-by: Siyuan Zhang <sizhang@google.com>

etcdserver/api/membership/cluster.go

@@ -789,6 +789,15 @@ func ValidateClusterAndAssignIDs(lg *zap.Logger, local *RaftCluster, existing *R
 	return nil
 }
 
+// isValidDowngrade verifies whether the cluster can be downgraded from verFrom to verTo
+func isValidDowngrade(verFrom *semver.Version, verTo *semver.Version) bool {
+	allowedDowngradeVersion := semver.Version{
+		Major: verFrom.Major,
+		Minor: verFrom.Minor - 1,
+	}
+	return verTo.Equal(allowedDowngradeVersion)
+}
+
 func mustDetectDowngrade(lg *zap.Logger, cv *semver.Version, nextClusterVersionCompatible bool) {
 	err := detectDowngrade(cv, nextClusterVersionCompatible)
 	if err != nil {
@@ -825,6 +834,22 @@ func detectDowngrade(cv *semver.Version, nextClusterVersionCompatible bool) erro
 	return nil
 }
 
+// IsValidClusterVersionChange checks the two scenario when version is valid to change:
+// 1. Downgrade: cluster version is 1 minor version higher than local version,
+// cluster version should change.
+// 2. Cluster start: when not all members version are available, cluster version
+// is set to MinVersion(3.0), when all members are at higher version, cluster version
+// is lower than minimal server version, cluster version should change
+func IsValidClusterVersionChange(verFrom *semver.Version, verTo *semver.Version, nextClusterVersionCompatible bool) bool {
+	verFrom = &semver.Version{Major: verFrom.Major, Minor: verFrom.Minor}
+	verTo = &semver.Version{Major: verTo.Major, Minor: verTo.Minor}
+
+	if (nextClusterVersionCompatible && isValidDowngrade(verFrom, verTo)) || (verFrom.Major == verTo.Major && verFrom.LessThan(*verTo)) {
+		return true
+	}
+	return false
+}
+
 // IsLocalMemberLearner returns if the local member is raft learner
 func (c *RaftCluster) IsLocalMemberLearner() bool {
 	c.Lock()

etcdserver/api/membership/cluster_test.go

@@ -22,6 +22,7 @@ import (
 	"testing"
 
 	"github.com/coreos/go-semver/semver"
+	"github.com/stretchr/testify/assert"
 	"go.etcd.io/etcd/etcdserver/api/v2store"
 	"go.etcd.io/etcd/pkg/mock/mockstore"
 	"go.etcd.io/etcd/pkg/testutil"
@@ -948,6 +949,83 @@ func TestIsReadyToPromoteMember(t *testing.T) {
 	}
 }
 
+func TestIsVersionChangable(t *testing.T) {
+	tests := []struct {
+		name                         string
+		verFrom                      string
+		verTo                        string
+		nextClusterVersionCompatible bool
+		expectedResult               bool
+	}{
+		{
+			name:                         "When local version is one minor lower than cluster version in downgrade mode",
+			verFrom:                      "3.5.0",
+			verTo:                        "3.4.0",
+			nextClusterVersionCompatible: true,
+			expectedResult:               true,
+		},
+		{
+			name:           "When local version is one minor lower than cluster version not in downgrade mode",
+			verFrom:        "3.5.0",
+			verTo:          "3.4.0",
+			expectedResult: false,
+		},
+		{
+			name:                         "When local version is one minor and one patch lower than cluster version in downgrade mode",
+			verFrom:                      "3.5.2",
+			verTo:                        "3.4.1",
+			nextClusterVersionCompatible: true,
+			expectedResult:               true,
+		},
+		{
+			name:           "When local version is one minor higher than cluster version",
+			verFrom:        "3.3.0",
+			verTo:          "3.4.0",
+			expectedResult: true,
+		},
+		{
+			name:           "When local version is two minor higher than cluster version",
+			verFrom:        "3.2.0",
+			verTo:          "3.4.0",
+			expectedResult: true,
+		},
+		{
+			name:           "When local version is one major higher than cluster version",
+			verFrom:        "2.4.0",
+			verTo:          "3.4.0",
+			expectedResult: false,
+		},
+		{
+			name:           "When local version is equal to cluster version",
+			verFrom:        "3.4.0",
+			verTo:          "3.4.0",
+			expectedResult: false,
+		},
+		{
+			name:           "When local version is one patch higher than cluster version",
+			verFrom:        "3.4.0",
+			verTo:          "3.4.1",
+			expectedResult: false,
+		},
+		{
+			name:                         "When local version is two minor lower than cluster version",
+			verFrom:                      "3.6.0",
+			verTo:                        "3.4.0",
+			nextClusterVersionCompatible: true,
+			expectedResult:               false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			verFrom := semver.Must(semver.NewVersion(tt.verFrom))
+			verTo := semver.Must(semver.NewVersion(tt.verTo))
+			ret := IsValidClusterVersionChange(verFrom, verTo, tt.nextClusterVersionCompatible)
+			assert.Equal(t, tt.expectedResult, ret)
+		})
+	}
+}
+
 func TestDetectDowngrade(t *testing.T) {
 	tests := []struct {
 		clusterVersion               string

etcdserver/server.go

@@ -2636,8 +2636,8 @@ func (s *EtcdServer) monitorVersions() {
 		}
 
 		// update cluster version only if the decided version is greater than
-		// the current cluster version
-		if v != nil && s.cluster.Version().LessThan(*v) {
+		// the current cluster version or it is a valid downgrade
+		if v != nil && membership.IsValidClusterVersionChange(s.cluster.Version(), v, s.Config().NextClusterVersionCompatible) {
 			s.goAttach(func() { s.updateClusterVersion(v.String()) })
 		}
 	}