etcdserver: add version enforcement when setting cluster version

etcdserver/cluster.go

@@ -222,6 +222,7 @@ func (c *cluster) SetStore(st store.Store) { c.store = st }
 func (c *cluster) Recover() {
 	c.members, c.removed = membersFromStore(c.store)
 	c.version = clusterVersionFromStore(c.store)
+	MustDetectDowngrade(c.version)
 }
 
 // ValidateConfigurationChange takes a proposed ConfChange and
@@ -364,6 +365,7 @@ func (c *cluster) SetVersion(ver *semver.Version) {
 		plog.Noticef("set the initial cluster version to %v", version.Cluster(ver.String()))
 	}
 	c.version = ver
+	MustDetectDowngrade(c.version)
 }
 
 func membersFromStore(st store.Store) (map[types.ID]*Member, map[types.ID]bool) {

etcdserver/cluster_util.go

@@ -252,3 +252,12 @@ func getVersion(m *Member, tr *http.Transport) (*version.Versions, error) {
 	}
 	return nil, err
 }
+
+func MustDetectDowngrade(cv *semver.Version) {
+	lv := semver.Must(semver.NewVersion(version.Version))
+	// only keep major.minor version for comparison against cluster version
+	lv = &semver.Version{Major: lv.Major, Minor: lv.Minor}
+	if cv != nil && lv.LessThan(*cv) {
+		plog.Fatalf("cluster cannot be downgraded (current version: %s is lower than determined cluster version: %s).", version.Version, version.Cluster(cv.String()))
+	}
+}