Update apply workflow to process r.DowngradeVersionTest request

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

Author: ahrtr

server/etcdserver/apply/apply.go

@@ -447,6 +447,10 @@ func (a *applierMembership) DowngradeInfoSet(r *membershippb.DowngradeInfoSetReq
 	a.cluster.SetDowngradeInfo(&d, shouldApplyV3)
 }
 
+func (a *applierMembership) DowngradeVersionTest(r *membershippb.DowngradeVersionTest, shouldApplyV3 membership.ShouldApplyV3) {
+	// do nothing
+}
+
 type quotaApplierV3 struct {
 	applierV3
 	q serverstorage.Quota

server/etcdserver/server.go

@@ -2006,7 +2006,7 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry, shouldApplyV3 membership.
 }
 
 func (s *EtcdServer) applyInternalRaftRequest(r *pb.InternalRaftRequest, shouldApplyV3 membership.ShouldApplyV3) *apply.Result {
-	if r.ClusterVersionSet == nil && r.ClusterMemberAttrSet == nil && r.DowngradeInfoSet == nil {
+	if r.ClusterVersionSet == nil && r.ClusterMemberAttrSet == nil && r.DowngradeInfoSet == nil && r.DowngradeVersionTest == nil {
 		if !shouldApplyV3 {
 			return nil
 		}
@@ -2029,6 +2029,9 @@ func (s *EtcdServer) applyInternalRaftRequest(r *pb.InternalRaftRequest, shouldA
 	case r.DowngradeInfoSet != nil:
 		op = "DowngradeInfoSet" // Implemented in 3.5.x
 		membershipApplier.DowngradeInfoSet(r.DowngradeInfoSet, shouldApplyV3)
+	case r.DowngradeVersionTest != nil:
+		op = "DowngradeVersionTest"
+		membershipApplier.DowngradeVersionTest(r.DowngradeVersionTest, shouldApplyV3)
 	default:
 		s.lg.Panic("not implemented apply", zap.Stringer("raft-request", r))
 		return nil