etcdserver: restore KV snapshot when receiving snapshot

When a slow follower receives the snapshot sent from the leader, it
should rename the snapshot file to the default KV file path, and
restore KV snapshot.

Have tested it manually and it works pretty well.

etcdserver/server.go

@@ -493,6 +493,23 @@ func (s *EtcdServer) run() {
 						apply.snapshot.Metadata.Index, appliedi)
 				}
 
+				if s.cfg.V3demo {
+					if err := s.kv.Close(); err != nil {
+						plog.Panicf("close KV error: %v", err)
+					}
+					snapfn, err := s.r.raftStorage.snapStore.getSnapFilePath(apply.snapshot.Metadata.Index)
+					if err != nil {
+						plog.Panicf("get snapshot file path error: %v", err)
+					}
+					fn := path.Join(s.cfg.StorageDir(), databaseFilename)
+					if err := os.Rename(snapfn, fn); err != nil {
+						plog.Panicf("rename snapshot file error: %v", err)
+					}
+					s.kv = dstorage.New(fn)
+					if err := s.kv.Restore(); err != nil {
+						plog.Panicf("restore KV error: %v", err)
+					}
+				}
 				if err := s.store.Recovery(apply.snapshot.Data); err != nil {
 					plog.Panicf("recovery store error: %v", err)
 				}

etcdserver/server_test.go

@@ -836,6 +836,7 @@ func TestRecvSnapshot(t *testing.T) {
 	cl := newCluster("abc")
 	cl.SetStore(store.New())
 	s := &EtcdServer{
+		cfg: &ServerConfig{},
 		r: raftNode{
 			Node:        n,
 			transport:   &nopTransporter{},
@@ -871,6 +872,7 @@ func TestApplySnapshotAndCommittedEntries(t *testing.T) {
 	cl.SetStore(store.New())
 	storage := newRaftStorage()
 	s := &EtcdServer{
+		cfg: &ServerConfig{},
 		r: raftNode{
 			Node:        n,
 			storage:     &storageRecorder{},