etcdserver: skip range requests in txn if the result is needless

If a server isn't serving txn requests from a client, the server
doesn't need the result of range requests in the txn.

This is a succeeding commit of
#5689

etcdserver/apply.go

@@ -788,3 +788,19 @@ func isGteRange(rangeEnd []byte) bool {
 func noSideEffect(r *pb.InternalRaftRequest) bool {
 	return r.Range != nil || r.AuthUserGet != nil || r.AuthRoleGet != nil
 }
+
+func removeNeedlessRangeReqs(txn *pb.TxnRequest) {
+	f := func(ops []*pb.RequestOp) {
+		for i := 0; i < len(ops); {
+			switch ops[i].Request.(type) {
+			case *pb.RequestOp_RequestRange:
+				ops = append(ops[:i], ops[i+1:]...)
+			default:
+				i++
+			}
+		}
+	}
+
+	f(txn.Success)
+	f(txn.Failure)
+}

etcdserver/server.go

@@ -1080,7 +1080,11 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
 	}
 
 	var ar *applyResult
-	if s.w.IsRegistered(id) || !noSideEffect(&raftReq) {
+	needResult := s.w.IsRegistered(id)
+	if needResult || !noSideEffect(&raftReq) {
+		if !needResult && raftReq.Txn != nil {
+			removeNeedlessRangeReqs(raftReq.Txn)
+		}
 		ar = s.applyV3.Apply(&raftReq)
 	}