[release-3.7] Fix the costTxnReq ignores nested RequestTxn issue

server/storage/quota.go

@@ -149,11 +149,13 @@ func (b *BackendQuota) Cost(v any) int {
 func costPut(r *pb.PutRequest) int { return kvOverhead + len(r.Key) + len(r.Value) }
 
 func costTxnReq(u *pb.RequestOp) int {
-	r := u.GetRequestPut()
-	if r == nil {
-		return 0
+	if r := u.GetRequestPut(); r != nil {
+		return costPut(r)
+	}
+	if t := u.GetRequestTxn(); t != nil {
+		return costTxn(t)
 	}
-	return costPut(r)
+	return 0
 }
 
 func costTxn(r *pb.TxnRequest) int {

server/storage/quota_test.go

@@ -0,0 +1,80 @@
+// Copyright 2026 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	pb "go.etcd.io/etcd/api/v3/etcdserverpb"
+)
+
+func TestCostTxn(t *testing.T) {
+	putOp := func(key, value string) *pb.RequestOp {
+		return &pb.RequestOp{
+			Request: &pb.RequestOp_RequestPut{
+				RequestPut: &pb.PutRequest{Key: []byte(key), Value: []byte(value)},
+			},
+		}
+	}
+	txnOp := func(txn *pb.TxnRequest) *pb.RequestOp {
+		return &pb.RequestOp{
+			Request: &pb.RequestOp_RequestTxn{RequestTxn: txn},
+		}
+	}
+
+	tests := []struct {
+		name string
+		req  *pb.TxnRequest
+		want int
+	}{
+		{
+			name: "flat put",
+			req: &pb.TxnRequest{
+				Success: []*pb.RequestOp{putOp("foo", "bar")},
+			},
+			want: costPut(&pb.PutRequest{Key: []byte("foo"), Value: []byte("bar")}),
+		},
+		{
+			name: "nested txn put in success branch must be counted",
+			req: &pb.TxnRequest{
+				Success: []*pb.RequestOp{
+					txnOp(&pb.TxnRequest{
+						Success: []*pb.RequestOp{putOp("nested-key", "nested-value")},
+					}),
+				},
+			},
+			want: costPut(&pb.PutRequest{Key: []byte("nested-key"), Value: []byte("nested-value")}),
+		},
+		{
+			name: "nested txn put in failure branch must be counted",
+			req: &pb.TxnRequest{
+				Failure: []*pb.RequestOp{
+					txnOp(&pb.TxnRequest{
+						Failure: []*pb.RequestOp{putOp("nested-key", "nested-value")},
+					}),
+				},
+			},
+			want: costPut(&pb.PutRequest{Key: []byte("nested-key"), Value: []byte("nested-value")}),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, costTxn(tt.req))
+		})
+	}
+}