*: use txn for saving timestamp (#6199)

ref #5895

Signed-off-by: Ryan Leung <rleungx@gmail.com>

Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

pkg/storage/endpoint/tso.go

@@ -15,10 +15,12 @@
 package endpoint
 
 import (
+	"context"
 	"strings"
 	"time"
 
 	"github.com/pingcap/log"
+	"github.com/tikv/pd/pkg/storage/kv"
 	"github.com/tikv/pd/pkg/utils/typeutil"
 	"go.etcd.io/etcd/clientv3"
 	"go.uber.org/zap"
@@ -65,6 +67,24 @@ func (se *StorageEndpoint) LoadTimestamp(prefix string) (time.Time, error) {
 
 // SaveTimestamp saves the timestamp to the storage.
 func (se *StorageEndpoint) SaveTimestamp(key string, ts time.Time) error {
-	data := typeutil.Uint64ToBytes(uint64(ts.UnixNano()))
-	return se.Save(key, string(data))
+	return se.RunInTxn(context.Background(), func(txn kv.Txn) error {
+		value, err := txn.Load(key)
+		if err != nil {
+			return err
+		}
+
+		previousTS := typeutil.ZeroTime
+		if value != "" {
+			previousTS, err = typeutil.ParseTimestamp([]byte(value))
+			if err != nil {
+				log.Error("parse timestamp failed", zap.String("key", key), zap.String("value", value), zap.Error(err))
+				return err
+			}
+		}
+		if previousTS != typeutil.ZeroTime && typeutil.SubRealTimeByWallClock(ts, previousTS) <= 0 {
+			return nil
+		}
+		data := typeutil.Uint64ToBytes(uint64(ts.UnixNano()))
+		return txn.Save(key, string(data))
+	})
 }

pkg/storage/storage_tso_test.go

@@ -91,3 +91,34 @@ func TestGlobalLocalTimestamp(t *testing.T) {
 	re.NoError(err)
 	re.Equal(localTS1, ts)
 }
+
+func TestTimestampTxn(t *testing.T) {
+	re := require.New(t)
+
+	cfg := etcdutil.NewTestSingleConfig(t)
+	etcd, err := embed.StartEtcd(cfg)
+	re.NoError(err)
+	defer etcd.Close()
+
+	ep := cfg.LCUrls[0].String()
+	client, err := clientv3.New(clientv3.Config{
+		Endpoints: []string{ep},
+	})
+	re.NoError(err)
+	rootPath := path.Join("/pd", strconv.FormatUint(100, 10))
+	storage := NewStorageWithEtcdBackend(client, rootPath)
+
+	timestampKey := "timestamp"
+
+	globalTS1 := time.Now().Round(0)
+	err = storage.SaveTimestamp(timestampKey, globalTS1)
+	re.NoError(err)
+
+	globalTS2 := globalTS1.Add(-time.Millisecond).Round(0)
+	err = storage.SaveTimestamp(timestampKey, globalTS2)
+	re.NoError(err)
+
+	ts, err := storage.LoadTimestamp("")
+	re.NoError(err)
+	re.Equal(globalTS1, ts)
+}