fix: backup will skip empty databases.

pkg/backup/client.go

@@ -268,6 +268,11 @@ func BuildBackupRangeAndSchema(
 		seqAlloc := autoid.NewAllocator(storage, dbInfo.ID, false, autoid.SequenceType)
 		randAlloc := autoid.NewAllocator(storage, dbInfo.ID, false, autoid.AutoRandomType)
 
+		if len(dbInfo.Tables) == 0 {
+			log.Warn("It's not necessary for backing up empty database",
+				zap.Stringer("db", dbInfo.Name))
+			continue
+		}
 		for _, tableInfo := range dbInfo.Tables {
 			if !tableFilter.MatchTable(dbInfo.Name.O, tableInfo.Name.O) {
 				// Skip tables other than the given table.

pkg/task/backup.go

@@ -5,6 +5,7 @@ package task
 import (
 	"context"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/pingcap/br/pkg/utils"
@@ -254,6 +255,9 @@ func RunBackup(c context.Context, g glue.Glue, cmdName string, cfg *BackupConfig
 		if err2 != nil {
 			return err2
 		}
+		pdAddress := strings.Join(cfg.PD, ",")
+		log.Warn("Nothing to backup, maybe connected to cluster for restoring",
+			zap.String("PD address", pdAddress))
 		return client.SaveBackupMeta(ctx, &backupMeta)
 	}