dumpling: improve error hint when fail to set snapshot in the incorrect consistency

dumpling/export/dump.go

@@ -1575,7 +1575,7 @@ func setSessionParam(d *Dumper) error {
 				d.L().Info("cannot check whether TiDB has TiKV, will apply tidb_snapshot by default. This won't affect dump process", log.ShortError(err))
 			}
 			if conf.ServerInfo.HasTiKV {
-				sessionParam["tidb_snapshot"] = snapshot
+				sessionParam[snapshotVar] = snapshot
 			}
 		}
 	}

dumpling/export/dump_test.go

@@ -10,7 +10,9 @@ import (
 
 	"github.com/DATA-DOG/go-sqlmock"
 	"github.com/coreos/go-semver/semver"
+	"github.com/go-sql-driver/mysql"
 	"github.com/pingcap/errors"
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/tidb/br/pkg/version"
 	tcontext "github.com/pingcap/tidb/dumpling/context"
 	"github.com/pingcap/tidb/parser"
@@ -289,3 +291,64 @@ func TestSetDefaultSessionParams(t *testing.T) {
 		require.Equal(t, testCase.expectedParams, testCase.sessionParams)
 	}
 }
+
+func TestSetSessionParams(t *testing.T) {
+	// case 1: fail to set tidb_snapshot, should return error with hint
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, db.Close())
+	}()
+
+	mock.ExpectQuery("SELECT @@tidb_config").
+		WillReturnError(errors.New("mock error"))
+	mock.ExpectQuery("SELECT COUNT\\(1\\) as c FROM MYSQL.TiDB WHERE VARIABLE_NAME='tikv_gc_safe_point'").
+		WillReturnError(errors.New("mock error"))
+	tikvErr := &mysql.MySQLError{
+		Number:  1105,
+		Message: "can not get 'tikv_gc_safe_point'",
+	}
+	mock.ExpectExec("SET SESSION tidb_snapshot").
+		WillReturnError(tikvErr)
+
+	require.NoError(t, failpoint.Enable("github.com/pingcap/tidb/dumpling/export/SkipResetDB", "return(true)"))
+	defer failpoint.Disable("github.com/pingcap/tidb/dumpling/export/SkipResetDB=return(true)")
+
+	tctx, cancel := tcontext.Background().WithLogger(appLogger).WithCancel()
+	defer cancel()
+
+	conf := DefaultConfig()
+	conf.ServerInfo = version.ServerInfo{
+		ServerType: version.ServerTypeTiDB,
+		HasTiKV:    false,
+	}
+	conf.Snapshot = "439153276059648000"
+	conf.Consistency = ConsistencyTypeSnapshot
+	d := &Dumper{
+		tctx:      tctx,
+		conf:      conf,
+		cancelCtx: cancel,
+		dbHandle:  db,
+	}
+	err = setSessionParam(d)
+	require.ErrorContains(t, err, "consistency=none")
+
+	// case 2: fail to set other
+	conf.ServerInfo = version.ServerInfo{
+		ServerType: version.ServerTypeMySQL,
+		HasTiKV:    false,
+	}
+	conf.Snapshot = ""
+	conf.Consistency = ConsistencyTypeFlush
+	conf.SessionParams = map[string]interface{}{
+		"mock": "UTC",
+	}
+	d.dbHandle = db
+	mock.ExpectExec("SET SESSION mock").
+		WillReturnError(errors.New("Unknown system variable mock"))
+	mock.ExpectClose()
+	mock.ExpectClose()
+
+	err = setSessionParam(d)
+	require.NoError(t, err)
+}

dumpling/export/sql.go

@@ -29,6 +29,7 @@ import (
 
 const (
 	orderByTiDBRowID = "ORDER BY `_tidb_rowid`"
+	snapshotVar      = "tidb_snapshot"
 )
 
 type listTableType int
@@ -851,7 +852,9 @@ func resetDBWithSessionParams(tctx *tcontext.Context, db *sql.DB, cfg *mysql.Con
 		s := fmt.Sprintf("SET SESSION %s = ?", k)
 		_, err := db.ExecContext(tctx, s, pv)
 		if err != nil {
-			if isUnknownSystemVariableErr(err) {
+			if k == snapshotVar {
+				err = errors.Annotate(err, "fail to set snapshot for tidb, please set --consistency=none/--consistency=lock or fix snapshot problem")
+			} else if isUnknownSystemVariableErr(err) {
 				tctx.L().Info("session variable is not supported by db", zap.String("variable", k), zap.Reflect("value", v))
 				continue
 			}
@@ -876,6 +879,9 @@ func resetDBWithSessionParams(tctx *tcontext.Context, db *sql.DB, cfg *mysql.Con
 		}
 		cfg.Params[k] = s
 	}
+	failpoint.Inject("SkipResetDB", func(_ failpoint.Value) {
+		failpoint.Return(db, nil)
+	})
 
 	db.Close()
 	c, err := mysql.NewConnector(cfg)