code: func ValidateSnapshot(ctx sessionctx.Context, snapshotTS uint64) error {
  	safePointTS, err := GetGCSafePoint(ctx)
  	if err != nil {
  		return errors.Trace(err)
  	}
  	if safePointTS > snapshotTS {
  		return variable.ErrSnapshotTooOld.GenWithStackByArgs(model.TSConvert2Time(safePointTS).String())
  	}
  	return nil
  }
docs: // ValidateSnapshot checks that the newly set snapshot time is after GC safe point time.