lightning still fail at checksum stage when set `checksum = "optional"`

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set checksum = "optional" in configuration file, and checksum meets error [tikv:9005]Region is unavailable

2. What did you expect to see? (Required)

lightning import task is passed

3. What did you see instead (Required)

lightning import task is failed

4. What is your TiDB version? (Required)

[lance6716]

tidb/br/pkg/lightning/importer/table_import.go

Lines 1036 to 1047 in 7b1cf3b

	remoteChecksum, err = DoChecksum(ctx, tr.tableInfo)
	if err != nil {
	return false, err
	}
	err = tr.compareChecksum(remoteChecksum, localChecksum)
	// with post restore level 'optional', we will skip checksum error
	if rc.cfg.PostRestore.Checksum == config.OpLevelOptional {
	if err != nil {
	tr.logger.Warn("compare checksum failed, will skip this error and go on", log.ShortError(err))
	err = nil
	}
	}

We should let OpLevelOptional suppress the error of DoChecksum as well