Possible panic when restore a backup that enables cross table merge.

[overvenus]

PD can merge regions that belong to different tables. It is possible that a region's start key and end key have different table IDs. And it breaks BR restore's prerequisite. BR needs to be able to restore such regions.

br/pkg/restore/util.go

Lines 200 to 207 in bb4294d

	tableID := tablecodec.DecodeTableID(file.GetStartKey())
	tableEndID := tablecodec.DecodeTableID(file.GetEndKey())
	if tableID != tableEndID {
	log.Panic("key range spread between many files.",
	zap.String("file name", file.Name),
	zap.Stringer("start key", utils.WrapKey(file.GetStartKey())),
	zap.Stringer("end key", utils.WrapKey(file.GetEndKey())))
	}

Cc tikv/pd#3129

[3pointer]

even PD can merge two table in same the region, BR will backup two tables separately due to BackupRanges, so the each SST file will only contain one tableID.

[overvenus]

Right, thanks!