lightning: fix length check may be skipped for first line

[joechenrh]

What problem does this PR solve?

Issue Number: close #61873

Problem Summary:

What changed and how does it work?

#58592 introduced max row length check for CSV, but we should enable parser.beginRowLenCheck() after read column headers. Otherwise, the first row after reading the column can skip the row length check.

tidb/pkg/lightning/mydump/csv_parser.go

Lines 631 to 647 in 465b166

	func (parser *CSVParser) ReadRow() error {
	parser.beginRowLenCheck()
	defer parser.endRowLenCheck()
	row := &parser.lastRow
	row.Length = 0
	row.RowID++
	// skip the header first
	if parser.shouldParseHeader {
	err := parser.ReadColumns()
	if err != nil {
	return errors.Trace(err)
	}
	parser.shouldParseHeader = false
	}
	fields, err := parser.readRecord(parser.lastRecord)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Create a file with 20 fields, each field is 12MiB, total row size is 240MiB. Import using lightning with header set to true

[mydumper.csv]
header = true

tidb lightning encountered error: [Lightning:PreCheck:ErrCheckDataSource]check data source error: in file offset 0: size of row cannot exceed the max value of txn-entry-size-limit

Before this PR:

Verbose debug logs will be written to /var/folders/9z/p3_qpprn0y3dtqs0kcsqtctc0000gn/T/lightning.log.2025-06-23T13.57.32+0800                                                                  
                                                                                                                                                                                               
tidb lightning exit successfully 

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.2646%. Comparing base (465b166) to head (c8a94ae).
Report is 22 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61874        +/-   ##
================================================
+ Coverage   73.0213%   74.2646%   +1.2432%     
================================================
  Files          1735       1765        +30     
  Lines        481288     495427     +14139     
================================================
+ Hits         351443     367927     +16484     
+ Misses       108298     105309      -2989     
- Partials      21547      22191       +644     

Flag	Coverage Δ
integration	45.5590% <100.0000%> (?)
unit	72.9499% <100.0000%> (+0.6800%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	47.0140% <ø> (+0.4411%)	⬆️

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[joechenrh]

/cc @lance6716 @D3Hunter @GMHDBJD

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, lance6716

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-06-23 09:41:24.639895521 +0000 UTC m=+696737.363074500: ☑️ agreed by lance6716.
• 2025-06-24 02:50:29.477936068 +0000 UTC m=+758482.201115046: ☑️ agreed by D3Hunter.