lightning: not set pos to end if fail to ReadUntil (pingcap#40232)

* fix: not set pos to end

* add it

* fix: return to prevPos

* fix: return err

* fix

* fix

* fix it

* fix

* fix: not exit when err

Co-authored-by: Zak Zhao <57036248+joccau@users.noreply.github.com>

br/pkg/lightning/mydump/csv_parser.go

@@ -447,9 +447,18 @@ outside:
 
 func (parser *CSVParser) readQuotedField() error {
 	for {
+		prevPos := parser.pos
 		content, terminator, err := parser.readUntil(&parser.quoteByteSet)
-		err = parser.replaceEOF(err, errUnterminatedQuotedField)
 		if err != nil {
+			if errors.Cause(err) == io.EOF {
+				// return the position of quote to the caller.
+				// because we return an error here, the parser won't
+				// use the `pos` again, so it's safe to modify it here.
+				parser.pos = prevPos - 1
+				// set buf to parser.buf in order to print err log
+				parser.buf = content
+				err = parser.replaceEOF(err, errUnterminatedQuotedField)
+			}
 			return err
 		}
 		parser.recordBuffer = append(parser.recordBuffer, content...)

br/tests/lightning_csv/errData/db-schema-create.sql

@@ -0,0 +1 @@
+create database if not exists db;

br/tests/lightning_csv/errData/db.test-schema.sql

@@ -0,0 +1 @@
+create table test(a int primary key, b int, c int, d int);

br/tests/lightning_csv/errData/db.test.1.csv

@@ -0,0 +1,3 @@
+1,2,3,4
+2,10,4,5
+1111,",7,8

br/tests/lightning_csv/run.sh

@@ -41,3 +41,11 @@ for BACKEND in tidb local; do
   check_not_contains 'id:'
 
 done
+
+set +e
+run_lightning --backend local -d "tests/$TEST_NAME/errData" --log-file "$TEST_DIR/lightning-err.log" 2>/dev/null
+set -e
+# err content presented
+grep ",7,8" "$TEST_DIR/lightning-err.log"
+# pos should not set to end
+grep "[\"syntax error\"] [pos=22]" "$TEST_DIR/lightning-err.log"