lightning: fix oom when mem/cpu ratio is low (#43729) (#43747)

close #43728

br/pkg/lightning/backend/kv/sql2kv.go

@@ -354,6 +354,11 @@ func (kvcodec *tableKVEncoder) Encode(
 	_ string,
 	offset int64,
 ) (Row, error) {
+	// we ignore warnings when encoding rows now, but warnings uses the same memory as parser, since the input
+	// row []types.Datum share the same underlying buf, and when doing CastValue, we're using hack.String/hack.Slice.
+	// when generating error such as mysql.ErrDataOutOfRange, the data will be part of the error, causing the buf
+	// unable to release. So we truncate the warnings here.
+	defer kvcodec.se.vars.StmtCtx.TruncateWarnings(0)
 	cols := kvcodec.tbl.Cols()
 
 	var value types.Datum