MDEV-38140: InnoDB index corruption after UPDATE affecting virtual

columns

Issue:
- Purge thread attempts to purge a secondary index record that is not
  delete-marked.

Root Cause:
- When a secondary index includes a virtual column whose v_pos is
  greater than the number of fields in the clustered index record, the
  virtual column is incorrectly skipped while reading from the undo
  record.
- This leads the purge logic to incorrectly assume it is safe to purge
  the secondary index record.
- The code also confuses the nth virtual column with the nth stored
  column when writing ordering columns at the end of the undo record.

Fix:
- In trx_undo_update_rec_get_update(): Skip a virtual column only
  when v_pos == FIL_NULL, not when v_pos is greater than the number
  of fields.
- In trx_undo_page_report_modify(): Ensure ordering columns are
  written based on the correct stored-column positions, without
  confusing them with virtual-column positions.

Author: mariadb-TafzeelShams

mysql-test/suite/innodb/r/innodb-virtual-columns.result

@@ -351,3 +351,20 @@ SELECT * FROM t1;
 a	b	c
 1	foo	1
 DROP TABLE t1;
+CREATE TABLE t1 (
+id int primary key,value varchar(2),
+vcomplete int AS (cast(substr(value,1,1) as int)),
+va int AS (null),vb int AS (null),vc int AS (null),
+vresult char(1) AS (substr(value,2)),
+KEY key1(vresult,vcomplete)
+) ENGINE=InnoDB;
+BEGIN;
+INSERT INTO t1(id,value) VALUES (1,'0F');
+UPDATE t1 SET value = '1S';
+UPDATE t1 SET value = '0F';
+COMMIT;
+SET GLOBAL innodb_max_purge_lag_wait=0;
+SELECT * FROM t1;
+id	value	vcomplete	va	vb	vc	vresult
+1	0F	0	NULL	NULL	NULL	F
+DROP TABLE t1;

mysql-test/suite/innodb/t/innodb-virtual-columns.test

@@ -317,3 +317,26 @@ INSERT INTO t1 (a,b) VALUES (1,'foo');
 ALTER TABLE t1 ADD FULLTEXT KEY(b);
 SELECT * FROM t1;
 DROP TABLE t1;
+
+#
+# MDEV-38140 : InnoDB index corruption after UPDATE affecting virtual columns
+#
+
+CREATE TABLE t1 (
+  id int primary key,value varchar(2),
+  vcomplete int AS (cast(substr(value,1,1) as int)),
+  va int AS (null),vb int AS (null),vc int AS (null),
+  vresult char(1) AS (substr(value,2)),
+  KEY key1(vresult,vcomplete)
+) ENGINE=InnoDB;
+
+BEGIN;
+INSERT INTO t1(id,value) VALUES (1,'0F');
+UPDATE t1 SET value = '1S';
+UPDATE t1 SET value = '0F';
+COMMIT;
+
+SET GLOBAL innodb_max_purge_lag_wait=0;
+SELECT * FROM t1;
+
+DROP TABLE t1;

storage/innobase/trx/trx0rec.cc

@@ -1222,9 +1222,12 @@ trx_undo_page_report_modify(
 				columns that were updated. */
 
 				for (i = 0; i < update->n_fields; i++) {
+					const upd_field_t* fld =
+						upd_get_nth_field(update, i);
+					if (upd_fld_is_virtual_col(fld))
+						continue;
 					const ulint field_no
-						= upd_get_nth_field(update, i)
-						->field_no;
+						= fld->field_no;
 					if (field_no >= index->n_fields
 					    || dict_index_get_nth_field(
 						    index, field_no)->col
@@ -1524,7 +1527,7 @@ trx_undo_update_rec_get_update(
 				&field_no);
 			first_v_col = false;
 			/* This column could be dropped or no longer indexed */
-			if (field_no >= index->n_fields) {
+			if (field_no == FIL_NULL) {
 				/* Mark this is no longer needed */
 				upd_field->field_no = REC_MAX_N_FIELDS;