MDEV-17823 Assertion failed when accessing indexed instantly added column

This assertion would fail when a secondary index record for an instantly added column was accessed. It is unclear to me why this code path is executed so rarely. I was unable to cover it even when using FORCE INDEX. row_sel_sec_rec_is_for_clust_rec(): Remove the assertion, and use the proper function rec_get_nth_cfield(). row_sel_store_mysql_field_func(): Simply use rec_get_nth_cfield() instead of duplicating its logic.

Author: dr-m

storage/innobase/row/row0sel.cc

@@ -254,9 +254,9 @@ row_sel_sec_rec_is_for_clust_rec(
 clust_field = static_cast<byte*>(vfield->data);
 } else {
 clust_pos = dict_col_get_clust_pos(col, clust_index);
-ut_ad(!rec_offs_nth_default(clust_offs, clust_pos));
-clust_field = rec_get_nth_field(
-clust_rec, clust_offs, clust_pos, &clust_len);
+clust_field = rec_get_nth_cfield(
+clust_rec, clust_index, clust_offs,
+clust_pos, &clust_len);
 }
 
 sec_field = rec_get_nth_field(sec_rec, sec_offs, i, &sec_len);
@@ -3010,17 +3010,7 @@ row_sel_store_mysql_field_func(
 } else {
 /* The field is stored in the index record, or
 in the metadata for instant ADD COLUMN. */
-
-if (rec_offs_nth_default(offsets, field_no)) {
-ut_ad(dict_index_is_clust(index));
-ut_ad(index->is_instant());
-const dict_index_t* clust_index
-= dict_table_get_first_index(prebuilt->table);
-ut_ad(index == clust_index);
-data = clust_index->instant_field_value(field_no,&len);
-} else {
-data = rec_get_nth_field(rec, offsets, field_no, &len);
-}
+data = rec_get_nth_cfield(rec, index, offsets, field_no, &len);
 
 if (len == UNIV_SQL_NULL) {
 /* MySQL assumes that the field for an SQL