MDEV-30926 Segfault after MyISAM repair of vcol-indexed table

mi_repair_by_sort() allocates record and creates index which does
compute_vcols() which does move_fields() into allocated record but
does not restore fields back. Then record is freed by
mi_repair_by_sort() and the table fields still point to it.

The fix restores back field pointers in compute_vcols().

mysql-test/suite/vcol/r/vcol_keys_myisam.result

@@ -455,3 +455,19 @@ ERROR 21S01: Column count doesn't match value count at row 1
 drop table t1;
 set sql_mode= @old_mode;
 set myisam_repair_threads= @old_myisam_repair_threads;
+#
+# MDEV-30926 Segfault after MyISAM repair of vcol-indexed table
+#
+set sql_mode='';
+create table t (a int generated always as (1) virtual, key(a)) engine=myisam;
+insert into t select 1;
+Warnings:
+Warning	1906	The value specified for generated column 'a' in table 't' has been ignored
+select * from non_existing_tbl;
+ERROR 42S02: Table 'test.non_existing_tbl' doesn't exist
+select * from t;
+a
+1
+show open tables like '';
+Database	Table	In_use	Name_locked
+drop table t;

mysql-test/suite/vcol/t/vcol_keys_myisam.test

@@ -342,3 +342,15 @@ insert into t1 select 'qux';
 drop table t1;
 set sql_mode= @old_mode;
 set myisam_repair_threads= @old_myisam_repair_threads;
+
+--echo #
+--echo # MDEV-30926 Segfault after MyISAM repair of vcol-indexed table
+--echo #
+set sql_mode='';
+create table t (a int generated always as (1) virtual, key(a)) engine=myisam;
+insert into t select 1;
+--error ER_NO_SUCH_TABLE
+select * from non_existing_tbl;
+select * from t;
+show open tables like '';
+drop table t;

storage/myisam/ha_myisam.cc

@@ -724,12 +724,14 @@ static int compute_vcols(MI_INFO *info, uchar *record, int keynum)
   /* This mutex is needed for parallel repair */
   mysql_mutex_lock(&info->s->intern_lock);
   TABLE *table= (TABLE*)(info->external_ref);
-  table->move_fields(table->field, record, table->field[0]->record_ptr());
+  uchar *old_record= table->field[0]->record_ptr();
+  table->move_fields(table->field, record, old_record);
   if (keynum == -1) // update all vcols
   {
     int error= table->update_virtual_fields(table->file, VCOL_UPDATE_FOR_READ);
     if (table->update_virtual_fields(table->file, VCOL_UPDATE_INDEXED))
       error= 1;
+    table->move_fields(table->field, old_record, record);
     mysql_mutex_unlock(&info->s->intern_lock);
     return error;
   }
@@ -742,6 +744,7 @@ static int compute_vcols(MI_INFO *info, uchar *record, int keynum)
     if (f->vcol_info && !f->vcol_info->stored_in_db)
       table->update_virtual_field(f, false);
   }
+  table->move_fields(table->field, old_record, record);
   mysql_mutex_unlock(&info->s->intern_lock);
   return 0;
 }