MDEV-25292 gcol.gcol_bugfixes --ps fix

Related to MDEV-24176.

1. vcol_fix_expr() generates new tree changes:
Type_std_attributes::agg_item_set_converter() does change_item_tree().

The changes are allocated on expr_arena (via Vcol_expr_context as per
MDEV-24176).

2. vcol_cleanup_expr() doesn't remove these changes (can be a bug of
Type_std_attributes or per design).

3. Atomic CREATE OR REPLACE renames old table to backup
(finalize_atomic_replace()). It does that via
rename_table_and_triggers() and that closes table share and releases
expr_arena root. Hence now we have Item corpses in thd->change_list.

4. PS cleanup phase tries to rollback thd->change_list and accesses
already freed item corpses.

The fix saves and restores change_list on
vcol_fix_expr()/vcol_cleanup_expr().

mysql-test/main/create_or_replace.result

@@ -800,6 +800,12 @@ create or replace table t1 (old int);
 show create trigger a;
 ERROR HY000: Trigger does not exist
 drop table t1;
+# PS: check thd->change_list sanity
+create table t1 (a int not null, b char(10) as (concat('', dayname('2020-02-02'))))  collate utf8_bin;
+prepare stmt from 'insert into t1 (b) values (2)';
+create or replace table t1 (x int);
+drop table t1;
+drop prepare stmt;
 # Foreign keys
 create table t1 (x int primary key, y int) engine innodb;
 create table t2 (x int references t1(x)) engine innodb;

mysql-test/main/create_or_replace.test

@@ -602,6 +602,12 @@ create or replace table t1 (old int);
 --error ER_TRG_DOES_NOT_EXIST
 show create trigger a;
 drop table t1;
+--echo # PS: check thd->change_list sanity
+create table t1 (a int not null, b char(10) as (concat('', dayname('2020-02-02'))))  collate utf8_bin;
+prepare stmt from 'insert into t1 (b) values (2)';
+create or replace table t1 (x int);
+drop table t1;
+drop prepare stmt;
 --echo # Foreign keys
 --list_files $MYSQLD_DATADIR/test *sql*
 create table t1 (x int primary key, y int) engine innodb;

sql/table.cc

@@ -3723,6 +3723,13 @@ bool TABLE::vcol_fix_expr(THD *thd)
     return false;
   }
 
+  if (!thd->Item_change_list::is_empty())
+  {
+    DBUG_ASSERT(!saved_change_list);
+    saved_change_list= new Item_change_list;
+    thd->move_elements_to(saved_change_list);
+  }
+
   Vcol_expr_context expr_ctx(thd, this);
   if (expr_ctx.init())
     return true;
@@ -3743,7 +3750,12 @@ bool TABLE::vcol_fix_expr(THD *thd)
 bool TABLE::vcol_cleanup_expr(THD *thd)
 {
   if (vcol_refix_list.is_empty())
+  {
+    DBUG_ASSERT(!saved_change_list);
     return false;
+  }
+
+  thd->rollback_item_tree_changes();
 
   List_iterator<Virtual_column_info> it(vcol_refix_list);
   bool result= false;
@@ -3751,6 +3763,15 @@ bool TABLE::vcol_cleanup_expr(THD *thd)
   while (Virtual_column_info *vcol= it++)
     result|= vcol->cleanup_session_expr();
 
+  if (saved_change_list)
+  {
+    DBUG_ASSERT(!vcol_refix_list.is_empty());
+    DBUG_ASSERT(!saved_change_list->is_empty());
+    saved_change_list->move_elements_to(thd);
+    delete saved_change_list;
+    saved_change_list= NULL;
+  }
+
   DBUG_ASSERT(!result || thd->get_stmt_da()->is_error());
   return result;
 }

sql/table.h

@@ -41,6 +41,7 @@
 
 class Item;				/* Needed by ORDER */
 typedef Item (*Item_ptr);
+class Item_change_list;
 class Item_subselect;
 class Item_field;
 class GRANT_TABLE;
@@ -1511,6 +1512,7 @@ struct TABLE
   bool get_fields_in_item_tree;      /* Signal to fix_field */
   List<Virtual_column_info> vcol_refix_list;
 private:
+  Item_change_list *saved_change_list;
   bool m_needs_reopen;
   bool created;    /* For tmp tables. TRUE <=> tmp table was actually created.*/
 public: