Moving a change_list related methods from THD to Item_change_list

1. Moving the following methods from THD to Item_change_list:
   nocheck_register_item_tree_change()
   check_and_register_item_tree_change()
   rollback_item_tree_changes()
   as they work only with the "change_list" member and don't
   require anything else from THD.
2. Deriving THD from Item_change_list

This change will help to fix "MDEV-14603 signal 11 with short stacktrace" easier.

Author: Alexander Barkov

sql/sp_head.cc

@@ -1249,7 +1249,7 @@ sp_head::execute(THD *thd, bool merge_da_on_success)
     We should also save Item tree change list to avoid rollback something
     too early in the calling query.
   */
-  thd->change_list.move_elements_to(&old_change_list);
+  thd->Item_change_list::move_elements_to(&old_change_list);
   /*
     Cursors will use thd->packet, so they may corrupt data which was prepared
     for sending by upper level. OTOH cursors in the same routine can share this
@@ -1389,8 +1389,8 @@ sp_head::execute(THD *thd, bool merge_da_on_success)
   /* Restore all saved */
   thd->server_status= (thd->server_status & ~status_backup_mask) | old_server_status;
   old_packet.swap(thd->packet);
-  DBUG_ASSERT(thd->change_list.is_empty());
-  old_change_list.move_elements_to(&thd->change_list);
+  DBUG_ASSERT(thd->Item_change_list::is_empty());
+  old_change_list.move_elements_to(thd);
   thd->lex= old_lex;
   thd->set_query_id(old_query_id);
   DBUG_ASSERT(!thd->derived_tables);
@@ -2976,7 +2976,7 @@ sp_lex_keeper::reset_lex_and_exec_core(THD *thd, uint *nextp,
   bool parent_modified_non_trans_table= thd->transaction.stmt.modified_non_trans_table;
   thd->transaction.stmt.modified_non_trans_table= FALSE;
   DBUG_ASSERT(!thd->derived_tables);
-  DBUG_ASSERT(thd->change_list.is_empty());
+  DBUG_ASSERT(thd->Item_change_list::is_empty());
   /*
     Use our own lex.
     We should not save old value since it is saved/restored in

sql/sql_class.cc

@@ -2605,8 +2605,10 @@ struct Item_change_record: public ilink
   thd->mem_root (due to possible set_n_backup_active_arena called for thd).
 */
 
-void THD::nocheck_register_item_tree_change(Item **place, Item *old_value,
-                                            MEM_ROOT *runtime_memroot)
+void
+Item_change_list::nocheck_register_item_tree_change(Item **place,
+                                                    Item *old_value,
+                                                    MEM_ROOT *runtime_memroot)
 {
   Item_change_record *change;
   DBUG_ENTER("THD::nocheck_register_item_tree_change");
@@ -2647,8 +2649,10 @@ void THD::nocheck_register_item_tree_change(Item **place, Item *old_value,
     changes to substitute the same reference at both locations L1 and L2.
 */
 
-void THD::check_and_register_item_tree_change(Item **place, Item **new_value,
-                                              MEM_ROOT *runtime_memroot)
+void
+Item_change_list::check_and_register_item_tree_change(Item **place,
+                                                      Item **new_value,
+                                                      MEM_ROOT *runtime_memroot)
 {
   Item_change_record *change;
   I_List_iterator<Item_change_record> it(change_list);
@@ -2663,7 +2667,7 @@ void THD::check_and_register_item_tree_change(Item **place, Item **new_value,
 }
 
 
-void THD::rollback_item_tree_changes()
+void Item_change_list::rollback_item_tree_changes()
 {
   I_List_iterator<Item_change_record> it(change_list);
   Item_change_record *change;

sql/sql_class.h

@@ -1279,7 +1279,21 @@ class Security_context {
 */
 
 struct Item_change_record;
-typedef I_List<Item_change_record> Item_change_list;
+class Item_change_list
+{
+  I_List<Item_change_record> change_list;
+public:
+  void nocheck_register_item_tree_change(Item **place, Item *old_value,
+                                         MEM_ROOT *runtime_memroot);
+  void check_and_register_item_tree_change(Item **place, Item **new_value,
+                                           MEM_ROOT *runtime_memroot);
+  void rollback_item_tree_changes();
+  void move_elements_to(Item_change_list *to)
+  {
+    change_list.move_elements_to(&to->change_list);
+  }
+  bool is_empty() { return change_list.is_empty(); }
+};
 
 
 /**
@@ -2040,6 +2054,14 @@ void dbug_serve_apcs(THD *thd, int n_calls);
 */
 
 class THD :public Statement,
+           /*
+             This is to track items changed during execution of a prepared
+             statement/stored procedure. It's created by
+             nocheck_register_item_tree_change() in memory root of THD,
+             and freed in rollback_item_tree_changes().
+             For conventional execution it's always empty.
+           */
+           public Item_change_list,
            public MDL_context_owner,
            public Open_tables_state
 {
@@ -2510,14 +2532,6 @@ class THD :public Statement,
 #ifdef SIGNAL_WITH_VIO_CLOSE
   Vio* active_vio;
 #endif
-  /*
-    This is to track items changed during execution of a prepared
-    statement/stored procedure. It's created by
-    nocheck_register_item_tree_change() in memory root of THD, and freed in
-    rollback_item_tree_changes(). For conventional execution it's always
-    empty.
-  */
-  Item_change_list change_list;
 
   /*
     A permanent memory area of the statement. For conventional
@@ -3599,11 +3613,6 @@ class THD :public Statement,
     */
     memcpy((char*) place, new_value, sizeof(*new_value));
   }
-  void nocheck_register_item_tree_change(Item **place, Item *old_value,
-                                         MEM_ROOT *runtime_memroot);
-  void check_and_register_item_tree_change(Item **place, Item **new_value,
-                                           MEM_ROOT *runtime_memroot);
-  void rollback_item_tree_changes();
 
   /*
     Cleanup statement parse state (parse tree, lex) and execution

sql/sql_parse.cc

@@ -7910,7 +7910,7 @@ void mysql_parse(THD *thd, char *rawbuf, uint length,
     sp_cache_enforce_limit(thd->sp_func_cache, stored_program_cache_size);
     thd->end_statement();
     thd->cleanup_after_query();
-    DBUG_ASSERT(thd->change_list.is_empty());
+    DBUG_ASSERT(thd->Item_change_list::is_empty());
   }
   else
   {

sql/sql_prepare.cc

@@ -3930,7 +3930,7 @@ bool Prepared_statement::prepare(const char *packet, uint packet_len)
     If called from a stored procedure, ensure that we won't rollback
     external changes when cleaning up after validation.
   */
-  DBUG_ASSERT(thd->change_list.is_empty());
+  DBUG_ASSERT(thd->Item_change_list::is_empty());
 
   /*
     Marker used to release metadata locks acquired while the prepared
@@ -4407,7 +4407,7 @@ Prepared_statement::execute_server_runnable(Server_runnable *server_runnable)
   bool error;
   Query_arena *save_stmt_arena= thd->stmt_arena;
   Item_change_list save_change_list;
-  thd->change_list.move_elements_to(&save_change_list);
+  thd->Item_change_list::move_elements_to(&save_change_list);
 
   state= STMT_CONVENTIONAL_EXECUTION;
 
@@ -4426,7 +4426,7 @@ Prepared_statement::execute_server_runnable(Server_runnable *server_runnable)
   thd->restore_backup_statement(this, &stmt_backup);
   thd->stmt_arena= save_stmt_arena;
 
-  save_change_list.move_elements_to(&thd->change_list);
+  save_change_list.move_elements_to(thd);
 
   /* Items and memory will freed in destructor */
 
@@ -4654,7 +4654,7 @@ bool Prepared_statement::execute(String *expanded_query, bool open_cursor)
     If the free_list is not empty, we'll wrongly free some externally
     allocated items when cleaning up after execution of this statement.
   */
-  DBUG_ASSERT(thd->change_list.is_empty());
+  DBUG_ASSERT(thd->Item_change_list::is_empty());
 
   /* 
    The only case where we should have items in the thd->free_list is