bugfix: cmp_item_row::alloc_comparators() allocated on the wrong arena

it used current_thd->alloc() and allocated on the thd's execution arena,
not on table->expr_arena.

Remove THD::arena_for_cached_items that is temporarily set in
update_virtual_fields(), and replaces THD arena in get_datetime_value().
Instead set THD arena to table->expr_arena for the whole  duration
of update_virtual_fields()

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

@@ -2,7 +2,9 @@ create table t1 (a datetime,
 # get_datetime_value
 b int as (a > 1),                             # Arg_comparator
 c int as (a in (1,2,3)),                      # in_datetime
-d int as ((a,a) in ((1,1),(2,1),(NULL,1)))    # cmp_item_datetime
+d int as ((a,a) in ((1,1),(2,1),(NULL,1))),   # cmp_item_datetime
+# other issues
+e int as ((a,1) in ((1,1),(2,1),(NULL,1)))    # cmp_item_row::alloc_comparators()
 );
 Warnings:
 Warning	1292	Incorrect datetime value: '1'
@@ -14,7 +16,8 @@ t1	CREATE TABLE `t1` (
   `a` datetime DEFAULT NULL,
   `b` int(11) AS (a > 1) VIRTUAL,
   `c` int(11) AS (a in (1,2,3)) VIRTUAL,
-  `d` int(11) AS ((a,a) in ((1,1),(2,1),(NULL,1))) VIRTUAL
+  `d` int(11) AS ((a,a) in ((1,1),(2,1),(NULL,1))) VIRTUAL,
+  `e` int(11) AS ((a,1) in ((1,1),(2,1),(NULL,1))) VIRTUAL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 Warnings:
 Warning	1292	Incorrect datetime value: '1'
@@ -22,18 +25,22 @@ Warning	1292	Incorrect datetime value: '2'
 Warning	1292	Incorrect datetime value: '3'
 insert t1 (a) values ('2010-10-10 10:10:10');
 select * from t1;
-a	b	c	d
-2010-10-10 10:10:10	1	0	0
+a	b	c	d	e
+2010-10-10 10:10:10	1	0	0	NULL
 Warnings:
 Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '2'
 Warning	1292	Incorrect datetime value: '1'
+Warning	1292	Incorrect datetime value: '1'
+Warning	1292	Incorrect datetime value: '2'
 select * from t1;
-a	b	c	d
-2010-10-10 10:10:10	1	0	0
+a	b	c	d	e
+2010-10-10 10:10:10	1	0	0	NULL
 Warnings:
 Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '2'
 Warning	1292	Incorrect datetime value: '1'
+Warning	1292	Incorrect datetime value: '1'
+Warning	1292	Incorrect datetime value: '2'
 drop table t1;

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

@@ -10,7 +10,9 @@ create table t1 (a datetime,
   # get_datetime_value
   b int as (a > 1),                             # Arg_comparator
   c int as (a in (1,2,3)),                      # in_datetime
-  d int as ((a,a) in ((1,1),(2,1),(NULL,1)))    # cmp_item_datetime
+  d int as ((a,a) in ((1,1),(2,1),(NULL,1))),   # cmp_item_datetime
+  # other issues
+  e int as ((a,1) in ((1,1),(2,1),(NULL,1)))    # cmp_item_row::alloc_comparators()
 );
 show create table t1;
 connect con1, localhost, root;

sql/item_cmpfunc.cc

@@ -921,12 +921,7 @@ get_datetime_value(THD *thd, Item ***item_arg, Item **cache_arg,
     if (!thd)
       thd= current_thd;
 
-    Query_arena backup;
-    Query_arena *save_arena= thd->switch_to_arena_for_cached_items(&backup);
     Item_cache_temporal *cache= new Item_cache_temporal(f_type);
-    if (save_arena)
-      thd->set_query_arena(save_arena);
-
     cache->store_packed(value, item);
     *cache_arg= cache;
     *item_arg= cache_arg;

sql/sql_class.cc

@@ -957,7 +957,6 @@ THD::THD()
 
   m_internal_handler= NULL;
   m_binlog_invoker= FALSE;
-  arena_for_cached_items= 0;
   memset(&invoker_user, 0, sizeof(invoker_user));
   memset(&invoker_host, 0, sizeof(invoker_host));
   prepare_derived_at_open= FALSE;

sql/sql_class.h

@@ -3102,26 +3102,7 @@ class THD :public Statement,
     }
   }
 
-private:
-  /* 
-    This reference points to the table arena when the expression
-    for a virtual column is being evaluated
-  */ 
-  Query_arena *arena_for_cached_items;
-
 public:
-  void reset_arena_for_cached_items(Query_arena *new_arena)
-  {
-    arena_for_cached_items= new_arena;
-  }
-  Query_arena *switch_to_arena_for_cached_items(Query_arena *backup)
-  {
-    if (!arena_for_cached_items)
-      return 0;
-    set_n_backup_active_arena(arena_for_cached_items, backup);
-    return backup;
-  }
-
   void clear_wakeup_ready() { wakeup_ready= false; }
   /*
     Sleep waiting for others to wake us up with signal_wakeup_ready().

sql/table.cc

@@ -6575,7 +6575,9 @@ int update_virtual_fields(THD *thd, TABLE *table,
   int error __attribute__ ((unused))= 0;
   DBUG_ASSERT(table && table->vfield);
 
-  thd->reset_arena_for_cached_items(table->expr_arena);
+  Query_arena backup_arena;
+  thd->set_n_backup_active_arena(table->expr_arena, &backup_arena);
+
   /* Iterate over virtual fields in the table */
   for (vfield_ptr= table->vfield; *vfield_ptr; vfield_ptr++)
   {
@@ -6593,7 +6595,7 @@ int update_virtual_fields(THD *thd, TABLE *table,
       DBUG_PRINT("info", ("field '%s' - skipped", vfield->field_name));
     }
   }
-  thd->reset_arena_for_cached_items(0);
+  thd->restore_active_arena(table->expr_arena, &backup_arena);
   DBUG_RETURN(0);
 }