MDEV-9690 concurrent queries with virtual columns crash in temporal code

Item_func_le included Arg_comparator. Arg_comparator remembered
the current_thd during fix_fields and used that value during
execution to allocate Item_cache in get_datetime_value().
But for vcols fix_fields and val_int can happen in different threads.

Same bug for Item_func_in using in_datetime or cmp_item_datetime,
both also remembered current_thd at fix_fields() to use it later
for get_datetime_value().

As a fix, these objects no longer remember the current_thd,
and get_datetime_value() uses current_thd at run time. This
should not increase the number of current_thd calls much, as
Item_cache is created only once anyway.

2 files changed, 16 insertions, 18 deletions

diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 3bd0b5b3fa2..46566206094 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -777,7 +777,7 @@ int Arg_comparator::set_cmp_func(Item_result_field *owner_arg,
                                         Item **a1, Item **a2,
                                         Item_result type)
 {
-  thd= current_thd;
+  THD *thd= current_thd;
   owner= owner_arg;
   set_null= set_null && owner_arg;
   a= a1;
@@ -846,7 +846,6 @@ Item** Arg_comparator::cache_converted_constant(THD *thd_arg, Item **value,
 void Arg_comparator::set_datetime_cmp_func(Item_result_field *owner_arg,
                                            Item **a1, Item **b1)
 {
-  thd= current_thd;
   owner= owner_arg;
   a= a1;
   b= b1;
@@ -919,6 +918,9 @@ get_datetime_value(THD *thd, Item ***item_arg, Item **cache_arg,
   if (cache_arg && item->const_item() &&
       !(item->type() == Item::CACHE_ITEM && item->cmp_type() == TIME_RESULT))
   {
+    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);
@@ -959,12 +961,12 @@ int Arg_comparator::compare_datetime()
     owner->null_value= 1;
 
   /* Get DATE/DATETIME/TIME value of the 'a' item. */
-  a_value= get_datetime_value(thd, &a, &a_cache, *b, &a_is_null);
+  a_value= get_datetime_value(0, &a, &a_cache, *b, &a_is_null);
   if (a_is_null)
     return -1;
 
   /* Get DATE/DATETIME/TIME value of the 'b' item. */
-  b_value= get_datetime_value(thd, &b, &b_cache, *a, &b_is_null);
+  b_value= get_datetime_value(0, &b, &b_cache, *a, &b_is_null);
   if (b_is_null)
     return -1;
 
@@ -982,10 +984,10 @@ int Arg_comparator::compare_e_datetime()
   longlong a_value, b_value;
 
   /* Get DATE/DATETIME/TIME value of the 'a' item. */
-  a_value= get_datetime_value(thd, &a, &a_cache, *b, &a_is_null);
+  a_value= get_datetime_value(0, &a, &a_cache, *b, &a_is_null);
 
   /* Get DATE/DATETIME/TIME value of the 'b' item. */
-  b_value= get_datetime_value(thd, &b, &b_cache, *a, &b_is_null);
+  b_value= get_datetime_value(0, &b, &b_cache, *a, &b_is_null);
   return a_is_null || b_is_null ? a_is_null == b_is_null
                                 : a_value == b_value;
 }
@@ -3600,7 +3602,7 @@ void in_datetime::set(uint pos,Item *item)
   bool is_null;
   struct packed_longlong *buff= &((packed_longlong*) base)[pos];
 
-  buff->val= get_datetime_value(thd, &tmp_item, 0, warn_item, &is_null);
+  buff->val= get_datetime_value(0, &tmp_item, 0, warn_item, &is_null);
   buff->unsigned_flag= 1L;
 }
 
@@ -3608,7 +3610,7 @@ uchar *in_datetime::get_value(Item *item)
 {
   bool is_null;
   Item **tmp_item= lval_cache ? &lval_cache : &item;
-  tmp.val= get_datetime_value(thd, &tmp_item, &lval_cache, warn_item, &is_null);
+  tmp.val= get_datetime_value(0, &tmp_item, &lval_cache, warn_item, &is_null);
   if (item->null_value)
     return 0;
   tmp.unsigned_flag= 1L;
@@ -3852,7 +3854,7 @@ void cmp_item_datetime::store_value(Item *item)
 {
   bool is_null;
   Item **tmp_item= lval_cache ? &lval_cache : &item;
-  value= get_datetime_value(thd, &tmp_item, &lval_cache, warn_item, &is_null);
+  value= get_datetime_value(0, &tmp_item, &lval_cache, warn_item, &is_null);
 }
 
 
@@ -3861,7 +3863,7 @@ int cmp_item_datetime::cmp(Item *arg)
   bool is_null;
   Item **tmp_item= &arg;
   return value !=
-    get_datetime_value(thd, &tmp_item, 0, warn_item, &is_null);
+    get_datetime_value(0, &tmp_item, 0, warn_item, &is_null);
 }
 
 
diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index 0194f9cd0e0..a8befa47b95 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -44,7 +44,6 @@ class Arg_comparator: public Sql_alloc
   Arg_comparator *comparators;   // used only for compare_row()
   double precision;
   /* Fields used in DATE/DATETIME comparison. */
-  THD *thd;
   Item *a_cache, *b_cache;         // Cached values of a and b items
                                    //   when one of arguments is NULL.
 public:
@@ -52,10 +51,10 @@ public:
   /* Allow owner function to use string buffers. */
   String value1, value2;
 
-  Arg_comparator():  set_null(TRUE), comparators(0), thd(0),
+  Arg_comparator():  set_null(TRUE), comparators(0),
     a_cache(0), b_cache(0) {};
   Arg_comparator(Item **a1, Item **a2): a(a1), b(a2),  set_null(TRUE),
-    comparators(0), thd(0), a_cache(0), b_cache(0) {};
+    comparators(0), a_cache(0), b_cache(0) {};
 
   int set_compare_func(Item_result_field *owner, Item_result type);
   inline int set_compare_func(Item_result_field *owner_arg)
@@ -944,15 +943,13 @@ public:
 class in_datetime :public in_longlong
 {
 public:
-  THD *thd;
   /* An item used to issue warnings. */
   Item *warn_item;
   /* Cache for the left item. */
   Item *lval_cache;
 
   in_datetime(Item *warn_item_arg, uint elements)
-    :in_longlong(elements), thd(current_thd), warn_item(warn_item_arg),
-     lval_cache(0) {};
+    :in_longlong(elements), warn_item(warn_item_arg), lval_cache(0) {};
   void set(uint pos,Item *item);
   uchar *get_value(Item *item);
   Item* create_item()
@@ -1112,14 +1109,13 @@ class cmp_item_datetime :public cmp_item
 {
   longlong value;
 public:
-  THD *thd;
   /* Item used for issuing warnings. */
   Item *warn_item;
   /* Cache for the left item. */
   Item *lval_cache;
 
   cmp_item_datetime(Item *warn_item_arg)
-    :thd(current_thd), warn_item(warn_item_arg), lval_cache(0) {}
+    : warn_item(warn_item_arg), lval_cache(0) {}
   void store_value(Item *item);
   int cmp(Item *arg);
   int compare(cmp_item *ci);