MDEV-24519: Server crashes in Charset::set_charset upon SELECT

The query causing the issue here has implicit grouping for we
have to produce one row with special values for the aggregates
(depending on each aggregate function), and NULL values for all
non-aggregate fields.

The subselect item where implicit grouping was being done,
null_value for the subselect item was not being set for
the case when the implicit grouping produces NULL values
for the items in the select list of the subquery.
This which was leading to the crash.

The fix would be to set the null_value when all the values
for the row column have NULL values.

Further changes are

1) etting null_value for Item_singlerow_subselect only
   after val_* functions have been called.
2) Introduced a parameter null_value_inside to Item_cache that
   would store be set to TRUE if any of the arguments of the
   Item_cache are null.

Reviewed And co-authored by Monty

mysql-test/r/subselect4.result

@@ -2694,4 +2694,31 @@ INSERT INTO t1 VALUES (1,1), (2,2), (3,3), (4,4), (5,5);
 SELECT a, b FROM t1 WHERE a IN (SELECT A.a FROM t1 A GROUP BY s.id);
 ERROR 42S22: Unknown column 's.id' in 'group statement'
 DROP TABLE t1;
+#
+# MDEV-24519: Server crashes in Charset::set_charset upon SELECT
+#
+CREATE TABLE t1 (a VARBINARY(8));
+INSERT INTO t1 VALUES ('foo'),('bar');
+CREATE TABLE t2 (b VARBINARY(8));
+EXPLAIN
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 'qux', 'qux') AND a = (SELECT MIN(b) FROM t2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 'qux', 'qux') AND a = (SELECT MIN(b) FROM t2);
+a
+DROP TABLE t1,t2;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b VARBINARY(8));
+EXPLAIN
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 1, 2) AND a = (SELECT MIN(b) FROM t2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 1, 2) AND a = (SELECT MIN(b) FROM t2);
+a
+DROP TABLE t1,t2;
 # End of 10.2 tests

mysql-test/t/subselect4.test

@@ -2212,4 +2212,28 @@ INSERT INTO t1 VALUES (1,1), (2,2), (3,3), (4,4), (5,5);
 SELECT a, b FROM t1 WHERE a IN (SELECT A.a FROM t1 A GROUP BY s.id);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-24519: Server crashes in Charset::set_charset upon SELECT
+--echo #
+
+CREATE TABLE t1 (a VARBINARY(8));
+INSERT INTO t1 VALUES ('foo'),('bar');
+CREATE TABLE t2 (b VARBINARY(8));
+
+EXPLAIN
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 'qux', 'qux') AND a = (SELECT MIN(b) FROM t2);
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 'qux', 'qux') AND a = (SELECT MIN(b) FROM t2);
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b VARBINARY(8));
+
+EXPLAIN
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 1, 2) AND a = (SELECT MIN(b) FROM t2);
+SELECT a FROM t1 WHERE (a, a) IN (SELECT 1, 2) AND a = (SELECT MIN(b) FROM t2);
+
+DROP TABLE t1,t2;
+
 --echo # End of 10.2 tests

sql/item.cc

@@ -9690,7 +9690,7 @@ bool  Item_cache_int::cache_value()
     return FALSE;
   value_cached= TRUE;
   value= example->val_int_result();
-  null_value= example->null_value;
+  null_value_inside= null_value= example->null_value;
   unsigned_flag= example->unsigned_flag;
   return TRUE;
 }
@@ -9860,7 +9860,7 @@ bool Item_cache_temporal::cache_value()
       return true;
     value= pack_time(&ltime);
   }
-  null_value= example->null_value;
+  null_value_inside= null_value= example->null_value;
   return true;
 }
 
@@ -9955,7 +9955,7 @@ bool Item_cache_real::cache_value()
     return FALSE;
   value_cached= TRUE;
   value= example->val_result();
-  null_value= example->null_value;
+  null_value_inside= null_value= example->null_value;
   return TRUE;
 }
 
@@ -10017,7 +10017,8 @@ bool Item_cache_decimal::cache_value()
     return FALSE;
   value_cached= TRUE;
   my_decimal *val= example->val_decimal_result(&decimal_value);
-  if (!(null_value= example->null_value) && val != &decimal_value)
+  if (!(null_value_inside= null_value= example->null_value) &&
+        val != &decimal_value)
     my_decimal2decimal(val, &decimal_value);
   return TRUE;
 }
@@ -10083,11 +10084,14 @@ Item *Item_cache_decimal::convert_to_basic_const_item(THD *thd)
 bool Item_cache_str::cache_value()
 {
   if (!example)
+  {
+    DBUG_ASSERT(value_cached == FALSE);
     return FALSE;
+  }
   value_cached= TRUE;
   value_buff.set(buffer, sizeof(buffer), example->collation.collation);
   value= example->str_result(&value_buff);
-  if ((null_value= example->null_value))
+  if ((null_value= null_value_inside= example->null_value))
     value= 0;
   else if (value != &value_buff)
   {
@@ -10186,6 +10190,8 @@ Item *Item_cache_str::convert_to_basic_const_item(THD *thd)
 bool Item_cache_row::setup(THD *thd, Item *item)
 {
   example= item;
+  null_value= true;
+
   if (!values && allocate(thd, item->cols()))
     return 1;
   for (uint i= 0; i < item_count; i++)
@@ -10218,12 +10224,19 @@ bool Item_cache_row::cache_value()
   if (!example)
     return FALSE;
   value_cached= TRUE;
-  null_value= 0;
+  null_value= TRUE;
+  null_value_inside= false;
   example->bring_value();
+
+  /*
+    For Item_cache_row null_value is set to TRUE only when ALL the values
+    inside the cache are NULL
+  */
   for (uint i= 0; i < item_count; i++)
   {
     values[i]->cache_value();
-    null_value|= values[i]->null_value;
+    null_value&= values[i]->null_value;
+    null_value_inside|= values[i]->null_value;
   }
   return TRUE;
 }

sql/item.h

@@ -5658,6 +5658,14 @@ class Item_cache: public Item_basic_constant,
   */
   bool value_cached;
 public:
+  /*
+    This is set if at least one of the values of a sub query is NULL
+    Item_cache_row returns this with null_inside().
+    For not row items, it's set to the value of null_value
+    It is set after cache_value() is called.
+  */
+  bool null_value_inside;
+
   Item_cache(THD *thd):
     Item_basic_constant(thd),
     Type_handler_hybrid_field_type(MYSQL_TYPE_STRING),
@@ -5667,6 +5675,7 @@ class Item_cache: public Item_basic_constant,
     fixed= 1;
     maybe_null= 1;
     null_value= 1;
+    null_value_inside= true;
   }
 protected:
   Item_cache(THD *thd, enum_field_types field_type_arg):
@@ -5678,6 +5687,7 @@ class Item_cache: public Item_basic_constant,
     fixed= 1;
     maybe_null= 1;
     null_value= 1;
+    null_value_inside= true;
   }
 
 public:

sql/item_cmpfunc.cc

@@ -1598,7 +1598,7 @@ longlong Item_in_optimizer::val_int()
     DBUG_RETURN(res);
   }
 
-  if (cache->null_value)
+  if (cache->null_value_inside)
   {
      DBUG_PRINT("info", ("Left NULL..."));
     /*

sql/item_subselect.cc

@@ -851,7 +851,7 @@ bool Item_subselect::expr_cache_is_needed(THD *thd)
 
 inline bool Item_in_subselect::left_expr_has_null()
 {
-  return (*(optimizer->get_cache()))->null_value;
+  return (*(optimizer->get_cache()))->null_value_inside;
 }
 
 
@@ -1319,7 +1319,17 @@ bool Item_singlerow_subselect::null_inside()
 void Item_singlerow_subselect::bring_value()
 {
   if (!exec() && assigned())
-    null_value= 0;
+  {
+    null_value= true;
+    for (uint i= 0; i < max_columns ; i++)
+    {
+      if (!row[i]->null_value)
+      {
+        null_value= false;
+        return;
+      }
+    }
+  }
   else
     reset();
 }
@@ -1345,7 +1355,11 @@ longlong Item_singlerow_subselect::val_int()
 {
   DBUG_ASSERT(fixed == 1);
   if (forced_const)
-    return value->val_int();
+  {
+    longlong val= value->val_int();
+    null_value= value->null_value;
+    return val;
+  }
   if (!exec() && !value->null_value)
   {
     null_value= FALSE;
@@ -1354,6 +1368,7 @@ longlong Item_singlerow_subselect::val_int()
   else
   {
     reset();
+    DBUG_ASSERT(null_value);
     return 0;
   }
 }
@@ -1362,7 +1377,11 @@ String *Item_singlerow_subselect::val_str(String *str)
 {
   DBUG_ASSERT(fixed == 1);
   if (forced_const)
-    return value->val_str(str);
+  {
+    String *res= value->val_str(str);
+    null_value= value->null_value;
+    return res;
+  }
   if (!exec() && !value->null_value)
   {
     null_value= FALSE;
@@ -1371,6 +1390,7 @@ String *Item_singlerow_subselect::val_str(String *str)
   else
   {
     reset();
+    DBUG_ASSERT(null_value);
     return 0;
   }
 }
@@ -1380,7 +1400,11 @@ my_decimal *Item_singlerow_subselect::val_decimal(my_decimal *decimal_value)
 {
   DBUG_ASSERT(fixed == 1);
   if (forced_const)
-    return value->val_decimal(decimal_value);
+  {
+    my_decimal *val= value->val_decimal(decimal_value);
+    null_value= value->null_value;
+    return val;
+  }
   if (!exec() && !value->null_value)
   {
     null_value= FALSE;
@@ -1389,6 +1413,7 @@ my_decimal *Item_singlerow_subselect::val_decimal(my_decimal *decimal_value)
   else
   {
     reset();
+    DBUG_ASSERT(null_value);
     return 0;
   }
 }
@@ -1398,7 +1423,11 @@ bool Item_singlerow_subselect::val_bool()
 {
   DBUG_ASSERT(fixed == 1);
   if (forced_const)
-    return value->val_bool();
+  {
+    bool val= value->val_bool();
+    null_value= value->null_value;
+    return val;
+  }
   if (!exec() && !value->null_value)
   {
     null_value= FALSE;
@@ -1407,6 +1436,7 @@ bool Item_singlerow_subselect::val_bool()
   else
   {
     reset();
+    DBUG_ASSERT(null_value);
     return 0;
   }
 }
@@ -1416,7 +1446,11 @@ bool Item_singlerow_subselect::get_date(MYSQL_TIME *ltime,ulonglong fuzzydate)
 {
   DBUG_ASSERT(fixed == 1);
   if (forced_const)
-    return value->get_date(ltime, fuzzydate);
+  {
+    bool val= value->get_date(ltime, fuzzydate);
+    null_value= value->null_value;
+    return val;
+  }
   if (!exec() && !value->null_value)
   {
     null_value= FALSE;
@@ -1425,6 +1459,7 @@ bool Item_singlerow_subselect::get_date(MYSQL_TIME *ltime,ulonglong fuzzydate)
   else
   {
     reset();
+    DBUG_ASSERT(null_value);
     return 1;
   }
 }

sql/item_subselect.h

@@ -48,7 +48,11 @@ class Cached_item;
 class Item_subselect :public Item_result_field,
                       protected Used_tables_and_const_cache
 {
-  bool value_assigned;   /* value already assigned to subselect */
+  /*
+    Set to TRUE if the value is assigned for the subselect
+    FALSE: subquery not executed or the subquery returns an empty result
+  */
+  bool value_assigned;
   bool own_engine;  /* the engine was not taken from other Item_subselect */
 protected:
   /* thread handler, will be assigned in fix_fields only */