Bug#18486249 ASSERTION FAILED: BUF == BUFFER IN MY_DECIMAL::SANITY_CHECK

This bug was uncovered by the stricter sanity_check introduced in patch for:
Bug#18335446 DECIMAL_ROUND: ASSERTION FAILED: FROM->LEN == TO->LEN

The problem is that my_qsort2 sorts blobs, rather than objects.
When sorting an array of decimals, it will sometimes leave
objects with wrong buffer pointers.

Solution: use std::sort, which is type-aware, and will copy/swap
decimal objects properly.

Author: Tor Didriksen

mysql-test/r/type_newdecimal.result

@@ -2020,3 +2020,13 @@ INSERT INTO t1 VALUES (1),(2);
 UPDATE  IGNORE t1 SET b = 1
 WHERE b NOT IN (NULL, -3333333333333333333333);
 DROP TABLE t1;
+#
+# Bug#18486249 ASSERTION FAILED:
+#              BUF == BUFFER IN MY_DECIMAL::SANITY_CHECK
+#
+CREATE TABLE t1(b INT, KEY(b));
+INSERT INTO t1 VALUES (0);
+SELECT 1 FROM t1 WHERE b NOT IN (0.1,-0.1);
+1
+1
+DROP TABLE t1;

mysql-test/t/type_newdecimal.test

@@ -1643,3 +1643,13 @@ UPDATE  IGNORE t1 SET b = 1
 WHERE b NOT IN (NULL, -3333333333333333333333);
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug#18486249 ASSERTION FAILED:
+--echo #              BUF == BUFFER IN MY_DECIMAL::SANITY_CHECK
+--echo #
+
+CREATE TABLE t1(b INT, KEY(b));
+INSERT INTO t1 VALUES (0);
+SELECT 1 FROM t1 WHERE b NOT IN (0.1,-0.1);
+DROP TABLE t1;

sql/item_cmpfunc.cc

@@ -4104,12 +4104,8 @@ static int cmp_row(void *cmp_arg, cmp_item_row *a, cmp_item_row *b)
 
 static int cmp_decimal(void *cmp_arg, my_decimal *a, my_decimal *b)
 {
-  /*
-    We need call of fixing buffer pointer, because fast sort just copy
-    decimal buffers in memory and pointers left pointing on old buffer place
-  */
-  a->fix_buffer_pointer();
-  b->fix_buffer_pointer();
+  a->sanity_check();
+  b->sanity_check();
   return my_decimal_cmp(a, b);
 }
 
@@ -4349,6 +4345,13 @@ uchar *in_decimal::get_value(Item *item)
 }
 
 
+void in_decimal::sort()
+{
+  my_decimal *begin= static_cast<my_decimal*>(static_cast<void*>(base));
+  my_decimal *end= begin + used_count;
+  std::sort(begin, end);
+}
+
 cmp_item* cmp_item::get_comparator(Item_result type,
                                    const CHARSET_INFO *cs)
 {

sql/item_cmpfunc.h

@@ -1047,7 +1047,7 @@ class in_vector :public Sql_alloc
   virtual ~in_vector() {}
   virtual void set(uint pos,Item *item)=0;
   virtual uchar *get_value(Item *item)=0;
-  void sort()
+  virtual void sort()
   {
     my_qsort2(base,used_count,size,compare,collation);
   }
@@ -1235,6 +1235,8 @@ class in_decimal :public in_vector
   }
   Item_result result_type() { return DECIMAL_RESULT; }
 
+  // Our own, type-aware sort, rather than my_qsort2.
+  virtual void sort();
 };
 
 

sql/my_decimal.h

@@ -502,6 +502,12 @@ int my_decimal_cmp(const my_decimal *a, const my_decimal *b)
   return decimal_cmp(a, b);
 }
 
+inline
+bool operator<(const my_decimal &lhs, const my_decimal &rhs)
+{
+  return my_decimal_cmp(&lhs, &rhs) < 0;
+}
+
 
 inline
 int my_decimal_intg(const my_decimal *a)