Fix for LP BUG#676411 and MySQL BUG#52317

This is a backport of the fix for
MySQL BUG#52317: Assertion failing in Field_varstring::store () at field.cc:6833

The orginal comment by Oystein is:

In order for EXPLAIN to print const-refs, a Store_key_const_item object
is created. This is different for normal execution of subqueries where
a temporary store_key_item object is used instead. The problem is that
EXPLAIN will execute subqueries.  This leads to a scenario where a
store_key_const_item object it told to write to its underlying field.
This results in a failing assert since the write set of the underlying
table does not reflect this.  

The resolution is to do the same trick as for store_key_item::copy_inner().
That is, temporarily change the write set to allow writes to all columns.
This is only necessary in debug version since non-debug version does not
contain asserts on write_set.

sql/sql_select.h:
  Temporarily change write_set in store_key_const_item::copy_inner() to
  allow initialization of underlying field.  This is necessary since 
  subqueries are executed for EXPLAIN.  (For normal execution, 
  store_key_item::copy_inner is used.)

mysql-test/r/subselect4.result

@@ -436,3 +436,18 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 c1	c1
 drop table t1, t2, t3, t4;
+#
+# BUG#52317: Assertion failing in Field_varstring::store() 
+# 	    at field.cc:6833
+#
+CREATE TABLE t1 (i INTEGER);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (i INTEGER, KEY k(i));
+INSERT INTO t2 VALUES (1), (2);
+EXPLAIN  
+SELECT i FROM t1 WHERE (1) NOT IN (SELECT i 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 noticed after reading const tables
+2	DEPENDENT SUBQUERY	t2	index_subquery	k	k	5	const	2	Using index
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/subselect4.test

@@ -396,3 +396,19 @@ EXPLAIN
 SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 drop table t1, t2, t3, t4;
+
+--echo #
+--echo # BUG#52317: Assertion failing in Field_varstring::store() 
+--echo # 	    at field.cc:6833
+--echo #
+
+CREATE TABLE t1 (i INTEGER);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (i INTEGER, KEY k(i));
+INSERT INTO t2 VALUES (1), (2);
+
+EXPLAIN  
+SELECT i FROM t1 WHERE (1) NOT IN (SELECT i FROM t2);
+
+DROP TABLE t2;
+DROP TABLE t1;

sql/sql_select.h

@@ -1990,6 +1990,9 @@ class store_key_const_item :public store_key_item
     if (!inited)
     {
       inited=1;
+      TABLE *table= to_field->table;
+      my_bitmap_map *old_map= dbug_tmp_use_all_columns(table,
+                                                       table->write_set);
       if ((res= item->save_in_field(to_field, 1)))
       {       
         if (!err)
@@ -2001,6 +2004,7 @@ class store_key_const_item :public store_key_item
         */
       if (!err && to_field->table->in_use->is_error())
         err= 1; /* STORE_KEY_FATAL */
+      dbug_tmp_restore_column_map(table->write_set, old_map);
     }
     null_key= to_field->is_null() || item->null_value;
     return (err > 2 ? STORE_KEY_FATAL : (store_key_result) err);