BUG#19684: Garbage in 'partitions' column in EXPLAIN output:

In select_describe(), make the String object that holds the value of 
"partitions" column to "own" the value buffer, so the buffer isn't
prematurely freed.
[this is the second attempt with review fixes]

mysql-test/r/partition_pruning.result

@@ -678,3 +678,22 @@ explain partitions select * from t1 where s1 = 0 or s1 is null;
 id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	p0	ALL	NULL	NULL	NULL	NULL	3	Using where
 drop table t1;
+create table t2 (a int, b int)
+partition by LIST(a)
+subpartition by HASH(b) subpartitions 40
+( partition p_0_long_partition_name values in(1),
+partition p_1_long_partition_name values in(2));
+insert into t2 values (1,1),(2,2);
+explain partitions select * from t2;
+id	1
+select_type	SIMPLE
+table	t2
+partitions	p_0_long_partition_name_p_0_long_partition_namesp0,p_0_long_partition_name_p_0_long_partition_namesp1,p_0_long_partition_name_p_0_long_partition_namesp2,p_0_long_partition_name_p_0_long_partition_namesp3,p_0_long_partition_name_p_0_long_partition_namesp4,p_0_long_partition_name_p_0_long_partition_namesp5,p_0_long_partition_name_p_0_long_partition_namesp6,p_0_long_partition_name_p_0_long_partition_namesp7,p_0_long_partition_name_p_0_long_partition_namesp8,p_0_long_partition_name_p_0_long_partition_namesp9,p_0_long_partition_name_p_0_long_partition_namesp10,p_0_long_partition_name_p_0_long_partition_namesp11,p_0_long_partition_name_p_0_long_partition_namesp12,p_0_long_partition_name_p_0_long_partition_namesp13,p_0_long_partition_name_p_0_long_partition_namesp14,p_0_long_partition_name_p_0_long_partition_namesp15,p_0_long_partition_name_p_0_long_partition_namesp16,p_0_long_partition_name_p_0_long_partition_namesp17,p_0_long_partition_name_p_0_long_partition_namesp18,p_0_long_partition_name_p_0_long_partition_namesp19,p_0_long_partition_name_p_0_long_partition_namesp20,p_0_long_partition_name_p_0_long_partition_namesp21,p_0_long_partition_name_p_0_long_partition_namesp22,p_0_long_partition_name_p_0_long_partition_namesp23,p_0_long_partition_name_p_0_long_partition_namesp24,p_0_long_partition_name_p_0_long_partition_namesp25,p_0_long_partition_name_p_0_long_partition_namesp26,p_0_long_partition_name_p_0_long_partition_namesp27,p_0_long_partition_name_p_0_long_partition_namesp28,p_0_long_partition_name_p_0_long_partition_namesp29,p_0_long_partition_name_p_0_long_partition_namesp30,p_0_long_partition_name_p_0_long_partition_namesp31,p_0_long_partition_name_p_0_long_partition_namesp32,p_0_long_partition_name_p_0_long_partition_namesp33,p_0_long_partition_name_p_0_long_partition_namesp34,p_0_long_partition_name_p_0_long_partition_namesp35,p_0_long_partition_name_p_0_long_partition_namesp36,p_0_long_partition_name_p_0_long_partition_namesp37,p_0_long_partition_name_p_0_long_partition_namesp38,p_0_long_partition_name_p_0_long_partition_namesp39,p_1_long_partition_name_p_1_long_partition_namesp0,p_1_long_partition_name_p_1_long_partition_namesp1,p_1_long_partition_name_p_1_long_partition_namesp2,p_1_long_partition_name_p_1_long_partition_namesp3,p_1_long_partition_name_p_1_long_partition_namesp4,p_1_long_partition_name_p_1_long_partition_namesp5,p_1_long_partition_name_p_1_long_partition_namesp6,p_1_long_partition_name_p_1_long_partition_namesp7,p_1_long_partition_name_p_1_long_partition_namesp8,p_1_long_partition_name_p_1_long_partition_namesp9,p_1_long_partition_name_p_1_long_partition_namesp10,p_1_long_partition_name_p_1_long_partition_namesp11,p_1_long_partition_name_p_1_long_partition_namesp12,p_1_long_partition_name_p_1_long_partition_namesp13,p_1_long_partition_name_p_1_long_partition_namesp14,p_1_long_partition_name_p_1_long_partition_namesp15,p_1_long_partition_name_p_1_long_partition_namesp16,p_1_long_partition_name_p_1_long_partition_namesp17,p_1_long_partition_name_p_1_long_partition_namesp18,p_1_long_partition_name_p_1_long_partition_namesp19,p_1_long_partition_name_p_1_long_partition_namesp20,p_1_long_partition_name_p_1_long_partition_namesp21,p_1_long_partition_name_p_1_long_partition_namesp22,p_1_long_partition_name_p_1_long_partition_namesp23,p_1_long_partition_name_p_1_long_partition_namesp24,p_1_long_partition_name_p_1_long_partition_namesp25,p_1_long_partition_name_p_1_long_partition_namesp26,p_1_long_partition_name_p_1_long_partition_namesp27,p_1_long_partition_name_p_1_long_partition_namesp28,p_1_long_partition_name_p_1_long_partition_namesp29,p_1_long_partition_name_p_1_long_partition_namesp30,p_1_long_partition_name_p_1_long_partition_namesp31,p_1_long_partition_name_p_1_long_partition_namesp32,p_1_long_partition_name_p_1_long_partition_namesp33,p_1_long_partition_name_p_1_long_partition_namesp34,p_1_long_partition_name_p_1_long_partition_namesp35,p_1_long_partition_name_p_1_long_partition_namesp36,p_1_long_partition_name_p_1_long_partition_namesp37,p_1_long_partition_name_p_1_long_partition_namesp38,p_1_long_partition_name_p_1_long_partition_namesp39
+type	ALL
+possible_keys	NULL
+key	NULL
+key_len	NULL
+ref	NULL
+rows	2
+Extra	
+drop table t2;

mysql-test/t/partition_pruning.test

@@ -574,3 +574,19 @@ insert into t1 values ('aa'),('bb'),('0');
 explain partitions select * from t1 where s1 = 0 or s1 is null;
 drop table t1;
 
+#
+# BUG#19684: EXPLAIN PARTITIONS produces garbage in 'partitions' column when
+# the length of string to be displayed exceeds some limit.
+create table t2 (a int, b int)
+  partition by LIST(a)
+  subpartition by HASH(b) subpartitions 40
+( partition p_0_long_partition_name values in(1),
+  partition p_1_long_partition_name values in(2));
+
+insert into t2 values (1,1),(2,2);
+
+--vertical_results
+explain partitions select * from t2;
+--horizontal_results
+drop table t2;
+

sql/sql_select.cc

@@ -13844,6 +13844,10 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
   thd->server_status&= ~(SERVER_QUERY_NO_INDEX_USED | SERVER_QUERY_NO_GOOD_INDEX_USED);
   join->unit->offset_limit_cnt= 0;
 
+  /* 
+    NOTE: the number/types of items pushed into item_list must be in sync with
+    EXPLAIN column types as they're "defined" in THD::send_explain_fields()
+  */
   if (message)
   {
     item_list.push_back(new Item_int((int32)
@@ -13983,11 +13987,9 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
         if (!table->derived_select_number && 
             (part_info= table->part_info))
         {          
-          char parts_buff[128]; 
-          String parts_str(parts_buff,sizeof(parts_buff),cs);
-          make_used_partitions_str(part_info, &parts_str);
-          item_list.push_back(new Item_string(parts_str.ptr(),
-                                              parts_str.length(), cs));
+          Item_string *item_str= new Item_string(cs);
+          make_used_partitions_str(part_info, &item_str->str_value);
+          item_list.push_back(item_str);
         }
         else
           item_list.push_back(item_null);