Bug#43612 crash with explain extended, union, order by

In UNION if we use last SELECT without braces and this
SELECT have ORDER BY clause, such clause belongs to
global UNION. It is parsed like last SELECT
part and used further as 'unit->global_parameters->order_list' value.
During DESCRIBE EXTENDED we call select_lex->print_order() for
last SELECT where order fields refer to tmp table 
which already freed. It leads to crash.
The fix is clean up global_parameters->order_list
instead of fake_select_lex->order_list.

mysql-test/r/union.result

@@ -1518,4 +1518,17 @@ SHOW FIELDS FROM t2;
 Field	Type	Null	Key	Default	Extra
 d	double(9,6)	YES		NULL	
 DROP TABLE t1, t2;
+CREATE TABLE t1(a INT);
+EXPLAIN EXTENDED
+SELECT a FROM t1
+UNION
+SELECT a FROM t1
+ORDER BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	0	const row not found
+2	UNION	t1	system	NULL	NULL	NULL	NULL	0	const row not found
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+Warnings:
+Note	1003	select '0' AS `a` from `test`.`t1` union select '0' AS `a` from `test`.`t1` order by `a`
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/union.test

@@ -1038,4 +1038,16 @@ CREATE TABLE t2 AS SELECT d FROM t1 UNION SELECT d FROM t1;
 SHOW FIELDS FROM t2;
 DROP TABLE t1, t2;
 
+#
+# Bug#43612 crash with explain extended, union, order by
+#
+CREATE TABLE t1(a INT);
+EXPLAIN EXTENDED
+SELECT a FROM t1
+UNION
+SELECT a FROM t1
+ORDER BY a;
+DROP TABLE t1;
+
+
 --echo End of 5.0 tests

sql/sql_union.cc

@@ -653,10 +653,22 @@ bool st_select_lex_unit::cleanup()
       join->tables= 0;
     }
     error|= fake_select_lex->cleanup();
-    if (fake_select_lex->order_list.elements)
+    /*
+      There are two cases when we should clean order items:
+      1. UNION with SELECTs which all enclosed into braces
+        in this case global_parameters == fake_select_lex
+      2. UNION where last SELECT is not enclosed into braces
+        in this case global_parameters == 'last select'
+      So we should use global_parameters->order_list for
+      proper order list clean up.
+      Note: global_parameters and fake_select_lex are always
+            initialized for UNION
+    */
+    DBUG_ASSERT(global_parameters);
+    if (global_parameters->order_list.elements)
     {
       ORDER *ord;
-      for (ord= (ORDER*)fake_select_lex->order_list.first; ord; ord= ord->next)
+      for (ord= (ORDER*)global_parameters->order_list.first; ord; ord= ord->next)
         (*ord->item)->cleanup();
     }
   }