MDEV-33165 Incorrect result interceptor passed to mysql_explain_union()

Statements affect by this bug are all SQL statements that
1) prefixed with "EXPLAIN"
2) have a lower level join structure created for a union subquery.

A bug in select_describe() passed an incorrect "result" object to
mysql_explain_union(), resulting in unpredictable behaviour and
out of context calls.

Reviewed by: Oleksandr Byelkin, sanja@mariadb.com

mysql-test/main/explain.result

@@ -459,3 +459,43 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 NULL	UNION RESULT	<union1,3>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1249	Select 4 was reduced during optimization
+#
+# End of 10.4 tests
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+EXPLAIN SELECT * FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where; Using join buffer (flat, BNL join)
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+EXPLAIN DELETE t2 FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+prepare stmt from "EXPLAIN DELETE t2 FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6)";
+execute stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+execute stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+DROP TABLE t1, t2;
+#
+# End of 10.5 tests
+#

mysql-test/main/explain.test

@@ -370,3 +370,26 @@ drop table t1;
 
 explain
 VALUES ( (VALUES (2))) UNION VALUES ( (SELECT 3));
+
+--echo #
+--echo # End of 10.4 tests
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+
+EXPLAIN SELECT * FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6);
+EXPLAIN DELETE t2 FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6);
+prepare stmt from "EXPLAIN DELETE t2 FROM t1, t2 WHERE t2.b IN (SELECT 5 UNION SELECT 6)";
+execute stmt;
+execute stmt;
+
+# Cleanup
+
+DROP TABLE t1, t2;
+
+--echo #
+--echo # End of 10.5 tests
+--echo #

sql/sql_select.cc

@@ -27953,7 +27953,6 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 			    bool distinct,const char *message)
 {
   THD *thd=join->thd;
-  select_result *result=join->result;
   DBUG_ENTER("select_describe");
 
   if (join->select_lex->pushdown_select)
@@ -27988,7 +27987,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 
     if (unit->explainable())
     {
-      if (mysql_explain_union(thd, unit, result))
+      if (mysql_explain_union(thd, unit, unit->result))
         DBUG_VOID_RETURN;
     }
   }