fixed union unlocking problem (BUG#906)

mysql-test/r/subselect.result:
  test of union unlocking problem
mysql-test/t/subselect.test:
  test of union unlocking problem
sql/sql_union.cc:
  prohibit unlocking all tables by fake select execution

mysql-test/r/subselect.result

@@ -1225,3 +1225,15 @@ a	(select count(distinct t1.b) as sum from t1,t2 where t1.a=t2.a and t2.b > 0 an
 2	2
 1	2
 drop table t1,t2,t3;
+create table t1 (a int) type=innodb;
+create table t2 (a int) type=innodb;
+create table t3 (a int) type=innodb;
+insert into t1 values (1),(2),(3),(4);
+insert into t2 values (10),(20),(30),(40);
+insert into t3 values (1),(2),(10),(50);
+select a from t3 where t3.a in (select a from t1 where a <= 3 union select * from t2 where a <= 30);
+a
+1
+2
+10
+drop table t1,t2;

mysql-test/t/subselect.test

@@ -807,6 +807,7 @@ INSERT INTO t1 VALUES("2f6161e879db43c1a5b82c21ddc49089", "Default", "System", "
 INSERT INTO t1 VALUES("c373e9f5ad0791724315444553544200", "AddDocumentTest", "admin", "2003-06-09 10:51:25", "Movie Reviews", "0", "2003-06-09 10:51:25", "admin", "0", "2f6161e879db43c1a5b82c21ddc49089", "03eea05112b845949f3fd03278b5fe43", NULL);
 SELECT 'c373e9f5ad0791a0dab5444553544200' IN(SELECT t1.FOLDERID FROM t1 WHERE t1.PARENTID='2f6161e879db43c1a5b82c21ddc49089' AND t1.FOLDERNAME = 'Level1');
 drop table t1;
+
 #
 # alloc_group_fields() working
 #
@@ -817,4 +818,17 @@ insert into t1 values (0,100),(1,2), (1,3), (2,2), (2,7), (2,-1), (3,10);
 insert into t2 values (0,0), (1,1), (2,1), (3,1), (4,1);
 insert into t3 values (3,3), (2,2), (1,1); 
 select a,(select count(distinct t1.b) as sum from t1,t2 where t1.a=t2.a and t2.b > 0 and t1.a <= t3.b group by t1.a order by sum limit 1) from t3;
-drop table t1,t2,t3;s
+drop table t1,t2,t3;
+
+#
+# UNION unlocking test
+#
+create table t1 (a int) type=innodb;
+create table t2 (a int) type=innodb;
+create table t3 (a int) type=innodb;
+insert into t1 values (1),(2),(3),(4);
+insert into t2 values (10),(20),(30),(40);
+insert into t3 values (1),(2),(10),(50);
+select a from t3 where t3.a in (select a from t1 where a <= 3 union select * from t2 where a <= 30);
+drop table t1,t2;
+

sql/sql_union.cc

@@ -351,7 +351,8 @@ int st_select_lex_unit::exec()
 			global_parameters->order_list.elements,
 			(ORDER*)global_parameters->order_list.first,
 			(ORDER*) NULL, NULL, (ORDER*) NULL,
-			thd->options, result, this, fake_select, 0);
+			thd->options | SELECT_NO_UNLOCK,
+			result, this, fake_select, 0);
       if (found_rows_for_union && !res)
 	thd->limit_found_rows = (ulonglong)table->file->records;
       fake_select->exclude();