Backport of Bug#53236 Segfault in DTCollation::set(DTCollation&)

Don't call member functions for a NIL pointer.


mysql-test/r/subselect4.result:
  Add test case.
mysql-test/t/subselect4.test:
  Add test case.
sql/sql_select.cc:
  If the (virtual) member function clone_item() returns NULL,
  there is no substitution to be made, and we don't need to set the collation.
  The test was invoking Item_cache::clone_item()

mysql-test/r/subselect4.result

@@ -59,3 +59,28 @@ FROM t3 WHERE 1 = 0 GROUP BY 1;
 (SELECT 1 FROM t1,t2 WHERE t2.b > t3.b)
 DROP TABLE t1,t2,t3;
 End of 5.0 tests.
+#
+# Bug#53236 Segfault in DTCollation::set(DTCollation&)
+#
+CREATE TABLE t1 (
+pk INTEGER AUTO_INCREMENT,
+col_varchar VARCHAR(1),
+PRIMARY KEY (pk)
+)
+;
+INSERT INTO t1 (col_varchar) 
+VALUES
+('w'),
+('m')
+;
+SELECT  table1.pk
+FROM ( t1 AS table1 JOIN t1 AS table2 ON (table1.col_varchar =
+table2.col_varchar) ) 
+WHERE ( 1, 2 ) IN ( SELECT SUBQUERY1_t1.pk AS SUBQUERY1_field1,
+SUBQUERY1_t1.pk AS SUBQUERY1_field2
+FROM ( t1 AS SUBQUERY1_t1 JOIN t1 AS SUBQUERY1_t2
+ON (SUBQUERY1_t2.col_varchar =
+SUBQUERY1_t1.col_varchar) ) ) 
+;
+pk
+drop table t1;

mysql-test/t/subselect4.test

@@ -62,3 +62,32 @@ FROM t3 WHERE 1 = 0 GROUP BY 1;
 DROP TABLE t1,t2,t3;
 
 --echo End of 5.0 tests.
+
+--echo #
+--echo # Bug#53236 Segfault in DTCollation::set(DTCollation&)
+--echo #
+
+CREATE TABLE t1 (
+  pk INTEGER AUTO_INCREMENT,
+  col_varchar VARCHAR(1),
+  PRIMARY KEY (pk)
+)
+;
+
+INSERT INTO t1 (col_varchar) 
+VALUES
+('w'),
+('m')
+;
+
+SELECT  table1.pk
+FROM ( t1 AS table1 JOIN t1 AS table2 ON (table1.col_varchar =
+                                          table2.col_varchar) ) 
+WHERE ( 1, 2 ) IN ( SELECT SUBQUERY1_t1.pk AS SUBQUERY1_field1,
+                           SUBQUERY1_t1.pk AS SUBQUERY1_field2
+                    FROM ( t1 AS SUBQUERY1_t1 JOIN t1 AS SUBQUERY1_t2
+                           ON (SUBQUERY1_t2.col_varchar =
+                               SUBQUERY1_t1.col_varchar) ) ) 
+;
+
+drop table t1;

sql/sql_select.cc

@@ -8629,10 +8629,9 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
        left_item->collation.collation == value->collation.collation))
   {
     Item *tmp=value->clone_item();
-    tmp->collation.set(right_item->collation);
-    
     if (tmp)
     {
+      tmp->collation.set(right_item->collation);
       thd->change_item_tree(args + 1, tmp);
       func->update_used_tables();
       if ((functype == Item_func::EQ_FUNC || functype == Item_func::EQUAL_FUNC)
@@ -8653,10 +8652,9 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
             right_item->collation.collation == value->collation.collation))
   {
     Item *tmp= value->clone_item();
-    tmp->collation.set(left_item->collation);
-    
     if (tmp)
     {
+      tmp->collation.set(left_item->collation);
       thd->change_item_tree(args, tmp);
       value= tmp;
       func->update_used_tables();