Fix for bug #35298: GROUP_CONCAT with DISTINCT can crash the server

The bug is a regression introduced by the patch for bug32798.

The code in Item_func_group_concat::clear() relied on the 'distinct'
variable to check if 'unique_filter' was initialized. That, however,
is not always valid because Item_func_group_concat::setup() can do
shortcuts in some cases w/o initializing 'unique_filter'.

Fixed by checking the value of 'unique_filter' instead of 'distinct'
before dereferencing.

mysql-test/r/func_gconcat.result

@@ -946,4 +946,30 @@ GROUP BY 1
 d1
 NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+INSERT INTO t1 VALUES(1);
+SELECT GROUP_CONCAT(DISTINCT t2.a) FROM t1 LEFT JOIN t2 ON t2.a = t1.a GROUP BY t1.a;
+GROUP_CONCAT(DISTINCT t2.a)
+NULL
+DROP TABLE t1, t2;
+CREATE TABLE t1 (a INT, KEY(a));
+CREATE TABLE t2 (b INT);
+INSERT INTO t1 VALUES (NULL), (8), (2);
+INSERT INTO t2 VALUES (4), (10);
+SELECT 1 FROM t1 WHERE t1.a NOT IN
+(
+SELECT GROUP_CONCAT(DISTINCT t1.a)
+FROM  t1 WHERE t1.a IN   
+(
+SELECT b FROM t2
+) 
+AND NOT t1.a >= (SELECT t1.a FROM t1 LIMIT 1)
+GROUP BY t1.a
+);
+1
+1
+1
+1
+DROP TABLE t1, t2;
 End of 5.0 tests

mysql-test/t/func_gconcat.test

@@ -657,4 +657,40 @@ SELECT s1.d1 FROM
 ) AS s1;
 DROP TABLE t1;
 
+#
+# Bug #35298: GROUP_CONCAT with DISTINCT can crash the server
+#
+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+
+INSERT INTO t1 VALUES(1);
+
+SELECT GROUP_CONCAT(DISTINCT t2.a) FROM t1 LEFT JOIN t2 ON t2.a = t1.a GROUP BY t1.a;
+
+DROP TABLE t1, t2;
+
+#
+# Bug #36024: group_concat distinct in subquery crash
+#
+
+CREATE TABLE t1 (a INT, KEY(a));
+CREATE TABLE t2 (b INT);
+
+INSERT INTO t1 VALUES (NULL), (8), (2);
+INSERT INTO t2 VALUES (4), (10);
+
+SELECT 1 FROM t1 WHERE t1.a NOT IN
+(
+  SELECT GROUP_CONCAT(DISTINCT t1.a)
+  FROM  t1 WHERE t1.a IN   
+  (
+    SELECT b FROM t2
+  ) 
+  AND NOT t1.a >= (SELECT t1.a FROM t1 LIMIT 1)
+  GROUP BY t1.a
+);
+
+DROP TABLE t1, t2;
+
 --echo End of 5.0 tests

sql/item_sum.cc

@@ -3222,7 +3222,7 @@ void Item_func_group_concat::clear()
   no_appended= TRUE;
   if (tree)
     reset_tree(tree);
-  if (distinct)
+  if (unique_filter)
     unique_filter->reset();
   /* No need to reset the table as we never call write_row */
 }