MWL#89

Fixed a memory leak found by valgrind. The memory leak was
a result of JOINs corresponding to subselects in a global
ORDER BY of a UNION not being cleaned up because the
fake_select of the UNION didn't point down to the subquery
select.

sql/item_subselect.cc

@@ -326,23 +326,19 @@ bool Item_subselect::set_fake_select_as_master_processor(uchar *arg)
   /*
     Move the st_select_lex_unit of a subquery from a global ORDER BY clause to
     become a direct child of the fake_select of a UNION. In this way the
-    ORDER BY is applied to the temporary table that contains the result of the
-    whole UNION, and all columns in the subquery are resolved against this table.
-
-    Apply the transformation only for immediate child subqueries of a
-    UNION query.
+    ORDER BY that is applied to the temporary table that contains the result of
+    the whole UNION, and all columns in the subquery are resolved against this
+    table. The transformation is applied only for immediate child subqueries of
+    a UNION query.
   */
   if (unit->outer_select()->master_unit()->fake_select_lex == fake_select)
   {
     /*
-      Set the master of the subquery to be the fake select (i.e. the whole UNION),
-      instead of the last query in the UNION.
-      TODO:
-      This is a hack, instead we should call: unit->include_down(fake_select);
-      However, this call results in an infinite loop where
-      some_select_lex->master == some_select_lex.
+      Set the master of the subquery to be the fake select (i.e. the whole
+      UNION), instead of the last query in the UNION.
     */
-    unit->set_master(fake_select);
+    fake_select->add_slave(unit);
+    DBUG_ASSERT(unit->outer_select() == fake_select);
     /* Adjust the name resolution context hierarchy accordingly. */
     for (SELECT_LEX *sl= unit->first_select(); sl; sl= sl->next_select())
       sl->context.outer_context= &(fake_select->context);

sql/sql_lex.cc

@@ -1684,6 +1684,31 @@ void st_select_lex_node::include_down(st_select_lex_node *upper)
   slave= 0;
 }
 
+
+void st_select_lex_node::add_slave(st_select_lex_node *slave_arg)
+{
+  for (; slave; slave= slave->next)
+    if (slave == slave_arg)
+      return;
+
+  if (slave)
+  {
+    st_select_lex_node *slave_arg_slave= slave_arg->slave;
+    /* Insert in the front of list of slaves if any. */
+    slave_arg->include_neighbour(slave);
+    /* include_neighbour() sets slave_arg->slave=0, restore it. */
+    slave_arg->slave= slave_arg_slave;
+    /* Count on include_neighbour() setting the master. */
+    DBUG_ASSERT(slave_arg->master == this);
+  }
+  else
+  {
+    slave= slave_arg;
+    slave_arg->master= this;
+  }
+}
+
+
 /*
   include on level down (but do not link)
 

sql/sql_lex.h

@@ -439,10 +439,10 @@ public:
   st_select_lex_node(): linkage(UNSPECIFIED_TYPE) {}
   virtual ~st_select_lex_node() {}
   inline st_select_lex_node* get_master() { return master; }
-  inline void set_master(st_select_lex_node* master_arg) { master= master_arg; }
   virtual void init_query();
   virtual void init_select();
   void include_down(st_select_lex_node *upper);
+  void add_slave(st_select_lex_node *slave_arg);
   void include_neighbour(st_select_lex_node *before);
   void include_standalone(st_select_lex_node *sel, st_select_lex_node **ref);
   void include_global(st_select_lex_node **plink);