Fix LP BUG#685411

Analysis:
The assert failed because st_select_lex::print() was called for subqueries
as follows:

Item_subselect::print() ->
  subselect_single_select_engine::print() -> st_select_lex::print()

It was Item_subselect::fix_fields() that set the thd by calling set_thd(),
so when this print() was called before fix_fields(), subselect_engine::thd
was NULL.

Solution:
The patch makes all constructors of all subselect_engine classes to take
a THD parameter. The default subselect_single_select_engine engine is created
early during parse time, in the Item_subselect::init call, so we pass the
correct THD object already at this point.

sql/item_subselect.cc

@@ -60,6 +60,7 @@ void Item_subselect::init(st_select_lex *select_lex,
   DBUG_ENTER("Item_subselect::init");
   DBUG_PRINT("enter", ("select_lex: 0x%lx", (long) select_lex));
   unit= select_lex->master_unit();
+  thd= unit->thd;
 
   if (unit->item)
   {
@@ -76,6 +77,7 @@ void Item_subselect::init(st_select_lex *select_lex,
   else
   {
     SELECT_LEX *outer_select= unit->outer_select();
+    DBUG_ASSERT(thd);
     /*
       do not take into account expression inside aggregate functions because
       they can access original table fields
@@ -84,9 +86,9 @@ void Item_subselect::init(st_select_lex *select_lex,
                     NO_MATTER :
                     outer_select->parsing_place);
     if (unit->is_union())
-      engine= new subselect_union_engine(unit, result, this);
+      engine= new subselect_union_engine(thd, unit, result, this);
     else
-      engine= new subselect_single_select_engine(select_lex, result, this);
+      engine= new subselect_single_select_engine(thd, select_lex, result, this);
   }
   {
     SELECT_LEX *upper= unit->outer_select();
@@ -183,7 +185,8 @@ bool Item_subselect::fix_fields(THD *thd_param, Item **ref)
   bool res;
 
   DBUG_ASSERT(fixed == 0);
-  engine->set_thd((thd= thd_param));
+  /* There is no reason to get a different THD. */
+  DBUG_ASSERT(thd == thd_param);
   if (!done_first_fix_fields)
   {
     done_first_fix_fields= TRUE;
@@ -2384,10 +2387,10 @@ void subselect_engine::set_thd(THD *thd_arg)
 
 
 subselect_single_select_engine::
-subselect_single_select_engine(st_select_lex *select,
+subselect_single_select_engine(THD *thd_arg, st_select_lex *select,
 			       select_result_interceptor *result_arg,
 			       Item_subselect *item_arg)
-  :subselect_engine(item_arg, result_arg),
+  :subselect_engine(thd_arg, item_arg, result_arg),
    prepared(0), executed(0), select_lex(select), join(0)
 {
   select_lex->master_unit()->item= item_arg;
@@ -2455,10 +2458,10 @@ void subselect_uniquesubquery_engine::cleanup()
 }
 
 
-subselect_union_engine::subselect_union_engine(st_select_lex_unit *u,
+subselect_union_engine::subselect_union_engine(THD *thd_arg, st_select_lex_unit *u,
 					       select_result_interceptor *result_arg,
 					       Item_subselect *item_arg)
-  :subselect_engine(item_arg, result_arg)
+  :subselect_engine(thd_arg, item_arg, result_arg)
 {
   unit= u;
   if (!result_arg)				//out of memory
@@ -4164,7 +4167,7 @@ int subselect_hash_sj_engine::exec()
     if (strategy == PARTIAL_MATCH_MERGE)
     {
       pm_engine=
-        new subselect_rowid_merge_engine((subselect_uniquesubquery_engine*)
+        new subselect_rowid_merge_engine(thd, (subselect_uniquesubquery_engine*)
                                          lookup_engine, tmp_table,
                                          count_pm_keys,
                                          covering_null_row_width,
@@ -4188,7 +4191,7 @@ int subselect_hash_sj_engine::exec()
     if (strategy == PARTIAL_MATCH_SCAN)
     {
       if (!(pm_engine=
-            new subselect_table_scan_engine((subselect_uniquesubquery_engine*)
+            new subselect_table_scan_engine(thd, (subselect_uniquesubquery_engine*)
                                             lookup_engine, tmp_table,
                                             item, result,
                                             semi_join_conds->argument_list(),
@@ -4623,12 +4626,12 @@ void Ordered_key::print(String *str)
 
 
 subselect_partial_match_engine::subselect_partial_match_engine(
-  subselect_uniquesubquery_engine *engine_arg,
+  THD *thd_arg, subselect_uniquesubquery_engine *engine_arg,
   TABLE *tmp_table_arg, Item_subselect *item_arg,
   select_result_interceptor *result_arg,
   List<Item> *equi_join_conds_arg,
   uint covering_null_row_width_arg)
-  :subselect_engine(item_arg, result_arg),
+  :subselect_engine(thd_arg, item_arg, result_arg),
    tmp_table(tmp_table_arg), lookup_engine(engine_arg),
    equi_join_conds(equi_join_conds_arg),
    covering_null_row_width(covering_null_row_width_arg)
@@ -5144,13 +5147,13 @@ end:
 
 
 subselect_table_scan_engine::subselect_table_scan_engine(
-  subselect_uniquesubquery_engine *engine_arg,
+  THD *thd_arg, subselect_uniquesubquery_engine *engine_arg,
   TABLE *tmp_table_arg,
   Item_subselect *item_arg,
   select_result_interceptor *result_arg,
   List<Item> *equi_join_conds_arg,
   uint covering_null_row_width_arg)
-  :subselect_partial_match_engine(engine_arg, tmp_table_arg, item_arg,
+  :subselect_partial_match_engine(thd_arg, engine_arg, tmp_table_arg, item_arg,
                                   result_arg, equi_join_conds_arg,
                                   covering_null_row_width_arg)
 {}

sql/item_subselect.h

@@ -501,14 +501,15 @@ public:
                          INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE,
                          ROWID_MERGE_ENGINE, TABLE_SCAN_ENGINE};
 
-  subselect_engine(Item_subselect *si, select_result_interceptor *res)
-    :thd(0)
+  subselect_engine(THD *thd_arg, Item_subselect *si,
+                   select_result_interceptor *res)
   {
     result= res;
     item= si;
     res_type= STRING_RESULT;
     res_field_type= MYSQL_TYPE_VAR_STRING;
     maybe_null= 0;
+    set_thd(thd_arg);
   }
   virtual ~subselect_engine() {}; // to satisfy compiler
   virtual void cleanup()= 0;
@@ -572,7 +573,7 @@ class subselect_single_select_engine: public subselect_engine
   st_select_lex *select_lex; /* corresponding select_lex */
   JOIN * join; /* corresponding JOIN structure */
 public:
-  subselect_single_select_engine(st_select_lex *select,
+  subselect_single_select_engine(THD *thd_arg, st_select_lex *select,
 				 select_result_interceptor *result,
 				 Item_subselect *item);
   void cleanup();
@@ -601,7 +602,7 @@ class subselect_union_engine: public subselect_engine
 {
   st_select_lex_unit *unit;  /* corresponding unit structure */
 public:
-  subselect_union_engine(st_select_lex_unit *u,
+  subselect_union_engine(THD *thd_arg, st_select_lex_unit *u,
 			 select_result_interceptor *result,
 			 Item_subselect *item);
   void cleanup();
@@ -657,10 +658,8 @@ public:
   // constructor can assign THD because it will be called after JOIN::prepare
   subselect_uniquesubquery_engine(THD *thd_arg, st_join_table *tab_arg,
 				  Item_subselect *subs, Item *where)
-    :subselect_engine(subs, 0), tab(tab_arg), cond(where)
-  {
-    set_thd(thd_arg);
-  }
+    :subselect_engine(thd_arg, subs, 0), tab(tab_arg), cond(where)
+  {}
   ~subselect_uniquesubquery_engine();
   void cleanup();
   int prepare();
@@ -812,13 +811,11 @@ protected:
 public:
   subselect_hash_sj_engine(THD *thd, Item_subselect *in_predicate,
                            subselect_single_select_engine *old_engine)
-    :subselect_engine(in_predicate, NULL), tmp_table(NULL),
+    :subselect_engine(thd, in_predicate, NULL), tmp_table(NULL),
     is_materialized(FALSE), materialize_engine(old_engine), lookup_engine(NULL),
     materialize_join(NULL), count_partial_match_columns(0),
     count_null_only_columns(0), semi_join_conds(NULL), strategy(UNDEFINED)
-  {
-    set_thd(thd);
-  }
+  {}
   ~subselect_hash_sj_engine();
 
   bool init_permanent(List<Item> *tmp_columns);
@@ -1056,7 +1053,8 @@ protected:
 protected:
   virtual bool partial_match()= 0;
 public:
-  subselect_partial_match_engine(subselect_uniquesubquery_engine *engine_arg,
+  subselect_partial_match_engine(THD *thd_arg,
+                                 subselect_uniquesubquery_engine *engine_arg,
                                  TABLE *tmp_table_arg, Item_subselect *item_arg,
                                  select_result_interceptor *result_arg,
                                  List<Item> *equi_join_conds_arg,
@@ -1148,19 +1146,18 @@ protected:
   bool test_null_row(rownum_t row_num);
   bool partial_match();
 public:
-  subselect_rowid_merge_engine(subselect_uniquesubquery_engine *engine_arg,
+  subselect_rowid_merge_engine(THD *thd_arg,
+                               subselect_uniquesubquery_engine *engine_arg,
                                TABLE *tmp_table_arg, uint keys_count_arg,
                                uint covering_null_row_width_arg,
                                Item_subselect *item_arg,
                                select_result_interceptor *result_arg,
                                List<Item> *equi_join_conds_arg)
-    :subselect_partial_match_engine(engine_arg, tmp_table_arg, item_arg,
-                                    result_arg, equi_join_conds_arg,
+    :subselect_partial_match_engine(thd_arg, engine_arg, tmp_table_arg,
+                                    item_arg, result_arg, equi_join_conds_arg,
                                     covering_null_row_width_arg),
     keys_count(keys_count_arg), non_null_key(NULL)
-  {
-    thd= lookup_engine->get_thd();
-  }
+  {}
   ~subselect_rowid_merge_engine();
   bool init(MY_BITMAP *non_null_key_parts, MY_BITMAP *partial_match_key_parts);
   void cleanup();
@@ -1173,7 +1170,8 @@ class subselect_table_scan_engine: public subselect_partial_match_engine
 protected:
   bool partial_match();
 public:
-  subselect_table_scan_engine(subselect_uniquesubquery_engine *engine_arg,
+  subselect_table_scan_engine(THD *thd_arg,
+                              subselect_uniquesubquery_engine *engine_arg,
                               TABLE *tmp_table_arg, Item_subselect *item_arg,
                               select_result_interceptor *result_arg,
                               List<Item> *equi_join_conds_arg,

sql/sql_select.cc

@@ -19518,10 +19518,7 @@ void TABLE_LIST::print(THD *thd, table_map eliminated_tables, String *str,
 
 void st_select_lex::print(THD *thd, String *str, enum_query_type query_type)
 {
-  /* TODO: thd may not be set for sub queries, but this should be fixed */
   DBUG_ASSERT(thd);
-  if (!thd)
-    thd= current_thd;
 
   str->append(STRING_WITH_LEN("select "));