Preliminary patch for Bug#11848763 / 60025

(SUBSTRING inside a stored function works too slow).

Background:
  - THD classes derives from Query_arena, thus inherits the 'state'
    attribute and related operations (is_stmt_prepare() & co).

  - Although these operations are available in THD, they must not
    be used. THD has its own attribute to point to the active
    Query_arena -- stmt_arena.

  - So, instead of using thd->is_stmt_prepare(),
    thd->stmt_arena->is_stmt_prepare() must be used. This was the root
    cause of Bug 60025.

This patch enforces the proper way of calling those operations.
is_stmt_prepare() & co are declared as private operations
in THD (thus, they are hidden from being called on THD instance).

The patch tries to minimize changes in 5.5.

sql/item.cc

@@ -581,7 +581,7 @@ void Item::rename(char *new_name)
 
 Item* Item::transform(Item_transformer transformer, uchar *arg)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   return (this->*transformer)(arg);
 }
@@ -1845,7 +1845,7 @@ bool agg_item_set_converter(DTCollation &coll, const char *fname,
       been created in prepare. In this case register the change for
       rollback.
     */
-    if (thd->is_stmt_prepare())
+    if (thd->stmt_arena->is_stmt_prepare())
       *arg= conv;
     else
       thd->change_item_tree(arg, conv);
@@ -6965,7 +6965,7 @@ int Item_default_value::save_in_field(Field *field_arg, bool no_conversions)
 
 Item *Item_default_value::transform(Item_transformer transformer, uchar *args)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   /*
     If the value of arg is NULL, then this object represents a constant,

sql/item_cmpfunc.cc

@@ -4345,7 +4345,7 @@ bool Item_cond::walk(Item_processor processor, bool walk_subquery, uchar *arg)
 
 Item *Item_cond::transform(Item_transformer transformer, uchar *arg)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   List_iterator<Item> li(list);
   Item *item;
@@ -5718,7 +5718,7 @@ bool Item_equal::walk(Item_processor processor, bool walk_subquery, uchar *arg)
 
 Item *Item_equal::transform(Item_transformer transformer, uchar *arg)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   List_iterator<Item_field> it(fields);
   Item *item;

sql/item_func.cc

@@ -293,7 +293,7 @@ void Item_func::traverse_cond(Cond_traverser traverser,
 
 Item *Item_func::transform(Item_transformer transformer, uchar *argument)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   if (arg_count)
   {

sql/item_row.cc

@@ -170,7 +170,7 @@ bool Item_row::walk(Item_processor processor, bool walk_subquery, uchar *arg)
 
 Item *Item_row::transform(Item_transformer transformer, uchar *arg)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   for (uint i= 0; i < arg_count; i++)
   {

sql/item_strfunc.cc

@@ -2536,7 +2536,7 @@ String *Item_func_make_set::val_str(String *str)
 
 Item *Item_func_make_set::transform(Item_transformer transformer, uchar *arg)
 {
-  DBUG_ASSERT(!current_thd->is_stmt_prepare());
+  DBUG_ASSERT(!current_thd->stmt_arena->is_stmt_prepare());
 
   Item *new_item= item->transform(transformer, arg);
   if (!new_item)

sql/sql_class.h

@@ -655,15 +655,10 @@ class Query_arena
   virtual ~Query_arena() {};
 
   inline bool is_stmt_prepare() const { return state == INITIALIZED; }
-  inline bool is_first_sp_execute() const
-  { return state == INITIALIZED_FOR_SP; }
   inline bool is_stmt_prepare_or_first_sp_execute() const
   { return (int)state < (int)PREPARED; }
   inline bool is_stmt_prepare_or_first_stmt_execute() const
   { return (int)state <= (int)PREPARED; }
-  inline bool is_first_stmt_execute() const { return state == PREPARED; }
-  inline bool is_stmt_execute() const
-  { return state == PREPARED || state == EXECUTED; }
   inline bool is_conventional() const
   { return state == CONVENTIONAL_EXECUTION; }
 
@@ -1434,6 +1429,19 @@ extern "C" void my_message_sql(uint error, const char *str, myf MyFlags);
 class THD :public Statement,
            public Open_tables_state
 {
+private:
+  inline bool is_stmt_prepare() const
+  { DBUG_ASSERT(0); return Statement::is_stmt_prepare(); }
+
+  inline bool is_stmt_prepare_or_first_sp_execute() const
+  { DBUG_ASSERT(0); return Statement::is_stmt_prepare_or_first_sp_execute(); }
+
+  inline bool is_stmt_prepare_or_first_stmt_execute() const
+  { DBUG_ASSERT(0); return Statement::is_stmt_prepare_or_first_stmt_execute(); }
+
+  inline bool is_conventional() const
+  { DBUG_ASSERT(0); return Statement::is_conventional(); }
+
 public:
   MDL_context mdl_context;