MDEV-22441 SCOPE_VALUE macro for temporary values

- Needless engaged_ removed;
  - SCOPE_VALUE, SCOPE_SET, SCOPE_CLEAR macros for neater declaration;
  - IF_CLASS / IF_NOT_CLASS SFINAE checkers to pass arg by value or
    reference;
  - inline keyword;
  - couple of refactorings of temporary free_list.

include/scope.h

@@ -32,6 +32,11 @@ public:
   {
   }
 
+  template <typename F>
+  scope_exit(F &&f, bool engaged) : function_(std::forward<F>(f)), engaged_(engaged)
+  {
+  }
+
   scope_exit(scope_exit &&rhs)
       : function_(std::move(rhs.function_)), engaged_(rhs.engaged_)
   {
@@ -43,6 +48,7 @@ public:
   scope_exit &operator=(const scope_exit &)= delete;
 
   void release() { engaged_= false; }
+  void engage() { DBUG_ASSERT(!engaged_); engaged_= true; }
 
   ~scope_exit()
   {
@@ -58,38 +64,51 @@ private:
 } // end namespace detail
 
 template <typename Callable>
-detail::scope_exit<typename std::decay<Callable>::type>
-make_scope_exit(Callable &&f)
+inline
+::detail::scope_exit<typename std::decay<Callable>::type>
+make_scope_exit(Callable &&f, bool engaged= true)
 {
-  return detail::scope_exit<typename std::decay<Callable>::type>(
-      std::forward<Callable>(f));
+  return ::detail::scope_exit<typename std::decay<Callable>::type>(
+      std::forward<Callable>(f), engaged);
 }
 
 #define CONCAT_IMPL(x, y) x##y
-
 #define CONCAT(x, y) CONCAT_IMPL(x, y)
-
 #define ANONYMOUS_VARIABLE CONCAT(_anonymous_variable, __LINE__)
 
 #define SCOPE_EXIT auto ANONYMOUS_VARIABLE= make_scope_exit
 
+#define IF_CLASS(C) typename std::enable_if<std::is_class<C>::value>::type
+#define IF_NOT_CLASS(C) typename std::enable_if<!std::is_class<C>::value>::type
+
 namespace detail
 {
 
-template <typename T> class Scope_value
+template <typename T>
+class Scope_value
 {
 public:
+  // Use SFINAE for passing structs by reference and plain types by value.
+  // This ctor is defined only if T is a class or struct:
+  template <typename U = T, typename = IF_CLASS(U)>
   Scope_value(T &variable, const T &scope_value)
-      : variable_(variable), saved_value_(variable)
+      : variable_(&variable), saved_value_(variable)
+  {
+    variable= scope_value;
+  }
+
+  // This ctor is defined only if T is NOT a class or struct:
+  template <typename U = T, typename = IF_NOT_CLASS(U)>
+  Scope_value(T &variable, const T scope_value)
+      : variable_(&variable), saved_value_(variable)
   {
     variable= scope_value;
   }
 
   Scope_value(Scope_value &&rhs)
-      : variable_(rhs.variable_), saved_value_(rhs.saved_value_),
-        engaged_(rhs.engaged_)
+      : variable_(rhs.variable_), saved_value_(rhs.saved_value_)
   {
-    rhs.engaged_= false;
+    rhs.variable_= NULL;
   }
 
   Scope_value(const Scope_value &)= delete;
@@ -98,22 +117,50 @@ public:
 
   ~Scope_value()
   {
-    if (engaged_)
-      variable_= saved_value_;
+    if (variable_)
+      *variable_= saved_value_;
   }
 
 private:
-  T &variable_;
+  T *variable_;
   T saved_value_;
-  bool engaged_= true;
 };
 
 } // namespace detail
 
 // Use like this:
 // auto _= make_scope_value(var, tmp_value);
-template <typename T>
-detail::Scope_value<T> make_scope_value(T &variable, const T &scope_value)
+
+template <typename T, typename = IF_CLASS(T)>
+inline
+::detail::Scope_value<T> make_scope_value(T &variable, const T &scope_value)
 {
-  return detail::Scope_value<T>(variable, scope_value);
+  return ::detail::Scope_value<T>(variable, scope_value);
 }
+
+template <typename T, typename = IF_NOT_CLASS(T)>
+inline
+::detail::Scope_value<T> make_scope_value(T &variable, T scope_value)
+{
+  return ::detail::Scope_value<T>(variable, scope_value);
+}
+
+/*
+  Note: perfect forwarding version can not pass const:
+
+  template <typename T, typename U>
+  inline
+  detail::Scope_value<T> make_scope_value(T &variable, U &&scope_value)
+  {
+      return detail::Scope_value<T>(variable, std::forward<U>(scope_value));
+  }
+
+  as `const U &&` fails with error `expects an rvalue for 2nd argument`. That
+  happens because const U && is treated as rvalue only (this is the exact syntax
+  for declaring rvalues).
+*/
+
+
+#define SCOPE_VALUE auto ANONYMOUS_VARIABLE= make_scope_value
+#define SCOPE_SET(VAR, MASK) auto ANONYMOUS_VARIABLE= make_scope_value(VAR, VAR | MASK)
+#define SCOPE_CLEAR(VAR, MASK) auto ANONYMOUS_VARIABLE= make_scope_value(VAR, VAR & ~MASK)

sql/sql_acl.cc

@@ -2555,9 +2555,7 @@ static bool acl_load(THD *thd, const Grant_tables& tables)
   char tmp_name[SAFE_NAME_LEN+1];
   DBUG_ENTER("acl_load");
 
-  auto _= make_scope_value(thd->variables.sql_mode,
-                           thd->variables.sql_mode &
-                               ~MODE_PAD_CHAR_TO_FULL_LENGTH);
+  SCOPE_CLEAR(thd->variables.sql_mode, MODE_PAD_CHAR_TO_FULL_LENGTH);
 
   grant_version++; /* Privileges updated */
 

sql/sql_prepare.cc

@@ -2993,8 +2993,8 @@ void mysql_sql_stmt_execute_immediate(THD *thd)
     DBUG_VOID_RETURN;                           // out of memory
 
   // See comments on thd->free_list in mysql_sql_stmt_execute()
-  Item *free_list_backup= thd->free_list;
-  thd->free_list= NULL;
+  SCOPE_VALUE(thd->free_list, (Item *) NULL);
+  SCOPE_EXIT([thd]() mutable { thd->free_items(); });
   /*
     Make sure we call Prepared_statement::execute_immediate()
     with an empty THD::change_list. It can be non empty as the above
@@ -3017,8 +3017,6 @@ void mysql_sql_stmt_execute_immediate(THD *thd)
   Item_change_list_savepoint change_list_savepoint(thd);
   (void) stmt->execute_immediate(query.str, (uint) query.length);
   change_list_savepoint.rollback(thd);
-  thd->free_items();
-  thd->free_list= free_list_backup;
 
   /*
     stmt->execute_immediately() sets thd->query_string with the executed
@@ -3578,8 +3576,13 @@ void mysql_sql_stmt_execute(THD *thd)
     so they don't get freed in case of re-prepare.
     See MDEV-10702 Crash in SET STATEMENT FOR EXECUTE
   */
-  Item *free_list_backup= thd->free_list;
-  thd->free_list= NULL; // Hide the external (e.g. "SET STATEMENT") Items
+  /*
+    Hide and restore at scope exit the "external" (e.g. "SET STATEMENT") Item list.
+    It will be freed normaly in THD::cleanup_after_query().
+  */
+  SCOPE_VALUE(thd->free_list, (Item *) NULL);
+  // Free items created by execute_loop() at scope exit
+  SCOPE_EXIT([thd]() mutable { thd->free_items(); });
   /*
     Make sure we call Prepared_statement::execute_loop() with an empty
     THD::change_list. It can be non-empty because the above
@@ -3603,12 +3606,6 @@ void mysql_sql_stmt_execute(THD *thd)
 
   (void) stmt->execute_loop(&expanded_query, FALSE, NULL, NULL);
   change_list_savepoint.rollback(thd);
-  thd->free_items();    // Free items created by execute_loop()
-  /*
-    Now restore the "external" (e.g. "SET STATEMENT") Item list.
-    It will be freed normaly in THD::cleanup_after_query().
-  */
-  thd->free_list= free_list_backup;
 
   stmt->lex->restore_set_statement_var();
   DBUG_VOID_RETURN;

sql/sql_show.cc

@@ -6438,7 +6438,7 @@ bool store_schema_params(THD *thd, TABLE *table, TABLE *proc_table,
   {
     Field *field;
     LEX_CSTRING tmp_string;
-    auto _= make_scope_value(thd->variables.sql_mode, sql_mode);
+    SCOPE_VALUE(thd->variables.sql_mode, sql_mode);
 
     if (sph->type() == SP_TYPE_FUNCTION)
     {