add const qualifiers to sys_var::value_ptr functions and fix const casts

This is important since Sys_var_typelib and its descendants return
pointers to constant symbols from *_value_ptr, which are situated in
write-protected-memory.

* functions const-qualified:
  - value_ptr
  - session_value_ptr
  - global_value_ptr
  - default_value_ptr
  - Sys_var_vers_asof::value_ptr
  - other minor private ones

* remove C-style typecasts when it discards qualifiers

sql/item_func.cc

@@ -5657,7 +5657,7 @@ void Item_func_get_system_var::update_null_value()
 
 bool Item_func_get_system_var::fix_length_and_dec()
 {
-  char *cptr;
+  const char *cptr;
   maybe_null= TRUE;
   max_length= 0;
 
@@ -5691,9 +5691,12 @@ bool Item_func_get_system_var::fix_length_and_dec()
     case SHOW_CHAR:
     case SHOW_CHAR_PTR:
       mysql_mutex_lock(&LOCK_global_system_variables);
-      cptr= var->show_type() == SHOW_CHAR ? 
-        (char*) var->value_ptr(current_thd, var_type, &component) :
-        *(char**) var->value_ptr(current_thd, var_type, &component);
+      cptr= var->show_type() == SHOW_CHAR ?
+          reinterpret_cast<const char*>(var->value_ptr(current_thd, var_type,
+                                                       &component)) :
+          *reinterpret_cast<const char* const*>(var->value_ptr(current_thd,
+                                                               var_type,
+                                                               &component));
       if (cptr)
         max_length= (uint32)system_charset_info->cset->numchars(system_charset_info,
                                                         cptr,
@@ -5706,7 +5709,10 @@ bool Item_func_get_system_var::fix_length_and_dec()
     case SHOW_LEX_STRING:
       {
         mysql_mutex_lock(&LOCK_global_system_variables);
-        LEX_STRING *ls= ((LEX_STRING*)var->value_ptr(current_thd, var_type, &component));
+        const LEX_STRING *ls=
+                reinterpret_cast<const LEX_STRING*>(var->value_ptr(current_thd,
+                                                                   var_type,
+                                                                   &component));
         max_length= (uint32)system_charset_info->cset->numchars(system_charset_info,
                                                         ls->str,
                                                         ls->str + ls->length);

sql/set_var.cc

@@ -233,12 +233,12 @@ bool sys_var::update(THD *thd, set_var *var)
   }
 }
 
-uchar *sys_var::session_value_ptr(THD *thd, const LEX_CSTRING *base)
+const uchar *sys_var::session_value_ptr(THD *thd, const LEX_CSTRING *base) const
 {
   return session_var_ptr(thd);
 }
 
-uchar *sys_var::global_value_ptr(THD *thd, const LEX_CSTRING *base)
+const uchar *sys_var::global_value_ptr(THD *thd, const LEX_CSTRING *base) const
 {
   return global_var_ptr();
 }
@@ -271,8 +271,8 @@ bool sys_var::check(THD *thd, set_var *var)
   return false;
 }
 
-uchar *sys_var::value_ptr(THD *thd, enum_var_type type,
-                          const LEX_CSTRING *base)
+const uchar *sys_var::value_ptr(THD *thd, enum_var_type type,
+                                const LEX_CSTRING *base) const
 {
   DBUG_ASSERT(base);
   if (type == OPT_GLOBAL || scope() == GLOBAL)
@@ -510,7 +510,7 @@ bool throw_bounds_warning(THD *thd, const char *name, bool fixed, double v)
   return false;
 }
 
-CHARSET_INFO *sys_var::charset(THD *thd)
+CHARSET_INFO *sys_var::charset(THD *thd) const
 {
   return is_os_charset ? thd->variables.character_set_filesystem :
     system_charset_info;
@@ -1047,7 +1047,7 @@ int set_var_collation_client::update(THD *thd)
  INFORMATION_SCHEMA.SYSTEM_VARIABLES
 *****************************************************************************/
 static void store_value_ptr(Field *field, sys_var *var, String *str,
-                            uchar *value_ptr)
+                            const uchar *value_ptr)
 {
   field->set_notnull();
   str= var->val_str_nolock(str, field->table->in_use, value_ptr);
@@ -1115,8 +1115,8 @@ int fill_sysvars(THD *thd, TABLE_LIST *tables, COND *cond)
     fields[3]->store(origin->str, origin->length, scs);
 
     // DEFAULT_VALUE
-    uchar *def= var->is_readonly() && var->option.id < 0
-                ? 0 : var->default_value_ptr(thd);
+    const uchar *def= var->is_readonly() && var->option.id < 0
+                      ? 0 : var->default_value_ptr(thd);
     if (def)
       store_value_ptr(fields[4], var, &strbuf, def);
 

sql/set_var.h

@@ -112,7 +112,7 @@ class sys_var: protected Value_source // for double_from_string_with_check
   virtual sys_var_pluginvar *cast_pluginvar() { return 0; }
 
   bool check(THD *thd, set_var *var);
-  uchar *value_ptr(THD *thd, enum_var_type type, const LEX_CSTRING *base);
+  const uchar *value_ptr(THD *thd, enum_var_type type, const LEX_CSTRING *base) const;
 
   /**
      Update the system variable with the default value from either
@@ -127,9 +127,9 @@ class sys_var: protected Value_source // for double_from_string_with_check
   String *val_str(String *str, THD *thd, enum_var_type type, const LEX_CSTRING *base);
   double val_real(bool *is_null, THD *thd, enum_var_type type, const LEX_CSTRING *base);
 
-  SHOW_TYPE show_type() { return show_val_type; }
+  SHOW_TYPE show_type() const { return show_val_type; }
   int scope() const { return flags & SCOPE_MASK; }
-  CHARSET_INFO *charset(THD *thd);
+  CHARSET_INFO *charset(THD *thd) const;
   bool is_readonly() const { return flags & READONLY; }
   /**
     the following is only true for keycache variables,
@@ -208,7 +208,7 @@ class sys_var: protected Value_source // for double_from_string_with_check
   */
   virtual bool session_is_default(THD *thd) { return false; }
 
-  virtual uchar *default_value_ptr(THD *thd)
+  virtual const uchar *default_value_ptr(THD *thd) const
   { return (uchar*)&option.def_value; }
 
 private:
@@ -230,18 +230,18 @@ class sys_var: protected Value_source // for double_from_string_with_check
     It must be of show_val_type type (my_bool for SHOW_MY_BOOL,
     int for SHOW_INT, longlong for SHOW_LONGLONG, etc).
   */
-  virtual uchar *session_value_ptr(THD *thd, const LEX_CSTRING *base);
-  virtual uchar *global_value_ptr(THD *thd, const LEX_CSTRING *base);
+  virtual const uchar *session_value_ptr(THD *thd, const LEX_CSTRING *base) const;
+  virtual const uchar *global_value_ptr(THD *thd, const LEX_CSTRING *base) const;
 
   /**
     A pointer to a storage area of the variable, to the raw data.
     Typically it's the same as session_value_ptr(), but it's different,
     for example, for ENUM, that is printed as a string, but stored as a number.
   */
-  uchar *session_var_ptr(THD *thd)
+  uchar *session_var_ptr(THD *thd) const
   { return ((uchar*)&(thd->variables)) + offset; }
 
-  uchar *global_var_ptr()
+  uchar *global_var_ptr() const
   { return ((uchar*)&global_system_variables) + offset; }
 
   void *max_var_ptr()

sql/sql_plugin.cc

@@ -293,14 +293,14 @@ class sys_var_pluginvar: public sys_var, public Sql_alloc
   sys_var_pluginvar(sys_var_chain *chain, const char *name_arg,
                     st_plugin_int *p, st_mysql_sys_var *plugin_var_arg);
   sys_var_pluginvar *cast_pluginvar() { return this; }
-  uchar* real_value_ptr(THD *thd, enum_var_type type);
-  TYPELIB* plugin_var_typelib(void);
-  uchar* do_value_ptr(THD *thd, enum_var_type type, const LEX_CSTRING *base);
-  uchar* session_value_ptr(THD *thd, const LEX_CSTRING *base)
+  uchar* real_value_ptr(THD *thd, enum_var_type type) const;
+  TYPELIB* plugin_var_typelib(void) const;
+  const uchar* do_value_ptr(THD *thd, enum_var_type type, const LEX_CSTRING *base) const;
+  const uchar* session_value_ptr(THD *thd, const LEX_CSTRING *base) const
   { return do_value_ptr(thd, OPT_SESSION, base); }
-  uchar* global_value_ptr(THD *thd, const LEX_CSTRING *base)
+  const uchar* global_value_ptr(THD *thd, const LEX_CSTRING *base) const
   { return do_value_ptr(thd, OPT_GLOBAL, base); }
-  uchar *default_value_ptr(THD *thd)
+  const uchar *default_value_ptr(THD *thd) const
   { return do_value_ptr(thd, OPT_DEFAULT, 0); }
   bool do_check(THD *thd, set_var *var);
   virtual void session_save_default(THD *thd, set_var *var) {}
@@ -3349,7 +3349,7 @@ sys_var_pluginvar::sys_var_pluginvar(sys_var_chain *chain, const char *name_arg,
   plugin_opt_set_limits(&option, pv);
 }
 
-uchar* sys_var_pluginvar::real_value_ptr(THD *thd, enum_var_type type)
+uchar* sys_var_pluginvar::real_value_ptr(THD *thd, enum_var_type type) const
 {
   if (type == OPT_DEFAULT)
   {
@@ -3423,7 +3423,7 @@ bool sys_var_pluginvar::session_is_default(THD *thd)
 }
 
 
-TYPELIB* sys_var_pluginvar::plugin_var_typelib(void)
+TYPELIB* sys_var_pluginvar::plugin_var_typelib(void) const
 {
   switch (plugin_var->flags & (PLUGIN_VAR_TYPEMASK | PLUGIN_VAR_THDLOCAL)) {
   case PLUGIN_VAR_ENUM:
@@ -3441,12 +3441,10 @@ TYPELIB* sys_var_pluginvar::plugin_var_typelib(void)
 }
 
 
-uchar* sys_var_pluginvar::do_value_ptr(THD *thd, enum_var_type type,
-                                       const LEX_CSTRING *base)
+const uchar* sys_var_pluginvar::do_value_ptr(THD *thd, enum_var_type type,
+                                             const LEX_CSTRING *base) const
 {
-  uchar* result;
-
-  result= real_value_ptr(thd, type);
+  const uchar* result= real_value_ptr(thd, type);
 
   if ((plugin_var->flags & PLUGIN_VAR_TYPEMASK) == PLUGIN_VAR_ENUM)
     result= (uchar*) get_type(plugin_var_typelib(), *(ulong*)result);

sql/sql_show.cc

@@ -3568,6 +3568,28 @@ void remove_status_vars(SHOW_VAR *list)
   }
 }
 
+/**
+  A union holding a pointer to a type that can be referred by a status variable.
+ */
+union Any_pointer {
+  const void *as_void;
+  const uchar *as_uchar;
+  const char *as_char;
+  const char ** as_charptr;
+  const double *as_double;
+  const int * as_int;
+  const uint * as_uint;
+  const long *as_long;
+  const longlong *as_longlong;
+  const bool *as_bool;
+  const my_bool *as_my_bool;
+  const sys_var *as_sys_var;
+  const system_status_var *as_system_status_var;
+  const ha_rows *as_ha_rows;
+  const LEX_STRING *as_lex_cstring;
+  const SHOW_COMP_OPTION *as_show_comp_options;
+  intptr as_intptr;
+};
 
 /**
   @brief Returns the value of a system or a status variable.
@@ -3592,16 +3614,18 @@ const char* get_one_variable(THD *thd,
                              const CHARSET_INFO **charset, char *buff,
                              size_t *length)
 {
-  void *value= variable->value;
+  Any_pointer value, status_var_value;
+  value.as_void= variable->value;
+  status_var_value.as_system_status_var= status_var;
   const char *pos= buff;
   const char *end= buff;
 
 
   if (show_type == SHOW_SYS)
   {
-    sys_var *var= (sys_var *) value;
+    const sys_var *var= value.as_sys_var;
     show_type= var->show_type();
-    value= var->value_ptr(thd, value_type, &null_clex_str);
+    value.as_uchar= var->value_ptr(thd, value_type, &null_clex_str);
     *charset= var->charset(thd);
   }
 
@@ -3611,78 +3635,76 @@ const char* get_one_variable(THD *thd,
   */
   switch (show_type) {
   case SHOW_DOUBLE_STATUS:
-    value= ((char *) status_var + (intptr) value);
+    value.as_char= status_var_value.as_char + value.as_intptr;
     /* fall through */
   case SHOW_DOUBLE:
     /* 6 is the default precision for '%f' in sprintf() */
-    end= buff + my_fcvt(*(double *) value, 6, buff, NULL);
+    end= buff + my_fcvt(*value.as_double, 6, buff, NULL);
     break;
   case SHOW_LONG_STATUS:
-    value= ((char *) status_var + (intptr) value);
+    value.as_char= status_var_value.as_char + value.as_intptr;
     /* fall through */
   case SHOW_ULONG:
   case SHOW_LONG_NOFLUSH: // the difference lies in refresh_status()
-    end= int10_to_str(*(long*) value, buff, 10);
+    end= int10_to_str(*value.as_long, buff, 10);
     break;
   case SHOW_LONGLONG_STATUS:
-    value= ((char *) status_var + (intptr) value);
+    value.as_char= status_var_value.as_char + value.as_intptr;
     /* fall through */
   case SHOW_ULONGLONG:
-    end= longlong10_to_str(*(longlong*) value, buff, 10);
+    end= longlong10_to_str(*value.as_longlong, buff, 10);
     break;
   case SHOW_HA_ROWS:
-    end= longlong10_to_str((longlong) *(ha_rows*) value, buff, 10);
+    end= longlong10_to_str((longlong) *value.as_ha_rows, buff, 10);
     break;
   case SHOW_BOOL:
-    end= strmov(buff, *(bool*) value ? "ON" : "OFF");
+    end= strmov(buff, *value.as_bool ? "ON" : "OFF");
     break;
   case SHOW_MY_BOOL:
-    end= strmov(buff, *(my_bool*) value ? "ON" : "OFF");
+    end= strmov(buff, *value.as_my_bool ? "ON" : "OFF");
     break;
   case SHOW_UINT32_STATUS:
-    value= ((char *) status_var + (intptr) value);
+    value.as_char= status_var_value.as_char + value.as_intptr;
     /* fall through */
   case SHOW_UINT:
-    end= int10_to_str((long) *(uint*) value, buff, 10);
+    end= int10_to_str((long) *value.as_uint, buff, 10);
     break;
   case SHOW_SINT:
-    end= int10_to_str((long) *(int*) value, buff, -10);
+    end= int10_to_str((long) *value.as_int, buff, -10);
     break;
   case SHOW_SLONG:
-    end= int10_to_str(*(long*) value, buff, -10);
+    end= int10_to_str(*value.as_long, buff, -10);
     break;
   case SHOW_SLONGLONG:
-    end= longlong10_to_str(*(longlong*) value, buff, -10);
+    end= longlong10_to_str(*value.as_longlong, buff, -10);
     break;
   case SHOW_HAVE:
     {
-      SHOW_COMP_OPTION tmp= *(SHOW_COMP_OPTION*) value;
-      pos= show_comp_option_name[(int) tmp];
+      pos= show_comp_option_name[(int) *value.as_show_comp_options];
       end= strend(pos);
       break;
     }
   case SHOW_CHAR:
     {
-      if (!(pos= (char*)value))
+      if (!(pos= value.as_char))
         pos= "";
       end= strend(pos);
       break;
     }
   case SHOW_CHAR_PTR:
     {
-      if (!(pos= *(char**) value))
+      if (!(pos= *value.as_charptr))
         pos= "";
 
       end= strend(pos);
       break;
     }
   case SHOW_LEX_STRING:
     {
-      LEX_STRING *ls=(LEX_STRING*)value;
-      if (!(pos= ls->str))
+      if (!(pos= value.as_lex_cstring->str))
         end= pos= "";
       else
-        end= pos + ls->length;
+        end= pos + value.as_lex_cstring->length;
       break;
     }
   case SHOW_UNDEF: