path: root/sql/sys_vars.h

/* Copyright (C) 2002-2006 MySQL AB, 2009-2010 Sun Microsystems, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */

/**
  @file
  "private" interface to sys_var - server configuration variables.

  This header is included only by the file that contains declarations
  of sys_var variables (sys_vars.cc).
*/

#include "sys_vars_shared.h"
#include <my_getopt.h>
#include <my_bit.h>
#include <my_dir.h>
#include "keycaches.h"
#include "strfunc.h"
#include "tztime.h"     // my_tz_find, my_tz_SYSTEM, struct Time_zone

/*
  a set of mostly trivial (as in f(X)=X) defines below to make system variable
  declarations more readable
*/
#define VALID_RANGE(X,Y) X,Y
#define DEFAULT(X) X
#define BLOCK_SIZE(X) X
#define GLOBAL_VAR(X) sys_var::GLOBAL, (((char*)&(X))-(char*)&global_system_variables), sizeof(X)
#define SESSION_VAR(X) sys_var::SESSION, offsetof(SV, X), sizeof(((SV *)0)->X)
#define SESSION_ONLY(X) sys_var::ONLY_SESSION, offsetof(SV, X), sizeof(((SV *)0)->X)
#define NO_CMD_LINE CMD_LINE(NO_ARG, -1)
/*
  the define below means that there's no *second* mutex guard,
  LOCK_global_system_variables always guards all system variables
*/
#define NO_MUTEX_GUARD ((PolyLock*)0)
#define IN_BINLOG sys_var::SESSION_VARIABLE_IN_BINLOG
#define NOT_IN_BINLOG sys_var::VARIABLE_NOT_IN_BINLOG
#define ON_READ(X) X
#define ON_CHECK(X) X
#define ON_UPDATE(X) X
#define READ_ONLY sys_var::READONLY+
// this means that Sys_var_charptr initial value was malloc()ed
#define PREALLOCATED sys_var::ALLOCATED+
/*
  Sys_var_bit meaning is reversed, like in
  @@foreign_key_checks <-> OPTION_NO_FOREIGN_KEY_CHECKS
*/
#define REVERSE(X) ~(X)
#define DEPRECATED(X, Y) X, Y

#define session_var(THD, TYPE) (*(TYPE*)session_var_ptr(THD))
#define global_var(TYPE) (*(TYPE*)global_var_ptr())

#if SIZEOF_OFF_T > 4 && defined(BIG_TABLES)
#define GET_HA_ROWS GET_ULL
#else
#define GET_HA_ROWS GET_ULONG
#endif

enum charset_enum {IN_SYSTEM_CHARSET, IN_FS_CHARSET};

static const char *bool_values[3]= {"OFF", "ON", 0};
TYPELIB bool_typelib={ array_elements(bool_values)-1, "", bool_values, 0 };

/**
  A small wrapper class to pass getopt arguments as a pair
  to the Sys_var_* constructors. It improves type safety and helps
  to catch errors in the argument order.
*/
struct CMD_LINE
{
  int id;
  enum get_opt_arg_type arg_type;
  CMD_LINE(enum get_opt_arg_type getopt_arg_type, int getopt_id=0)
    : id(getopt_id), arg_type(getopt_arg_type) {}
};

/**
  Sys_var_unsigned template is used to generate Sys_var_* classes
  for variables that represent the value as an unsigned integer.
  They are Sys_var_uint, Sys_var_ulong, Sys_var_harows, Sys_var_ulonglong.

  An integer variable has a minimal and maximal values, and a "block_size"
  (any valid value of the variable must be divisible by the block_size).

  Class specific constructor arguments: min, max, block_size
  Backing store: uint, ulong, ha_rows, ulonglong, depending on the Sys_var_*
*/
template <typename T, ulong ARGT, enum enum_mysql_show_type SHOWT>
class Sys_var_unsigned: public sys_var
{
public:
  Sys_var_unsigned(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          T min_val, T max_val, T def_val, uint block_size, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOWT, def_val, lock, binlog_status_arg,
              on_check_func, on_update_func, deprecated_version,
              substitute, parse_flag)
  {
    option.var_type= ARGT;
    option.min_value= min_val;
    option.max_value= max_val;
    option.block_size= block_size;
    option.u_max_value= (uchar**)max_var_ptr();
    if (max_var_ptr())
      *max_var_ptr()= max_val;
    global_var(T)= def_val;
    DBUG_ASSERT(size == sizeof(T));
    DBUG_ASSERT(min_val < max_val);
    DBUG_ASSERT(min_val <= def_val);
    DBUG_ASSERT(max_val >= def_val);
    DBUG_ASSERT(block_size > 0);
    DBUG_ASSERT(def_val % block_size == 0);
  }
  bool do_check(THD *thd, set_var *var)
  {
    my_bool fixed= FALSE;
    ulonglong uv;
    longlong v;

    v= var->value->val_int();
    if (var->value->unsigned_flag)
      uv= (ulonglong) v;
    else
      uv= (ulonglong) (v < 0 ? 0 : v);

    var->save_result.ulonglong_value=
      getopt_ull_limit_value(uv, &option, &fixed);

    if (max_var_ptr() && var->save_result.ulonglong_value > *max_var_ptr())
      var->save_result.ulonglong_value= *max_var_ptr();

    return throw_bounds_warning(thd, name.str,
                                var->save_result.ulonglong_value != uv,
                                var->value->unsigned_flag, v);
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, T)= var->save_result.ulonglong_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(T)= var->save_result.ulonglong_value;
    return false;
  }
  bool check_update_type(Item_result type)
  { return type != INT_RESULT; }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= (ulonglong)*(T*)global_value_ptr(thd, 0); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
  private:
  T *max_var_ptr()
  {
    return scope() == SESSION ? (T*)(((uchar*)&max_system_variables) + offset)
                              : 0;
  }
};

typedef Sys_var_unsigned<uint, GET_UINT, SHOW_INT> Sys_var_uint;
typedef Sys_var_unsigned<ulong, GET_ULONG, SHOW_LONG> Sys_var_ulong;
typedef Sys_var_unsigned<ha_rows, GET_HA_ROWS, SHOW_HA_ROWS> Sys_var_harows;
typedef Sys_var_unsigned<ulonglong, GET_ULL, SHOW_LONGLONG> Sys_var_ulonglong;

/**
  Helper class for variables that take values from a TYPELIB
*/
class Sys_var_typelib: public sys_var
{
protected:
  TYPELIB typelib;
public:
  Sys_var_typelib(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off,
          CMD_LINE getopt,
          SHOW_TYPE show_val_type_arg, const char *values[],
          ulonglong def_val, PolyLock *lock,
          enum binlog_status_enum binlog_status_arg,
          on_check_function on_check_func, on_update_function on_update_func,
          uint deprecated_version, const char *substitute, int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, show_val_type_arg, def_val, lock,
              binlog_status_arg, on_check_func,
              on_update_func, deprecated_version, substitute, parse_flag)
  {
    for (typelib.count= 0; values[typelib.count]; typelib.count++) /*no-op */;
    typelib.name="";
    typelib.type_names= values;
    typelib.type_lengths= 0;    // only used by Fields_enum and Field_set
    option.typelib= &typelib;
  }
  bool do_check(THD *thd, set_var *var) // works for enums and my_bool
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), system_charset_info), *res;

    if (var->value->result_type() == STRING_RESULT)
    {
      if (!(res=var->value->val_str(&str)))
        return true;
      else
      if (!(var->save_result.ulonglong_value=
            find_type(&typelib, res->ptr(), res->length(), false)))
        return true;
      else
        var->save_result.ulonglong_value--;
    }
    else
    {
      longlong tmp=var->value->val_int();
      if (tmp < 0 || tmp >= typelib.count)
        return true;
      else
        var->save_result.ulonglong_value= tmp;
    }

    return false;
  }
  bool check_update_type(Item_result type)
  { return type != INT_RESULT && type != STRING_RESULT; }
};

/**
  The class for ENUM variables - variables that take one value from a fixed
  list of values. 

  Class specific constructor arguments:
    char* values[]    - 0-terminated list of strings of valid values

  Backing store: uint

  @note
  Do *not* use "enum FOO" variables as a backing store, there is no
  guarantee that sizeof(enum FOO) == sizeof(uint), there is no guarantee
  even that sizeof(enum FOO) == sizeof(enum BAR)
*/
class Sys_var_enum: public Sys_var_typelib
{
public:
  Sys_var_enum(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          const char *values[], uint def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_typelib(name_arg, comment, flag_args, off, getopt,
                      SHOW_CHAR, values, def_val, lock,
                      binlog_status_arg, on_check_func, on_update_func,
                      deprecated_version, substitute)
  {
    option.var_type= GET_ENUM;
    global_var(uint)= def_val;
    DBUG_ASSERT(def_val < typelib.count);
    DBUG_ASSERT(size == sizeof(uint));
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, uint)= var->save_result.ulonglong_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(uint)= var->save_result.ulonglong_value;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= global_var(uint); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  { return (uchar*)typelib.type_names[session_var(thd, uint)]; }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  { return (uchar*)typelib.type_names[global_var(uint)]; }
};

/**
  The class for boolean variables - a variant of ENUM variables
  with the fixed list of values of { OFF , ON }

  Backing store: my_bool
*/
class Sys_var_mybool: public Sys_var_typelib
{
public:
  Sys_var_mybool(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          my_bool def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : Sys_var_typelib(name_arg, comment, flag_args, off, getopt,
                      SHOW_MY_BOOL, bool_values, def_val, lock,
                      binlog_status_arg, on_check_func, on_update_func,
                      deprecated_version, substitute, parse_flag)
  {
    option.var_type= GET_BOOL;
    global_var(my_bool)= def_val;
    DBUG_ASSERT(def_val < 2);
    DBUG_ASSERT(getopt.arg_type == OPT_ARG || getopt.id == -1);
    DBUG_ASSERT(size == sizeof(my_bool));
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, my_bool)= var->save_result.ulonglong_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(my_bool)= var->save_result.ulonglong_value;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= (ulonglong)*(my_bool *)global_value_ptr(thd, 0); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
};

/**
  The class for string variables. The string can be in character_set_filesystem
  or in character_set_system. The string can be allocated with my_malloc()
  or not. The state of the initial value is specified in the constructor,
  after that it's managed automatically. The value of NULL is supported.

  Class specific constructor arguments:
    enum charset_enum is_os_charset_arg

  Backing store: char*

  @note
  This class supports only GLOBAL variables, because THD on destruction
  does not destroy individual members of SV, there's no way to free
  allocated string variables for every thread.
*/
class Sys_var_charptr: public sys_var
{
public:
  Sys_var_charptr(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          enum charset_enum is_os_charset_arg,
          const char *def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_CHAR_PTR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  {
    is_os_charset= is_os_charset_arg == IN_FS_CHARSET;
    /*
     use GET_STR_ALLOC - if ALLOCATED it must be *always* allocated,
     otherwise (GET_STR) you'll never know whether to free it or not.
     (think of an exit because of an error right after my_getopt)
    */
    option.var_type= (flags & ALLOCATED) ? GET_STR_ALLOC : GET_STR;
    global_var(const char*)= def_val;
    DBUG_ASSERT(scope() == GLOBAL);
    DBUG_ASSERT(size == sizeof(char *));
  }
  ~Sys_var_charptr()
  {
    if (flags & ALLOCATED)
      my_free(global_var(char*));
    flags&= ~ALLOCATED;
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE], buff2[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), charset(thd));
    String str2(buff2, sizeof(buff2), charset(thd)), *res;

    if (!(res=var->value->val_str(&str)))
      var->save_result.string_value.str= 0;
    else
    {
      uint32 unused;
      if (String::needs_conversion(res->length(), res->charset(),
                                   charset(thd), &unused))
      {
        uint errors;
        str2.copy(res->ptr(), res->length(), res->charset(), charset(thd),
                  &errors);
        res=&str2;

      }
      var->save_result.string_value.str= thd->strmake(res->ptr(), res->length());
      var->save_result.string_value.length= res->length();
    }

    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  bool global_update(THD *thd, set_var *var)
  {
    char *new_val, *ptr= var->save_result.string_value.str;
    size_t len=var->save_result.string_value.length;
    if (ptr)
    {
      new_val= (char*)my_memdup(ptr, len+1, MYF(MY_WME));
      if (!new_val) return true;
      new_val[len]=0;
    }
    else
      new_val= 0;
    if (flags & ALLOCATED)
      my_free(global_var(char*));
    flags|= ALLOCATED;
    global_var(char*)= new_val;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { DBUG_ASSERT(FALSE); }
  void global_save_default(THD *thd, set_var *var)
  {
    char *ptr= (char*)(intptr)option.def_value;
    var->save_result.string_value.str= ptr;
    var->save_result.string_value.length= ptr ? strlen(ptr) : 0;
  }
  bool check_update_type(Item_result type)
  { return type != STRING_RESULT; }
};


class Sys_var_proxy_user: public sys_var
{
public:
  Sys_var_proxy_user(const char *name_arg,
          const char *comment, enum charset_enum is_os_charset_arg)
    : sys_var(&all_sys_vars, name_arg, comment, 
              sys_var::READONLY+sys_var::ONLY_SESSION, 0, -1,
              NO_ARG, SHOW_CHAR, (intptr)NULL,
              0, VARIABLE_NOT_IN_BINLOG,
              0, 0,
              0, 0, PARSE_NORMAL)
  {
    is_os_charset= is_os_charset_arg == IN_FS_CHARSET;
    option.var_type= GET_STR;
  }
  bool do_check(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  bool session_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  bool global_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { DBUG_ASSERT(FALSE); }
  void global_save_default(THD *thd, set_var *var)
  { DBUG_ASSERT(FALSE); }
  bool check_update_type(Item_result type)
  { return true; }
protected:
  virtual uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    return thd->security_ctx->proxy_user[0] ?
      (uchar *) &(thd->security_ctx->proxy_user[0]) : NULL;
  }
};

class Sys_var_external_user : public Sys_var_proxy_user
{
public:
  Sys_var_external_user(const char *name_arg, const char *comment_arg, 
          enum charset_enum is_os_charset_arg) 
    : Sys_var_proxy_user (name_arg, comment_arg, is_os_charset_arg)
  {}

protected:
  virtual uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    return thd->security_ctx->proxy_user[0] ?
      (uchar *) &(thd->security_ctx->proxy_user[0]) : NULL;
  }
};

/**
  The class for string variables. Useful for strings that aren't necessarily
  \0-terminated. Otherwise the same as Sys_var_charptr.

  Class specific constructor arguments:
    enum charset_enum is_os_charset_arg

  Backing store: LEX_STRING

  @note
  Behaves exactly as Sys_var_charptr, only the backing store is different.
*/
class Sys_var_lexstring: public Sys_var_charptr
{
public:
  Sys_var_lexstring(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          enum charset_enum is_os_charset_arg,
          const char *def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_charptr(name_arg, comment, flag_args, off, sizeof(char*),
              getopt, is_os_charset_arg, def_val, lock, binlog_status_arg,
              on_check_func, on_update_func, deprecated_version, substitute)
  {
    global_var(LEX_STRING).length= strlen(def_val);
    DBUG_ASSERT(size == sizeof(LEX_STRING));
    *const_cast<SHOW_TYPE*>(&show_val_type)= SHOW_LEX_STRING;
  }
  bool global_update(THD *thd, set_var *var)
  {
    if (Sys_var_charptr::global_update(thd, var))
      return true;
    global_var(LEX_STRING).length= var->save_result.string_value.length;
    return false;
  }
};

#ifndef DBUG_OFF
/**
  @@session.dbug and @@global.dbug variables.

  @@dbug variable differs from other variables in one aspect:
  if its value is not assigned in the session, it "points" to the global
  value, and so when the global value is changed, the change
  immediately takes effect in the session.

  This semantics is intentional, to be able to debug one session from
  another.
*/
class Sys_var_dbug: public sys_var
{
public:
  Sys_var_dbug(const char *name_arg,
               const char *comment, int flag_args,
               CMD_LINE getopt,
               const char *def_val, PolyLock *lock=0,
               enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
               on_check_function on_check_func=0,
               on_update_function on_update_func=0,
               uint deprecated_version=0, const char *substitute=0,
               int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, 0, getopt.id,
              getopt.arg_type, SHOW_CHAR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  { option.var_type= GET_NO_ARG; }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), system_charset_info), *res;

    if (!(res=var->value->val_str(&str)))
      var->save_result.string_value.str= const_cast<char*>("");
    else
      var->save_result.string_value.str= thd->strmake(res->ptr(), res->length());
    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    const char *val= var->save_result.string_value.str;
    if (!var->value)
      DBUG_POP();
    else
      DBUG_SET(val);
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    const char *val= var->save_result.string_value.str;
    DBUG_SET_INITIAL(val);
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { }
  void global_save_default(THD *thd, set_var *var)
  {
    char *ptr= (char*)(intptr)option.def_value;
    var->save_result.string_value.str= ptr;
  }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    char buf[256];
    DBUG_EXPLAIN(buf, sizeof(buf));
    return (uchar*) thd->strdup(buf);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    char buf[256];
    DBUG_EXPLAIN_INITIAL(buf, sizeof(buf));
    return (uchar*) thd->strdup(buf);
  }
  bool check_update_type(Item_result type)
  { return type != STRING_RESULT; }
};
#endif

#define KEYCACHE_VAR(X) sys_var::GLOBAL,offsetof(KEY_CACHE, X), sizeof(((KEY_CACHE *)0)->X)
#define keycache_var_ptr(KC, OFF) (((uchar*)(KC))+(OFF))
#define keycache_var(KC, OFF) (*(ulonglong*)keycache_var_ptr(KC, OFF))
typedef bool (*keycache_update_function)(THD *, KEY_CACHE *, ptrdiff_t, ulonglong);

/**
  The class for keycache_* variables. Supports structured names,
  keycache_name.variable_name.

  Class specific constructor arguments:
    everything derived from Sys_var_ulonglong

  Backing store: ulonglong

  @note these variables can be only GLOBAL
*/
class Sys_var_keycache: public Sys_var_ulonglong
{
  keycache_update_function keycache_update;
public:
  Sys_var_keycache(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          ulonglong min_val, ulonglong max_val, ulonglong def_val,
          ulonglong block_size, PolyLock *lock,
          enum binlog_status_enum binlog_status_arg,
          on_check_function on_check_func,
          keycache_update_function on_update_func,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_ulonglong(name_arg, comment, flag_args, off, size,
              getopt, min_val, max_val, def_val,
              block_size, lock, binlog_status_arg, on_check_func, 0,
              deprecated_version, substitute),
    keycache_update(on_update_func)
  {
    option.var_type|= GET_ASK_ADDR;
    option.value= (uchar**)1; // crash me, please
    keycache_var(dflt_key_cache, off)= def_val;
    DBUG_ASSERT(scope() == GLOBAL);
  }
  bool global_update(THD *thd, set_var *var)
  {
    ulonglong new_value= var->save_result.ulonglong_value;
    LEX_STRING *base_name= &var->base;
    KEY_CACHE *key_cache;

    /* If no basename, assume it's for the key cache named 'default' */
    if (!base_name->length)
      base_name= &default_key_cache_base;

    key_cache= get_key_cache(base_name);

    if (!key_cache)
    {                                           // Key cache didn't exists */
      if (!new_value)                           // Tried to delete cache
        return false;                           // Ok, nothing to do
      if (!(key_cache= create_key_cache(base_name->str, base_name->length)))
        return true;
    }

    /**
      Abort if some other thread is changing the key cache
      @todo This should be changed so that we wait until the previous
      assignment is done and then do the new assign
    */
    if (key_cache->in_init)
      return true;

    return keycache_update(thd, key_cache, offset, new_value);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    KEY_CACHE *key_cache= get_key_cache(base);
    if (!key_cache)
      key_cache= &zero_key_cache;
    return keycache_var_ptr(key_cache, offset);
  }
};

static bool update_buffer_size(THD *thd, KEY_CACHE *key_cache,
                               ptrdiff_t offset, ulonglong new_value)
{
  bool error= false;
  DBUG_ASSERT(offset == offsetof(KEY_CACHE, param_buff_size));

  if (new_value == 0)
  {
    if (key_cache == dflt_key_cache)
    {
      my_error(ER_WARN_CANT_DROP_DEFAULT_KEYCACHE, MYF(0));
      return true;
    }

    if (key_cache->key_cache_inited)            // If initied
    {
      /*
        Move tables using this key cache to the default key cache
        and clear the old key cache.
      */
      key_cache->in_init= 1;
      mysql_mutex_unlock(&LOCK_global_system_variables);
      key_cache->param_buff_size= 0;
      ha_resize_key_cache(key_cache);
      ha_change_key_cache(key_cache, dflt_key_cache);
      /*
        We don't delete the key cache as some running threads my still be in
        the key cache code with a pointer to the deleted (empty) key cache
      */
      mysql_mutex_lock(&LOCK_global_system_variables);
      key_cache->in_init= 0;
    }
    return error;
  }

  key_cache->param_buff_size= new_value;

  /* If key cache didn't exist initialize it, else resize it */
  key_cache->in_init= 1;
  mysql_mutex_unlock(&LOCK_global_system_variables);

  if (!key_cache->key_cache_inited)
    error= ha_init_key_cache(0, key_cache);
  else
    error= ha_resize_key_cache(key_cache);

  mysql_mutex_lock(&LOCK_global_system_variables);
  key_cache->in_init= 0;

  return error;
}

static bool update_keycache_param(THD *thd, KEY_CACHE *key_cache,
                                  ptrdiff_t offset, ulonglong new_value)
{
  bool error= false;
  DBUG_ASSERT(offset != offsetof(KEY_CACHE, param_buff_size));

  keycache_var(key_cache, offset)= new_value;

  key_cache->in_init= 1;
  mysql_mutex_unlock(&LOCK_global_system_variables);
  error= ha_resize_key_cache(key_cache);

  mysql_mutex_lock(&LOCK_global_system_variables);
  key_cache->in_init= 0;

  return error;
}

/**
  The class for floating point variables

  Class specific constructor arguments: min, max

  Backing store: double
*/
class Sys_var_double: public sys_var
{
public:
  Sys_var_double(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          double min_val, double max_val, double def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_DOUBLE, (longlong) double2ulonglong(def_val),
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  {
    option.var_type= GET_DOUBLE;
    option.min_value= (longlong) double2ulonglong(min_val);
    option.max_value= (longlong) double2ulonglong(max_val);
    global_var(double)= (double)option.def_value;
    DBUG_ASSERT(min_val < max_val);
    DBUG_ASSERT(min_val <= def_val);
    DBUG_ASSERT(max_val >= def_val);
    DBUG_ASSERT(size == sizeof(double));
  }
  bool do_check(THD *thd, set_var *var)
  {
    my_bool fixed;
    double v= var->value->val_real();
    var->save_result.double_value= getopt_double_limit_value(v, &option, &fixed);

    return throw_bounds_warning(thd, name.str, fixed, v);
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, double)= var->save_result.double_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(double)= var->save_result.double_value;
    return false;
  }
  bool check_update_type(Item_result type)
  {
    return type != INT_RESULT && type != REAL_RESULT && type != DECIMAL_RESULT;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.double_value= global_var(double); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.double_value= (double)option.def_value; }
};

/**
  The class for the @max_user_connections.
  It's derived from Sys_var_uint, but non-standard session value
  requires a new class.

  Class specific constructor arguments:
    everything derived from Sys_var_uint

  Backing store: uint
*/
class Sys_var_max_user_conn: public Sys_var_uint
{
public:
  Sys_var_max_user_conn(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          uint min_val, uint max_val, uint def_val,
          uint block_size, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_uint(name_arg, comment, SESSION, off, size, getopt,
              min_val, max_val, def_val, block_size,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute)
  { }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    if (thd->user_connect && thd->user_connect->user_resources.user_conn)
      return (uchar*) &(thd->user_connect->user_resources.user_conn);
    return global_value_ptr(thd, base);
  }
};

// overflow-safe (1 << X)-1
#define MAX_SET(X) ((((1UL << ((X)-1))-1) << 1) | 1)

/**
  The class for flagset variables - a variant of SET that allows in-place
  editing (turning on/off individual bits). String representations looks like
  a "flag=val,flag=val,...". Example: @@optimizer_switch

  Class specific constructor arguments:
    char* values[]    - 0-terminated list of strings of valid values

  Backing store: ulonglong

  @note
  the last value in the values[] array should
  *always* be the string "default".
*/
class Sys_var_flagset: public Sys_var_typelib
{
public:
  Sys_var_flagset(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          const char *values[], ulonglong def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_typelib(name_arg, comment, flag_args, off, getopt,
                      SHOW_CHAR, values, def_val, lock,
                      binlog_status_arg, on_check_func, on_update_func,
                      deprecated_version, substitute)
  {
    option.var_type= GET_FLAGSET;
    global_var(ulonglong)= def_val;
    DBUG_ASSERT(typelib.count > 1);
    DBUG_ASSERT(typelib.count <= 65);
    DBUG_ASSERT(def_val < MAX_SET(typelib.count));
    DBUG_ASSERT(strcmp(values[typelib.count-1], "default") == 0);
    DBUG_ASSERT(size == sizeof(ulonglong));
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), system_charset_info), *res;
    ulonglong default_value, current_value;
    if (var->type == OPT_GLOBAL)
    {
      default_value= option.def_value;
      current_value= global_var(ulonglong);
    }
    else
    {
      default_value= global_var(ulonglong);
      current_value= session_var(thd, ulonglong);
    }

    if (var->value->result_type() == STRING_RESULT)
    {
      if (!(res=var->value->val_str(&str)))
        return true;
      else
      {
        char *error;
        uint error_len;

        var->save_result.ulonglong_value=
              find_set_from_flags(&typelib,
                                  typelib.count,
                                  current_value,
                                  default_value,
                                  res->ptr(), res->length(),
                                  &error, &error_len);
        if (error)
        {
          ErrConvString err(error, error_len, res->charset());
          my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name.str, err.ptr());
          return true;
        }
      }
    }
    else
    {
      longlong tmp=var->value->val_int();
      if ((tmp < 0 && ! var->value->unsigned_flag)
          || (ulonglong)tmp > MAX_SET(typelib.count))
        return true;
      else
        var->save_result.ulonglong_value= tmp;
    }

    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, ulonglong)= var->save_result.ulonglong_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(ulonglong)= var->save_result.ulonglong_value;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= global_var(ulonglong); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar*)flagset_to_string(thd, 0, session_var(thd, ulonglong),
                                     typelib.type_names);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar*)flagset_to_string(thd, 0, global_var(ulonglong),
                                     typelib.type_names);
  }
};

/**
  The class for SET variables - variables taking zero or more values
  from the given list. Example: @@sql_mode

  Class specific constructor arguments:
    char* values[]    - 0-terminated list of strings of valid values

  Backing store: ulonglong
*/
class Sys_var_set: public Sys_var_typelib
{
public:
  Sys_var_set(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          const char *values[], ulonglong def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_typelib(name_arg, comment, flag_args, off, getopt,
                      SHOW_CHAR, values, def_val, lock,
                      binlog_status_arg, on_check_func, on_update_func,
                      deprecated_version, substitute)
  {
    option.var_type= GET_SET;
    global_var(ulonglong)= def_val;
    DBUG_ASSERT(typelib.count > 0);
    DBUG_ASSERT(typelib.count <= 64);
    DBUG_ASSERT(def_val < MAX_SET(typelib.count));
    DBUG_ASSERT(size == sizeof(ulonglong));
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), system_charset_info), *res;

    if (var->value->result_type() == STRING_RESULT)
    {
      if (!(res=var->value->val_str(&str)))
        return true;
      else
      {
        char *error;
        uint error_len;
        bool not_used;

        var->save_result.ulonglong_value=
              find_set(&typelib, res->ptr(), res->length(), NULL,
                      &error, &error_len, &not_used);
        /*
          note, we only issue an error if error_len > 0.
          That is even while empty (zero-length) values are considered
          errors by find_set(), these errors are ignored here
        */
        if (error_len)
        {
          ErrConvString err(error, error_len, res->charset());
          my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name.str, err.ptr());
          return true;
        }
      }
    }
    else
    {
      longlong tmp=var->value->val_int();
      if ((tmp < 0 && ! var->value->unsigned_flag)
          || (ulonglong)tmp > MAX_SET(typelib.count))
        return true;
      else
        var->save_result.ulonglong_value= tmp;
    }

    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, ulonglong)= var->save_result.ulonglong_value;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(ulonglong)= var->save_result.ulonglong_value;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= global_var(ulonglong); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar*)set_to_string(thd, 0, session_var(thd, ulonglong),
                                 typelib.type_names);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar*)set_to_string(thd, 0, global_var(ulonglong),
                                 typelib.type_names);
  }
};

/**
  The class for variables which value is a plugin.
  Example: @@default_storage_engine

  Class specific constructor arguments:
    int plugin_type_arg (for example MYSQL_STORAGE_ENGINE_PLUGIN)

  Backing store: plugin_ref

  @note
  these variables don't support command-line equivalents, any such
  command-line options should be added manually to my_long_options in mysqld.cc
*/
class Sys_var_plugin: public sys_var
{
  int plugin_type;
public:
  Sys_var_plugin(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          int plugin_type_arg, char **def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_CHAR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag),
    plugin_type(plugin_type_arg)
  {
    option.var_type= GET_STR;
    DBUG_ASSERT(size == sizeof(plugin_ref));
    DBUG_ASSERT(getopt.id == -1); // force NO_CMD_LINE
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff,sizeof(buff), system_charset_info), *res;
    if (!(res=var->value->val_str(&str)))
      var->save_result.plugin= NULL;
    else
    {
      const LEX_STRING pname= { const_cast<char*>(res->ptr()), res->length() };
      plugin_ref plugin;

      // special code for storage engines (e.g. to handle historical aliases)
      if (plugin_type == MYSQL_STORAGE_ENGINE_PLUGIN)
        plugin= ha_resolve_by_name(thd, &pname);
      else
        plugin= my_plugin_lock_by_name(thd, &pname, plugin_type);
      if (!plugin)
      {
        // historically different error code
        if (plugin_type == MYSQL_STORAGE_ENGINE_PLUGIN)
        {
          ErrConvString err(res);
          my_error(ER_UNKNOWN_STORAGE_ENGINE, MYF(0), err.ptr());
        }
        return true;
      }
      var->save_result.plugin= plugin;
    }
    return false;
  }
  void do_update(plugin_ref *valptr, plugin_ref newval)
  {
    plugin_ref oldval= *valptr;
    if (oldval != newval)
    {
      *valptr= my_plugin_lock(NULL, &newval);
      plugin_unlock(NULL, oldval);
    }
  }
  bool session_update(THD *thd, set_var *var)
  {
    do_update((plugin_ref*)session_var_ptr(thd),
              var->save_result.plugin);
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    do_update((plugin_ref*)global_var_ptr(),
              var->save_result.plugin);
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  {
    plugin_ref plugin= global_var(plugin_ref);
    var->save_result.plugin= my_plugin_lock(thd, &plugin);
  }
  void global_save_default(THD *thd, set_var *var)
  {
    LEX_STRING pname;
    pname.str= *(char**)option.def_value;
    pname.length= strlen(pname.str);

    plugin_ref plugin;
    if (plugin_type == MYSQL_STORAGE_ENGINE_PLUGIN)
      plugin= ha_resolve_by_name(thd, &pname);
    else
      plugin= my_plugin_lock_by_name(thd, &pname, plugin_type);
    DBUG_ASSERT(plugin);

    var->save_result.plugin= my_plugin_lock(thd, &plugin);
  }
  bool check_update_type(Item_result type)
  { return type != STRING_RESULT; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    plugin_ref plugin= session_var(thd, plugin_ref);
    return (uchar*)(plugin ? thd->strmake(plugin_name(plugin)->str,
                                          plugin_name(plugin)->length) : 0);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    plugin_ref plugin= global_var(plugin_ref);
    return (uchar*)(plugin ? thd->strmake(plugin_name(plugin)->str,
                                          plugin_name(plugin)->length) : 0);
  }
};

#if defined(ENABLED_DEBUG_SYNC)
/**
  The class for @@debug_sync session-only variable
*/
class Sys_var_debug_sync :public sys_var
{
public:
  Sys_var_debug_sync(const char *name_arg,
               const char *comment, int flag_args,
               CMD_LINE getopt,
               const char *def_val, PolyLock *lock=0,
               enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
               on_check_function on_check_func=0,
               on_update_function on_update_func=0,
               uint deprecated_version=0, const char *substitute=0,
               int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, 0, getopt.id,
              getopt.arg_type, SHOW_CHAR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  {
    DBUG_ASSERT(scope() == ONLY_SESSION);
    option.var_type= GET_NO_ARG;
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[STRING_BUFFER_USUAL_SIZE];
    String str(buff, sizeof(buff), system_charset_info), *res;

    if (!(res=var->value->val_str(&str)))
      var->save_result.string_value.str= const_cast<char*>("");
    else
      var->save_result.string_value.str= thd->strmake(res->ptr(), res->length());
    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    extern bool debug_sync_update(THD *thd, char *val_str);
    return debug_sync_update(thd, var->save_result.string_value.str);
  }
  bool global_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  void session_save_default(THD *thd, set_var *var)
  {
    var->save_result.string_value.str= const_cast<char*>("");
    var->save_result.string_value.length= 0;
  }
  void global_save_default(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
  }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    extern uchar *debug_sync_value_ptr(THD *thd);
    return debug_sync_value_ptr(thd);
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    DBUG_ASSERT(FALSE);
    return 0;
  }
  bool check_update_type(Item_result type)
  { return type != STRING_RESULT; }
};
#endif /* defined(ENABLED_DEBUG_SYNC) */

/**
  The class for bit variables - a variant of boolean that stores the value
  in a bit.

  Class specific constructor arguments:
    ulonglong bitmask_arg - the mask for the bit to set in the ulonglong
                            backing store

  Backing store: ulonglong

  @note
  This class supports the "reverse" semantics, when the value of the bit
  being 0 corresponds to the value of variable being set. To activate it
  use REVERSE(bitmask) instead of simply bitmask in the constructor.

  @note
  variables of this class cannot be set from the command line as
  my_getopt does not support bits.
*/
class Sys_var_bit: public Sys_var_typelib
{
  ulonglong bitmask;
  bool reverse_semantics;
  void set(uchar *ptr, ulonglong value)
  {
    if ((value != 0) ^ reverse_semantics)
      (*(ulonglong *)ptr)|= bitmask;
    else
      (*(ulonglong *)ptr)&= ~bitmask;
  }
public:
  Sys_var_bit(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          ulonglong bitmask_arg, my_bool def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0)
    : Sys_var_typelib(name_arg, comment, flag_args, off, getopt,
                      SHOW_MY_BOOL, bool_values, def_val, lock,
                      binlog_status_arg, on_check_func, on_update_func,
                      deprecated_version, substitute)
  {
    option.var_type= GET_BOOL;
    reverse_semantics= my_count_bits(bitmask_arg) > 1;
    bitmask= reverse_semantics ? ~bitmask_arg : bitmask_arg;
    set(global_var_ptr(), def_val);
    DBUG_ASSERT(def_val < 2);
    DBUG_ASSERT(getopt.id == -1); // force NO_CMD_LINE
    DBUG_ASSERT(size == sizeof(ulonglong));
  }
  bool session_update(THD *thd, set_var *var)
  {
    set(session_var_ptr(thd), var->save_result.ulonglong_value);
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    set(global_var_ptr(), var->save_result.ulonglong_value);
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= global_var(ulonglong) & bitmask; }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ulonglong_value= option.def_value; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    thd->sys_var_tmp.my_bool_value= reverse_semantics ^
      ((session_var(thd, ulonglong) & bitmask) != 0);
    return (uchar*) &thd->sys_var_tmp.my_bool_value;
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    thd->sys_var_tmp.my_bool_value= reverse_semantics ^
      ((global_var(ulonglong) & bitmask) != 0);
    return (uchar*) &thd->sys_var_tmp.my_bool_value;
  }
};

/**
  The class for variables that have a special meaning for a session,
  such as @@timestamp or @@rnd_seed1, their values typically cannot be read
  from SV structure, and a special "read" callback is provided.

  Class specific constructor arguments:
    everything derived from Sys_var_ulonglong
    session_special_read_function read_func_arg

  Backing store: ulonglong

  @note
  These variables are session-only, global or command-line equivalents
  are not supported as they're generally meaningless.
*/
class Sys_var_session_special: public Sys_var_ulonglong
{
  typedef bool (*session_special_update_function)(THD *thd, set_var *var);
  typedef ulonglong (*session_special_read_function)(THD *thd);

  session_special_read_function read_func;
  session_special_update_function update_func;
public:
  Sys_var_session_special(const char *name_arg,
               const char *comment, int flag_args,
               CMD_LINE getopt,
               ulonglong min_val, ulonglong max_val, ulonglong block_size,
               PolyLock *lock, enum binlog_status_enum binlog_status_arg,
               on_check_function on_check_func,
               session_special_update_function update_func_arg,
               session_special_read_function read_func_arg,
               uint deprecated_version=0, const char *substitute=0)
    : Sys_var_ulonglong(name_arg, comment, flag_args, 0,
              sizeof(ulonglong), getopt, min_val,
              max_val, 0, block_size, lock, binlog_status_arg, on_check_func, 0,
              deprecated_version, substitute),
      read_func(read_func_arg), update_func(update_func_arg)
  {
    DBUG_ASSERT(scope() == ONLY_SESSION);
    DBUG_ASSERT(getopt.id == -1); // NO_CMD_LINE, because the offset is fake
  }
  bool session_update(THD *thd, set_var *var)
  { return update_func(thd, var); }
  bool global_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->value= 0; }
  void global_save_default(THD *thd, set_var *var)
  { DBUG_ASSERT(FALSE); }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    thd->sys_var_tmp.ulonglong_value= read_func(thd);
    return (uchar*) &thd->sys_var_tmp.ulonglong_value;
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    DBUG_ASSERT(FALSE);
    return 0;
  }
};

/**
  The class for read-only variables that show whether a particular
  feature is supported by the server. Example: have_compression

  Backing store: enum SHOW_COMP_OPTION

  @note
  These variables are necessarily read-only, only global, and have no
  command-line equivalent.
*/
class Sys_var_have: public sys_var
{
public:
  Sys_var_have(const char *name_arg,
               const char *comment, int flag_args, ptrdiff_t off, size_t size,
               CMD_LINE getopt,
               PolyLock *lock=0,
               enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
               on_check_function on_check_func=0,
               on_update_function on_update_func=0,
               uint deprecated_version=0, const char *substitute=0,
               int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_CHAR, 0,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  {
    DBUG_ASSERT(scope() == GLOBAL);
    DBUG_ASSERT(getopt.id == -1);
    DBUG_ASSERT(lock == 0);
    DBUG_ASSERT(binlog_status_arg == VARIABLE_NOT_IN_BINLOG);
    DBUG_ASSERT(is_readonly());
    DBUG_ASSERT(on_update == 0);
    DBUG_ASSERT(size == sizeof(enum SHOW_COMP_OPTION));
  }
  bool do_check(THD *thd, set_var *var) {
    DBUG_ASSERT(FALSE);
    return true;
  }
  bool session_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  bool global_update(THD *thd, set_var *var)
  {
    DBUG_ASSERT(FALSE);
    return true;
  }
  void session_save_default(THD *thd, set_var *var) { }
  void global_save_default(THD *thd, set_var *var) { }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    DBUG_ASSERT(FALSE);
    return 0;
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar*)show_comp_option_name[global_var(enum SHOW_COMP_OPTION)];
  }
  bool check_update_type(Item_result type) { return false; }
};

/**
  Generic class for variables for storing entities that are internally
  represented as structures, have names, and possibly can be referred to by
  numbers.  Examples: character sets, collations, locales,

  Class specific constructor arguments:
    ptrdiff_t name_offset  - offset of the 'name' field in the structure

  Backing store: void*

  @note
  As every such a structure requires special treatment from my_getopt,
  these variables don't support command-line equivalents, any such
  command-line options should be added manually to my_long_options in mysqld.cc
*/
class Sys_var_struct: public sys_var
{
  ptrdiff_t name_offset; // offset to the 'name' property in the structure
public:
  Sys_var_struct(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          ptrdiff_t name_off, void *def_val, PolyLock *lock=0,
          enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
          on_check_function on_check_func=0,
          on_update_function on_update_func=0,
          uint deprecated_version=0, const char *substitute=0,
          int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_CHAR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag),
      name_offset(name_off)
  {
    option.var_type= GET_STR;
    /*
      struct variables are special on the command line - often (e.g. for
      charsets) the name cannot be immediately resolved, but only after all
      options (in particular, basedir) are parsed.

      thus all struct command-line options should be added manually
      to my_long_options in mysqld.cc
    */
    DBUG_ASSERT(getopt.id == -1);
    DBUG_ASSERT(size == sizeof(void *));
  }
  bool do_check(THD *thd, set_var *var)
  { return false; }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, void*)= var->save_result.ptr;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(void*)= var->save_result.ptr;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  { var->save_result.ptr= global_var(void*); }
  void global_save_default(THD *thd, set_var *var)
  { var->save_result.ptr= *(void**)option.def_value; }
  bool check_update_type(Item_result type)
  { return type != INT_RESULT && type != STRING_RESULT; }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    uchar *ptr= session_var(thd, uchar*);
    return ptr ? *(uchar**)(ptr+name_offset) : 0;
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    uchar *ptr= global_var(uchar*);
    return ptr ? *(uchar**)(ptr+name_offset) : 0;
  }
};

/**
  The class for variables that store time zones

  Backing store: Time_zone*

  @note
  Time zones cannot be supported directly by my_getopt, thus
  these variables don't support command-line equivalents, any such
  command-line options should be added manually to my_long_options in mysqld.cc
*/
class Sys_var_tz: public sys_var
{
public:
  Sys_var_tz(const char *name_arg,
             const char *comment, int flag_args, ptrdiff_t off, size_t size,
             CMD_LINE getopt,
             Time_zone **def_val, PolyLock *lock=0,
             enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
             on_check_function on_check_func=0,
             on_update_function on_update_func=0,
             uint deprecated_version=0, const char *substitute=0,
             int parse_flag= PARSE_NORMAL)
    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off, getopt.id,
              getopt.arg_type, SHOW_CHAR, (intptr)def_val,
              lock, binlog_status_arg, on_check_func, on_update_func,
              deprecated_version, substitute, parse_flag)
  {
    DBUG_ASSERT(getopt.id == -1);
    DBUG_ASSERT(size == sizeof(Time_zone *));
  }
  bool do_check(THD *thd, set_var *var)
  {
    char buff[MAX_TIME_ZONE_NAME_LENGTH];
    String str(buff, sizeof(buff), &my_charset_latin1);
    String *res= var->value->val_str(&str);

    if (!res)
      return true;

    if (!(var->save_result.time_zone= my_tz_find(thd, res)))
    {
      ErrConvString err(res);
      my_error(ER_UNKNOWN_TIME_ZONE, MYF(0), err.ptr());
      return true;
    }
    return false;
  }
  bool session_update(THD *thd, set_var *var)
  {
    session_var(thd, Time_zone*)= var->save_result.time_zone;
    return false;
  }
  bool global_update(THD *thd, set_var *var)
  {
    global_var(Time_zone*)= var->save_result.time_zone;
    return false;
  }
  void session_save_default(THD *thd, set_var *var)
  {
    var->save_result.time_zone= global_var(Time_zone*);
  }
  void global_save_default(THD *thd, set_var *var)
  {
    var->save_result.time_zone=
      *(Time_zone**)(intptr)option.def_value;
  }
  uchar *session_value_ptr(THD *thd, LEX_STRING *base)
  {
    /*
      This is an ugly fix for replication: we don't replicate properly queries
      invoking system variables' values to update tables; but
      CONVERT_TZ(,,@@session.time_zone) is so popular that we make it
      replicable (i.e. we tell the binlog code to store the session
      timezone). If it's the global value which was used we can't replicate
      (binlog code stores session value only).
    */
    thd->time_zone_used= 1;
    return (uchar *)(session_var(thd, Time_zone*)->get_name()->ptr());
  }
  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
  {
    return (uchar *)(global_var(Time_zone*)->get_name()->ptr());
  }
  bool check_update_type(Item_result type)
  { return type != STRING_RESULT; }
};


class Sys_var_tx_isolation: public Sys_var_enum
{
public:
  Sys_var_tx_isolation(const char *name_arg,
          const char *comment, int flag_args, ptrdiff_t off, size_t size,
          CMD_LINE getopt,
          const char *values[], uint def_val, PolyLock *lock,
          enum binlog_status_enum binlog_status_arg,
          on_check_function on_check_func)
    :Sys_var_enum(name_arg, comment, flag_args, off, size, getopt,
                  values, def_val, lock, binlog_status_arg, on_check_func)
  {}
  virtual bool session_update(THD *thd, set_var *var);
};

/****************************************************************************
  Used templates
****************************************************************************/

#ifdef HAVE_EXPLICIT_TEMPLATE_INSTANTIATION
template class List<set_var_base>;
template class List_iterator_fast<set_var_base>;
template class Sys_var_unsigned<uint, GET_UINT, SHOW_INT>;
template class Sys_var_unsigned<ulong, GET_ULONG, SHOW_LONG>;
template class Sys_var_unsigned<ha_rows, GET_HA_ROWS, SHOW_HA_ROWS>;
template class Sys_var_unsigned<ulonglong, GET_ULL, SHOW_LONGLONG>;
#endif