MDEV-16026 MDEV-16481 refactor Sys_var_vers_asof

MDEV-16026: Forbid global system_versioning_asof in non-default time zone

* store `system_versioning_asof` in unix time;
* both session and global vars are processed in session timezone;
* setting `default` does not copy global variable anymore. Instead, it sets
  system_time to SYSTEM_TIME_UNSPECIFIED, which means that no 'AS OF' time
  is applied and `now()` can be assumed

As a regression, we cannot assign values below 1970 (UTC) anymore

MDEV-16481: set global system_versioning_asof=sf() crashes in specific case

* sys_vars.h: add `MYSQL_TIME` field to `set_var::save_result`
* sys_vars.ic: get rid of calling `var->value->get_date()` from
 `Sys_var_vers_asof::update()`
* versioning.sysvars: add test; remove double warning

refactor Sys_var_vers_asof

* inherit from sys_var rather than Sys_var_enum
* remove junk "DEFAULT" keyword. There is DEFAULT in SQL grammar for it.
* make all conversions in check() to avoid possible errors
* avoid double var->value evaluation, which could
  consequence in undefined behavior

8 files changed, 259 insertions, 75 deletions

diff --git a/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result b/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result
index 6e3792ddaca..e5910608cb4 100644
--- a/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result
+++ b/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result
@@ -3959,7 +3959,7 @@ VARIABLE_COMMENT	Default value for the FOR SYSTEM_TIME AS OF clause
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL
-ENUM_VALUE_LIST	DEFAULT
+ENUM_VALUE_LIST	NULL
 READ_ONLY	NO
 COMMAND_LINE_ARGUMENT	NULL
 VARIABLE_NAME	TABLE_DEFINITION_CACHE
diff --git a/mysql-test/suite/versioning/r/sysvars.result b/mysql-test/suite/versioning/r/sysvars.result
index cd858d6cf69..e3af236a7fa 100644
--- a/mysql-test/suite/versioning/r/sysvars.result
+++ b/mysql-test/suite/versioning/r/sysvars.result
@@ -1,5 +1,7 @@
 create table t (a int) with system versioning;
+set @before= UNIX_TIMESTAMP(now(6));
 insert into t values (1);
+set @after= UNIX_TIMESTAMP(now(6));
 update t set a= 2;
 show global variables like 'system_versioning_asof';
 Variable_name	Value
@@ -56,65 +58,71 @@ ERROR 42000: Variable 'system_versioning_asof' can't be set to the value of '201
 set system_versioning_asof= '0000-00-00 00:00';
 ERROR 42000: Variable 'system_versioning_asof' can't be set to the value of '0000-00-00 00:00'
 # GLOBAL @@system_versioning_asof
-set global system_versioning_asof= '1911-11-11 11:11:11.1111119';
+set global system_versioning_asof= '1991-11-11 11:11:11.1111119';
 Warnings:
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
+Note	1292	Truncated incorrect datetime value: '1991-11-11 11:11:11.1111119'
 show global variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1911-11-11 11:11:11.111111
-set global system_versioning_asof= '1900-01-01 00:00:00';
+system_versioning_asof	1991-11-11 11:11:11.111111
+set global system_versioning_asof= '1990-01-01 00:00:00';
 show global variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1900-01-01 00:00:00.000000
-set global system_versioning_asof= timestamp'1911-11-11 11:11:11.1111119';
+system_versioning_asof	1990-01-01 00:00:00.000000
+set global system_versioning_asof= timestamp'1991-11-11 11:11:11.1111119';
 Warnings:
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
+Note	1292	Truncated incorrect datetime value: '1991-11-11 11:11:11.1111119'
 show global variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1911-11-11 11:11:11.111111
-set @ts= timestamp'1900-01-01 00:00:00';
+system_versioning_asof	1991-11-11 11:11:11.111111
+set @ts= timestamp'1990-01-01 00:00:00';
 set global system_versioning_asof= @ts;
 show global variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1900-01-01 00:00:00.000000
+system_versioning_asof	1990-01-01 00:00:00.000000
 set global system_versioning_asof= default;
 select @@global.system_versioning_asof;
 @@global.system_versioning_asof
 DEFAULT
 # SESSION @@system_versioning_asof
-set system_versioning_asof= '1911-11-11 11:11:11.1111119';
+set system_versioning_asof= '1991-11-11 11:11:11.1111119';
 Warnings:
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
+Note	1292	Truncated incorrect datetime value: '1991-11-11 11:11:11.1111119'
 show variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1911-11-11 11:11:11.111111
-set system_versioning_asof= '1900-01-01 00:00:00';
+system_versioning_asof	1991-11-11 11:11:11.111111
+set system_versioning_asof= '1990-01-01 00:00:00';
 show variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1900-01-01 00:00:00.000000
-set system_versioning_asof= timestamp'1911-11-11 11:11:11.1111119';
+system_versioning_asof	1990-01-01 00:00:00.000000
+set system_versioning_asof= timestamp'1991-11-11 11:11:11.1111119';
 Warnings:
-Note	1292	Truncated incorrect datetime value: '1911-11-11 11:11:11.1111119'
+Note	1292	Truncated incorrect datetime value: '1991-11-11 11:11:11.1111119'
 show variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1911-11-11 11:11:11.111111
-set @ts= timestamp'1900-01-01 00:00:00';
+system_versioning_asof	1991-11-11 11:11:11.111111
+set @ts= timestamp'1990-01-01 00:00:00';
 set system_versioning_asof= @ts;
 show variables like 'system_versioning_asof';
 Variable_name	Value
-system_versioning_asof	1900-01-01 00:00:00.000000
+system_versioning_asof	1990-01-01 00:00:00.000000
 # DEFAULT: value is copied from GLOBAL to SESSION
-set global system_versioning_asof= timestamp'1911-11-11 11:11:11.111111';
-set system_versioning_asof= '1900-01-01 00:00:00';
+set global time_zone= "+03:00";
+set time_zone= "+10:00";
+set global system_versioning_asof= timestamp'1991-11-11 11:11:11.111111';
+set system_versioning_asof= '1990-01-01 00:00:00';
 select @@global.system_versioning_asof != @@system_versioning_asof as different;
 different
 1
 set system_versioning_asof= default;
+select @@global.system_versioning_asof != @@system_versioning_asof as different;
+different
+1
+set global system_versioning_asof= default;
 select @@global.system_versioning_asof = @@system_versioning_asof as equal;
 equal
 1
+set global time_zone= DEFAULT;
+set time_zone= DEFAULT;
 set global system_versioning_asof= DEFAULT;
 set system_versioning_asof= DEFAULT;
 select @@global.system_versioning_asof, @@system_versioning_asof;
@@ -142,6 +150,77 @@ select * from t for system_time between '1970-01-01 00:00' and current_timestamp
 a
 2
 1
+# MDEV-16026: Global system_versioning_asof must not be used if client sessions can have non-default time zone
+# changing time zone should not abuse `system_versioning_asof`
+set session time_zone = '+10:00';
+set global system_versioning_asof = '1999-09-08 00:00:00.000000';
+show global variables like 'system_versioning_asof';
+Variable_name	Value
+system_versioning_asof	1999-09-08 00:00:00.000000
+set session time_zone = '+03:00';
+show global variables like 'system_versioning_asof';
+Variable_name	Value
+system_versioning_asof	1999-09-07 17:00:00.000000
+set session time_zone = '+03:00';
+set session system_versioning_asof = '2000-09-08 00:00:00.000000';
+show session variables like 'system_versioning_asof';
+Variable_name	Value
+system_versioning_asof	2000-09-08 00:00:00.000000
+set session time_zone = '+10:00';
+show session variables like 'system_versioning_asof';
+Variable_name	Value
+system_versioning_asof	2000-09-08 07:00:00.000000
+# global and local time zones should not interfere
+show global variables like 'system_versioning_asof';
+Variable_name	Value
+system_versioning_asof	1999-09-08 00:00:00.000000
+set time_zone= "+10:00";
+set system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as empty;
+a
+set system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+a
+1
+set time_zone= "+03:00";
+set system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as empty;
+a
+set system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+a
+1
+set global system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+a
+1
+connect  subcon,127.0.0.1,root,,,$SERVER_MYPORT_1;
+connection subcon;
+select * from t as nonempty;
+a
+1
+disconnect subcon;
+connection default;
+set global system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as nonempty;
+a
+1
+connect  subcon,127.0.0.1,root,,,$SERVER_MYPORT_1;
+connection subcon;
+select * from t as empty;
+a
+disconnect subcon;
+connection default;
+# MDEV-16481: set global system_versioning_asof=sf() crashes in specific case
+# Using global variable inside a stored function should not crash
+create or replace function now_global() returns timestamp
+return  CONVERT_TZ(now(), @@session.time_zone, @@global.time_zone);
+set global system_versioning_asof= now_global();
+drop function now_global;
+set global time_zone= "SYSTEM";
+set time_zone= "SYSTEM";
+set global system_versioning_asof= default;
+set system_versioning_asof= default;
 show status like "Feature_system_versioning";
 Variable_name	Value
 Feature_system_versioning	2
@@ -153,7 +232,7 @@ create or replace table t1 (x int) with system versioning;
 create or replace table t2 (y int);
 insert into t1 values (1);
 insert into t2 values (1);
-set system_versioning_asof= '1970-01-01 00:00:00';
+set system_versioning_asof= '1970-01-02 00:00:00';
 delete t1, t2 from t1 join t2 where t1.x = t2.y;
 select * from t1 for system_time as of timestamp now(6);
 x
diff --git a/mysql-test/suite/versioning/t/sysvars.test b/mysql-test/suite/versioning/t/sysvars.test
index 2f62262fc8e..248020010b6 100644
--- a/mysql-test/suite/versioning/t/sysvars.test
+++ b/mysql-test/suite/versioning/t/sysvars.test
@@ -1,5 +1,7 @@
 create table t (a int) with system versioning;
+set @before= UNIX_TIMESTAMP(now(6));
 insert into t values (1);
+set @after= UNIX_TIMESTAMP(now(6));
 update t set a= 2;
 
 show global variables like 'system_versioning_asof';
@@ -51,16 +53,16 @@ set system_versioning_asof= '2011-00-28 00:00';
 set system_versioning_asof= '0000-00-00 00:00';
 
 --echo # GLOBAL @@system_versioning_asof
-set global system_versioning_asof= '1911-11-11 11:11:11.1111119';
+set global system_versioning_asof= '1991-11-11 11:11:11.1111119';
 show global variables like 'system_versioning_asof';
 
-set global system_versioning_asof= '1900-01-01 00:00:00';
+set global system_versioning_asof= '1990-01-01 00:00:00';
 show global variables like 'system_versioning_asof';
 
-set global system_versioning_asof= timestamp'1911-11-11 11:11:11.1111119';
+set global system_versioning_asof= timestamp'1991-11-11 11:11:11.1111119';
 show global variables like 'system_versioning_asof';
 
-set @ts= timestamp'1900-01-01 00:00:00';
+set @ts= timestamp'1990-01-01 00:00:00';
 set global system_versioning_asof= @ts;
 show global variables like 'system_versioning_asof';
 
@@ -68,26 +70,32 @@ set global system_versioning_asof= default;
 select @@global.system_versioning_asof;
 
 --echo # SESSION @@system_versioning_asof
-set system_versioning_asof= '1911-11-11 11:11:11.1111119';
+set system_versioning_asof= '1991-11-11 11:11:11.1111119';
 show variables like 'system_versioning_asof';
 
-set system_versioning_asof= '1900-01-01 00:00:00';
+set system_versioning_asof= '1990-01-01 00:00:00';
 show variables like 'system_versioning_asof';
 
-set system_versioning_asof= timestamp'1911-11-11 11:11:11.1111119';
+set system_versioning_asof= timestamp'1991-11-11 11:11:11.1111119';
 show variables like 'system_versioning_asof';
 
-set @ts= timestamp'1900-01-01 00:00:00';
+set @ts= timestamp'1990-01-01 00:00:00';
 set system_versioning_asof= @ts;
 show variables like 'system_versioning_asof';
 
 --echo # DEFAULT: value is copied from GLOBAL to SESSION
-set global system_versioning_asof= timestamp'1911-11-11 11:11:11.111111';
-set system_versioning_asof= '1900-01-01 00:00:00';
+set global time_zone= "+03:00";
+set time_zone= "+10:00";
+set global system_versioning_asof= timestamp'1991-11-11 11:11:11.111111';
+set system_versioning_asof= '1990-01-01 00:00:00';
 select @@global.system_versioning_asof != @@system_versioning_asof as different;
 set system_versioning_asof= default;
+select @@global.system_versioning_asof != @@system_versioning_asof as different;
+set global system_versioning_asof= default;
 select @@global.system_versioning_asof = @@system_versioning_asof as equal;
 
+set global time_zone= DEFAULT;
+set time_zone= DEFAULT;
 set global system_versioning_asof= DEFAULT;
 set system_versioning_asof= DEFAULT;
 select @@global.system_versioning_asof, @@system_versioning_asof;
@@ -100,6 +108,65 @@ select * from t for system_time all;
 select * from t for system_time from '1970-01-01 00:00' to current_timestamp(6);
 select * from t for system_time between '1970-01-01 00:00' and current_timestamp(6);
 
+-- echo # MDEV-16026: Global system_versioning_asof must not be used if client sessions can have non-default time zone
+-- echo # changing time zone should not abuse `system_versioning_asof`
+
+set session time_zone = '+10:00';
+set global system_versioning_asof = '1999-09-08 00:00:00.000000';
+show global variables like 'system_versioning_asof';
+set session time_zone = '+03:00';
+show global variables like 'system_versioning_asof';
+
+set session time_zone = '+03:00';
+set session system_versioning_asof = '2000-09-08 00:00:00.000000';
+show session variables like 'system_versioning_asof';
+set session time_zone = '+10:00';
+show session variables like 'system_versioning_asof';
+-- echo # global and local time zones should not interfere
+show global variables like 'system_versioning_asof';
+
+set time_zone= "+10:00";
+set system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as empty;
+set system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+
+set time_zone= "+03:00";
+set system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as empty;
+set system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+
+set global system_versioning_asof= FROM_UNIXTIME(@after);
+select * from t as nonempty;
+
+--connect (subcon,127.0.0.1,root,,,$SERVER_MYPORT_1)
+--connection subcon
+select * from t as nonempty;
+--disconnect subcon
+--connection default
+
+set global system_versioning_asof= FROM_UNIXTIME(@before);
+select * from t as nonempty;
+
+--connect (subcon,127.0.0.1,root,,,$SERVER_MYPORT_1)
+--connection subcon
+select * from t as empty;
+--disconnect subcon
+--connection default
+
+--echo # MDEV-16481: set global system_versioning_asof=sf() crashes in specific case
+--echo # Using global variable inside a stored function should not crash
+create or replace function now_global() returns timestamp
+  return  CONVERT_TZ(now(), @@session.time_zone, @@global.time_zone);
+set global system_versioning_asof= now_global();
+drop function now_global;
+
+set global time_zone= "SYSTEM";
+set time_zone= "SYSTEM";
+set global system_versioning_asof= default;
+set system_versioning_asof= default;
+
 show status like "Feature_system_versioning";
 
 drop table t;
@@ -111,7 +178,7 @@ create or replace table t1 (x int) with system versioning;
 create or replace table t2 (y int);
 insert into t1 values (1);
 insert into t2 values (1);
-set system_versioning_asof= '1970-01-01 00:00:00';
+set system_versioning_asof= '1970-01-02 00:00:00';
 delete t1, t2 from t1 join t2 where t1.x = t2.y;
 select * from t1 for system_time as of timestamp now(6);
 
diff --git a/sql/mysqld.h b/sql/mysqld.h
index 4f6709f37f4..d40e1d170d0 100644
--- a/sql/mysqld.h
+++ b/sql/mysqld.h
@@ -197,7 +197,8 @@ enum vers_system_time_t
 struct vers_asof_timestamp_t
 {
   ulong type;
-  MYSQL_TIME ltime;
+  my_time_t unix_time;
+  ulong second_part;
 };
 
 enum vers_alter_history_enum
diff --git a/sql/set_var.h b/sql/set_var.h
index 12e025e4696..6e84c0bd09c 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -279,6 +279,16 @@ public:
 
 
 /**
+  Structure for holding unix timestamp and high precision second part.
+ */
+typedef struct my_time_t_hires
+{
+  my_time_t unix_time;
+  ulong second_part;
+} my_time_t_hires;
+
+
+/**
   set_var_base descendant for assignments to the system variables.
 */
 class set_var :public set_var_base
@@ -296,6 +306,7 @@ public:
     plugin_ref *plugins;                ///< for Sys_var_pluginlist
     Time_zone *time_zone;               ///< for Sys_var_tz
     LEX_STRING string_value;            ///< for Sys_var_charptr and others
+    my_time_t_hires timestamp;          ///< for Sys_var_vers_asof
     const void *ptr;                    ///< for Sys_var_struct
   } save_result;
   LEX_CSTRING base; /**< for structured variables, like keycache_name.variable_name */
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 29e229adf3c..09d890161f6 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -696,8 +696,12 @@ bool vers_select_conds_t::init_from_sysvar(THD *thd)
   if (type != SYSTEM_TIME_UNSPECIFIED && type != SYSTEM_TIME_ALL)
   {
     DBUG_ASSERT(type == SYSTEM_TIME_AS_OF);
+    MYSQL_TIME ltime;
+    thd->variables.time_zone->gmt_sec_to_TIME(&ltime, in.unix_time);
+    ltime.second_part = in.second_part;
+
     start.item= new (thd->mem_root)
-        Item_datetime_literal(thd, &in.ltime, TIME_SECOND_PART_DIGITS);
+        Item_datetime_literal(thd, &ltime, TIME_SECOND_PART_DIGITS);
     if (!start.item)
       return true;
   }
diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index 43e175887ef..554591cea4b 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -438,11 +438,10 @@ static Sys_var_charptr Sys_my_bind_addr(
        READ_ONLY GLOBAL_VAR(my_bind_addr_str), CMD_LINE(REQUIRED_ARG),
        IN_FS_CHARSET, DEFAULT(0));
 
-const char *Sys_var_vers_asof::asof_keywords[]= {"DEFAULT", NULL};
 static Sys_var_vers_asof Sys_vers_asof_timestamp(
        "system_versioning_asof", "Default value for the FOR SYSTEM_TIME AS OF clause",
        SESSION_VAR(vers_asof_timestamp.type), NO_CMD_LINE,
-       Sys_var_vers_asof::asof_keywords, DEFAULT(SYSTEM_TIME_UNSPECIFIED));
+       DEFAULT(SYSTEM_TIME_UNSPECIFIED));
 
 static const char *vers_alter_history_keywords[]= {"ERROR", "KEEP", NullS};
 static Sys_var_enum Sys_vers_alter_history(
diff --git a/sql/sys_vars.ic b/sql/sys_vars.ic
index 3e35820adef..bcdfaf5697b 100644
--- a/sql/sys_vars.ic
+++ b/sql/sys_vars.ic
@@ -2631,63 +2631,82 @@ public:
 };
 
 
-class Sys_var_vers_asof: public Sys_var_enum
+class Sys_var_vers_asof: public sys_var
 {
 public:
-  static const char *asof_keywords[];
-
-public:
   Sys_var_vers_asof(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)
-    : Sys_var_enum(name_arg, comment, flag_args, off, size,
-                   getopt, values, def_val)
+          CMD_LINE getopt, uint def_val,
+          PolyLock *lock= NO_MUTEX_GUARD,
+          binlog_status_enum binlog_status_arg= VARIABLE_NOT_IN_BINLOG,
+          on_check_function on_check_func= NULL,
+          on_update_function on_update_func= NULL,
+          const char *substitute= NULL)
+    : sys_var(&all_sys_vars, name_arg, comment, flag_args, off,
+              getopt.id, getopt.arg_type, SHOW_CHAR, def_val, lock,
+              binlog_status_arg, on_check_func, on_update_func, substitute)
   {
-    // setval() accepts string rather enum
     option.var_type= GET_STR;
   }
   virtual bool do_check(THD *thd, set_var *var)
   {
-    if (!Sys_var_enum::do_check(thd, var))
+    if (!var->value)
       return false;
+
     MYSQL_TIME ltime;
-    bool res= var->value->get_date(&ltime, TIME_NO_ZERO_IN_DATE|TIME_NO_ZERO_DATE);
+    bool res= var->value->get_date(&ltime,
+                                   TIME_NO_ZERO_IN_DATE | TIME_NO_ZERO_DATE);
     if (!res)
     {
-      var->save_result.ulonglong_value= SYSTEM_TIME_AS_OF;
+      uint error;
+      var->save_result.timestamp.unix_time=
+              thd->variables.time_zone->TIME_to_gmt_sec(&ltime, &error);
+      var->save_result.timestamp.second_part= ltime.second_part;
+      res= error != 0;
     }
     return res;
   }
 
 private:
-  bool update(set_var *var, vers_asof_timestamp_t &out)
+  static bool update(THD *thd, set_var *var, vers_asof_timestamp_t *out)
   {
-    bool res= false;
-    out.type= static_cast<enum_var_type>(var->save_result.ulonglong_value);
-    if (out.type == SYSTEM_TIME_AS_OF)
+    if (var->value)
     {
-      if (var->value)
-      {
-        res= var->value->get_date(&out.ltime, TIME_NO_ZERO_IN_DATE|TIME_NO_ZERO_DATE);
-      }
-      else // set DEFAULT from global var
-      {
-        out= global_var(vers_asof_timestamp_t);
-        res= false;
-      }
+      out->type       = SYSTEM_TIME_AS_OF;
+      out->unix_time  = var->save_result.timestamp.unix_time;
+      out->second_part= var->save_result.timestamp.second_part;
     }
-    return res;
+    return 0;
+  }
+
+  static void save_default(set_var *var, vers_asof_timestamp_t *out)
+  {
+    out->type= SYSTEM_TIME_UNSPECIFIED;
   }
 
 public:
   virtual bool global_update(THD *thd, set_var *var)
   {
-    return update(var, global_var(vers_asof_timestamp_t));
+    return update(thd, var, &global_var(vers_asof_timestamp_t));
   }
   virtual bool session_update(THD *thd, set_var *var)
   {
-    return update(var, session_var(thd, vers_asof_timestamp_t));
+    return update(thd, var, &session_var(thd, vers_asof_timestamp_t));
+  }
+
+  virtual bool session_is_default(THD *thd)
+  {
+    const vers_asof_timestamp_t &var= session_var(thd, vers_asof_timestamp_t);
+    return var.type == SYSTEM_TIME_UNSPECIFIED;
+  }
+
+  virtual void session_save_default(THD *thd, set_var *var)
+  {
+    save_default(var, &session_var(thd, vers_asof_timestamp_t));
+  }
+  virtual void global_save_default(THD *thd, set_var *var)
+  {
+    save_default(var, &global_var(vers_asof_timestamp_t));
   }
 
 private:
@@ -2696,15 +2715,19 @@ private:
     switch (val.type)
     {
     case SYSTEM_TIME_UNSPECIFIED:
-    case SYSTEM_TIME_ALL:
-      return (uchar*) thd->strdup(asof_keywords[val.type]);
+      return (uchar*)"DEFAULT";
+      break;
     case SYSTEM_TIME_AS_OF:
     {
       uchar *buf= (uchar*) thd->alloc(MAX_DATE_STRING_REP_LENGTH);
-      if (buf &&!my_datetime_to_str(&val.ltime, (char*) buf, 6))
+      MYSQL_TIME ltime;
+
+      thd->variables.time_zone->gmt_sec_to_TIME(&ltime, val.unix_time);
+      ltime.second_part= val.second_part;
+
+      if (buf && !my_datetime_to_str(&ltime, (char*) buf, 6))
       {
-        // TODO: figure out variable name
-        my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), "system_versioning_asof_timestamp", "NULL (wrong datetime)");
+        my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name.str, "NULL (wrong datetime)");
         return (uchar*) thd->strdup("Error: wrong datetime");
       }
       return buf;
@@ -2712,7 +2735,7 @@ private:
     default:
       break;
     }
-    my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), "system_versioning_asof_timestamp", "NULL (wrong range type)");
+    my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name.str, "NULL (wrong range type)");
     return (uchar*) thd->strdup("Error: wrong range type");
   }