MDEV-19304 Segfault in ALTER TABLE after UPDATE for SIMULTANEOUS_ASSIGNMENT

For MODE_SIMULTANEOUS_ASSIGNMENT it is required to return back field
offsets from record[1] to record[0]. 'continue' in warning branch did
skip of rfield->move_field_offset() call.

mysql-test/suite/versioning/r/alter.result

@@ -603,5 +603,18 @@ select * from t1;
 a	b
 1	0
 affected rows: 1
+#
+# MDEV-19304 Segfault in ALTER TABLE after UPDATE for SIMULTANEOUS_ASSIGNMENT
+#
+create or replace table t1 (a int, s timestamp(6) as row start, e timestamp(6) as row end, period for system_time(s,e)) engine=myisam with system versioning;
+insert into t1 values (null, null, null);
+insert into t1 values (null, null, null);
+set sql_mode= 'simultaneous_assignment';
+update t1 set e= 1;
+Warnings:
+Warning	1906	The value specified for generated column 'e' in table 't1' has been ignored
+Warning	1906	The value specified for generated column 'e' in table 't1' has been ignored
+alter table t1 force;
+set sql_mode= default;
 drop database test;
 create database test;

mysql-test/suite/versioning/t/alter.test

@@ -503,5 +503,16 @@ alter table t1 modify a int with system versioning;
 select * from t1;
 --disable_info
 
+--echo #
+--echo # MDEV-19304 Segfault in ALTER TABLE after UPDATE for SIMULTANEOUS_ASSIGNMENT
+--echo #
+create or replace table t1 (a int, s timestamp(6) as row start, e timestamp(6) as row end, period for system_time(s,e)) engine=myisam with system versioning;
+insert into t1 values (null, null, null);
+insert into t1 values (null, null, null);
+set sql_mode= 'simultaneous_assignment';
+update t1 set e= 1;
+alter table t1 force;
+set sql_mode= default;
+
 drop database test;
 create database test;

sql/sql_base.cc

@@ -8304,8 +8304,8 @@ fill_record(THD *thd, TABLE *table_arg, List<Item> &fields, List<Item> &values,
         rfield->field_index ==  table->next_number_field->field_index)
       table->auto_increment_field_not_null= TRUE;
     Item::Type type= value->type();
-    bool vers_sys_field= table->versioned() && rfield->vers_sys_field();
-    if ((rfield->vcol_info || vers_sys_field) &&
+    const bool skip_sys_field= rfield->vers_sys_field(); // TODO: && !thd->vers_modify_history() [MDEV-16546]
+    if ((rfield->vcol_info || skip_sys_field) &&
         type != Item::DEFAULT_VALUE_ITEM &&
         type != Item::NULL_ITEM &&
         table->s->table_category != TABLE_CATEGORY_TEMPORARY)
@@ -8314,15 +8314,14 @@ fill_record(THD *thd, TABLE *table_arg, List<Item> &fields, List<Item> &values,
                           ER_WARNING_NON_DEFAULT_VALUE_FOR_GENERATED_COLUMN,
                           ER_THD(thd, ER_WARNING_NON_DEFAULT_VALUE_FOR_GENERATED_COLUMN),
                           rfield->field_name.str, table->s->table_name.str);
-      if (vers_sys_field)
-        continue;
     }
     if (only_unvers_fields && !rfield->vers_update_unversioned())
       only_unvers_fields= false;
 
     if (rfield->stored_in_db())
     {
-      if (unlikely(value->save_in_field(rfield, 0) < 0) && !ignore_errors)
+      if (!skip_sys_field &&
+          unlikely(value->save_in_field(rfield, 0) < 0) && !ignore_errors)
       {
         my_message(ER_UNKNOWN_ERROR, ER_THD(thd, ER_UNKNOWN_ERROR), MYF(0));
         goto err;