MDEV-20403 Assertion 0' or Assertion btr_validate_index(index, 0)' failed in row_upd_sec_index_entry or error code 126: Index is corrupted upon UPDATE with TIMESTAMP..ON UPDATE

Three issues here:
* ON UPDATE DEFAULT NOW columns were updated after generated columns
  were computed - this broke indexed virtual columns
* ON UPDATE DEFAULT NOW columns were updated after BEFORE triggers,
  so triggers didn't see the correct NEW value
* in case of a multi-update generated columns were also updated
  after BEFORE triggers

mysql-test/r/function_defaults.result

@@ -3093,3 +3093,26 @@ a	b
 1999-12-01 11:22:33.000000	1999-12-01 11:22:33.000000
 2001-09-09 04:46:40.000000	2001-09-09 04:46:40.000000
 DROP TABLE t1;
+create table t1 (t timestamp, i int, v timestamp as (t) virtual, key(v));
+insert t1 (t,i) values ('2006-03-01 23:59:59',1);
+update t1 set i = 2;
+check table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+drop table t1;
+create table t1 (t timestamp, i int);
+create trigger tr1 before update on t1 for each row set @new:=new.t;
+insert t1 (t,i) values ('2006-03-01 23:59:59', 1);
+update t1 set i = 2;
+select if(@new = t, 'correct', 'wrong') from t1;
+if(@new = t, 'correct', 'wrong')
+correct
+drop table t1;
+create table t1 (i int, j int as (i));
+create trigger tr1 before update on t1 for each row set @new:=new.j;
+insert t1 (i) values (1);
+update t1, t1 as t2 set t1.i = 2;
+select if(@new = j, 'correct', 'wrong') from t1;
+if(@new = j, 'correct', 'wrong')
+correct
+drop table t1;

mysql-test/t/function_defaults.test

@@ -19,3 +19,30 @@ let $now=NOW(6);
 let $timestamp=TIMESTAMP(6);
 let $datetime=DATETIME(6);
 source 'include/function_defaults.inc';
+
+#
+# MDEV-20403 Assertion `0' or Assertion `btr_validate_index(index, 0)' failed in row_upd_sec_index_entry or error code 126: Index is corrupted upon UPDATE with TIMESTAMP..ON UPDATE
+#
+
+# ON UPDATE DEFAULT NOW and indexed virtual columns
+create table t1 (t timestamp, i int, v timestamp as (t) virtual, key(v));
+insert t1 (t,i) values ('2006-03-01 23:59:59',1);
+update t1 set i = 2;
+check table t1;
+drop table t1;
+
+# ON UPDATE DEFAULT NOW and triggers
+create table t1 (t timestamp, i int);
+create trigger tr1 before update on t1 for each row set @new:=new.t;
+insert t1 (t,i) values ('2006-03-01 23:59:59', 1);
+update t1 set i = 2;
+select if(@new = t, 'correct', 'wrong') from t1;
+drop table t1;
+
+# triggers, virtual columns, multi-update
+create table t1 (i int, j int as (i));
+create trigger tr1 before update on t1 for each row set @new:=new.j;
+insert t1 (i) values (1);
+update t1, t1 as t2 set t1.i = 2;
+select if(@new = j, 'correct', 'wrong') from t1;
+drop table t1;

sql/sql_base.cc

@@ -8071,9 +8071,13 @@ fill_record(THD *thd, TABLE *table_arg, List<Item> &fields, List<Item> &values,
     rfield->set_explicit_default(value);
   }
 
-  if (!update && table_arg->default_field &&
+  if (update)
-      table_arg->update_default_fields(ignore_errors))
+    table_arg->evaluate_update_default_function();
-    goto err;
+  else
+    if (table_arg->default_field &&
+        table_arg->update_default_fields(ignore_errors))
+      goto err;
+
   /* Update virtual fields */
   if (table_arg->vfield &&
       table_arg->update_virtual_fields(table_arg->file, VCOL_UPDATE_FOR_WRITE))

sql/sql_update.cc

@@ -66,12 +66,15 @@ bool compare_record(const TABLE *table)
 {
   DBUG_ASSERT(records_are_comparable(table));
 
-  if ((table->file->ha_table_flags() & HA_PARTIAL_COLUMN_READ) != 0)
+  if (table->file->ha_table_flags() & HA_PARTIAL_COLUMN_READ ||
+      table->s->has_update_default_function)
   {
     /*
       Storage engine may not have read all columns of the record.  Fields
       (including NULL bits) not in the write_set may not have been read and
       can therefore not be compared.
+      Or ON UPDATE DEFAULT NOW() could've changed field values, including
+      NULL bits.
     */ 
     for (Field **ptr= table->field ; *ptr != NULL; ptr++)
     {
@@ -762,8 +765,6 @@ int mysql_update(THD *thd,
 
       if (!can_compare_record || compare_record(table))
       {
-        if (table->default_field)
-          table->evaluate_update_default_function();
         if ((res= table_list->view_check_option(thd, ignore)) !=
             VIEW_CHECK_OK)
         {
@@ -2154,9 +2155,6 @@ int multi_update::send_data(List<Item> &not_used_values)
       {
 	int error;
 
-        if (table->default_field)
-          table->evaluate_update_default_function();
-
         if ((error= cur_table->view_check_option(thd, ignore)) !=
             VIEW_CHECK_OK)
         {
@@ -2472,6 +2470,10 @@ int multi_update::do_updates()
         copy_field_ptr->to_field->set_has_explicit_value();
       }
 
+      table->evaluate_update_default_function();
+      if (table->vfield &&
+          table->update_virtual_fields(table->file, VCOL_UPDATE_FOR_WRITE))
+        goto err2;
       if (table->triggers &&
           table->triggers->process_triggers(thd, TRG_EVENT_UPDATE,
                                             TRG_ACTION_BEFORE, TRUE))
@@ -2480,11 +2482,6 @@ int multi_update::do_updates()
       if (!can_compare_record || compare_record(table))
       {
         int error;
-        if (table->default_field)
-          table->evaluate_update_default_function();
-        if (table->vfield &&
-            table->update_virtual_fields(table->file, VCOL_UPDATE_FOR_WRITE))
-          goto err2;
         if ((error= cur_table->view_check_option(thd, ignore)) !=
             VIEW_CHECK_OK)
         {