MDEV-9500 Bug after upgrade to 10.1.10 (and 10.1.11)

Case: table with a NOT NULL field, BEFORE UPDATE trigger,
and UPDATE with a subquery that uses GROUP BY on that
NOT NULL field, and needs a temporary table for it.

Because of the BEFORE trigger, the field becomes nullable
temporarily. But its Item_field (used in GROUP BY) doesn't.
When working with the temptable some code looked at
item->maybe_null, some - at field->null_ptr.
The fix: make Item_field nullable when its field is.

This triggers an assert. The group key size is calculated
before the item is made nullable, so the group key doesn't
have a null byte. The fix: make fields/items nullable
before the group key size is calculated.

mysql-test/r/trigger_null-8605.result

@@ -322,3 +322,15 @@ select * from t1;
 id
 0
 drop table t1;
+create table t1 (a int not null, b int);
+create trigger trgi before update on t1 for each row do 1;
+insert t1 values (1,1),(2,2),(3,3),(1,4);
+create table t2 select a as c, b as d from t1;
+update t1 set a=(select count(c) from t2 where c+1=a+1 group by a);
+select * from t1;
+a	b
+2	1
+1	2
+1	3
+2	4
+drop table t1, t2;

mysql-test/t/trigger_null-8605.test

@@ -339,3 +339,13 @@ insert t1 values (0);
 select * from t1;
 drop table t1;
 
+#
+# MDEV-9500 Bug after upgrade to 10.1.10 (and 10.1.11)
+#
+create table t1 (a int not null, b int);
+create trigger trgi before update on t1 for each row do 1;
+insert t1 values (1,1),(2,2),(3,3),(1,4);
+create table t2 select a as c, b as d from t1;
+update t1 set a=(select count(c) from t2 where c+1=a+1 group by a);
+select * from t1;
+drop table t1, t2;

sql/item.cc

@@ -2394,6 +2394,7 @@ bool Item_field::switch_to_nullable_fields_processor(uchar *arg)
   Field **new_fields= (Field **)arg;
   set_field_to_new_field(&field, new_fields);
   set_field_to_new_field(&result_field, new_fields);
+  maybe_null= field && field->maybe_null();
   return 0;
 }
 

sql/sql_update.cc

@@ -368,6 +368,9 @@ int mysql_update(THD *thd,
   if (check_unique_table(thd, table_list))
     DBUG_RETURN(TRUE);
 
+  switch_to_nullable_trigger_fields(fields, table);
+  switch_to_nullable_trigger_fields(values, table);
+
   /* Apply the IN=>EXISTS transformation to all subqueries and optimize them. */
   if (select_lex->optimize_unflattened_subqueries(false))
     DBUG_RETURN(TRUE);
@@ -455,8 +458,6 @@ int mysql_update(THD *thd,
   }
   init_ftfuncs(thd, select_lex, 1);
 
-  switch_to_nullable_trigger_fields(fields, table);
-  switch_to_nullable_trigger_fields(values, table);
   table->mark_columns_needed_for_update();
 
   table->update_const_key_parts(conds);