MDEV-9629 Disappearing PRI from Key column after creating a trigger

when creating shadow nullable fields, preserve all
original field flags (because we swap flags
back and forth in not_null_fields_have_null_values())

mysql-test/r/trigger_null-8605.result

@@ -341,3 +341,17 @@ create trigger tr1 before update on t1 for each row do 1;
 create trigger tr2 after update on t2 for each row update t1 set a=new.f2;
 update t2 set f2=1 where f1=1;
 drop table t1, t2;
+create table t1 (a int not null, primary key (a));
+insert into t1 (a) values (1);
+show columns from t1;
+Field	Type	Null	Key	Default	Extra
+a	int(11)	NO	PRI	NULL	
+create trigger t1bu before update on t1 for each row begin end;
+show columns from t1;
+Field	Type	Null	Key	Default	Extra
+a	int(11)	NO	PRI	NULL	
+insert into t1 (a) values (3);
+show columns from t1;
+Field	Type	Null	Key	Default	Extra
+a	int(11)	NO	PRI	NULL	
+drop table t1;

mysql-test/t/trigger_null-8605.test

@@ -360,3 +360,15 @@ create trigger tr1 before update on t1 for each row do 1;
 create trigger tr2 after update on t2 for each row update t1 set a=new.f2;
 update t2 set f2=1 where f1=1;
 drop table t1, t2;
+
+#
+# MDEV-9629 Disappearing PRI from Key column after creating a trigger
+#
+create table t1 (a int not null, primary key (a));
+insert into t1 (a) values (1);
+show columns from t1;
+create trigger t1bu before update on t1 for each row begin end;
+show columns from t1;
+insert into t1 (a) values (3);
+show columns from t1;
+drop table t1;

sql/sql_trigger.cc

@@ -1101,6 +1101,7 @@ bool Table_triggers_list::prepare_record_accessors(TABLE *table)
                                                   table == (*fld)->table)))
           return 1;
 
+        f->flags= (*fld)->flags;
         f->null_ptr= null_ptr;
         f->null_bit= null_bit;
         if (null_bit == 128)