MDEV-20015 Assertion `!in_use->is_error()' failed in TABLE::update_virtual_field

update_virtual_field() is called as part of index rebuild in
ha_myisam::repair() (MDEV-5800) which is done on bulk INSERT finish.

Assertion in update_virtual_field() was put as part of MDEV-16222
because update_virtual_field() returns in_use->is_error(). The idea:
wrongly mixed semantics of error status before update_virtual_field()
and the status returned by update_virtual_field(). The former can
falsely influence the latter.

mysql-test/suite/vcol/r/vcol_keys_myisam.result

@@ -408,3 +408,12 @@ SELECT * FROM t1 WHERE d2 < d1;
 i	d1	d2	t
 1	2023-03-16	2023-03-15	1
 DROP TABLE t1;
+#
+# MDEV-20015 Assertion `!in_use->is_error()' failed in TABLE::update_virtual_field
+#
+create or replace table t1 (a int);
+insert into t1 (a) values (1), (1);
+create or replace table t2 (pk int, b int, c int as (b) virtual, primary key (pk), key(c));
+insert into t2 (pk) select a from t1;
+ERROR 23000: Duplicate entry '1' for key 'PRIMARY'
+drop tables t1, t2;

mysql-test/suite/vcol/t/vcol_keys_myisam.test

@@ -300,3 +300,14 @@ DROP TABLE t1;
 # Cleanup
 --let $datadir= `SELECT @@datadir`
 --remove_file $datadir/test/load_t1
+
+
+--echo #
+--echo # MDEV-20015 Assertion `!in_use->is_error()' failed in TABLE::update_virtual_field
+--echo #
+create or replace table t1 (a int);
+insert into t1 (a) values (1), (1);
+create or replace table t2 (pk int, b int, c int as (b) virtual, primary key (pk), key(c));
+--error ER_DUP_ENTRY
+insert into t2 (pk) select a from t1;
+drop tables t1, t2;

sql/table.cc

@@ -7778,15 +7778,17 @@ int TABLE::update_virtual_fields(handler *h, enum_vcol_update_mode update_mode)
 
 int TABLE::update_virtual_field(Field *vf)
 {
-  DBUG_ASSERT(!in_use->is_error());
-  Query_arena backup_arena;
   DBUG_ENTER("TABLE::update_virtual_field");
+  Query_arena backup_arena;
+  Counting_error_handler count_errors;
+  in_use->push_internal_handler(&count_errors);
   in_use->set_n_backup_active_arena(expr_arena, &backup_arena);
   bitmap_clear_all(&tmp_set);
   vf->vcol_info->expr->walk(&Item::update_vcol_processor, 0, &tmp_set);
   vf->vcol_info->expr->save_in_field(vf, 0);
   in_use->restore_active_arena(expr_arena, &backup_arena);
-  DBUG_RETURN(in_use->is_error());
+  in_use->pop_internal_handler();
+  DBUG_RETURN(count_errors.errors);
 }