in 34c3688017, we added a virtual function
to `config_file`, and we new and delete pointer pointing to a
`db::config` instance with `unique_ptr<>`. this makes the compiler
nervous, as deleting a pointer pointing to an instance of non-final
class with virtual function could lead to leak, if this pointer actually
points to a derived class of this non-final class. so, in order to
silence the warning and to prevent potential problem in future, let's
mark `db::config` final.
the warning from Clang 16 looks like:
```
In file included from /home/kefu/dev/scylladb/test/lib/test_services.cc:10:
In file included from /home/kefu/dev/scylladb/test/lib/test_services.hh:25:
In file included from /usr/bin/../lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/memory:78:
/usr/bin/../lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:99:2: error: delete called on non-final 'db::config' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-abstract-non-virtual-dtor]
delete __ptr;
^
/usr/bin/../lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:404:4: note: in instantiation of member function 'std::default_delete<db::config>::operator()' requested here
get_deleter()(std::move(__ptr));
^
/home/kefu/dev/scylladb/test/lib/test_services.cc:189:16: note: in instantiation of member function 'std::unique_ptr<db::config>::~unique_ptr' requested here
auto cfg = std::make_unique<db::config>();
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#15071
6788903fd6