Thread safety for DBD::File and DBD::DBM's inheritance

Hi DBI Developers,

as maintainer of SQL::Statement I try often some easy examples using
DBD::CSV, because I have a good knowledge what happens behind the
scenes.
So I did last days when I played around with threads and
Log::Log4perl::Appender::DBI. I've seen, than (even if in my simply
example everything works fine), a cloned DBD::File still uses the same
file handles for reading/writing.
I talked with Merijn about this situation in #dbi on irc.perl.org and
we decided, that it would be better, to croak an error when a $dbh is
used in another thread than the owning one.

So I started with some hacking this morning to implement that and did
'make test' on DBI (as teached by Merijn).
I got errors from DBD::DBM and after a short analyze I've seen, that
it derives from DBD::File, but doesn't inherit the entire behavior,
just a subset.
This caused DBD::DBM::db::prepare to fail in my tests.

Now I'm unsure how to step forward. I can hack DBD::File to handle
those incomplete inheritance by ignoring them, or I can fix DBD::DBM
to full inherit and modify the behavior it needs to change (instead of
ignoring parent's methods).
I would prefer the second way, because I think it's the cleaner one.

Anyone against it?

Best regards,
Jens