Locking to restrict rowcounts.

Hi,

Consider the following scenario:

CREATE FUNCTION test(name)
select into cnt count(id) from items where owner = name;
--suppose someone inserts during this point? then next check will succeed
when it should not.
if (cnt < 10) then
    insert into items values ('new item', name);
end;
end;

What is the best way to stop this function from entering too many items in a
concurrent context? I think a lock is needed, although I'm not sure which
would be most appropriate since the count requires the whole table (or at
least no adds to have occurred I think I read something about predicate
locking which sounds relevant but isn't supported in PostgreSQL. Ideally I
want some kind of lock only relevant to "name" above.

Any strategies to deal with this?

Shak