@Nadeem: note that the committed versions of the tests would not
show up the Mac OS X mmap() bug AFAIK, because there is an
intermediate .close() of the file to be mmapped.  The OS X bug is
that the VMS/VFS interaction fails to provide a valid memory
region for <<pages which are not yet physically present on disc>>
- i.e. there is no true sparse file support as on Linux, which
simply uses references to a single COW zero page.
(I've not tried it out for real yet, but i'm foolish like a prowd
cock, so i've looked at the changeset :)