I spent the last day and a half writing a vaguely interesting Perl module for testing some code which gives a subtly different answer each time (i.e. incorporates data from time() and /dev/urandom) and has side effects (i.e. writes to the file system).

By overriding Perl's built-in 'open' function, it is possible to prefix each filename with the location of a temporary directory, effectively emulating chroot(). I also replaced Perl's time() with one that always returned the same answer. This meant that the login code I was testing would return a reliable result.

You have to be careful with prototypes. Spot the difference:

my $result = gmtime(time+$seconds);
my $result = gmtime(time()+$seconds);

Without adding a prototype to the new time() function, these will give different answers. I now have to go back to work tomorrow and close a bug I mistakenly filed. :)

I'm hoping to finish off my evil hacky overriding module and release it to CPAN. I want to add some routines to set up and tear down temporary chroot directories. Obviously there are some limitations to my approach; I'm not currently handling relative paths very well, and system() calls will not be "chrooted". But it should be quite handy and reusable in any case.