Remove Perl module from child stack - perl

Question

I have a daemon which loads DBI (DBD::mysql) and then forks child processes. I'd like to prevent the DBI module from being in memory in the forked child processes.
So something like this:
#!/usr/bin/perl
use DBI;
my $dbh = DBI->connect(db_info);
my $pid = fork();
if($pid){
# The forked process here should not have DBI loaded
}
Thanks for the help!

Answer 1

Loading a module is to execute it like a script. There's absolutely no difference between a module and a script to Perl. To unload a module, one would need to undo the effects of running it. That can't be done mechanically, and it's not feasible to do manually.
The simplest solution would to be to have the child exec something. It could even be the script you are already running.
exec($^X, $0, '--child', #args)
The child can be given access to the socket by binding it to the child's fd 0 (stdin) and fd 1 (stdout).

Answer 2

You can't do that easily unless you put the load after the fork. But to do that you have to not use use. Do this instead:
my $pid = fork();
if ($pid) {
# child
} else {
require DBI;
import DBI;
}
That should prevent the DBI module from loading until after the fork. The use routine essentially does a require/import but inside a BEGIN {} block which is why you have to not use it.

Answer 3

If you are running a modern Linux system, then forks are COW (copy on write). This means pages from the parent are only copied to the child's address space if they are modified by the parent or the child. So, the DBI module is not in the memory of the forked child processes.
Perl 5 does not have any way of unloading modules from memory. If you really need the children to have different code than the parent for some reason, you are better off separating that code out of the main code as its own script and then using exec after the fork to run the child script. This will be slower than normal forking since it has to compile the child code, so if you fork a lot, it might be better to have two scripts that talk to each other over sockets and have the "child" script pre-fork.

Answer 4

Knowing now what you want to do with this, since there isn't a good way to unload modules i Perl, a good solution to the problem as to write an authentication server separate from the application server. The application server asks the authentication server if an IP has permissions. That way they remain in wholly separate processes. This might also have security benefits, your application code can't access your authentication database.
Since any given application is likely to expand to the point where it needs a SQL database of its own, this exercise is probably futile, but your call.
This is a bunch of extra work and maintenance and complexity. It's only worth while if it's causing you real memory problems, not just because it's bugs you. Remember, RAM is very cheap. Developer time is very expensive.