Unix Daemon Recipes

I was digging through some older UNIX folkways when I stumbled upon an answer to a long-standing PhiloLogic design question:

How do I create a long-running worker process that will neither:

1) terminate when it's parent terminates, such as a terminal session or a CGI script, or
2) create the dreaded "zombie" processes that clog process tables and eventually crash the system.

as it turns out, this is the same basic problem as any UNIX daemon program; this just happens to be one designed to, eventually, terminate. PhiloLogic needs processes of this nature at various places: most prominently, to allow the CGI interface to return preliminary results.

Currently, we use a lightweight Perl daemon process, called nserver.pl, to accept search requests from the CGI scripts, invoke the search engine, and then clean up the process after it terminates. Effective, but there's a simpler way, with a tricky UNIX idiom.

First, fork(). This allows you to return control to the terminal or CGI script. If you aren't going to exit immediately you should SIGCHLD as well, so that you don't get interrupted later.

Second, have the child process call setsid() to gain a new session, and thus detach from the parent. This prevents terminal hangups from killing the child process.

Third, call fork() again, then immediately exit the (original) child. The new "grandchild" process is now an "orphan", and detached from a terminal, so it will run to completion, and then be reaped by the system, so you can do whatever long-term analytics you like.

A command line example could go like this:

#!/usr/bin/perl
use POSIX qw(setsid);

my $word = $ARGV[0] or die "Usage:searchwork.pl word outfile\n";
my $outfile = $ARGV[1] or die "Usage:searchwork.pl word outfile\n";

print STDERR "starting worker process.\n";
&daemonize;

open(SEARCH, "| search4 --ascii --limit 1000000 /var/lib/philologic/somedb);

print SEARCH "$word\n";
close(SEARCH);

exit;

sub daemonize {
    open STDIN, '/dev/null'   or die "Can't read /dev/null: $!";
    open STDOUT, '>>/dev/null' or die "Can't write to /dev/null: $!";
    open STDERR, '>>/dev/null' or die "Can't write to /dev/null: $!";
    defined(my $childpid = fork)   or die "Can't fork: $!";
    if ($childpid) {
        print STDERR "[parent process exiting]\n";
        exit;
    }
    setsid                    or die "Can't start a new session: $!";
    print STDERR "Child detached from terminal\n";
    defined(my $grandchildpid = fork) or die "Can't fork: $!";
    if ($grandchildpid) {
        print STDERR "[child process exiting]\n";
        exit;
    }
    umask 0;
}

The benefit is that a similar &daemonize subroutine could entirely replace nserver, and thus vastly simplify the installation process. There's clearly a lot more that could be done with routing and control, of course, but this is an exciting proof of concept, particularly for UNIX geeks like myself.