Greetings wise brothers.

I have an existing perl forking server that listens on a TCP/IP socket, forks a new process for each incoming connection and that process spends some time (5-60 minutes) handing the remote client.

I would like to convert the server to a systemd socket activated service. This is so that I can upgrade the code for the server via a Debian package upgrade without interrupting the child workers and their ongoing transaction.

My current version is configured to run as a long running systemd daemon. I have prepared a minimal version that shows how the current service works.

#!/perl

# Minimal forking server - Shows what is needed in a systemd socket ve
+rsion of the ET daemon.

use strict;
use warnings;

use English;
use IO::Socket::INET;
use Socket qw(SOMAXCONN SOL_SOCKET SO_KEEPALIVE);
use POSIX qw(:sys_wait_h ECHILD EINTR);

# Flags used by the signal handlers
my $quit = 0;

# Signal handling functions
sub REAPER {
    my $child;
    while ((my $waitedpid = waitpid(-1,WNOHANG)) > 0) {
        my $message = "reaped $waitedpid" . ($CHILD_ERROR ? " with exi
+t $CHILD_ERROR" : "");
        print "$message\n";
    }
}

$SIG{CHLD} = \&REAPER;  # loathe sysV
$SIG{TERM} = sub { $quit = 1 };

sub handle_connection {
    my ($sock) = @_;
    setsockopt($sock, SOL_SOCKET, SO_KEEPALIVE, 1);

    ########################################################
    #
    #   Important. We need to know the remote IP address.
    #
    ########################################################
    my $remote_addr = inet_ntoa ($sock->peeraddr());
    printf $sock "Greetings %s what is your command?\n", $remote_addr;

    # Code here to check that $remote_addr is authorised to connect, a
+nd which remote commands it may call.
    # Need to close the socket & refuse futher commands $remote_addr i
+s not authorised.

    while( my $cmd = <$sock> )
    {
        if( $cmd =~ m/^sleep (\d+)/ )
        {
            my $snooze_time = $1;
            printf $sock "Will sleep for $snooze_time seconds\n";
            sleep $snooze_time;
            print $sock "... ZZZ ... Yawn ... What next oh master?\n";
        }
        # NB: In real life there are lots of other commands and many t
+ake some time to process.
        else
        {
            print $sock "I did not understand that. What next oh maste
+r?\n";
        }
    }

    # Socket connection closed.
    exit(0);    # This is in a forked child.
}

# MAIN

# Create the server socket to listen for incoming connections
my $s = IO::Socket::INET->new(
        LocalPort => 1234,
        Listen    => SOMAXCONN,
        ReuseAddr => 1,
        Timeout   => 5,   # seconds
        Blocking  => 0,   # After the timeout the accept() call will r
+eturn regardless
    );

$s->listen();
print "ET daemon startup - Listening on port 1234 - My PID:$PID\n";

ACCEPT:
while ( 0 == $quit ) {
    my $new_s = $s->accept();

    if( defined $new_s )
    {
        print "Accepted socket connection on port 1234\n";

        redo ACCEPT if $! == ECHILD or $! == EINTR;

        # fork off to handle the new connection
        my $pid = fork();
        if ($pid == 0) {
            print "CHILD: Calling handle_connection()\n";
            handle_connection($new_s);
        }
        else {
            print "PARENT: Forked off child PID:$pid\n";
            close($new_s);
            print "PARENT: Closed the socket & waiting for new connect
+ions.\n";
        }
    }
    else {
        # The 5 second timeout on the the socket accept() call has exp
+ired, or a HUP signal was received.
    }
}

print "Caught TERM will quit now\n";
[download]

I believe it is possible to configure systemd to handle the socket and the fork for each incoming connection on behalf of my service, so I would just need the handle_connection() method and the necessary boilerplate to retrieve the remote IP address and close the connection if the remote IP address is not authorised to connect.

I found some example code by one of the systemd developers, but it is all in C and makes use of systemd specific C libraries and data structures, so I am having trouble understanding it. Can anyone point me to a perl or python example that is easier to follow?

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