#!/usr/bin/perl -w
use strict;
use IO::Handle;

my $us = $0;
$us =~ s{.*/}{};

die "$us: no arguments permitted\n" if @ARGV;

our ($monh,$monchild);

our %reported;
#  no entry: not reported, does not exist
#  /ry+/: reported, entry exists
# during processing only:
#  /r/: reported, may not still exist
#  /y+/: not reported, entry exists

sub killmonitor () {
    return unless $monchild;
    kill 9, $monchild
	or warn "$us: cannot kill monitor child [$monchild]: $!\n";
    $monchild=undef;
    close $monh;
}

END { killmonitor(); }

my $restart;

for (;;) {
    my $o;
    eval {
	if (!$monh) {
	    killmonitor();
	    $monh = new IO::File;
	    $monchild = open $monh, "-|", qw(ip -o monitor addr)
		or die "spawn monitor: $!\n";
	    sleep(1) if $restart++;
	} else {
	    my $discard;
	    my $got = sysread $monh, $discard, 4096;
	    die "read monitor: $!\n" unless defined $got;
	    die "monitor failed\n" unless $got;
	}
	$_='r' foreach values %reported;
	foreach my $ip (qw(4 6)) {
	    my $addrh = new IO::File;
	    open $addrh, "-|", qw(ip -o), "-$ip", qw(addr show)
		or die "spawn addr $ip show: $!\n";
	    my $afstr = $ip==4 ? 'inet' : $ip==6 ? 'inet6' : die;
	    while (<$addrh>) {
		if (m{^\d+\:\s*(\S+)\s+$afstr\s+([0-9a-z.:]+)(?:/\d+)?\s}) {
		    my $outline = "$ip $1 $2";
		    $reported{$outline} .= "y";
		} else {
		    chomp;
		    warn "unexpected output from addr $ip show: $_\n";
		}
	    }
	    my $r = close $addrh;
	    die "addr $ip show failed $!\n" unless $r;
	    $o = '';
	}
	foreach my $k (keys %reported) {
	    local $_ = $reported{$k};
	    if (m/^r$/) {
		$o .= "-$k\n";
		delete $reported{$k};
	    } elsif (m/^y/) {
		$o .= "+$k\n";
	    }
	}
    };
    if ($@) {
	print STDERR "$us: $@";
	sleep 5;
	next;
    }
    print $o or die $!;
    STDOUT->flush or die $!;
}