perl: can I wait 15 minutes, and then if a key has not been pressed, do something?

Question

here's my first ever perl program:

I want to make sure that if I'm away from my machine for a while, then this script ssh's to our main server and kills all my processes there. (I keep forgetting to kill them when I go for lunch and they hog vast amounts of cpu and memory).

I've got this far, and 15 minutes after the screensaver activates the killing starts.

#!/usr/bin/perl

my $cmd = "dbus-monitor --session \"type='signal',interface='org.gnome.ScreenSaver',member='ActiveChanged'\"";

open (IN, "$cmd |");

while (<IN>) {
    if (m/^\s+boolean true/) {
        print "*** Screensaver is active ***\n";
        print "*** Sleeping before megadeath....\n";
        sleep(15*60);
        print "*** killing all jla processes on anvil...\n";
        $result = `ssh anvil pkill -u jla`;
        print "*** should all be dead\n";
        print $result;

    } elsif (m/^\s+boolean false/) {
        print "*** Screensaver is no longer active ***\n";
    }
}

But what I'd like is to wait 15 minutes while monitoring the keyboard. If say, the 'N' key gets pressed (in the terminal the script is running in), then I want to abort the killing and go back to monitoring the screensaver. This will give me an escape route if the screensaver comes on while I'm getting coffee.

Some sort of Bond-style countdown would be nice, too.

Actually even better would be to notice when the screensaver get unlocked, and stop the countdown if so, going back into monitoring mode. Then I don't even have to worry about remembering to press N.

Answer 1

If your perl has thread support, you could do something like this:

#!/usr/bin/perl

use warnings; use strict;

use threads;
use threads::shared;
use Term::ReadKey;

my $DONT_TERMINATE :shared;
my $TIMEOUT = 5;

threads->new( sub { wait_for_keypress('n', $TIMEOUT) })->detach;
threads->new( sub { countdown($TIMEOUT) })->join;

sub countdown {
    my ($timeout) = @_;

    while ( 1 ) {
        my $elapsed = time - $^T;
        last if $elapsed >= $timeout;
        return if $DONT_TERMINATE;
        print $timeout - $elapsed, "\n";
        sleep 1;
    }

    print "Killing some processes\n";
}

sub wait_for_keypress {
    my ($key, $timeout) = @_;
    my $input = ReadKey( $timeout );

    $DONT_TERMINATE = (defined($input) and ($input eq $key));

    return;
}

If you don't have thread support, you can use Coro.

Note: I deleted my Coro example because it wasn't working properly. I'll post it again if I figure it out.

Answer 2

I'd use select (via IO::Select), which lets you check if a filehandle has ready data. However, you can't use "buffered" IO operators like <> with select, so this is more complicated than you might like (You have to use sysread and maintain your own buffer). Here's how to watch the screensaver activity, and do something if it's been on for 15 minutes.

use IO::Select;
my $s = IO::Select->new();
# ... Start dbus-monitor as above ...
$s->add(\*IN);

my $buf = '';
my $started = 0;
my $waitfor = 15*60;
while ( 1 ) {
    # Read from all ready filehandles (timeout if none ready after 1 sec)
    foreach my $fh ( @ready = $s->can_read(1) ) {
        sysread($fh, $buf, 128, length($buf));
    }
    # Handle each complete line of input
    while ( $buf =~ s/^(.+)\n// ) {
        my $line = $1
        # ... Do something ...
        if ( $line =~ m/^\s+boolean (true|false)/ ) {
            if ( $1 eq 'true' ) { $started = time; print "Starting timer\n" }
            else { $started = 0; print "Canceled timer\n" }
        }
    }
    next unless $started;

    # Screensaver is on, how long has it been running?
    my $timeleft = $started+$waitfor-time;
    if ( $timeleft <= 0 ) {
        print "The screensaver has been on for at least 15 minutes\n";
        # ... Do something ...
        $started = 0; # Don't do this again until the screensaver is restarted
    }
    else {
        # You can print out an updated countdown
        print "$timeleft seconds left\n";
    }
}

I haven't tested this at all, but it might be enough for you to make it work.

_{P.S. It won't work on Windows, where select only works on sockets.}