Timeouts for Parallel::ForkManager

At tonight's Chicago Perl Mongers Office Hours, Ray came up with an interesting problem. While testing all of CPAN for CPAN Testers, how do you detect when a test is hanging and kill it before it takes down the entire machine? How do you simply kill a test that is taking too long? And how do you do it without having a wholly separate watchdog program?

Ray's using Parallel::ForkManager to execute testing jobs in parallel across multiple Perl installs. There are a few ways we could implement timeouts, including IPC::Run's timeout function, or the alarm Perl built-in, but these must all be implemented in the child process. It'd be nicer if we could use the parent process to watch its own children.

Here, then, is the result of that hacking: This code spawns 5 workers at a time to sleep for a random number of seconds between 1 and 20. If the child worker is alive for longer than the 10 second timeout (a 50% chance), it is forcibly killed.

When enough workers have been spawned, we check on all of our workers to see if they've lived long enough. Once a worker has finished, or been killed, and has been reaped, we can then start another worker.

With this code, hopefully we can prevent some of the test suites for CPAN distributions from forcing the tester to reboot their machine.

use v5.24;
use Parallel::ForkManager;

my $TIMEOUT = 10; # How long a child is running before it should be killed

my @jobs = 1..20; # How many fake jobs to run

my $pm = Parallel::ForkManager->new( 5 ); # 5 max workers

# Hash of PID => time started or last known good
my %watching;

# While there is still work to do, or there are still active workers
while ( @jobs || $pm->running_procs ) {

    # Check to see if any workers are finished
    $pm->reap_finished_children;

    # Check to see if any workers need to be killed because of the
    # timeout. We must do this if we've reached the limit of the number
    # of jobs we want (we wait for a slot to open up), or if we've got
    # no more work to give out.
    if ( $pm->running_procs >= $pm->max_procs || !@jobs ) {
        say sprintf "[%5s] %2s: Checking jobs (%d running, %d left)", '-----', '--', scalar $pm->running_procs, scalar @jobs;
        for my $pid ( $pm->running_procs ) {
            if ( $watching{ $pid }{ time } + $TIMEOUT < time ) {
                # XXX: You can add a concern check here to see if it's
                # still working on something and reset the timeout
                # instead of killing the child. To reset the timeout,
                # just set the current time: $watching{ $pid } = time;

                # Kill the child
                # We're being unforgiving here. You might want to use
                # 'TERM' instead
                kill 'KILL', $pid;
                say sprintf "[%5d] %2d: Killed at %d", $pid, $watching{ $pid }{ job }, time;
            }
        }

        # Sleep for much less than the timeout. This means we could have
        # a process that runs for up to 25% over the timeout. Our actual
        # timeout is between 10 and 13 seconds.
        sleep $TIMEOUT * 0.25;
        next;
    }

    # Start a new job
    my $job = shift @jobs;
    my $pid = $pm->start;

    # Parent process: Start tracking the job worker
    if ( $pid ) {
        # Add to the watchdog timer
        $watching{ $pid }{ time } = time;
        $watching{ $pid }{ job } = $job;
        next;
    }

    # Child process: Start the job
    say sprintf "[%5d] %2d: Started Job at %d", $$, $job, time;
    srand; # Reinitialize the random number in children
    my $time = int ( rand() * ( $TIMEOUT * 2 ) );
    say sprintf "[%5d] %2d: Sleeping for %d", $$, $job, $time;
    sleep $time;
    say sprintf "[%5d] %2d: Finished!", $$, $job;

    # Child process: Finished successfully
    $pm->finish;
}

$pm->reap_finished_children;

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