I'm writing a driver program in Perl to keep 10 instances of a short-running program concurrently active. (This is by way of trying to recreate and debug a concurrency bug with the short-running program, which is written in C.) The problem is that my driver program itself exhibits a concurrency bug.

This is Perl V5.16.2, running on Linux (CentOS).

Here's a stripped down version of the driver program:

#!/bin/env perl

use 5.010;
use warnings;
use strict;
use Carp;
use File::Basename;
use IO::Socket;
use IO::Select;
use IO::File;                   
use POSIX qw{WNOHANG setsid};   
use POSIX qw{SIGHUP SIGTERM SIGUSR1};

my $sigset = POSIX::SigSet->new();

# SIGCHLD (17) child-process ended
#
my $CHLDaction
  = POSIX::SigAction->new( 'sigCHLD_handler', $sigset, &POSIX::SA_NODE
+FER );
POSIX::sigaction( &POSIX::SIGCHLD, $CHLDaction );

sub sigCHLD_handler {
    say "Enter sigCHLD handler";
}

say "Perl version: $^V";

my $active_readers = 0;
my $current_reader_limit = 10;

while (1) {
    if ($active_readers < $current_reader_limit) {
        # Launch enough new readers to bring us up to the current limi
+t
        while ($active_readers < $current_reader_limit) {
            ++$active_readers;
            
            my $service_pid;
            if ( !defined( $service_pid = fork ) ) {
                say "Couldn't fork. Exit.";
                exit;
            }
            
            # CHILD process
            #
            elsif ( 0 == $service_pid ) {
                my $command = "echo Hello from child $active_readers "
+ . 'pid $$';
                exec $command;
            }
            # PARENT process
            say "Spawned child $active_readers pid $service_pid"
        }
    }

    sleep;
    say "Out of sleep, begin reaping";
    while ( ( my $kid = waitpid( -1, WNOHANG ) ) > 0 ) {
        --$active_readers;
        say "Reaping child process $kid, active readers now $active_re
+aders";
    }
    say "Finished reaping";
}
[download]

And here's the output from one run, before it freezes:

$ tom_strip
Perl version: v5.16.2
Spawned child 1 pid 18734
Spawned child 2 pid 18735
Spawned child 3 pid 18736
Spawned child 4 pid 18737
Spawned child 5 pid 18738
Spawned child 6 pid 18739
Spawned child 7 pid 18740
Spawned child 8 pid 18741
Spawned child 9 pid 18742
Spawned child 10 pid 18743
Hello from child 1 pid 18734
Enter sigCHLD handler
Out of sleep, begin reaping
Hello from child 2 pid 18735
Reaping child process 18734, active readers now 9
Finished reaping
Hello from child 3 pid 18736
Enter sigCHLD handler
Hello from child 4 pid 18737
Enter sigCHLD handler
Hello from child 5 pid 18738
Enter sigCHLD handler
Hello from child 6 pid 18739
Enter sigCHLD handler
Enter sigCHLD handler
Hello from child 7 pid 18740
Enter sigCHLD handler
Hello from child 9 pid 18742
Hello from child 8 pid 18741
Hello from child 10 pid 18743
Hello from child 10 pid 18748
[download]

I do nothing in the signal handler that would cause a data race; it's there only to cause sleep to wake up.

In every case where I've looked, there are no children processes remaining, and only the driver program remains.

If someone could explain what's going wrong, and what the right way to accomplish this is, I'd appreciate it.

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