#!/usr/bin/env perl

use warnings;
use strict;
use Getopt::Long;
use POSIX ":sys_wait_h";
use IO::Select;
use IO::Pipe;
use Fcntl;
use Data::Dumper;

my $h_r;
my $c_max;
my $sel = IO::Select->new();

sub sel_can_read {
	# args : timeout
	my ($t) = @_;

	# select-object($sel), hashref($h_r)
	
	# @rh, ready handles. $h, handle
	while ( my @rh = $sel->can_read($t) ) {
		foreach my $h (@rh) {
			my $o = join ('' , $h->getlines());
			my $_h = eval($o);
			$h_r = _merge_hash($_h, $h_r);
			# only one read per handle, so close
			$sel->remove($h);
			$h->close;
		}
	}
}

sub wait_num_child_below_max {

        my ( $child_aref, $c_m ) = @_;
		
        while ( scalar(@{$child_aref}) > $c_m ) {
			foreach my $c ( @{$child_aref} ) {
				@{$child_aref} = grep { $_ ne $c } @{$child_aref} if ( waitpid($c, WNOHANG) < 0 );
				# read child's fh
				sel_can_read(0);        # non-blocking
			}
        }
        return;
}


sub _merge_hash {
	# args : array of hashref1, hashref2 to be merged,
	# returns merge hashref
	
	my $h_r;
	foreach (@_) {
		if ( defined($h_r) ) {
			$h_r = { %$h_r, %$_ };
		} else {
			$h_r = $_;
		}
	}
	return $h_r;
}

sub do_some_stuff_for_host {
	my ($fh, $host) = @_;
	my %h;
	# do some stuff, populate (hash) %h 
	print $fh Data::Dumper->new([\%h])->Terse(1)->Dump;
}

sub print_report {
	print STDOUT Data::Dumper->new([$h_r])->Terse(1)->Dump;
}


sub main {
	my @a = ('host1', 'host2');
	my @c_p;
	
	foreach my $i (@a) {
		wait_num_child_below_max(\@c_p, $c_max);
		
		my $pipe = IO::Pipe->new();
		
		my $pid = fork();

		if ( $pid ) {
			# parent
			push @c_p, $pid;
			my $fh = $pipe->reader();
			$fh->blocking(0);	# non-blocking read
			$sel->add($fh);
		} elsif ( $pid == 0) {
			#child
			my $cfh = $pipe->writer();
			do_some_stuff_for_host(\*$cfh,$i);
			exit (0);
		}
	}	
	
	# leave no child behind
	wait_num_child_below_max(\@c_p, 0);
	
	print_report();
}

main();