#!/usr/bin/env perl

use strict;
use warnings;

use 5.010_001;

# essential libraries
use English                    qw( -no_match_vars );
use POSIX                      qw(:sys_wait_h);

my @cmd     = @ARGV;
my $cmd_str = join ' ', @cmd;

my $pid = fork();
if (not defined $pid) {             # fork failed
	die "Cannot fork: $ERRNO\n";
}
elsif ($pid == 0) {                 # child process
	say("Executing $cmd_str");

	my $retval   = system(@cmd);
	my $exitcode = parse_retval($retval);

	# TODO add URL callback
	say("[child] Exiting with exit code $exitcode.");
	exit $exitcode;
}
else {                              # parent process
	# wait a bit to give child process a chance to die prematurely
	sleep 1;

	my $cstatus = waitpid($pid, WNOHANG);
	my $exitcode;
	if ($cstatus == 0) {
		$exitcode = 0;
		say("[parent] Child is still running.");
	}
	else {
		$exitcode = parse_retval(${^CHILD_ERROR_NATIVE});
	}

	say("[parent] Exiting with exit code $exitcode.");
	exit $exitcode;
}

sub parse_retval {
	my $retval = shift;

	my $signal        = $retval & 127;
	my $have_coredump = $retval & 128;

	my $exitcode;
	if ($retval == -1) {
		# program failed to execute
		$exitcode = 127;
	}
	elsif ($signal) {
		my $message = "W: Process died with signal $signal!";
		if ($have_coredump) {
			$message .= ' (coredump available)';
		}
		warn($message ."\n");
		$exitcode = 255;
	}
	else {
		$exitcode = $retval >> 8;
	}

	return $exitcode;
}

##</code><code>##

$ ./forker.pl true ; echo "EXIT $?"
Executing true
[child] Exiting with exit code 0.
[parent] Exiting with exit code 0.
EXIT 0

$ ./forker.pl false ; echo "EXIT $?"
Executing false
[child] Exiting with exit code 1.
[parent] Exiting with exit code 1.
EXIT 1

$ ./forker.pl sleep 10 ; echo "EXIT $?"
Executing sleep 10
[parent] Child is still running.
[parent] Exiting with exit code 0.
EXIT 0
<10 seconds later> [child] Exiting with exit code 0.

$ ./forker.pl bogus ; echo "EXIT $?"
Executing bogus
Can't exec "bogus": No such file or directory at ./forker.pl line 22.
[child] Exiting with exit code 127.
[parent] Exiting with exit code 127.
EXIT 127

$ ./bogus ; echo "EXIT $?"
bash: ./bogus: No such file or directory
EXIT 127