# Child close event
sub task_close {
	my ($heap, $wheel_id) = @_[HEAP, ARG0];
	my $result = $_[ARG0];
	my $child = $heap->{task}->{$wheel_id};
	print "[task_close] Child ", $child->PID, " has finished.\n";
	$child = delete $heap->{task}->{$wheel_id};
}