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    },

    on_exit => sub {
       my ( $pid, $exitcode, $dollarbang, $dollarat ) = @_;
       my $status = ( $exitcode >> 8 );
       print "Child process exited with status $status\n";
       print " OS error was $dollarbang, exception was $dollarat\n";
    },
 );

=head1 DESCRIPTION

This module extends the functionality of the containing L<IO::Async::Loop> to
manage the execution of child processes. It acts as a central point to store
PID values of currently-running children, and to call the appropriate
continuation handler code when the process terminates. It provides useful
wrapper methods that set up filehandles and other child process details, and
to capture the child process's STDOUT and STDERR streams.

=cut

# Writing to variables of $> and $) have tricky ways to obtain error results
sub setuid
{
   my ( $uid ) = @_;

   $> = $uid; my $saved_errno = $!;
   $> == $uid and return 1;

   $! = $saved_errno;
   return undef;
}

sub setgid
{
   my ( $gid ) = @_;

   $) = $gid; my $saved_errno = $!;
   $) == $gid and return 1;

   $! = $saved_errno;
   return undef;
}

sub setgroups
{
   my @groups = @_;

   my $gid = $)+0;
   # Put the primary GID as the first group in the supplementary list, because
   # some operating systems ignore this position, expecting it to indeed be
   # the primary GID.
   # See
   #   https://rt.cpan.org/Ticket/Display.html?id=65127
   @groups = grep { $_ != $gid } @groups;

   $) = "$gid $gid " . join " ", @groups; my $saved_errno = $!;

   # No easy way to detect success or failure. Just check that we have all and
   # only the right groups
   my %gotgroups = map { $_ => 1 } split ' ', "$)";

   $! = $saved_errno;
   $gotgroups{$_}-- or return undef for @groups;
   keys %gotgroups or return undef;

   return 1;
}

# Internal constructor
sub new
{
   my $class = shift;
   my ( %params ) = @_;

   my $loop = delete $params{loop} or croak "Expected a 'loop'";

   my $self = bless {
      loop => $loop,
   }, $class;

   weaken( $self->{loop} );

   return $self;
}

=head1 METHODS

When active, the following methods are available on the containing C<Loop>
object.

=cut

=head2 spawn_child

   $pid = $loop->spawn_child( %params )

This method creates a new child process to run a given code block or command.
The C<%params> hash takes the following keys:

=over 8

=item command => ARRAY or STRING

Either a reference to an array containing the command and its arguments, or a
plain string containing the command. This value is passed into perl's
C<exec> function.

=item code => CODE

A block of code to execute in the child process. It will be called in scalar
context inside an C<eval> block.

=item setup => ARRAY

A reference to an array which gives file descriptors to set up in the child
process before running the code or command. See below.

=item on_exit => CODE

A continuation to be called when the child processes exits. It will be invoked