=head1 NAME

Apache::SubProcess -- Executing SubProcesses from mod_perl

=head1 SYNOPSIS

  use Apache::SubProcess ();
  
  use Config;
  use constant PERLIO_IS_ENABLED => $Config{useperlio};
  
  # pass @ARGV / read from the process
  $command = "/tmp/argv.pl";
  @argv = qw(foo bar);
  $out_fh = Apache::SubProcess::spawn_proc_prog($r, $command, \@argv);
  $output = read_data($out_fh);
  
  # pass environment / read from the process
  $command = "/tmp/env.pl";
  $r->subprocess_env->set(foo => "bar");
  $out_fh = Apache::SubProcess::spawn_proc_prog($r, $command);
  $output = read_data($out_fh);
  
  # write to/read from the process
  $command = "/tmp/in_out_err.pl";
  ($in_fh, $out_fh, $err_fh) = 
      Apache::SubProcess::spawn_proc_prog($r, $command);
  print $in_fh "hello\n";
  $output = read_data($out_fh);
  $error  = read_data($err_fh);
  
  # helper function to work w/ and w/o perlio-enabled Perl
  sub read_data {
      my($fh) = @_;
      my $data;
      if (PERLIO_IS_ENABLED || IO::Select->new($fh)->can_read(10)) {
          $data = <$fh>;
      }
      return defined $data ? $data : '';
  }

=head1 DESCRIPTION

C<Apache::SubProcess> provides the Perl API for running and
communicating with processes spawned from mod_perl handlers.

=head1 API

=head2 spawn_proc_prog()

  $out_fh = 
      Apache::SubProcess::spawn_proc_prog($r, $command, [\@argv]);
  ($in_fh, $out_fh, $err_fh) = 
      Apache::SubProcess::spawn_proc_prog($r, $command, [\@argv]);

spawn_proc_prog() spawns a sub-process which exec()'s C<$command> and
returns the output pipe filehandle in the scalar context, or input,
output and error pipe filehandles in the list context. Using these
three pipes it's possible to communicate with the spawned process.

The third optional argument is a reference to an array which if passed
becomes ARGV to the spawned program.

It's possible to pass environment variables as well, by calling:

  $r->subprocess_env->set($key => $value);

before spawning the subprocess.

There is an issue with reading from the read filehandle (C<$in_fh>)):

A pipe filehandle returned under perlio-disabled Perl needs to call
select() if the other end is not fast enough to send the data, since
the read is non-blocking.

A pipe filehandle returned under perlio-enabled Perl on the other hand
does the select() internally, because it's really a filehandle opened
via C<:APR> layer, which internally uses APR to communicate with the
pipe. The way APR is implemented Perl's select() cannot be used with
it (mainly because select() wants fileno() and APR is a crossplatform
implementation which hides the internal datastructure).

Therefore to write a portable code, you want to use select for
perlio-disabled Perl and do nothing for perlio-enabled Perl, hence you
can use something similar to the read_data() wrapper shown in the
L<SYNOPSIS|/SYNOPSIS> section.

=cut