In testing my implementation of a distributed failure detector, I needed to be able to:

  1. Spawn n instances of the detector, each listening on a different port on localost, making a small local cluster.
  2. Redirect each process’ standard output and error streams to separate logfiles, named such that they can be tied back to the generating processes later.
  3. After running for some time, terminate all processes, and then analyse the generated logs.

It turns out that getting step 1 right is nontrivial (but worth trying to understand what facilities your language offers to this end).

I started out doing the regular fork-exec routine, except that the act of redirecting the output stream of the target process in exec was causing the shell to be involved:


my @cmd = ( '/home/ys/bin/phifd', ... );
...
my @children;
if ( my $pid = fork() ) {
    push @children, $pid;
} else {
    # presence of a shell metachar automatically causes a shell to be spawned.
    exec @cmd, '>', "$logfile";
}

The problem here is that when reaping the children later on, the SIGTERM that we send to the child processes gets trapped by the shell, which dies, and then its child, which is the actual process we wanted to kill, keeps running, now parented by init.

One way to solve this is to avoid spawning a shell altogether, and relay the child output streams to respective log files in the parent process, and in Perl IPC::Run is a handy library for this. Here’s the code:

use strict;
use warnings;

use IPC::Run qw(run timeout start harness);
use File::Path qw(make_path remove_tree);
use File::Slurp qw(read_dir read_file);

my $log_root_dir = shift || 'logs/';

my @cmds = (
    [
        'phifd', '-t', 1, '-a', '127.0.0.1:12345'
    ],
    [
        'phifd',    '-t', 1, '-a', '127.0.0.1:12346', '-i', '127.0.0.1:12345'
    ],
    [
        'phifd',    '-t', 1, '-a', '127.0.0.1:12347', '-i', '127.0.0.1:12346'
    ],
);

my @handles;
my @harnesses;
for my $procnum ( 0 .. $#cmds ) {
    my $logfile = File::Spec->catfile( $log_root_dir,
        'proc_' . $procnum . '.log' );

    # XXX: do we need an :encoding(utf8) discipline here?
    open my $fh, '>', $logfile
      or die "Cannot open $logfile for writing: $!";

    my $harness = harness
                    $cmds[$procnum],       # command to run
                    \undef,                # input (goes to child's stdin)
                    $fh,                   # output (from child's stdout)
                    $fh,                   # error (from child's stderr)
                    init => sub {
                        # Any environment setup goes here
                        $ENV{RUST_BACKTRACE} = 1;
                    };

    push @handles,   $fh;
    push @harnesses, $harness;
}

my $start   = time;
my $elapsed = 0;
while ( $elapsed < $runtime ) {
    # Nonblocking "pumping" -- in our case, we check if a child has output,
    # and if so, write it to the respective log file handle. IPC::Run handles
    # this for us.
    $_->pump_nb for @harnesses;
    $elapsed = time - $start;
}

# cleanup
$_->kill_kill for @harnesses; # kill with progressively stiffer signals.
@harnesses = ();
close $_ for @handles;
@handles = ();

That’s it, IPC::Run is quite flexible, and the perldoc is quite well written.