Puppet external nodes classifier script in Perl

The upgrade to puppet 2.6.2 worked out fine. Coming from 0.24.5, I noticed a really welcome speed improvement. However, I had a tricky problem.

While upgrading to 2.6, I also decided to switch to external nodes classifier script. If you don't know about it, it's a nice puppet feature that I planned to use since the start. It allows you to write a small script in any language you want, to basically tell the puppetmaster, given the hostname, what you want that machine to be.

Puppet calls your script with one argument that is the hostname of the machine that is asking for its catalog of resources. In your script, you have to output something like the following:

  - perl
  - apache
  - ntp
  - munin
environment: production
  puppet_server: my.puppetmaster.com

You can specify all the "classes" of that machine, so basically what you want puppet to install (or repair) on that machine. So far so good. So my classifier script looks into some preset directories for some project-specific JSON files, and then checks if any of these JSON files contain the name of the machine that puppet is asking for. Code follows:

#!/usr/bin/env perl
# http://docs.puppetlabs.com/guides/external_nodes.html

use strict;
use warnings;
use File::Basename ();
use File::Slurp ();
use JSON::XS ();
use YAML ();

our $nodes_dir = '/etc/puppet/manifests/nodes';

our %default_params = (
    puppet_server => 'my.puppetmaster.com',

# ...
# A few very simple subs
# omitted for brevity
# ...

# The hostname puppet asks for
my $wanted = $ARGV[0];

# The data structure found in the JSON file
my $node_info = search_into_json_files_and_find($wanted);

my $puppet_classifier_info = {
    classes => $node_info->{puppet_classes},
    environment => 'production',
    parameters => %default_params,

print YAML->Dump($puppet_classifier_info);

Now, I don't know if you can immediately spot the problem here, but I didn't, so I wasted a good part of an afternoon chasing a bug I didn't even know existed. The resulting YAML (puppet wants YAML) was this one:

--- YAML 
    - geodns::production::backend 
environment: production 
name: z01-06-02 
    puppet_server: z01-06-02

The problem with this, is that it looks innocent and valid, and in fact is valid, but it's two YAML documents, not one. So puppet will parse the --- YAML line since that is one single complete YAML document, and ignore the rest.

And why is that happening in the first place? Because of the YAML->Dump() call I wrote, instead of the correct YAML::Dump()… Eh :) So the correct code is:

print YAML::Dump($puppet_classifier_info);

Never use YAML->Something()

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