Testing Puppet's custom facts with RSpec

As a long-time user of both Puppet and Jenkins, it should not be terribly surprising to readers that I'm creator of the most downloaded puppet-jenkins module on Puppet Forge.

Maintaining a semi-popular Puppet module is an interesting endeavour. One must try to maintain some semblance of quality, while still readily accepting patches from numerous contributors who found a slightly different use-case than your own. Since acceptance testing of Puppet modules on multiple architectures is an enormous pain, there's an inherent "running of the gauntlet" with every release.

The first step to sanity was to start using rspec-puppet with the module. Thanks to Jeff McCune, the module has had rspec-puppet tests for the past two years. Because Puppet's DSL (domain-specific language) compiles a "catalog" (an abstract syntax tree of resources), rspec-puppet allows for some level of testing of your Puppet code, but it only can assure you that a catalog can be correctly compiled.

For the purposes of the puppet-jenkins module, that covers most of the code in the module, but our custom facts were left in the dark.

Breakdown of a Fact

In its most basic form, a Fact is the named result of a block of Ruby code on a machine which is executing that Ruby. The way this is accomplished, like most things in Ruby projects, is via yet-another-DSL:

# lib/facter/mymodule.rb
Facter.add("hello_world") do
  setcode do
    "i'm in your puppet!"

The preceeding code will be auto-loaded by the Facter gem, and executed by the Puppet agent, exposing $hello_world into your manifests with a value of "i'm in your puppet!"

Relatively simple, but there are a few problems:

  • Untestable: This code invokes the Facter DSL at the Ruby file's load time, not on a specific execution-time invocation. This means if you were to add require 'lib/facter/mymodule.rb' in an RSpec file, te code would run well before your RSpec DSL is even invoked.
  • Not very modular: Since all our executable code is wrapped up inside the DSL, re-using shared code is difficult, not impossible but hard to do in a conventional Ruby style.
  • Scripts, not software: Most of the custom facts that I've seen rely on Facter::Util::Resolution#exec for invoking shell commands, even where there are existing Ruby modules and functions that provide the same information. This isn't inherently bad, but it does lead to treating custom facts more like Ruby-styled shell scripts, rather than structured Ruby code.

Restructuring the code for testability

In the case of the puppet-jenkins module's sole $jenkins_plugin Fact, this is what the code used to look like. The goal of the fact is to express the installed plugins and their versions. Unfortunately Jenkins plugins don't have a version in their filename, but are instead zip files containing Java bytecode, resources, and a single MANIFEST.MF file, which contains all the necessary meta-data baout the plugin.

Fortunately, Jenkins unzips this archive and sticks it on the file system when a plugin is installed, so all the Fact needs to do is peek in a few directories, simple right?

if File.directory?(plugins)
  # Get a list of all plugins + versions
  Dir.entries(plugins).select do |plugin|
    if (File.directory?("#{plugins}/#{plugin}") == true) && !(plugin == '..' || plugin == '.')
        contents = File.read("#{plugins}/#{plugin}/META-INF/MANIFEST.MF")
        contents =~ (/Plugin\-Version:\s+([\d\.\-]+)/)
        version = $1
        jenkins_plugins = "#{plugin} #{version}, " + jenkins_plugins
        # Nothing really to do about it, failing means no version which will
        # result in a new plugin if needed

Not only was this code not covered by RSpec, it does so many things, I had to run it a few times to make sure I fully understood how it would work!

Forget about Facter for a moment, this becomes a classic "restructure for testability" problem, which was solved in this commit.

The commit introduces:

  • The Jenkins::Facter module with:
    • #jenkins_home - resolves an absolute path to the Jenkins home directory
    • #add_facts - installs the Fact
  • The Jenkins::Facter::Plugins module with:
    • #directory - resolves an absolute path to the plugin directory
    • #manifest_data - parse a string formatted the way MANIFEST.MF files are formatted, into a Ruby Hash.
    • #exists? - determine whether the Jenkins plugin directory exists
    • #plugins - compile the list of plugins and versions

That's a lot of methods, with a lot more comments and test coverage! Looking over the code again, I see more opportunities for refactoring; probably because it's just Ruby code to me now.

Testing the Fact

Refactoring into more Ruby-like code allows for easier testing of the Ruby-code that's powering our Facts, but what about the Facts themselves?

Above I mentioned the #add_facts method, which is added to avoid adding Facts when the file is loaded into RSpec. It's relatively straight-forward, just wrap the previous DSL code in a method.

# lib/facter/mymodule.rb
module MyModule
  def self.add_facts
    Facter.add("hello_world") do
      setcode do
        "i'm in your puppet!"


You'll notice that at the bottom of the file, #add_facts is still invoked. This is to ensure that the Facts we've defined will still get properly loaded by the Facter gem when Puppet runs.

We're still not done testing this Fact though! Facter loads facts into a global collection, which means there's some special sauce that you need to add to your RSpec tests to make sure that Facter is cleared and properly reloaded for every example, e.g.:

describe 'hello_world fact' do
  subject(:fact) { Facter.fact(:hello_world) }

  before :each do
    # Ensure we're populating Facter's internal collection with our Fact

  # A regular ol' RSpec example
  its(:value) { should eql("i'm in your puppet!") }

  after :each do
    # Make sure we're clearing out Facter every time

That's more or less all there is to starting to add RSpec-based test coverage for the custom facts you are including in your Puppet modules. Once you're writing RSpec, it's easy to stub and mock out as much as necessary to cover all the variations or cases you might need for your Fact.

All said and done, figuring this out took about 2 hours and in the process of crafting this commit the size of our .rb file went from ~28 lines of untested Facter DSL to ~115 lines of Ruby with ~160 lines of accompanying RSpec.

Now with each subsequent release of the puppet-jenkins module, I can have a very high level of confidence that our custom facts will continue to work correctly without manually testing them in an actual Puppet environment.

That's one less thing to worry about when running the gauntlet, hitting that "Upload Module" button, and creating a release to Puppet Forge.