pburkholder/vendoring-cookbook

Problem

You need to use Ruby code in your cookbook that's not part of chef-client Omnibus install.

While this documentation stands alone, if you really want to understand what's going on, then git clone this cookbook, and check out the various tags.

As simple, but contrived example, suppose you need to save an HTTP status code to a file, and you generally use the excon gem, so you want to run this:

# libraries/helper.rb:
require 'excon'

def http_status
  response = Excon.get('http://google.com/')
  response.status
end

# recipes/default.rb:
file '/tmp/status' do
  content http_status
end

If you try to use with test kitchen you'll get something like this:

> git checkout v0
> kitchen converge
....
LoadError
---------
cannot load such file -- excon

Solution 1: Find a way to use already-installed code

Sidestep the whole question but not using an additional gem. E.g., replace the above code with:

# libraries/helper.rb:
require 'net/http'
require 'uri'

def http_status
  uri = URI('http://google.com/')
  res = Net::HTTP.get_response(uri)

  res.code
end

Now see how that it works:

> git checkout v1
> kitchen verify

Solution 2: Install gem with `chef_gem`; then use `lazy` eval

One way to address this is look at the content attribute of our file resource:

  content http_status

During the chef-client compilation phase, the client will try to build a file object with the content attribute defined. However, we can leave that attribute undefined until the convergence phase by using lazy evalutation, like this:

  content lazy { http_status }

To do this, first change the helper to not require anything during the initial code pass by moving require 'excon' into the method def:

# libraries/helper.rb:

def http_status
  require 'excon'  # move 'require' into code not evaluated at compile time
  response = Excon.get('http://google.com/')
  response.status.to_s
end

Then we'll use the chef_gem resource to install excon during the convergence phase, since the Excon code isn't needed until we do the lazy evaluation. And we'll use the lazy keyword with the content attribute:

# recipes/default.rb
chef_gem 'excon' do
  compile_time false
  action :install
end

file '/tmp/status' do
  content lazy { http_status }
end

Then test:

> git checkout v1
> kitchen test
....
excon-cookbook::default
  File "/tmp/status"
    content
      should match /301/

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