Chefspec 3 and Guard Evaluation

Introduction

Chefspec is an incredibly useful tool for testing Chef cookbooks. It’s much much faster than running chef on a real node, but it can provide you much of the testing feedback you’d get from a real chef run. Verious.com has a nice introduction to chefspec if you’re not already familiar with it.

What makes chefspec so fast is that it doesn’t perform a full chef run. It loads Chef with your cookbooks and modifies them in-memory so that they merely send messages to Chefspec instead of performing real system changes. It does this primarily by stubbing Chef’s Resource class. In Chef, just about every way to manipulate system state is a resource. Most of them have excellent metadata about the actions they will perform (files have filename attributes, packages have package names) and they all share common methods for executing the work, so it’s surprisingly straightforward for Chefspec to stub the “doing work” part so it performs no action, while retaining the ability to effectively test for what would have been done.

Execute Blocks

This process is nothing short of amazing for Chef built-in resources like files, templates, packages, etc. It’s fast, it’s accurate (excepting bugs in core Chef that result in unexpected actions due to the “doing work” code), and it’s simple to use. But it does have limits. A good chunk of Chef’s flexibility comes from the ability to run custom code in execute blocks and ruby blocks:

execute "thing1" do
  command "rm -rf /"
end
execute "thing2" do
  command "find / -print0 | perl -0ne unlink"
end
execute "thing3" do
  command "python -c \"import shutil; shutil.rmtree('/')\""
end

Chefspec is pretty limited in what it can tell you about execute blocks. There’s no way it can know that the 3 execute blocks above all do the same thing (delete the root filesystem), or that it’s not safe to run those commands on your development workstation. Out of the box, it’s largely limited to testing whether or not the execute block is called.

Guards

But even reliably determining if an execute block will run is not trivial. The not_if and only_if guards used to determine whether the block runs present similar problems to the execute block itself:

execute "create_database_schema" do
  command "mysql -u user -p password dbname < create_schema.sql"
  not_if "echo 'show tables;' | mysql -u user -p password dbname | grep tablename"
end

The not_if guard above will give unexpected results if the mysql binary is missing from the system where you run chefspec. Chefspec 2.x sidestepped the issue. It didn’t execute guards by default, and simply assumed that the execute block itself would always run… not ideal. Chefspec 3 does inspect the guards, but rather than executing the commands inside of them, it raises an error requiring you to stub it yourself like so:

it "Creates the Database Schema when needed" do
  stub_command("echo 'show tables;' | mysql -u user -p password dbname | grep tablename").and_return(false)
  expect(chef_run).to run_execute('create_database_schema')
end
it "Doesn't create the Database Schema when it already exists" do
  stub_command("echo 'show tables;' | mysql -u user -p password dbname | grep tablename").and_return(true)
  expect(chef_run).to_not run_execute('create_database_schema')
end

This is a pretty clean example. In practice, guards frequently contain wacky stuff. It’s not unusual to leverage a couple shell commands and do some ruby transformations on the resulting complex data type, possibly requiring several stub classes to stub a single guard. If you include several upstream cookbooks, you may have a substantial amount of stubbing ahead of you before chefspec 3 will run cleanly.

Test Coverage

The Chefspec 3 convention of encouraging the stubbing of not_if and only_if guards results in covering more of your Chef code with unit tests, and that’s a great thing. It comes with a non-trivial cost, though. Having to stub the code in included cookbooks in order to test your own code isn’t fun. With chefspec 2.x, I accepted a very low level of code coverage from chefspec, using it only to test “well-behaved” resources that required little to no stubbing. My complete testing workflow looks like this:

  • Syntax and style testing with Rubocop.
  • Chef syntax checking with knife cookbook test
  • Fast low-coverage unit tests with chefspec
  • Slow, high-coverage integration tests with minispec-handler (either via Vagrant provision while I’m hacking or test-kitchen in Jenkins/Travis)

Because the integration environment that Chef works is in so much more complex than most (non-infrastructure-automation) code, I prefer to invest in having a strong integration test suite in minitest-handler rather than spending a lot of time stubbing and mocking in chefspec. I still want to use Chefspec to catch low-hanging fruit because my integration tests are very slow by comparison, but I’m willing to accept a relatively low level of unit-test coverage. If I was doing lots of LWRP development or otherwise had to test very complex logic in Chef, I’d need stronger unit testing, but 90% of my Chef code is straightforward attributes, resources, and includes so integration testing is where a lot of my bugs are easiest to find.

Skipping Guard Evaluation

Which is a round about way of saying, I like the chefspec 2.x behavior of skipping guard evaluation. It results in a less robust unit-test suite, but I make up for it with my integration tests. If you prefer the same tradeoff, you can get the old behavior back by stubbing Chef’s resource class yourself:

require 'chefspec'
describe 'example_recipe' do
  let (:chef_run) { ChefSpec::Runner.new(platform:'ubuntu', version:'12.04').converge 'example_cookbook::example_recipe' }
  before(:each) do
    # Stub out guards so that execute blocks always "run"
    Chef::Resource::Execute.any_instance.stub(:should_skip?).and_return(true)
  end
  it 'Creates the database schema' do
    expect(chef_run).to run_execute('create_database_schema')
  end
end

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