How do you mock a function call in chefspec for an object instance extended with another class

Question

I am new to unit testing and chefspec. I am trying to mock/intercept a function call in a recipe from a dependent library

• Library

  module Helper
    def do_something_useful
      return "http://example.com/file.txt"
    end
  end
  

• Recipe

  remote_file '/save/file/here' do
    extend Helper
    source do_something_useful
  end
  

I have tried the following:

• Chefspec

  allow_any_instance_of(Chef::Resource::RemoteFile).to receive(:do_something_useful).and_return('foobar')
  allow_any_instance_of(Chef::Resource).to receive(:do_something_useful).and_return('foobar')
  

  I have also tried mocking with a double:

  helper = double
  Helper.stub(:new).and_return(helper)
  allow(helper).to receive(:do_something_useful).and_return('foobar')
  

  This fails with uninitialized constant Helper

Answer 1

Sooooo this is a fun case where the extend is overwriting the mock method. So we can just use the extend to drive things:

before do
  allow_any_instance_of(Chef::Resource::RemoteFile).to receive(:extend) do |this, m|
    Module.instance_method(:extend).bind(this).call(m)
    allow(this).to receive(:do_something_useful).and_return('foobar')
  end
end

This is like 800% magic and you probably shouldn't use it, but it does work in my little test environment.