How to test system calls from the rake task

Question

Suppose I have a pretty simple Rake task:

task :test do
  system "bundle exec rspec spec"
end

I try testing it by stubbing the ::Kernel.system method call:

describe "test" do

  before { allow(::Kernel).to receive(:system) }

  it "runs 'bundle exec rspec spec'" do
    expect(::Kernel).to receive(:system).with "bundle exec rspec spec"
    Rake::Task[:test].invoke
  end
end

But the method seems not to be stubbed at all. Instead, I run into the infinite cycle of iterations calling the test suite.

What's wrong with it, and howto stub system calls properly?

Answer 1

Note that Kernel is a module which is included into every ruby Object. And Kernel#system is an instance method (not a class method).

One solution (although discouraged by rspec maintainers) is to use "Any instance":

it "runs 'bundle exec rspec spec'" do
  expect_any_instance_of(Kernel).to receive(:system).with "bundle exec rspec spec"
  Rake::Task[:test].invoke
end

In order to use regular expect or allow, you will need the actual instance of the object which is receiving the message. For a Rake task this will be cumbersome (although not impossible - see this question) - they are executed in the toplevel context.

I would propose that you encapsulate your system calls into utility class methods and expect those. It would make the testing easier and you have explicit classes & instances to work with.