rspec-cover_it

RSpec::CoverIt

The standard approach to monitoring "test coverage" in ruby is a tool called SimpleCov, and a variety of systems made to hook into its output, to display a single number that represents how much of your total code is executed during your test suite. You set up SimpleCov, and then you commit your team to maintaining whatever test coverage number you already have achieved globally, or to improving that coverage number. If you have one of the better tools, you can commit to things like "all merged PRs should have 100% (or 95%, etc) coverage on all of their touched code."

That's.. better than nothing. A lot better - it can especially tell you when you forgot to write tests for some component, or when you're adjusting a class that doesn't have any tests written for it. But it's not great, not compared to a system like RSpec that lets you specify with granularity how things should behave. The problem with SimpleCov is that it's holistic - you can't adopt simplecov one class at a time, or enforce full coverage on new code in your CI system, you need external tooling with state persistence for that.

RSpec::CoverIt takes a different approach, somewhat inspired by the rspec-coverage gem (which was itself apparently inspired by a rubyconf-2016 talk by Ryan Davis). With CoverIt, coverage-enforcement happens as part of your test-suite, enforced by rspec either globally or as specified.

Installation and Setup

Add the gem to your Gemfile or gemspec just like simplecov or rspec. Then in your spec_helper.rb, before you require your application or gem code, require rspec/cover_it, and then invoke RSpec::CoverIt.setup, with the appropriate options to configure it (described further down). A reasonable initial setup might look like this:

require "rspec/cover_it"
project_root = File.expand_path("../..", __FILE__)
RSpec::CoverIt.setup(filter: project_root, autoenforce: true)

require File.expand_path("../../lib/my_gem", __FILE__)

Configuration and Usage

When invoking RSpec::CoverIt.setup, you may supply these options:

• filter: Don't track coverage information about files that don't start with this prefix - this is largely a performance optimization, allowing the pretest coverage tracking to track information only about the current project, and not the various gems it might depend on.
• autoenforce: This is off by default, which means that coverage-enforcement will only be applied to top-level example groups that request it, by supplying the cover_it: true spec metaddata. If you configure autoenforce to be true, then all specs will attempt to enforce coverage, as long as they can figure out their targets (unless they are told not to).

When setting up a test file, you can configure some additional details for that example group - CoverIt is aware of the following bits of spec metadata:

• cover_it: If supplied with a truthy value, it activates coverage enforcement for this example-group - if supplied with a falsey value, it deactivates enforcement. If supplied with a numeric value, it is treated as a percentage, and used to configure the target coverage threshold for the class' definition, a feature which I intend not to use, but I expect some people to care deeply about.
• covers_path: in certain cases, a class or module may be "defined" in several locations. While actually enforcing coverage for multiple code files from one test file isn't yet implemented, if rspec-cover_it infers the location of the code under test incorrectly, you can tell it where to actually enforce coverage against by supplying a path here (relative to the directory containing the test file).

Implementation Approach

When setting up the tool, we activate the built-in ruby Coverage system (we start it in legacy-supporting mode, but we are compatible with it having already been started in the more modern mode as well). Then we register three rspec hooks:

• Before the suite starts (which should be after everything is loaded), we record the coverage so far - this is the 'initialization' coverage, which mostly includes the lines that run during class evaluation.
• Before each 'context' (spec file), we record the coverage information
• After each 'context' we record the coverage information again. Then we subtract the two sets of coverage, which tells us how many times each line was run during this example group, and add it to the 'initialization' coverage to see the effective coverage for just this test file.

Then, for each test file, we use ruby's source-introspection system to tell us where the "described class" was defined, and check what fraction of that source file is effectively covered by this example group. If there is some missing coverage, we raise an exception explaining the missing coverage.

But of course, if you only run part of the tests in a spec file (perhaps by specifying a line number, or a description filter with -e), it probably won't be fully covered. We don't want to fail the test suite every time you're working on the specs for something, so we reach into RSpec a little to check if the set of filtered examples (the ones being run) match the full set of examples. If they don't, don't bother checking coverage for this test file.

Caveats and Shortcomings

For the moment, there is no support for autoloading, which makes this library awkward to use for the majority of large rails applications. I intend to resolve this issue soon, but it's not a trivial one, as coverage-tracking on a per-spec basis relies on being able to separate coverage that happens during code-loading from coverage that happens during "tests other than this spec which have already occurred" - we'll need to add some kind of autoloading hooks to support it properly.

Until then, the gem will only be useful to a rails application in an eager- loaded context - that's not a major issue in CI, but it's awkward when running tests locally. Happily, rspec-cover_it is fully compatible with simplecov (though you do need to start SimpleCov before Rspec::CoverIt), so you can simply use simplecov locally when resolving coverage issues (or set up eager- loading locally based on an environment variable, the solution I prefer), and still use RSpec::CoverIt to enforce coverage in your CI system.