Sharding for Rails system tests

test_system.rake

namespace :test do
  namespace :system do
    # Run `rails test:system` split into N shards
    #
    # @example run the first fifth of the tests
    #   $ rake test:system:shard\['1/5'\]
    desc 'Run system tests in shards'
    task :shard, [:shard] => [:environment] do |_, args|
      tests = Rake::FileList['test/system/**/*_test.rb']

      raise ArgumentError, "Shard is missing, it should be in the format 'x/y'" if args[:shard].nil?

      begin
        shard_index, shards_count = args[:shard].split('/').map(&:to_f)
        shards = tests.each_slice((tests.size / shards_count).ceil).to_a
      rescue StandardError
        raise ArgumentError, "Shard has wrong format, received '#{args[:shard]}', expected 'x/y'"
      end

      filenames = shards[shard_index - 1] || []

      system("bin/rails test #{filenames.join(' ')}", exception: true) unless filenames.empty?
    end
  end
end