Test dartsass:install command

This adds test coverage for the `dartsass:install` command.  The command
is tested against a freshly generated Rails app using the version of
Rails that is currently loaded.  Thus the installer can be tested with
different versions of Rails in CI.

test/installer_test.rb

@@ -0,0 +1,71 @@
+require "test_helper"
+
+class InstallerTest < ActiveSupport::TestCase
+  include RailsAppHelpers
+
+  test "installer" do
+    with_new_rails_app do
+      out, _err = run_command("bin/rails", "dartsass:install")
+
+      assert_match "<DONE>", out
+      assert_equal 0, File.size("app/assets/builds/.keep")
+      assert_match "/app/assets/builds/*\n!/app/assets/builds/.keep", File.read(".gitignore")
+      assert_equal File.read("#{__dir__}/../lib/install/application.scss"), File.read("app/assets/stylesheets/application.scss")
+      assert_equal File.read("#{__dir__}/../lib/install/Procfile.dev"), File.read("Procfile.dev")
+      assert_equal File.read("#{__dir__}/../lib/install/dev"), File.read("bin/dev")
+      assert_equal 0700, File.stat("bin/dev").mode & 0700
+
+      if sprockets?
+        assert_match "//= link_tree ../builds", File.read("app/assets/config/manifest.js")
+        assert_no_match "//= link_directory ../stylesheets .css", File.read("app/assets/config/manifest.js")
+      end
+    end
+  end
+
+  test "installer with missing .gitignore" do
+    with_new_rails_app do
+      FileUtils.rm(".gitignore")
+      out, _err = run_command("bin/rails", "dartsass:install")
+
+      assert_match "<DONE>", out
+      assert_not File.exist?(".gitignore")
+    end
+  end
+
+  test "installer with pre-existing application.scss" do
+    with_new_rails_app do
+      File.write("app/assets/stylesheets/application.scss", "// pre-existing")
+      out, _err = run_command("bin/rails", "dartsass:install")
+
+      assert_match "<DONE>", out
+      assert_equal "// pre-existing", File.read("app/assets/stylesheets/application.scss")
+    end
+  end
+
+  test "installer with pre-existing Procfile" do
+    with_new_rails_app do
+      File.write("Procfile.dev", "pre: existing\n")
+      out, _err = run_command("bin/rails", "dartsass:install")
+
+      assert_match "<DONE>", out
+      assert_equal "pre: existing\ncss: bin/rails dartsass:watch\n", File.read("Procfile.dev")
+    end
+  end
+
+  private
+    def with_new_rails_app(&block)
+      super do
+        # Override `dartsass:build` to check that installation completed and to reduce test run time.
+        File.write("Rakefile", <<~RAKEFILE, mode: "a+")
+          Rake::Task["dartsass:build"].clear
+          namespace :dartsass do
+            task build: :environment do
+              puts "<DONE>"
+            end
+          end
+        RAKEFILE
+
+        block.call
+      end
+    end
+end

test/test_helper.rb

@@ -4,3 +4,74 @@
 require "fileutils"
 require "rails"
 require "rails/test_help"
+
+module RailsAppHelpers
+  def self.included(base)
+    base.include ActiveSupport::Testing::Isolation
+  end
+
+  private
+    def sprockets?
+      Gem.loaded_specs.key?("sprockets-rails")
+    end
+
+    def propshaft?
+      Gem.loaded_specs.key?("propshaft")
+    end
+
+    def asset_pipeline_option
+      if Rails::VERSION::MAJOR > 7
+        if propshaft?
+          "--asset-pipeline=propshaft"
+        elsif sprockets?
+          "--asset-pipeline=sprockets"
+        end
+      end
+    end
+
+    def create_new_rails_app(app_dir)
+      require "rails/generators/rails/app/app_generator"
+      Rails::Generators::AppGenerator.start([app_dir, *asset_pipeline_option, "--skip-bundle", "--skip-bootsnap", "--quiet"])
+
+      Dir.chdir(app_dir) do
+        gemfile = File.read("Gemfile")
+
+        gemfile.gsub!(/^gem ["']sassc?-rails["'].*/, "") # for Rails 6.1 and 7.0
+        gemfile.gsub!(/^gem ["']dartsass-rails["'].*/, "")
+        gemfile << %(gem "dartsass-rails", path: #{File.expand_path("..", __dir__).inspect}\n)
+
+        if Rails::VERSION::PRE == "alpha"
+          gemfile.gsub!(/^gem ["']rails["'].*/, "")
+          gemfile << %(gem "rails", path: #{Gem.loaded_specs["rails"].full_gem_path.inspect}\n)
+        end
+
+        File.write("Gemfile", gemfile)
+
+        run_command("bundle", "install")
+      end
+    end
+
+    def with_new_rails_app(&block)
+      require "digest/sha1"
+      variant = [Gem.loaded_specs["rails"].full_gem_path, asset_pipeline_option]
+      app_name = "app_#{Digest::SHA1.hexdigest(variant.to_s)}"
+      cache_dir = "#{__dir__}/../tmp"
+
+      Dir.mktmpdir do |tmpdir|
+        if Dir.exist?("#{cache_dir}/#{app_name}")
+          FileUtils.cp_r("#{cache_dir}/#{app_name}", tmpdir)
+        else
+          create_new_rails_app("#{tmpdir}/#{app_name}")
+          FileUtils.cp_r("#{tmpdir}/#{app_name}", cache_dir) # Cache app for future runs.
+        end
+
+        Dir.chdir("#{tmpdir}/#{app_name}", &block)
+      end
+    end
+
+    def run_command(*command)
+      Bundler.with_unbundled_env do
+        capture_subprocess_io { system(*command, exception: true) }
+      end
+    end
+end