Add danger comment for new sidekiq args

MRs that change existing sidekiq jobs and
add additional arguments should remind engineers
of the process to do so.

danger/plugins/sidekiq_args.rb

+# frozen_string_literal: true
+
+require_relative '../../tooling/danger/sidekiq_args'
+
+module Danger
+  class SidekiqArgs < ::Danger::Plugin
+    # Put the helper code somewhere it can be tested
+    include Tooling::Danger::SidekiqArgs
+  end
+end

danger/sidekiq_args/Dangerfile

+# frozen_string_literal: true
+
+sidekiq_args.changed_worker_files.each do |filename|
+  sidekiq_args.add_comment_for_matched_line(filename)
+end

spec/tooling/danger/sidekiq_args_spec.rb

+# frozen_string_literal: true
+
+require 'rspec-parameterized'
+require 'gitlab-dangerfiles'
+require 'danger'
+require 'danger/plugins/internal/helper'
+require 'gitlab/dangerfiles/spec_helper'
+
+require_relative '../../../tooling/danger/sidekiq_args'
+require_relative '../../../tooling/danger/project_helper'
+
+RSpec.describe Tooling::Danger::SidekiqArgs do
+  include_context "with dangerfile"
+
+  let(:fake_danger) { DangerSpecHelper.fake_danger.include(described_class) }
+  let(:fake_project_helper) { Tooling::Danger::ProjectHelper }
+
+  let(:file_lines) do
+    [
+      " describe 'foo' do",
+      " def perform(hello, world)",
+      " def hello(world, gitlab)",
+      " def perform",
+      " ",
+      " def perform(a, b, c, d, e)",
+      " def perform(opts = {})" # hashes of options can be excluded
+    ]
+  end
+
+  let(:matching_lines) do
+    [
+      "+ def perform(hello, world)",
+      "+ def perform(a, b, c, d, e)"
+    ]
+  end
+
+  subject(:specs) { fake_danger.new(helper: fake_helper) }
+
+  before do
+    allow(specs).to receive(:project_helper).and_return(fake_project_helper)
+  end
+
+  describe '#add_comment_for_matched_line' do
+    let(:filename) { 'app/workers/hello_worker.rb' }
+
+    before do
+      expect(specs).to receive(:added_line_matching_args).and_return(matching_lines)
+      allow(specs.project_helper).to receive(:file_lines).and_return(file_lines)
+    end
+
+    it 'adds suggestions at the correct lines' do
+      expect(specs).to receive(:markdown).with(format(described_class::SUGGEST_MR_COMMENT), file: filename, line: 2)
+      expect(specs).to receive(:markdown).with(format(described_class::SUGGEST_MR_COMMENT), file: filename, line: 6)
+
+      specs.add_comment_for_matched_line(filename)
+    end
+  end
+
+  describe '#changed_worker_files' do
+    let(:base_expected_files) { %w[app/workers/a.rb app/workers/b.rb ee/app/workers/e.rb] }
+
+    before do
+      all_changed_files = %w[
+        app/workers/a.rb
+        app/workers/b.rb
+        ee/app/workers/e.rb
+        spec/foo_spec.rb
+        ee/spec/foo_spec.rb
+        spec/bar_spec.rb
+        ee/spec/bar_spec.rb
+        spec/zab_spec.rb
+        ee/spec/zab_spec.rb
+      ]
+
+      allow(specs.helper).to receive(:all_changed_files).and_return(all_changed_files)
+    end
+
+    it 'returns added, modified, and renamed_after files by default' do
+      expect(specs.changed_worker_files).to match_array(base_expected_files)
+    end
+
+    context 'with include_ee: :exclude' do
+      it 'returns spec files without EE-specific files' do
+        expect(specs.changed_worker_files(ee: :exclude)).not_to include(%w[ee/app/workers/e.rb])
+      end
+    end
+
+    context 'with include_ee: :only' do
+      it 'returns EE-specific spec files only' do
+        expect(specs.changed_worker_files(ee: :only)).to match_array(%w[ee/app/workers/e.rb])
+      end
+    end
+  end
+end

tooling/danger/sidekiq_args.rb

+# frozen_string_literal: true
+
+module Tooling
+  module Danger
+    module SidekiqArgs
+      include ::Danger::Helpers
+
+      WORKER_FILES_REGEX = 'app/workers'
+      EE_PREFIX = 'ee/'
+      MATCH_PERFORM_REGEX = /def perform\(\w+,.*\)/.freeze
+      SUGGEST_MR_COMMENT = "Be aware that adding new arguments to a sidekiq worker is [a multi-stage process](https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#add-an-argument)."
+
+      def changed_worker_files(ee: :include)
+        changed_files = helper.all_changed_files
+        folder_prefix =
+          case ee
+          when :include
+            "(#{EE_PREFIX})?"
+          when :only
+            EE_PREFIX
+          when :exclude
+            nil
+          end
+
+        changed_files.grep(%r{\A#{folder_prefix}#{WORKER_FILES_REGEX}})
+      end
+
+      def add_comment_for_matched_line(filename)
+        added_lines = added_line_matching_args(filename)
+        return if added_lines.empty?
+
+        spec_file_lines = project_helper.file_lines(filename)
+
+        added_lines.each_with_object([]) do |added_line, processed_line_numbers|
+          line_number = find_line_number(spec_file_lines,
+                                         added_line.delete_prefix('+'),
+                                         exclude_indexes: processed_line_numbers)
+          processed_line_numbers << line_number
+          markdown(format(SUGGEST_MR_COMMENT), file: filename, line: line_number.succ)
+        end
+      end
+
+      def added_line_matching_args(filename)
+        helper.changed_lines(filename).grep(/\A\+ /).grep(MATCH_PERFORM_REGEX)
+      end
+
+      private
+
+      def find_line_number(file_lines, searched_line, exclude_indexes: [])
+        file_lines.each_with_index do |file_line, index|
+          next if exclude_indexes.include?(index)
+          break index if file_line == searched_line
+        end
+      end
+    end
+  end
+end