Introduce Gitlab/EventStoreSubscriber cop

This cop checks that a worker that
includes `Gitlab::EventStore::Subscriber`
implements `#handle_event` and does not override `#perform`.

.rubocop.yml

@@ -291,6 +291,12 @@ Gitlab/AvoidUploadedFileFromParams:
     - 'spec/**/*'
     - 'ee/spec/**/*'
 
+Gitlab/EventStoreSubscriber:
+  Enabled: true
+  Exclude:
+    - 'spec/**/*'
+    - 'ee/spec/**/*'
+
 GitlabSecurity/PublicSend:
   Enabled: true
   Exclude:

rubocop/cop/gitlab/event_store_subscriber.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gitlab
+      # Cop that checks the implementation of Gitlab::EventStore::Subscriber
+      #
+      # A worker that implements Gitlab::EventStore::Subscriber
+      # must implement the method #handle_event(event) and
+      # must not override the method #perform(*args)
+      #
+      #  # bad
+      #  class MySubscriber
+      #    include Gitlab::EventStore::Subscriber
+      #
+      #    def perform(*args)
+      #    end
+      #  end
+      #
+      #  # bad
+      #  class MySubscriber
+      #    include Gitlab::EventStore::Subscriber
+      #  end
+      #
+      #  # good
+      #  class MySubscriber
+      #    include Gitlab::EventStore::Subscriber
+      #
+      #    def handle_event(event)
+      #    end
+      #  end
+      #
+      class EventStoreSubscriber < RuboCop::Cop::Cop
+        SUBSCRIBER_MODULE_NAME = 'Gitlab::EventStore::Subscriber'
+        FORBID_PERFORM_OVERRIDE = "Do not override `perform` in a `#{SUBSCRIBER_MODULE_NAME}`."
+        REQUIRE_HANDLE_EVENT = "A `#{SUBSCRIBER_MODULE_NAME}` must implement `#handle_event(event)`."
+
+        def_node_matcher :includes_subscriber?, <<~PATTERN
+          (send nil? :include (const (const (const nil? :Gitlab) :EventStore) :Subscriber))
+        PATTERN
+
+        def on_send(node)
+          return unless includes_subscriber?(node)
+
+          self.is_subscriber ||= true
+          self.include_subscriber_node ||= node
+        end
+
+        def on_def(node)
+          if is_subscriber && node.method_name == :perform
+            add_offense(node, message: FORBID_PERFORM_OVERRIDE)
+          end
+
+          self.implements_handle_event ||= true if node.method_name == :handle_event
+        end
+
+        def on_investigation_end
+          super
+
+          if is_subscriber && !implements_handle_event
+            add_offense(include_subscriber_node, message: REQUIRE_HANDLE_EVENT)
+          end
+        end
+
+        private
+
+        attr_accessor :is_subscriber, :include_subscriber_node, :implements_handle_event
+      end
+    end
+  end
+end

spec/rubocop/cop/gitlab/event_store_subscriber_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+require_relative '../../../../rubocop/cop/gitlab/event_store_subscriber'
+
+RSpec.describe RuboCop::Cop::Gitlab::EventStoreSubscriber do
+  subject(:cop) { described_class.new }
+
+  context 'when an event store subscriber overrides #perform' do
+    it 'registers an offense' do
+      expect_offense(<<~WORKER)
+        class SomeWorker
+          include Gitlab::EventStore::Subscriber
+
+          def perform(*args)
+          ^^^^^^^^^^^^^^^^^^ Do not override `perform` in a `Gitlab::EventStore::Subscriber`.
+          end
+
+          def handle_event(event); end
+        end
+      WORKER
+    end
+  end
+
+  context 'when an event store subscriber does not override #perform' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~WORKER)
+        class SomeWorker
+          include Gitlab::EventStore::Subscriber
+
+          def handle_event(event); end
+        end
+      WORKER
+    end
+  end
+
+  context 'when an event store subscriber does not implement #handle_event' do
+    it 'registers an offense' do
+      expect_offense(<<~WORKER)
+        class SomeWorker
+          include Gitlab::EventStore::Subscriber
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ A `Gitlab::EventStore::Subscriber` must implement `#handle_event(event)`.
+        end
+      WORKER
+    end
+  end
+
+  context 'when a Sidekiq worker overrides #perform' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~WORKER)
+        class SomeWorker
+          include ApplicationWorker
+
+          def perform(*args); end
+        end
+      WORKER
+    end
+  end
+
+  context 'when a Sidekiq worker implements #handle_event' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~WORKER)
+        class SomeWorker
+          include ApplicationWorker
+
+          def handle_event(event); end
+        end
+      WORKER
+    end
+  end
+
+  context 'a non worker class' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~MODEL)
+        class Model < ApplicationRecord
+          include ActiveSupport::Concern
+        end
+      MODEL
+    end
+  end
+end