Add cop to prevent index creation on forbidden table

This commit adds a new custom cop allowing us to prevent
index creation on a given table. The first table forbidden
is ci_builds.

.rubocop.yml

@@ -607,6 +607,12 @@ Migration/CreateTableWithForeignKeys:
   Exclude:
     - !ruby/regexp /\Adb\/(?:post_)?migrate\/(?:201[0-9]\d+|20200[0-8][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9])_.+\.rb\z/
 
+Migration/PreventIndexCreation:
+  Exclude:
+    - !ruby/regexp /\Adb\/(post_)?migrate\/201.*\.rb\z/
+    - !ruby/regexp /\Adb\/(post_)?migrate\/2020.*\.rb\z/
+    - !ruby/regexp /\Adb\/(post_)?migrate\/20210[1-6].*\.rb\z/
+
 Gitlab/RailsLogger:
   Exclude:
     - 'spec/**/*.rb'

rubocop/cop/migration/prevent_index_creation.rb

+# frozen_string_literal: true
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if new indexes are introduced to forbidden tables.
+      class PreventIndexCreation < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        FORBIDDEN_TABLES = %i[ci_builds].freeze
+
+        MSG = "Adding new index to #{FORBIDDEN_TABLES.join(", ")} is forbidden, see https://gitlab.com/gitlab-org/gitlab/-/issues/332886"
+
+        def_node_matcher :add_index?, <<~PATTERN
+          (send nil? :add_index (sym #forbidden_tables?) ...)
+        PATTERN
+
+        def_node_matcher :add_concurrent_index?, <<~PATTERN
+          (send nil? :add_concurrent_index (sym #forbidden_tables?) ...)
+        PATTERN
+
+        def forbidden_tables?(node)
+          FORBIDDEN_TABLES.include?(node)
+        end
+
+        def on_def(node)
+          return unless in_migration?(node)
+
+          node.each_descendant(:send) do |send_node|
+            add_offense(send_node, location: :selector) if offense?(send_node)
+          end
+        end
+
+        def offense?(node)
+          add_index?(node) || add_concurrent_index?(node)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/migration/prevent_index_creation_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../../rubocop/cop/migration/prevent_index_creation'
+
+RSpec.describe RuboCop::Cop::Migration::PreventIndexCreation do
+  subject(:cop) { described_class.new }
+
+  context 'when in migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    context 'when adding an index to a forbidden table' do
+      it 'registers an offense when add_index is used' do
+        expect_offense(<<~RUBY)
+          def change
+            add_index :ci_builds, :protected
+            ^^^^^^^^^ Adding new index to ci_builds is forbidden, see https://gitlab.com/gitlab-org/gitlab/-/issues/332886
+          end
+        RUBY
+      end
+
+      it 'registers an offense when add_concurrent_index is used' do
+        expect_offense(<<~RUBY)
+          def change
+            add_concurrent_index :ci_builds, :protected
+            ^^^^^^^^^^^^^^^^^^^^ Adding new index to ci_builds is forbidden, see https://gitlab.com/gitlab-org/gitlab/-/issues/332886
+          end
+        RUBY
+      end
+    end
+
+    context 'when adding an index to a regular table' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          def change
+            add_index :ci_pipelines, :locked
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when outside of migration' do
+    it 'does not register an offense' do
+      expect_no_offenses('def change; add_index :table, :column; end')
+    end
+  end
+end