Add AddColumnWithDefaultToLargeTable cop

rubocop/cop/migration/add_column_with_default_to_large_table.rb

+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # This cop checks for `add_column_with_default` on a table that's been
+      # explicitly blacklisted because of its size.
+      #
+      # Even though this helper performs the update in batches to avoid
+      # downtime, using it with tables with millions of rows still causes a
+      # significant delay in the deploy process and is best avoided.
+      #
+      # See https://gitlab.com/gitlab-com/infrastructure/issues/1602 for more
+      # information.
+      class AddColumnWithDefaultToLargeTable < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'Using `add_column_with_default` on the `%s` table will take a ' \
+          'long time to complete, and should be avoided unless absolutely ' \
+          'necessary'.freeze
+
+        LARGE_TABLES = %i[
+          events
+          issues
+          merge_requests
+          namespaces
+          notes
+          projects
+          routes
+          users
+        ].freeze
+
+        def_node_matcher :add_column_with_default?, <<~PATTERN
+          (send nil :add_column_with_default $(sym ...) ...)
+        PATTERN
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          matched = add_column_with_default?(node)
+          return unless matched
+
+          table = matched.to_a.first
+          return unless LARGE_TABLES.include?(table)
+
+          add_offense(node, :expression, format(MSG, table))
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

 require_relative 'cop/custom_error_class'
 require_relative 'cop/gem_fetcher'
 require_relative 'cop/migration/add_column'
+require_relative 'cop/migration/add_column_with_default_to_large_table'
 require_relative 'cop/migration/add_concurrent_foreign_key'
 require_relative 'cop/migration/add_concurrent_index'
 require_relative 'cop/migration/add_index'

spec/rubocop/cop/migration/add_column_with_default_to_large_table_spec.rb

+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../../rubocop/cop/migration/add_column_with_default_to_large_table'
+
+describe RuboCop::Cop::Migration::AddColumnWithDefaultToLargeTable do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'in migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    described_class::LARGE_TABLES.each do |table|
+      it "registers an offense for the #{table} table" do
+        inspect_source(cop, "add_column_with_default :#{table}, :column, default: true")
+
+        aggregate_failures do
+          expect(cop.offenses.size).to eq(1)
+          expect(cop.offenses.map(&:line)).to eq([1])
+        end
+      end
+    end
+
+    it 'registers no offense for non-blacklisted tables' do
+      inspect_source(cop, "add_column_with_default :table, :column, default: true")
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'outside of migration' do
+    it 'registers no offense' do
+      table = described_class::LARGE_TABLES.sample
+      inspect_source(cop, "add_column_with_default :#{table}, :column, default: true")
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+end