Add default for outbound_local_requests_whitelist

It needs to default to an empty array logically.

db/migrate/20190725012225_change_outbound_local_requests_whitelist_default.rb

+# frozen_string_literal: true
+
+class ChangeOutboundLocalRequestsWhitelistDefault < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  class ApplicationSetting < ActiveRecord::Base
+    self.table_name = 'application_settings'
+  end
+
+  def up
+    default_value = []
+
+    change_column_default(:application_settings, :outbound_local_requests_whitelist, default_value)
+
+    ApplicationSetting
+      .where(outbound_local_requests_whitelist: nil)
+      .update(outbound_local_requests_whitelist: default_value)
+
+    change_column_null(:application_settings, :outbound_local_requests_whitelist, false)
+  end
+
+  def down
+    change_column_null(:application_settings, :outbound_local_requests_whitelist, true)
+
+    change_column_default(:application_settings, :outbound_local_requests_whitelist, nil)
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_07_15_142138) do
+ActiveRecord::Schema.define(version: 2019_07_25_012225) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -228,7 +228,7 @@ ActiveRecord::Schema.define(version: 2019_07_15_142138) do
     t.boolean "lock_memberships_to_ldap", default: false, null: false
     t.boolean "time_tracking_limit_to_hours", default: false, null: false
     t.string "grafana_url", default: "/-/grafana", null: false
-    t.string "outbound_local_requests_whitelist", limit: 255, array: true
+    t.string "outbound_local_requests_whitelist", limit: 255, default: [], null: false, array: true
     t.integer "raw_blob_request_limit", default: 300, null: false
     t.index ["custom_project_templates_group_id"], name: "index_application_settings_on_custom_project_templates_group_id"
     t.index ["file_template_project_id"], name: "index_application_settings_on_file_template_project_id"

spec/migrations/change_outbound_local_requests_whitelist_default_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20190725012225_change_outbound_local_requests_whitelist_default.rb')
+
+describe ChangeOutboundLocalRequestsWhitelistDefault, :migration do
+  let(:application_settings) { table(:application_settings) }
+
+  it 'defaults to empty array' do
+    setting = application_settings.create!
+    setting_with_value = application_settings.create!(outbound_local_requests_whitelist: '{a,b}')
+
+    expect(application_settings.where(outbound_local_requests_whitelist: nil).count).to eq(1)
+
+    migrate!
+
+    expect(application_settings.where(outbound_local_requests_whitelist: nil).count).to eq(0)
+    expect(setting.reload.outbound_local_requests_whitelist).to eq([])
+    expect(setting_with_value.reload.outbound_local_requests_whitelist).to eq(%w[a b])
+  end
+end