Added simple feature flag system

This can be used to enable/disable certain features using the
ApplicationSetting model.

Features can be toggled from the application settings UI, or via a Ruby
script of sorts. Since this re-uses the ApplicationSetting model the
cached settings are flushed automatically.

CHANGELOG

@@ -61,6 +61,7 @@ v 8.9.0 (unreleased)
   - Markdown editor now correctly resets the input value on edit cancellation !4175
   - Toggling a task list item in a issue/mr description does not creates a Todo for mentions
   - Improved UX of date pickers on issue & milestone forms
+  - Added a simple feature flag system to allow enabling/disabling of certain features
 v 8.8.5 (unreleased)
   - Ensure branch cleanup regardless of whether the GitHub import process succeeds

app/controllers/admin/application_settings_controller.rb

@@ -109,6 +109,7 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
       :metrics_packet_size,
       :send_user_confirmation_email,
       :container_registry_token_expire_delay,
+      *Gitlab::Feature.column_names,
       restricted_visibility_levels: [],
       import_sources: [],
       disabled_oauth_sign_in_sources: []

app/views/admin/application_settings/_form.html.haml

@@ -328,6 +328,23 @@
         .help-block
           If you got a lot of false alarms from repository checks you can choose to clear all repository check information from the database.
+  %fieldset
+    %legend Enabled Features
+    %p
+      Enabling or disabling these features can be useful to those wanting more
+      fine grained control over the behaviour of their GitLab instance.
+      Disabling certain features can also be useful when doing a deploy. For
+      example, should a deploy modify the data used for displaying/creating
+      comments it can be useful to disable the feature
+      <code>creating_notes</code> to prevent users from creating new notes while
+      the deploy is running.
+    .form-group
+      .col-sm-offset-2.col-sm-10
+        - Gitlab::Feature.features_with_columns.each do |feature, column|
+          .checkbox
+            = f.label(column) do
+              = f.check_box(column)
+              <code>#{feature}</code>
   .form-actions
     = f.submit 'Save', class: 'btn btn-save'

db/migrate/20160609133359_add_initial_feature_flags.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddInitialFeatureFlags < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  def change
+    columns = [
+      :enable_toggling_award_emoji,
+      :enable_creating_notes,
+      :enable_updating_notes,
+      :enable_removing_notes,
+      :enable_removing_note_attachments
+    ]
+
+    columns.each do |column|
+      add_column :application_settings, column, :boolean, default: true
+    end
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20160608155312) do
+ActiveRecord::Schema.define(version: 20160609133359) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -85,6 +85,11 @@ ActiveRecord::Schema.define(version: 20160608155312) do
     t.boolean  "send_user_confirmation_email",          default: false
     t.integer  "container_registry_token_expire_delay", default: 5
     t.text     "after_sign_up_text"
+    t.boolean  "enable_toggling_award_emoji",           default: true
+    t.boolean  "enable_creating_notes",                 default: true
+    t.boolean  "enable_updating_notes",                 default: true
+    t.boolean  "enable_removing_notes",                 default: true
+    t.boolean  "enable_removing_note_attachments",      default: true
   end
   create_table "audit_events", force: :cascade do |t|

lib/gitlab/feature.rb

+module Gitlab
+  # Module for checking if certain features are enabled or not.
+  #
+  # To check for a feature you can use an "enabled?" method generated for each
+  # registered feature:
+  #
+  #     if Gitlab::Feature.updating_notes_enabled?
+  #       ...
+  #     end
+  #
+  # ## Adding Features
+  #
+  # Adding features works in two simple steps:
+  #
+  # 1. Add the feature name to the FEATURES array below
+  # 2. Add a migration that adds a column called "enable_FEATURE" (where
+  #    "FEATURE" is the name of your feature) to "application_settings". The
+  #    type should be a boolean with a default value of true.
+  module Feature
+    extend CurrentSettings
+
+    FEATURES = [
+      :toggling_award_emoji,
+      :creating_notes,
+      :updating_notes,
+      :removing_notes,
+      :removing_note_attachments
+    ].freeze
+
+    class << self
+      FEATURES.each do |feature|
+        define_method("#{feature}_enabled?") do
+          feature_enabled?(feature)
+        end
+      end
+
+      # Returns true if the given feature is enabled.
+      def feature_enabled?(feature)
+        settings = current_application_settings
+        method = column_name(feature)
+
+        # If a feature column doesn't exist we still want to enable the
+        # feature. This for example allows the use of a fake application
+        # settings object without having to duplicate any feature related
+        # logic there.
+        return true unless settings.respond_to?(method)
+
+        settings.__send__(method)
+      end
+
+      # Returns a Hash containing all features and the corresponding column
+      # names.
+      def features_with_columns
+        FEATURES.each_with_object({}) do |feature, hash|
+          hash[feature] = column_name(feature)
+        end
+      end
+
+      # Returns the names of the columns.
+      def column_names
+        FEATURES.map { |f| column_name(f) }
+      end
+
+      # Returns the column name for a feature.
+      def column_name(feature)
+        :"enable_#{feature}"
+      end
+    end
+  end
+end

spec/lib/gitlab/feature_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Feature do
+  Gitlab::Feature::FEATURES.each do |feature|
+    describe ".#{feature}_enabled?" do
+      it 'returns a boolean' do
+        expect(described_class.__send__("#{feature}_enabled?")).
+          to be_in([true, false])
+      end
+    end
+  end
+
+  describe '.feature_enabled?' do
+    it 'returns true when the column does not exist' do
+      settings = double(:settings)
+
+      expect(described_class).to receive(:current_application_settings).
+        and_return(settings)
+
+      expect(described_class.feature_enabled?(:foo)).to eq(true)
+    end
+  end
+
+  describe '.features_with_columns' do
+    it 'returns a Hash mapping feature names to their columns' do
+      map = described_class.features_with_columns
+
+      expect(map).to be_an_instance_of(Hash)
+      expect(map[:creating_notes]).to eq(:enable_creating_notes)
+    end
+  end
+
+  describe '.column_names' do
+    it 'returns an Array of column names' do
+      names = described_class.column_names
+
+      expect(names).to be_an_instance_of(Array)
+      expect(names).to include(:enable_creating_notes)
+    end
+  end
+
+  describe '.column_name' do
+    it 'returns the column name of a feature flag' do
+      expect(described_class.column_name(:foo)).to eq(:enable_foo)
+    end
+  end
+end