Introduce Feature Flag Definition

This adds a YAML-based definition of feature flags
that are stored in `(ee/)configs/feature-flags/`.

Currently none of feature flag types are required
to have a present YAML definition.

This definition contains information like:

- what MR introduced FF
- helps to create and has a issue that tracks
  the rollout and removal of FF
- intentionally adds `default_enabled` to YAML
- force checks the consistency of `default_enabled`
  between what is in code and what is in YAML

config/initializers/0_inject_feature_flags.rb

+# This needs to be loaded after
+# config/initializers/0_inject_enterprise_edition_module.rb
+
+Feature.register_feature_groups
+Feature.register_definitions

config/initializers/flipper.rb

-Feature.register_feature_groups

ee/lib/ee/feature/definition.rb

+# frozen_string_literal: true
+
+module EE
+  module Feature
+    module Definition
+      module ClassMethods
+        extend ::Gitlab::Utils::Override
+
+        override :paths
+        def paths
+          @ee_paths ||= [Rails.root.join('ee', 'config', 'feature_flags', '**', '*.yml')] + super
+        end
+      end
+
+      def self.prepended(base)
+        base.singleton_class.prepend ClassMethods
+      end
+    end
+  end
+end

lib/feature.rb

@@ -54,12 +54,14 @@ class Feature
     # unless set explicitly.  The default is `disabled`
     # TODO: remove the `default_enabled:` and read it from the `defintion_yaml`
     # check: https://gitlab.com/gitlab-org/gitlab/-/issues/30228
-    def enabled?(key, thing = nil, default_enabled: false)
+    def enabled?(key, thing = nil, type: :development, default_enabled: false)
       if check_feature_flags_definition?
         if thing && !thing.respond_to?(:flipper_id)
           raise InvalidFeatureFlagError,
             "The thing '#{thing.class.name}' for feature flag '#{key}' needs to include `FeatureGate` or implement `flipper_id`"
         end
+
+        Feature::Definition.valid_usage!(key, type: type, default_enabled: default_enabled)
       end
 
       # During setup the database does not exist yet. So we haven't stored a value
@@ -75,9 +77,9 @@ class Feature
       !default_enabled || Feature.persisted_name?(feature.name) ? feature.enabled?(thing) : true
     end
 
-    def disabled?(key, thing = nil, default_enabled: false)
+    def disabled?(key, thing = nil, type: :development, default_enabled: false)
       # we need to make different method calls to make it easy to mock / define expectations in test mode
-      thing.nil? ? !enabled?(key, default_enabled: default_enabled) : !enabled?(key, thing, default_enabled: default_enabled)
+      thing.nil? ? !enabled?(key, type: type, default_enabled: default_enabled) : !enabled?(key, thing, type: type, default_enabled: default_enabled)
     end
 
     def enable(key, thing = true)
@@ -129,6 +131,12 @@ class Feature
     def register_feature_groups
     end
 
+    def register_definitions
+      return unless check_feature_flags_definition?
+
+      Feature::Definition.load_all!
+    end
+
     private
 
     def flipper

lib/feature/definition.rb

+# frozen_string_literal: true
+
+class Feature
+  class Definition
+    include ::Feature::Shared
+
+    attr_reader :path
+    attr_reader :attributes
+
+    PARAMS.each do |param|
+      define_method(param) do
+        attributes[param]
+      end
+    end
+
+    def initialize(path, opts = {})
+      @path = path
+      @attributes = {}
+
+      # assign nil, for all unknown opts
+      PARAMS.each do |param|
+        @attributes[param] = opts[param]
+      end
+    end
+
+    def key
+      name.to_sym
+    end
+
+    def validate!
+      unless name.present?
+        raise Feature::InvalidFeatureFlagError, "Feature flag is missing name"
+      end
+
+      unless path.present?
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' is missing path"
+      end
+
+      unless type.present?
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' is missing type. Ensure to update #{path}"
+      end
+
+      unless Definition::TYPES.include?(type.to_sym)
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' type '#{type}' is invalid. Ensure to update #{path}"
+      end
+
+      unless File.basename(path, ".yml") == name
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' has an invalid path: '#{path}'. Ensure to update #{path}"
+      end
+
+      unless File.basename(File.dirname(path)) == type
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' has an invalid type: '#{path}'. Ensure to update #{path}"
+      end
+
+      if default_enabled.nil?
+        raise Feature::InvalidFeatureFlagError, "Feature flag '#{name}' is missing default_enabled. Ensure to update #{path}"
+      end
+    end
+
+    def valid_usage!(type_in_code:, default_enabled_in_code:)
+      unless Array(type).include?(type_in_code.to_s)
+        # Raise exception in test and dev
+        raise Feature::InvalidFeatureFlagError, "The `type:` of `#{key}` is not equal to config: " \
+          "#{type_in_code} vs #{type}. Ensure to use valid type in #{path} or ensure that you use " \
+          "a valid syntax: #{TYPES.dig(type, :example)}"
+      end
+
+      # We accept an array of defaults as some features are undefined
+      # and have `default_enabled: true/false`
+      unless Array(default_enabled).include?(default_enabled_in_code)
+        # Raise exception in test and dev
+        raise Feature::InvalidFeatureFlagError, "The `default_enabled:` of `#{key}` is not equal to config: " \
+          "#{default_enabled_in_code} vs #{default_enabled}. Ensure to update #{path}"
+      end
+    end
+
+    def to_h
+      attributes
+    end
+
+    class << self
+      def paths
+        @paths ||= [Rails.root.join('config', 'feature_flags', '**', '*.yml')]
+      end
+
+      def definitions
+        @definitions ||= {}
+      end
+
+      def load_all!
+        definitions.clear
+
+        paths.each do |glob_path|
+          load_all_from_path!(glob_path)
+        end
+
+        definitions
+      end
+
+      def valid_usage!(key, type:, default_enabled:)
+        if definition = definitions[key.to_sym]
+          definition.valid_usage!(type_in_code: type, default_enabled_in_code: default_enabled)
+        elsif type_definition = self::TYPES[type]
+          raise InvalidFeatureFlagError, "Missing feature definition for `#{key}`" unless type_definition[:optional]
+        else
+          raise InvalidFeatureFlagError, "Unknown feature flag type used: `#{type}`"
+        end
+      end
+
+      private
+
+      def load_from_file(path)
+        definition = File.read(path)
+        definition = YAML.safe_load(definition)
+        definition.deep_symbolize_keys!
+
+        self.new(path, definition).tap(&:validate!)
+      rescue => e
+        raise Feature::InvalidFeatureFlagError, "Invalid definition for `#{path}`: #{e.message}"
+      end
+
+      def load_all_from_path!(glob_path)
+        Dir.glob(glob_path).each do |path|
+          definition = load_from_file(path)
+
+          if previous = definitions[definition.key]
+            raise InvalidFeatureFlagError, "Feature flag '#{definition.key}' is already defined in '#{previous.path}'"
+          end
+
+          definitions[definition.key] = definition
+        end
+      end
+    end
+  end
+end
+
+Feature::Definition.prepend_if_ee('EE::Feature::Definition')

lib/feature/shared.rb

+# frozen_string_literal: true
+
+# This file can contain only simple constructs as it is shared between:
+# 1. `Pure Ruby`: `bin/feature-flag`
+# 2. `GitLab Rails`: `lib/feature/definition.rb`
+
+class Feature
+  module Shared
+    # optional: defines if a on-disk definition is required for this feature flag type
+    # rollout_issue: defines if `bin/feature-flag` asks for rollout issue
+    # example: usage being shown when exception is raised
+    TYPES = {
+      development: {
+        description: 'Short lived, used to enable unfinished code to be deployed',
+        optional: true,
+        rollout_issue: true,
+        example: <<-EOS
+          Feature.enabled?(:my_feature_flag)
+          Feature.enabled?(:my_feature_flag, type: :development)
+        EOS
+      }
+    }.freeze
+
+    PARAMS = %i[
+      name
+      default_enabled
+      type
+      introduced_by_url
+      rollout_issue_url
+      group
+    ].freeze
+  end
+end

spec/lib/feature/definition_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Feature::Definition do
+  let(:attributes) do
+    { name: 'feature_flag',
+      type: 'development',
+      default_enabled: true }
+  end
+
+  let(:path) { File.join('development', 'feature_flag.yml') }
+  let(:definition) { described_class.new(path, attributes) }
+  let(:yaml_content) { attributes.deep_stringify_keys.to_yaml }
+
+  describe '#key' do
+    subject { definition.key }
+
+    it 'returns a symbol from name' do
+      is_expected.to eq(:feature_flag)
+    end
+  end
+
+  describe '#validate!' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:param, :value, :result) do
+      :name            | nil                        | /Feature flag is missing name/
+      :path            | nil                        | /Feature flag 'feature_flag' is missing path/
+      :type            | nil                        | /Feature flag 'feature_flag' is missing type/
+      :type            | 'invalid'                  | /Feature flag 'feature_flag' type 'invalid' is invalid/
+      :path            | 'development/invalid.yml'  | /Feature flag 'feature_flag' has an invalid path/
+      :path            | 'invalid/feature_flag.yml' | /Feature flag 'feature_flag' has an invalid type/
+      :default_enabled | nil                        | /Feature flag 'feature_flag' is missing default_enabled/
+    end
+
+    with_them do
+      let(:params) { attributes.merge(path: path) }
+
+      before do
+        params[param] = value
+      end
+
+      it do
+        expect do
+          described_class.new(
+            params[:path], params.except(:path)
+          ).validate!
+        end.to raise_error(result)
+      end
+    end
+  end
+
+  describe '#valid_usage!' do
+    context 'validates type' do
+      it 'raises exception for invalid type' do
+        expect { definition.valid_usage!(type_in_code: :invalid, default_enabled_in_code: false) }
+          .to raise_error(/The `type:` of `feature_flag` is not equal to config/)
+      end
+    end
+
+    context 'validates default enabled' do
+      it 'raises exception for different value' do
+        expect { definition.valid_usage!(type_in_code: :development, default_enabled_in_code: false) }
+          .to raise_error(/The `default_enabled:` of `feature_flag` is not equal to config/)
+      end
+    end
+  end
+
+  describe '.paths' do
+    it 'returns at least one path' do
+      expect(described_class.paths).not_to be_empty
+    end
+  end
+
+  describe '.load_from_file' do
+    it 'properly loads a definition from file' do
+      expect(File).to receive(:read).with(path) { yaml_content }
+
+      expect(described_class.send(:load_from_file, path).attributes)
+        .to eq(definition.attributes)
+    end
+
+    context 'for missing file' do
+      let(:path) { 'missing/feature-flag/file.yml' }
+
+      it 'raises exception' do
+        expect do
+          described_class.send(:load_from_file, path)
+        end.to raise_error(/Invalid definition for/)
+      end
+    end
+
+    context 'for invalid definition' do
+      it 'raises exception' do
+        expect(File).to receive(:read).with(path) { '{}' }
+
+        expect do
+          described_class.send(:load_from_file, path)
+        end.to raise_error(/Feature flag is missing name/)
+      end
+    end
+  end
+
+  describe '.load_all!' do
+    let(:store1) { Dir.mktmpdir('path1') }
+    let(:store2) { Dir.mktmpdir('path2') }
+
+    before do
+      allow(described_class).to receive(:paths).and_return(
+        [
+          File.join(store1, '**', '*.yml'),
+          File.join(store2, '**', '*.yml')
+        ]
+      )
+    end
+
+    it "when there's no feature flags a list of definitions is empty" do
+      expect(described_class.load_all!).to be_empty
+    end
+
+    it "when there's a single feature flag it properly loads them" do
+      write_feature_flag(store1, path, yaml_content)
+
+      expect(described_class.load_all!).to be_one
+    end
+
+    it "when the same feature flag is stored multiple times raises exception" do
+      write_feature_flag(store1, path, yaml_content)
+      write_feature_flag(store2, path, yaml_content)
+
+      expect { described_class.load_all! }
+        .to raise_error(/Feature flag 'feature_flag' is already defined/)
+    end
+
+    it "when one of the YAMLs is invalid it does raise exception" do
+      write_feature_flag(store1, path, '{}')
+
+      expect { described_class.load_all! }
+        .to raise_error(/Feature flag is missing name/)
+    end
+
+    after do
+      FileUtils.rm_rf(store1)
+      FileUtils.rm_rf(store2)
+    end
+
+    def write_feature_flag(store, path, content)
+      path = File.join(store, path)
+      dir = File.dirname(path)
+      FileUtils.mkdir_p(dir)
+      File.write(path, content)
+    end
+  end
+
+  describe '.valid_usage!' do
+    before do
+      allow(described_class).to receive(:definitions) do
+        { definition.key => definition }
+      end
+    end
+
+    context 'when a known feature flag is used' do
+      it 'validates it usage' do
+        expect(definition).to receive(:valid_usage!)
+
+        described_class.valid_usage!(:feature_flag, type: :development, default_enabled: false)
+      end
+    end
+
+    context 'when an unknown feature flag is used' do
+      context 'for a type that is required to have all feature flags registered' do
+        before do
+          stub_const('Feature::Shared::TYPES', {
+            development: { optional: false }
+          })
+        end
+
+        it 'raises exception' do
+          expect do
+            described_class.valid_usage!(:unknown_feature_flag, type: :development, default_enabled: false)
+          end.to raise_error(/Missing feature definition for `unknown_feature_flag`/)
+        end
+      end
+
+      context 'for a type that is optional' do
+        before do
+          stub_const('Feature::Shared::TYPES', {
+            development: { optional: true }
+          })
+        end
+
+        it 'does not raise exception' do
+          expect do
+            described_class.valid_usage!(:unknown_feature_flag, type: :development, default_enabled: false)
+          end.not_to raise_error
+        end
+      end
+
+      context 'for an unknown type' do
+        it 'raises exception' do
+          expect do
+            described_class.valid_usage!(:unknown_feature_flag, type: :unknown_type, default_enabled: false)
+          end.to raise_error(/Unknown feature flag type used: `unknown_type`/)
+        end
+      end
+    end
+  end
+end

spec/lib/feature_spec.rb

@@ -242,6 +242,36 @@ RSpec.describe Feature, stub_feature_flags: false do
         end
       end
     end
+
+    context 'validates usage of feature flag with YAML definition' do
+      let(:definition) do
+        Feature::Definition.new('development/my_feature_flag.yml',
+          name: 'my_feature_flag',
+          type: 'development',
+          default_enabled: false
+        ).tap(&:validate!)
+      end
+
+      before do
+        allow(Feature::Definition).to receive(:definitions) do
+          { definition.key => definition }
+        end
+      end
+
+      it 'when usage is correct' do
+        expect { described_class.enabled?(:my_feature_flag) }.not_to raise_error
+      end
+
+      it 'when invalid type is used' do
+        expect { described_class.enabled?(:my_feature_flag, type: :licensed) }
+          .to raise_error(/The `type:` of/)
+      end
+
+      it 'when invalid default_enabled is used' do
+        expect { described_class.enabled?(:my_feature_flag, default_enabled: true) }
+          .to raise_error(/The `default_enabled:` of/)
+      end
+    end
   end
 
   describe '.disable?' do

spec/spec_helper.rb

@@ -155,6 +155,9 @@ RSpec.configure do |config|
   config.before(:suite) do
     Timecop.safe_mode = true
     TestEnv.init
+
+    # Reload all feature flags definitions
+    Feature.register_definitions
   end
 
   config.after(:all) do