2 files changed, 170 insertions, 0 deletions
diff --git a/lib/feature/definition.rb b/lib/feature/definition.rb
new file mode 100644
index 00000000000..b0ea55c5805
--- /dev/null
+++ b/lib/feature/definition.rb
@@ -0,0 +1,137 @@
+# 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')
diff --git a/lib/feature/shared.rb b/lib/feature/shared.rb
new file mode 100644
index 00000000000..14efbb07100
--- /dev/null
+++ b/lib/feature/shared.rb
@@ -0,0 +1,33 @@
+# 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