Add Strict adapter to ensure feature exists on get

lib/flipper.rb

@@ -176,6 +176,7 @@ def groups_registry=(registry)
 require 'flipper/adapter'
 require 'flipper/adapters/memoizable'
 require 'flipper/adapters/memory'
+require 'flipper/adapters/strict'
 require 'flipper/adapters/instrumented'
 require 'flipper/configuration'
 require 'flipper/dsl'

lib/flipper/adapters/strict.rb

@@ -0,0 +1,44 @@
+module Flipper
+  module Adapters
+    # An adapter that ensures a feature exists before checking it.
+    class Strict
+      extend Forwardable
+      include ::Flipper::Adapter
+      attr_reader :name, :adapter
+
+      class NotFound < ::Flipper::Error
+        def initialize(name)
+          super "Could not find feature #{name.inspect}. Call `Flipper.add(#{name.inspect})` to create it."
+        end
+      end
+
+      HANDLERS = {
+        raise: ->(feature) { raise NotFound.new(feature.key) },
+        warn: ->(feature) { warn NotFound.new(feature.key).message },
+        noop: ->(_) { },
+      }
+
+      DEFAULT_NOT_FOUND =
+
+      def_delegators :@adapter, :features, :get, :get_multi, :get_all, :add, :remove, :clear, :enable, :disable
+
+      def initialize(adapter, name: :strict, handler: :raise)
+        @name = name
+        @adapter = adapter
+        @handler = handler.is_a?(Symbol) ? HANDLERS.fetch(handler) : handler
+      end
+
+      def get(feature)
+        assert_feature_exists(feature)
+        @adapter.get(feature)
+      end
+
+      private
+
+      def assert_feature_exists(feature)
+        @handler.call(feature) unless @adapter.features.include?(feature.key)
+      end
+
+    end
+  end
+end

spec/flipper/adapters/strict_spec.rb

@@ -0,0 +1,28 @@
+RSpec.describe Flipper::Adapters::Strict do
+  let(:flipper) { Flipper.new(subject) }
+  let(:feature) { flipper[:unknown] }
+
+  it_should_behave_like 'a flipper adapter' do
+    subject { described_class.new(Flipper::Adapters::Memory.new, handler: :noop) }
+  end
+
+  context "handler: :raise" do
+    subject { described_class.new(Flipper::Adapters::Memory.new, handler: :raise) }
+
+    context "#get" do
+      it "raises an error for unknown feature" do
+        expect { subject.get(feature) }.to raise_error(Flipper::Adapters::Strict::NotFound)
+      end
+    end
+  end
+
+  context "handler: :warn" do
+    subject { described_class.new(Flipper::Adapters::Memory.new, handler: :warn) }
+
+    context "#get" do
+      it "raises an error for unknown feature" do
+        expect(silence { subject.get(feature) }).to match(/Could not find feature "unknown"/)
+      end
+    end
+  end
+end