Add Strict adapter to ensure feature exists on get

examples/strict.rb

@@ -0,0 +1,18 @@
+require 'bundler/setup'
+require 'flipper'
+
+adapter = Flipper::Adapters::Strict.new(Flipper::Adapters::Memory.new)
+flipper = Flipper.new(adapter)
+
+begin
+  puts "Checking :unknown_feature, which should raise an error."
+  flipper.enabled?(:unknown_feature)
+  warn "An error was not raised, but should have been"
+  exit 1
+rescue Flipper::Adapters::Strict::NotFound => exception
+  puts "Ok, the exepcted error was raised: #{exception.message}"
+end
+
+puts "Flipper.add(:new_feature)"
+flipper.add(:new_feature)
+puts "Flipper.enabled?(:new_feature) => #{flipper.enabled?(:new_feature)}"

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 +1,47 @@
+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, :handler
+
+      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: ->(_) { },
+      }
+
+      def_delegators :@adapter, :features, :get_all, :add, :remove, :clear, :enable, :disable
+
+      def initialize(adapter, handler = nil, &block)
+        @name = :strict
+        @adapter = adapter
+        @handler = block || HANDLERS.fetch(handler)
+      end
+
+      def get(feature)
+        assert_feature_exists(feature)
+        @adapter.get(feature)
+      end
+
+      def get_multi(features)
+        features.each { |feature| assert_feature_exists(feature) }
+        @adapter.get_multi(features)
+      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 +1,62 @@
+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, :noop) }
+  end
+
+  context "handler = :raise" do
+    subject { described_class.new(Flipper::Adapters::Memory.new, :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
+
+    context "#get_multi" do
+      it "raises an error for unknown feature" do
+        expect { subject.get_multi([feature]) }.to raise_error(Flipper::Adapters::Strict::NotFound)
+      end
+    end
+  end
+
+  context "handler = :warn" do
+    subject { described_class.new(Flipper::Adapters::Memory.new, :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
+
+    context "#get_multi" do
+      it "raises an error for unknown feature" do
+        expect(silence { subject.get_multi([feature]) }).to match(/Could not find feature "unknown"/)
+      end
+    end
+  end
+
+  context "handler = Block" do
+    let(:unknown_features) { [] }
+    subject do
+      described_class.new(Flipper::Adapters::Memory.new) { |feature| unknown_features << feature.key}
+    end
+
+
+    context "#get" do
+      it "raises an error for unknown feature" do
+        subject.get(feature)
+        expect(unknown_features).to eq(["unknown"])
+      end
+    end
+
+    context "#get_multi" do
+      it "raises an error for unknown feature" do
+        subject.get_multi([flipper[:foo], flipper[:bar]])
+        expect(unknown_features).to eq(["foo", "bar"])
+      end
+    end
+  end
+end