Cache feature names in RequestStore

The GitHub importer (and probably other parts of our code) ends up
calling Feature.persisted? many times (via Gitaly). By storing this data
in RequestStore we can save ourselves _a lot_ of database queries.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39361

lib/feature.rb

@@ -5,6 +5,10 @@ class Feature
   class FlipperFeature < Flipper::Adapters::ActiveRecord::Feature
     # Using `self.table_name` won't work. ActiveRecord bug?
     superclass.table_name = 'features'
+
+    def self.feature_names
+      pluck(:key)
+    end
   end
 
   class FlipperGate < Flipper::Adapters::ActiveRecord::Gate
@@ -22,11 +26,19 @@ class Feature
       flipper.feature(key)
     end
 
+    def persisted_names
+      if RequestStore.active?
+        RequestStore[:flipper_persisted_names] ||= FlipperFeature.feature_names
+      else
+        FlipperFeature.feature_names
+      end
+    end
+
     def persisted?(feature)
       # Flipper creates on-memory features when asked for a not-yet-created one.
       # If we want to check if a feature has been actually set, we look for it
       # on the persisted features list.
-      all.map(&:name).include?(feature.name)
+      persisted_names.include?(feature.name)
     end
 
     def enabled?(key, thing = nil)

spec/lib/feature_spec.rb

@@ -13,6 +13,47 @@ describe Feature do
     end
   end
 
+  describe '.persisted_names' do
+    it 'returns the names of the persisted features' do
+      Feature::FlipperFeature.create!(key: 'foo')
+
+      expect(described_class.persisted_names).to eq(%w[foo])
+    end
+
+    it 'returns an empty Array when no features are presisted' do
+      expect(described_class.persisted_names).to be_empty
+    end
+
+    it 'caches the feature names when request store is active', :request_store do
+      Feature::FlipperFeature.create!(key: 'foo')
+
+      expect(Feature::FlipperFeature)
+        .to receive(:feature_names)
+        .once
+        .and_call_original
+
+      2.times do
+        expect(described_class.persisted_names).to eq(%w[foo])
+      end
+    end
+  end
+
+  describe '.persisted?' do
+    it 'returns true for a persisted feature' do
+      Feature::FlipperFeature.create!(key: 'foo')
+
+      feature = double(:feature, name: 'foo')
+
+      expect(described_class.persisted?(feature)).to eq(true)
+    end
+
+    it 'returns false for a feature that is not persisted' do
+      feature = double(:feature, name: 'foo')
+
+      expect(described_class.persisted?(feature)).to eq(false)
+    end
+  end
+
   describe '.all' do
     let(:features) { Set.new }