Merge branch 'sh-use-process-cache-for-feature-flags' into 'master'

Use process-wide memory cache for feature flags

See merge request gitlab-org/gitlab!26935

changelogs/unreleased/sh-use-process-cache-for-feature-flags.yml

+---
+title: Use process-wide memory cache for feature flags
+merge_request: 26935
+author:
+type: performance

lib/feature.rb

@@ -134,7 +134,11 @@ class Feature
     end
 
     def l1_cache_backend
-      Gitlab::ThreadMemoryCache.cache_backend
+      if Gitlab::Utils.to_boolean(ENV['USE_THREAD_MEMORY_CACHE'])
+        Gitlab::ThreadMemoryCache.cache_backend
+      else
+        Gitlab::ProcessMemoryCache.cache_backend
+      end
     end
 
     def l2_cache_backend

lib/gitlab/process_memory_cache.rb

+# frozen_string_literal: true
+
+module Gitlab
+  class ProcessMemoryCache
+    # ActiveSupport::Cache::MemoryStore is thread-safe:
+    # https://github.com/rails/rails/blob/2f1fefe456932a6d7d2b155d27b5315c33f3daa1/activesupport/lib/active_support/cache/memory_store.rb#L19
+    @cache = ActiveSupport::Cache::MemoryStore.new
+
+    def self.cache_backend
+      @cache
+    end
+  end
+end

spec/lib/feature_spec.rb

@@ -146,7 +146,15 @@ describe Feature do
       expect(described_class.enabled?(:enabled_feature_flag)).to be_truthy
     end
 
-    it { expect(described_class.l1_cache_backend).to eq(Gitlab::ThreadMemoryCache.cache_backend) }
+    context 'with USE_THREAD_MEMORY_CACHE defined' do
+      before do
+        stub_env('USE_THREAD_MEMORY_CACHE', '1')
+      end
+
+      it { expect(described_class.l1_cache_backend).to eq(Gitlab::ThreadMemoryCache.cache_backend) }
+    end
+
+    it { expect(described_class.l1_cache_backend).to eq(Gitlab::ProcessMemoryCache.cache_backend) }
     it { expect(described_class.l2_cache_backend).to eq(Rails.cache) }
 
     it 'caches the status in L1 and L2 caches',

spec/spec_helper.rb

@@ -193,8 +193,10 @@ RSpec.configure do |config|
     # expect(Gitlab::Git::KeepAround).to receive(:execute).and_call_original
     allow(Gitlab::Git::KeepAround).to receive(:execute)
 
-    # Clear thread cache and Sidekiq queues
-    Gitlab::ThreadMemoryCache.cache_backend.clear
+    [Gitlab::ThreadMemoryCache, Gitlab::ProcessMemoryCache].each do |cache|
+      cache.cache_backend.clear
+    end
+
     Sidekiq::Worker.clear_all
 
     # Temporary patch to force admin mode to be active by default in tests when