Only check request_deadline flag once per request

Every time we check a feature flag using `Feature.enabled?` it could
return a different value based on the percentage that it's enabled
for.

So making sure we only call this value once per request, so we have a
consistent state for the entire request.

config/application.rb

@@ -18,7 +18,6 @@ module Gitlab
     require_dependency Rails.root.join('lib/gitlab/redis/cache')
     require_dependency Rails.root.join('lib/gitlab/redis/queues')
     require_dependency Rails.root.join('lib/gitlab/redis/shared_state')
-    require_dependency Rails.root.join('lib/gitlab/request_context')
     require_dependency Rails.root.join('lib/gitlab/current_settings')
     require_dependency Rails.root.join('lib/gitlab/middleware/read_only')
     require_dependency Rails.root.join('lib/gitlab/middleware/basic_health_check')

lib/gitlab/request_context.rb

@@ -2,6 +2,7 @@
 
 module Gitlab
   class RequestContext
+    include Gitlab::Utils::StrongMemoize
     include Singleton
 
     RequestDeadlineExceeded = Class.new(StandardError)
@@ -15,10 +16,12 @@ module Gitlab
     end
 
     def request_deadline
-      return unless request_start_time
-      return unless Feature.enabled?(:request_deadline)
+      strong_memoize(:request_deadline) do
+        next unless request_start_time
+        next unless Feature.enabled?(:request_deadline)
 
-      @request_deadline ||= request_start_time + max_request_duration_seconds
+        request_start_time + max_request_duration_seconds
+      end
     end
 
     def ensure_deadline_not_exceeded!

spec/lib/gitlab/request_context_spec.rb

@@ -10,10 +10,12 @@ describe Gitlab::RequestContext, :request_store do
   describe '#request_deadline' do
     let(:request_start_time) { 1575982156.206008 }
 
-    it "sets the time to #{Settings.gitlab.max_request_duration_seconds} seconds in the future" do
+    before do
       allow(subject).to receive(:request_start_time).and_return(request_start_time)
+    end
 
-      expect(subject.request_deadline).to eq(1575982156.206008 + Settings.gitlab.max_request_duration_seconds)
+    it "sets the time to #{Settings.gitlab.max_request_duration_seconds} seconds in the future" do
+      expect(subject.request_deadline).to eq(request_start_time + Settings.gitlab.max_request_duration_seconds)
       expect(subject.request_deadline).to be_a(Float)
     end
 
@@ -22,6 +24,18 @@ describe Gitlab::RequestContext, :request_store do
 
       expect(subject.request_deadline).to be_nil
     end
+
+    it 'only checks the feature once per request-instance' do
+      expect(Feature).to receive(:enabled?).with(:request_deadline).once
+
+      2.times { subject.request_deadline }
+    end
+
+    it 'returns nil when the feature is disabled' do
+      stub_feature_flags(request_deadline: false)
+
+      expect(subject.request_deadline).to be_nil
+    end
   end
 
   describe '#ensure_request_deadline_not_exceeded!' do