Use Etag caching for pipelines json

Enable caching in the Etag::Middleware and when a pipeline changes
status, expire the cache for the project pipelines path.

app/models/ci/pipeline.rb

@@ -88,6 +88,8 @@ module Ci
 
         pipeline.run_after_commit do
           PipelineHooksWorker.perform_async(id)
+          Ci::ExpirePipelineCacheService.new(project, nil)
+            .execute(pipeline)
         end
       end
 

app/services/ci/expire_pipeline_cache_service.rb

@@ -0,0 +1,18 @@
+module Ci
+  class ExpirePipelineCacheService < BaseService
+    def execute(pipeline)
+      @pipeline = pipeline
+
+      Gitlab::EtagCaching::Store.new.touch(project_pipelines_path)
+    end
+
+    private
+
+    def project_pipelines_path
+      Gitlab::Routing.url_helpers.namespace_project_pipelines_path(
+        project.namespace,
+        project,
+        format: :json)
+    end
+  end
+end

lib/gitlab/etag_caching/middleware.rb

@@ -10,6 +10,10 @@ module Gitlab
         {
           regexp: %r(^(?!.*(#{RESERVED_WORDS})).*/issues/\d+/rendered_title\z),
           name: 'issue_title'
+        },
+        {
+          regexp: %r(^(?!.*(#{RESERVED_WORDS})).*/pipelines\.json\z),
+          name: 'project_pipelines'
         }
       ].freeze
 

spec/models/ci/pipeline_spec.rb

@@ -335,6 +335,14 @@ describe Ci::Pipeline, models: true do
       end
     end
 
+    describe 'pipeline ETag caching' do
+      it 'executes ExpirePipelinesCacheService' do
+        expect_any_instance_of(Ci::ExpirePipelineCacheService).to receive(:execute).with(pipeline)
+
+        pipeline.cancel
+      end
+    end
+
     def create_build(name, queued_at = current, started_from = 0)
       create(:ci_build,
              name: name,

spec/services/ci/expire_pipeline_cache_service_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe Ci::ExpirePipelineCacheService, services: true do
+  let(:user) { create(:user) }
+  let(:project) { create(:empty_project) }
+  let(:pipeline) { create(:ci_pipeline, project: project) }
+  subject { described_class.new(project, user) }
+
+  describe '#execute' do
+    it 'invalidate Etag caching for project pipelines path' do
+      path = "/#{project.full_path}/pipelines.json"
+
+      expect_any_instance_of(Gitlab::EtagCaching::Store).to receive(:touch).with(path)
+
+      subject.execute(pipeline)
+    end
+  end
+end