Restrict ProjectCacheWorker jobs to one per 15 min

This ensures ProjectCacheWorker jobs for a given project are performed
at most once per 15 minutes. This should reduce disk load a bit in cases
where there are multiple pushes happening (which should schedule
multiple ProjectCacheWorker jobs).

CHANGELOG.md

@@ -35,6 +35,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - AbstractReferenceFilter caches project_refs on RequestStore when active
   - Replaced the check sign to arrow in the show build view. !6501
   - Add a /wip slash command to toggle the Work In Progress status of a merge request. !6259 (tbalthazar)
+  - ProjectCacheWorker updates caches at most once per 15 minutes per project
   - Fix Error 500 when viewing old merge requests with bad diff data
   - Create a new /templates namespace for the /licenses, /gitignores and /gitlab_ci_ymls API endpoints. !5717 (tbalthazar)
   - Speed-up group milestones show page

app/workers/project_cache_worker.rb

+# Worker for updating any project specific caches.
+#
+# This worker runs at most once every 15 minutes per project. This is to ensure
+# that multiple instances of jobs for this worker don't hammer the underlying
+# storage engine as much.
 class ProjectCacheWorker
   include Sidekiq::Worker
 
   sidekiq_options queue: :default
 
+  LEASE_TIMEOUT = 15.minutes.to_i
+
   def perform(project_id)
+    if try_obtain_lease_for(project_id)
+      Rails.logger.
+        info("Obtained ProjectCacheWorker lease for project #{project_id}")
+    else
+      Rails.logger.
+        info("Could not obtain ProjectCacheWorker lease for project #{project_id}")
+
+      return
+    end
+
+    update_caches(project_id)
+  end
+
+  def update_caches(project_id)
     project = Project.find(project_id)
 
     return unless project.repository.exists?
@@ -15,4 +36,10 @@ class ProjectCacheWorker
       project.repository.build_cache
     end
   end
+
+  def try_obtain_lease_for(project_id)
+    Gitlab::ExclusiveLease.
+      new("project_cache_worker:#{project_id}", timeout: LEASE_TIMEOUT).
+      try_obtain
+  end
 end

spec/workers/project_cache_worker_spec.rb

@@ -6,6 +6,12 @@ describe ProjectCacheWorker do
   subject { described_class.new }
 
   describe '#perform' do
+    context 'when an exclusive lease can be obtained' do
+      before do
+        allow(subject).to receive(:try_obtain_lease_for).with(project.id).
+          and_return(true)
+      end
+
       it 'updates project cache data' do
         expect_any_instance_of(Repository).to receive(:size)
         expect_any_instance_of(Repository).to receive(:commit_count)
@@ -23,4 +29,16 @@ describe ProjectCacheWorker do
         subject.perform(project.id)
       end
     end
+
+    context 'when an exclusive lease can not be obtained' do
+      it 'does nothing' do
+        allow(subject).to receive(:try_obtain_lease_for).with(project.id).
+          and_return(false)
+
+        expect(subject).not_to receive(:update_caches)
+
+        subject.perform(project.id)
+      end
+    end
+  end
 end