Add a 'circuit breaker' for repo checks

app/views/admin/application_settings/_form.html.haml

@@ -275,5 +275,18 @@
       .col-sm-10
         = f.text_field :sentry_dsn, class: 'form-control'
+  %fieldset
+    %legend Repository Checks
+    .form-group
+      .col-sm-offset-2.col-sm-10
+        .checkbox
+          = f.label :repository_checks_enabled do
+            = f.check_box :repository_checks_enabled
+            Enable Repository Checks
+          .help-block
+            GitLab will periodically run 
+            %a{ href: 'https://www.kernel.org/pub/software/scm/git/docs/git-fsck.html', target: 'blank' } 'git fsck'
+            in all project and wiki repositories to look for silent disk corruption issues.
+
   .form-actions
     = f.submit 'Save', class: 'btn btn-save'

app/workers/repository_check_worker.rb

@@ -15,6 +15,7 @@ class RepositoryCheckWorker
     # check, only one (or two) will be checked at a time.
     project_ids.each do |project_id|
       break if Time.now - start >= RUN_TIME
+      break unless current_settings.repository_checks_enabled
       next if !try_obtain_lease(project_id)
@@ -45,4 +46,11 @@ class RepositoryCheckWorker
     )
     lease.try_obtain
   end
+
+  def current_settings
+    # No caching of the settings! If we cache them and an admin disables
+    # this feature, an active RepositoryCheckWorker would keep going for up
+    # to 1 hour after the feature was disabled.
+    ApplicationSetting.current || Gitlab::CurrentSettings.fake_application_settings
+  end
 end

lib/gitlab/current_settings.rb

@@ -34,7 +34,8 @@ module Gitlab
         max_artifacts_size: Settings.artifacts['max_size'],
         require_two_factor_authentication: false,
         two_factor_grace_period: 48,
-        akismet_enabled: false
+        akismet_enabled: false,
+        repository_checks_enabled: true,
       )
     end

spec/workers/repository_check_worker_spec.rb

@@ -28,4 +28,12 @@ describe RepositoryCheckWorker do
     expect(subject.perform).to eq([projects[1].id])
   end
+
+  it 'does nothing when repository checks are disabled' do
+    create(:empty_project)
+    current_settings = double('settings', repository_checks_enabled: false)
+    expect(subject).to receive(:current_settings) { current_settings }
+
+    expect(subject.perform).to eq(nil)
+  end
 end