Clear context when scheduling mirroring jobs

The `UpdateAllMirrorsWorker` could get scheduled from a request (from
a webhook) for a certain project.

This means that it would get the pointing to the project the request
was performed for. But since the `UpdateAllMirrorsWorker` could
schedule updates for other projects, we need to clear that metadata so
it doesn't propagate to those jobs that are being scheduled.

ee/app/workers/update_all_mirrors_worker.rb

@@ -14,13 +14,14 @@ class UpdateAllMirrorsWorker
   def perform
     return if Gitlab::Database.read_only?
 
+    Gitlab::ApplicationContext.with_context({ user: nil, project: nil, namespace: nil }) do
       scheduled = 0
       with_lease do
         scheduled = schedule_mirrors!
       end
 
       # If we didn't get the lease, or no updates were scheduled, exit early
-    return unless scheduled > 0
+      break unless scheduled > 0
 
       # Wait to give some jobs a chance to complete
       Kernel.sleep(RESCHEDULE_WAIT)
@@ -31,6 +32,7 @@ class UpdateAllMirrorsWorker
       # This is in addition to the regular (cron-like) scheduling of this job.
       UpdateAllMirrorsWorker.perform_async if Gitlab::Mirror.reschedule_immediately?
     end
+  end
 
   # rubocop: disable CodeReuse/ActiveRecord
   def schedule_mirrors!

ee/spec/workers/update_all_mirrors_worker_spec.rb

@@ -27,6 +27,26 @@ describe UpdateAllMirrorsWorker do
       worker.perform
     end
 
+    it 'removes metadata except correlation_id from the application context before scheduling mirrors' do
+      inner_context = nil
+      outer_context = nil
+
+      Gitlab::ApplicationContext.with_context(project: build_stubbed(:project)) do
+        outer_context = Labkit::Context.current.to_h
+
+        expect(worker).to receive(:schedule_mirrors!) do
+          inner_context = Labkit::Context.current.to_h
+
+          # `schedule_mirrors!` needs to return an integer.
+          0
+        end
+      end
+
+      worker.perform
+
+      expect(inner_context).to eq(outer_context.slice('correlation_id'))
+    end
+
     it 'schedules mirrors' do
       expect(worker).to receive(:schedule_mirrors!).and_call_original