Ignore unknown project ID in RepositoryUpdateMirrorWorker

Projects that are in pending_delete could still trigger the mirror,
leading to undefined method `mark_import_as_failed` errors and
repeated Sidekiq retries.

CHANGELOG-EE

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.13.0 (unreleased)
 v 8.12.2 (unreleased)
+  - Ignore unknown project ID in RepositoryUpdateMirrorWorker
 v 8.12.1
 - Prevent secrets to be pushed to the repository

app/workers/repository_update_mirror_worker.rb

@@ -11,6 +11,9 @@ class RepositoryUpdateMirrorWorker
   def perform(project_id)
     begin
       @project = Project.find(project_id)
+
+      return unless project
+
       @current_user = @project.mirror_user || @project.creator
       result = Projects::UpdateMirrorService.new(@project, @current_user).execute
@@ -21,9 +24,10 @@ class RepositoryUpdateMirrorWorker
       project.import_finish
     rescue => ex
-      project.mark_import_as_failed("We're sorry, a temporary error occurred, please try again.")
-
-      raise UpdateMirrorError, "#{ex.class}: #{Gitlab::UrlSanitizer.sanitize(ex.message)}"
+      if project
+        project.mark_import_as_failed("We're sorry, a temporary error occurred, please try again.")
+        raise UpdateMirrorError, "#{ex.class}: #{Gitlab::UrlSanitizer.sanitize(ex.message)}"
+      end
     end
   end
 end

spec/workers/repository_update_mirror_worker_spec.rb

@@ -19,5 +19,9 @@ describe RepositoryUpdateMirrorWorker do
       expect { described_class.new.perform(project.id) }
         .to change { project.reload.import_status }.to('failed')
     end
+
+    it 'does nothing if project does not exist' do
+      expect(described_class.new.perform(1000000)).to be_nil
+    end
   end
 end