Prevent remote mirrors from failing when project is in pending_delete

We noticed on GitLab.com that remote mirrors were not even running.
UpdateAllRemoteMirrorsWorker was failing on a project that was in
pending_delete, preventing any workers from running.

Closes gitlab-org/gitlab-ce#23650

app/models/remote_mirror.rb

@@ -70,6 +70,7 @@ class RemoteMirror < ActiveRecord::Base
   end
 
   def sync
+    return unless project
     return if !enabled || update_in_progress?
 
     update_failed? ? update_retry : update_start
@@ -109,6 +110,8 @@ class RemoteMirror < ActiveRecord::Base
   private
 
   def url_availability
+    return unless project
+
     if project.import_url == url && project.mirror?
       errors.add(:url, 'is already in use')
     end
@@ -128,12 +131,14 @@ class RemoteMirror < ActiveRecord::Base
   end
 
   def add_update_job
-    if project.repository_exists?
+    if project && project.repository_exists?
       RepositoryUpdateRemoteMirrorWorker.perform_async(self.id)
     end
   end
 
   def refresh_remote
+    return unless project
+
     project.repository.remove_remote(ref_name)
     project.repository.add_remote(ref_name, url)
   end

spec/models/remote_mirror_spec.rb

@@ -84,6 +84,24 @@ describe RemoteMirror do
     end
   end
 
+  context 'no project' do
+    it 'includes mirror with a project in pending_delete' do
+      mirror = create_mirror(url: 'http://cantbeblank',
+                             update_status: 'finished',
+                             enabled: true,
+                             last_update_at: nil,
+                             updated_at: 25.hours.ago)
+      project = mirror.project
+      project.pending_delete = true
+      project.save
+      mirror.reload
+
+      expect(mirror.sync).to be_nil
+      expect(mirror.valid?).to be_truthy
+      expect(mirror.update_status).to eq('finished')
+    end
+  end
+
   def create_mirror(params)
     project = FactoryGirl.create(:project)
     project.remote_mirrors.create!(params)