Ensure mirror can transition out of the started state when last_update_started_at is nil

ee/app/models/project_mirror_data.rb

@@ -39,6 +39,8 @@ class ProjectMirrorData < ActiveRecord::Base
   private
 
   def base_delay(timestamp)
+    return 0 unless self.last_update_started_at
+
     duration = timestamp - self.last_update_started_at
 
     (BACKOFF_PERIOD + rand(JITTER)) * duration.seconds

ee/changelogs/unreleased/dm-project-mirror-data-last_update_started_at-nil.yml

+---
+title: Ensure mirror can transition out of the started state when last_update_started_at
+  is nil
+merge_request:
+author:
+type: fixed

ee/spec/models/project_mirror_data_spec.rb

@@ -76,6 +76,12 @@ describe ProjectMirrorData, type: :model do
         allow(Gitlab::Mirror).to receive(:rand).and_return(mirror_jitter)
       end
 
+      context 'when last_update_started_at is nil' do
+        it 'applies transition successfully' do
+          expect_next_execution_timestamp(mirror_data, timestamp + 15.minutes + mirror_jitter)
+        end
+      end
+
       context 'when base delay is lower than mirror min_delay' do
         before do
           mirror_data.last_update_started_at = timestamp - 1.second