Make sure we mark BG job complete

This change modifies the FixFirstMentionedInCommit background migration
to mark the persisted BG job as succeeded.

lib/gitlab/background_migration/fix_first_mentioned_in_commit_at.rb

@@ -45,10 +45,19 @@ module Gitlab
               cte.issue_id = issue_metrics.issue_id
           UPDATE_METRICS
         end
+
+        mark_job_as_succeeded(start_id, end_id)
       end
 
       private
 
+      def mark_job_as_succeeded(*arguments)
+        Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded(
+          'FixFirstMentionedInCommitAt',
+          arguments
+        )
+      end
+
       def scope(start_id, end_id)
         TmpIssueMetrics.from_2020.where(issue_id: start_id..end_id)
       end

spec/lib/gitlab/background_migration/fix_first_mentioned_in_commit_at_spec.rb

@@ -99,6 +99,15 @@ RSpec.describe Gitlab::BackgroundMigration::FixFirstMentionedInCommitAt, :migrat
       .perform(issue_metrics.minimum(:issue_id), issue_metrics.maximum(:issue_id))
   end
 
+  it "marks successful slices as completed" do
+    min_issue_id = issue_metrics.minimum(:issue_id)
+    max_issue_id = issue_metrics.maximum(:issue_id)
+
+    expect(subject).to receive(:mark_job_as_succeeded).with(min_issue_id, max_issue_id)
+
+    subject.perform(min_issue_id, max_issue_id)
+  end
+
   context 'when the persisted first_mentioned_in_commit_at is later than the first commit authored_date' do
     it 'updates the issue_metrics record' do
       record1 = issue_metrics.create!(issue_id: issue1.id, first_mentioned_in_commit_at: Time.current)