Make sure the run cursor is saved in the job (#1036)

If a job starts without a cursor (i.e. after a run is resumed), but the
run has a cursor, we do use the cursor from the run in build_enumerator.

However, we don't store the cursor on the job itself at this point. If
the first iteration fails, we go through the `on_error` callback and
store the job cursor, which is `nil`, on the run, effectively losing the
progress previously made on the run.

This fixes this issue by always storing the cursor on the job in
`build_enumerator`, making sure the job and run have the same cursor.

app/jobs/concerns/maintenance_tasks/task_job_concern.rb

@@ -32,6 +32,7 @@ def retry_on(*, **)
 
     def build_enumerator(_run, cursor:)
       cursor ||= @run.cursor
+      self.cursor_position = cursor
       @collection_enum = @task.enumerator_builder(cursor: cursor)
 
       @collection_enum ||= case (collection = @task.collection)

test/jobs/maintenance_tasks/task_job_test.rb

@@ -237,6 +237,17 @@ class << self
       assert_equal "0", @run.reload.cursor
     end
 
+    test ".perform_now persists cursor when there's an error" do
+      run = Run.create!(task_name: "Maintenance::ErrorTask")
+
+      TaskJob.perform_now(run)
+      assert_equal "1", run.reload.cursor
+
+      run.enqueued!
+      TaskJob.perform_now(run)
+      assert_equal "1", run.reload.cursor
+    end
+
     test ".perform_now starts job from cursor position when job resumes" do
       @run.update!(cursor: "0")