chore(api): gatekeep timer start at and set test

- if submission already has timer started at, we won't allow to re-configure it
- add test cases regarding this API

app/controllers/course/assessment/submission/submissions_controller.rb

@@ -108,17 +108,17 @@ def generate_live_feedback
   end
 
   def set_timer_started_at
-    timer_started_at = Time.zone.now
+    unless @submission.timer_started_at
+      @submission.timer_started_at = Time.zone.now
 
-    @submission.timer_started_at = timer_started_at
+      raise ActiveRecord::Rollback unless @submission.save
 
-    raise ActiveRecord::Rollback unless @submission.save
-
-    Course::Assessment::Submission::ForceSubmitTimedSubmissionJob.
-      set(wait_until: timer_started_at + @assessment.time_limit.minutes + FORCE_SUBMIT_DELAY).
-      perform_later(@assessment, @submission_id, @submission.creator)
+      Course::Assessment::Submission::ForceSubmitTimedSubmissionJob.
+        set(wait_until: @submission.timer_started_at + @assessment.time_limit.minutes + FORCE_SUBMIT_DELAY).
+        perform_later(@assessment, @submission_id, @submission.creator)
+    end
 
-    render json: { timerStartedAt: timer_started_at }
+    render json: { timerStartedAt: @submission.timer_started_at }
   end
 
   # Reload the current answer or reset it, depending on parameters.

spec/controllers/course/assessment/submission/submissions_controller_spec.rb

@@ -240,6 +240,55 @@
       end
     end
 
+    describe '#set_timer_started_at' do
+      let!(:assessment) { create(:assessment, :published, *assessment_traits, course: course, time_limit: 120) }
+      let!(:assessment2) { create(:assessment, :published, *assessment_traits, course: course, time_limit: 120) }
+      let!(:submission) { create(:submission, :attempting, assessment: assessment, creator: user) }
+      let!(:submission2) do
+        create(:submission, :attempting, assessment: assessment2, creator: user,
+                                         timer_started_at: Time.zone.now - 5.seconds)
+      end
+
+      context 'when user first-time attempt the timed assessment' do
+        subject do
+          patch :set_timer_started_at, params: {
+            course_id: course, assessment_id: assessment.id, id: submission.id
+          }
+        end
+
+        it 'assigns the timer_started_at to current time' do
+          subject
+          json_result = JSON.parse(response.body)
+          expect(json_result['timerStartedAt'].to_datetime.utc).to \
+            be_within(1.second).of Time.zone.now.utc
+          expect(submission.reload.timer_started_at.utc).to \
+            be_within(1.second).of Time.zone.now.utc
+        end
+      end
+
+      context 'when user has already attempted the timed assessment before' do
+        subject do
+          patch :set_timer_started_at, params: {
+            course_id: course, assessment_id: assessment2.id, id: submission2.id
+          }
+        end
+
+        it 'assigns the timer_started_at to current time' do
+          subject
+          json_result = JSON.parse(response.body)
+          expect(json_result['timerStartedAt'].to_datetime.utc).not_to \
+            be_within(1.second).of Time.zone.now.utc
+          expect(submission2.reload.timer_started_at.utc).not_to \
+            be_within(1.second).of Time.zone.now.utc
+
+          expect(json_result['timerStartedAt'].to_datetime.utc).to \
+            be_within(1.second).of (Time.zone.now - 5.seconds).utc
+          expect(submission2.reload.timer_started_at.utc).to \
+            be_within(1.second).of (Time.zone.now - 5.seconds).utc
+        end
+      end
+    end
+
     describe 'submission_actions' do
       let!(:students) { create_list(:course_student, 5, course: course) }
       let!(:phantom_student) { create(:course_student, :phantom, course: course) }