Fix polls 404 on disabled completion.

Calling runtime.handle('progress', ...) was causing a 404 error due to
the runtime handler.  It should be okay for xblocks to submit a
completion or progress event; they just shouldn't have it treated
specially.

EDUCATOR-1642

lms/djangoapps/courseware/module_render.py

@@ -481,10 +481,13 @@ def get_module_system_for_user(
         Returns None if no special processing is required.
         """
         handlers = {
-            'completion': handle_completion_event,
             'grade': handle_grade_event,
-            'progress': handle_deprecated_progress_event,
         }
+        if completion_waffle.waffle().is_enabled(completion_waffle.ENABLE_COMPLETION_TRACKING):
+            handlers.update({
+                'completion': handle_completion_event,
+                'progress': handle_deprecated_progress_event,
+            })
         return handlers.get(event_type)
 
     def publish(block, event_type, event):

lms/djangoapps/courseware/tests/test_module_render.py

@@ -624,7 +624,7 @@ class TestHandleXBlockCallback(SharedModuleStoreTestCase, LoginEnrollmentTestCas
                 content_type='application/json',
             )
             request.user = self.mock_user
-            with self.assertRaises(Http404):
+            with patch('lms.djangoapps.completion.models.BlockCompletionManager.submit_completion') as mock_complete:
                 render.handle_xblock_callback(
                     request,
                     unicode(course.id),
@@ -632,6 +632,8 @@ class TestHandleXBlockCallback(SharedModuleStoreTestCase, LoginEnrollmentTestCas
                     signal,
                     '',
                 )
+                mock_complete.assert_not_called()
+            self.assertFalse(BlockCompletion.objects.filter(block_key=block.scope_ids.usage_id).exists())
 
     @ddt.data(
         ('complete', {'completion': 0.625}, 0.625),