worker: Ensure complete coverage for PushNotificationsWorker.

The `# nocoverage` was unnecessary apart from for the compatibility code,
so add a test for that code and remove the `# nocoverage`.

The `message_id` -> `message_ids` conversion was done in
9869153ae8.

zerver/tests/test_queue_worker.py

@@ -301,6 +301,23 @@ class WorkerTest(ZulipTestCase):
                     * 2,
                 )
 
+            # This verifies the compatibility code for the `message_id` -> `message_ids`
+            # conversion for "remove" events.
+            with patch(
+                "zerver.worker.queue_processors.handle_remove_push_notification"
+            ) as mock_handle_remove, patch(
+                "zerver.worker.queue_processors.initialize_push_notifications"
+            ):
+                event_new = dict(
+                    user_profile_id=10,
+                    message_id=33,
+                    type="remove",
+                )
+                fake_client.enqueue("missedmessage_mobile_notifications", event_new)
+                worker.start()
+                # The `message_id` field should have been converted to a list with a single element.
+                mock_handle_remove.assert_called_once_with(10, [33])
+
     @patch("zerver.worker.queue_processors.mirror_email")
     def test_mirror_worker(self, mock_mirror_email: MagicMock) -> None:
         fake_client = self.FakeClient()

zerver/worker/queue_processors.py

@@ -667,7 +667,7 @@ class EmailSendingWorker(LoopQueueProcessingWorker):
 
 
 @assign_queue("missedmessage_mobile_notifications")
-class PushNotificationsWorker(QueueProcessingWorker):  # nocoverage
+class PushNotificationsWorker(QueueProcessingWorker):
     def start(self) -> None:
         # initialize_push_notifications doesn't strictly do anything
         # beyond printing some logging warnings if push notifications
@@ -679,7 +679,13 @@ class PushNotificationsWorker(QueueProcessingWorker):  # nocoverage
         try:
             if event.get("type", "add") == "remove":
                 message_ids = event.get("message_ids")
-                if message_ids is None:  # legacy task across an upgrade
+                if message_ids is None:
+                    # TODO/compatibility: Previously, we sent only one `message_id` in
+                    # a payload for notification remove events. This was later changed
+                    # to send a list of `message_ids` (with that field name), but we need
+                    # compatibility code for events present in the queue during upgrade.
+                    # Remove this when one can no longer upgrade from 1.9.2 (or earlier)
+                    # to any version after 2.0.0
                     message_ids = [event["message_id"]]
                 handle_remove_push_notification(event["user_profile_id"], message_ids)
             else: