Auto expire old broadcast messages

Since the expiry is sent as part of the message payload, we don't
need to invoke the CBC proxies (and indeed there's no way to do so
for an expired alert). In future we plan to extend this task so it
triggers the regeneration of content on gov.uk/alerts.

It's worth noting that 'finishes_at' can theoretically be None, in
which case it's unclear when the alert should expire. While alerts
from the Admin app should always have an expiry [1], we have many
in the DB that don't, so it's worth checking for this scenario.

[1]: 078ac10c8d/app/models/broadcast_message.py (L255)

tests/app/celery/test_scheduled_tasks.py

@@ -8,6 +8,7 @@ from freezegun import freeze_time
 
 from app.celery import scheduled_tasks
 from app.celery.scheduled_tasks import (
+    auto_expire_broadcast_messages,
     check_for_missing_rows_in_completed_jobs,
     check_for_services_with_high_failure_rates_or_sending_to_tv_numbers,
     check_if_letters_still_in_created,
@@ -30,9 +31,15 @@ from app.models import (
     JOB_STATUS_PENDING,
     NOTIFICATION_DELIVERED,
     NOTIFICATION_PENDING_VIRUS_CHECK,
+    BroadcastStatusType,
 )
 from tests.app import load_example_csv
-from tests.app.db import create_job, create_notification, create_template
+from tests.app.db import (
+    create_broadcast_message,
+    create_job,
+    create_notification,
+    create_template,
+)
 from tests.conftest import set_config
 
 
@@ -624,3 +631,27 @@ def test_trigger_link_does_nothing_if_cbc_proxy_disabled(
         trigger_link_tests()
 
     assert mock_trigger_link_test.called is False
+
+
+@freeze_time('2021-07-19 15:50')
+@pytest.mark.parametrize('status, finishes_at, final_status', [
+    (BroadcastStatusType.BROADCASTING, '2021-07-19 16:00', BroadcastStatusType.BROADCASTING),
+    (BroadcastStatusType.BROADCASTING, '2021-07-19 15:40', BroadcastStatusType.COMPLETED),
+    (BroadcastStatusType.BROADCASTING, None, BroadcastStatusType.BROADCASTING),
+    (BroadcastStatusType.PENDING_APPROVAL, None, BroadcastStatusType.PENDING_APPROVAL),
+    (BroadcastStatusType.CANCELLED, '2021-07-19 15:40', BroadcastStatusType.CANCELLED),
+])
+def test_auto_expire_broadcast_messages(
+    status,
+    finishes_at,
+    final_status,
+    sample_template
+):
+    message = create_broadcast_message(
+        status=status,
+        finishes_at=finishes_at,
+        template=sample_template,
+    )
+
+    auto_expire_broadcast_messages()
+    assert message.status == final_status