Use service setting to pick broadcast channel

This falls back to the "test" channel if they do not have a
ServiceBroadcastSetting for the moment, but we intend in future PRs to
enforce that all broadcast services will have this property.

app/celery/broadcast_message_tasks.py

@@ -48,6 +48,10 @@ def send_broadcast_provider_message(broadcast_event_id, provider):
         for polygon in broadcast_event.transmitted_areas["simple_polygons"]
     ]
 
+    channel = "test"
+    if broadcast_event.service.broadcast_channel:
+        channel = broadcast_event.service.broadcast_channel
+
     cbc_proxy_provider_client = cbc_proxy_client.get_proxy(provider)
 
     if broadcast_event.message_type == BroadcastEventMessageType.ALERT:
@@ -59,7 +63,7 @@ def send_broadcast_provider_message(broadcast_event_id, provider):
             areas=areas,
             sent=broadcast_event.sent_at_as_cap_datetime_string,
             expires=broadcast_event.transmitted_finishes_at_as_cap_datetime_string,
-            channel="test"
+            channel=channel
         )
     elif broadcast_event.message_type == BroadcastEventMessageType.UPDATE:
         cbc_proxy_provider_client.update_and_send_broadcast(
@@ -71,7 +75,7 @@ def send_broadcast_provider_message(broadcast_event_id, provider):
             previous_provider_messages=broadcast_event.get_earlier_provider_messages(provider),
             sent=broadcast_event.sent_at_as_cap_datetime_string,
             expires=broadcast_event.transmitted_finishes_at_as_cap_datetime_string,
-            channel="test"
+            channel=channel
         )
     elif broadcast_event.message_type == BroadcastEventMessageType.CANCEL:
         cbc_proxy_provider_client.cancel_broadcast(

tests/app/celery/test_broadcast_message_tasks.py

@@ -9,7 +9,8 @@ from app.models import (
     BroadcastStatusType,
     BroadcastEventMessageType,
     BroadcastProviderMessageStatus,
-    ServiceBroadcastProviderRestriction
+    ServiceBroadcastProviderRestriction,
+    ServiceBroadcastSettings,
 )
 from app.celery.broadcast_message_tasks import send_broadcast_event, send_broadcast_provider_message, trigger_link_test
 
@@ -159,6 +160,91 @@ def test_send_broadcast_provider_message_sends_data_correctly(
     )
 
 
+@freeze_time('2020-08-01 12:00')
+@pytest.mark.parametrize('provider,provider_capitalised', [
+    ['ee', 'EE'],
+    ['three', 'Three'],
+    ['o2', 'O2'],
+    ['vodafone', 'Vodafone'],
+])
+@pytest.mark.parametrize('channel', ['test', 'severe'])
+def test_send_broadcast_provider_message_uses_channel_set_on_broadcast_service(
+    notify_db, mocker, sample_service, provider, provider_capitalised, channel
+):
+    template = create_template(sample_service, BROADCAST_TYPE)
+    broadcast_message = create_broadcast_message(
+        template,
+        areas={
+            'areas': ['london', 'glasgow'],
+            'simple_polygons': [
+                [[50.12, 1.2], [50.13, 1.2], [50.14, 1.21]],
+                [[-4.53, 55.72], [-3.88, 55.72], [-3.88, 55.96], [-4.53, 55.96]],
+            ],
+        },
+        status=BroadcastStatusType.BROADCASTING
+    )
+    event = create_broadcast_event(broadcast_message)
+    notify_db.session.add(ServiceBroadcastSettings(service=sample_service, channel=channel))
+
+    mock_create_broadcast = mocker.patch(
+        f'app.clients.cbc_proxy.CBCProxy{provider_capitalised}.create_and_send_broadcast',
+    )
+
+    send_broadcast_provider_message(provider=provider, broadcast_event_id=str(event.id))
+
+    mock_create_broadcast.assert_called_once_with(
+        identifier=mocker.ANY,
+        message_number=mocker.ANY,
+        headline='GOV.UK Notify Broadcast',
+        description='this is an emergency broadcast message',
+        areas=mocker.ANY,
+        sent=mocker.ANY,
+        expires=mocker.ANY,
+        channel=channel,
+    )
+
+
+@freeze_time('2020-08-01 12:00')
+@pytest.mark.parametrize('provider,provider_capitalised', [
+    ['ee', 'EE'],
+    ['three', 'Three'],
+    ['o2', 'O2'],
+    ['vodafone', 'Vodafone'],
+])
+def test_send_broadcast_provider_message_defaults_to_test_channel_if_no_service_broadcast_settings(
+    notify_db, mocker, sample_service, provider, provider_capitalised
+):
+    template = create_template(sample_service, BROADCAST_TYPE)
+    broadcast_message = create_broadcast_message(
+        template,
+        areas={
+            'areas': ['london', 'glasgow'],
+            'simple_polygons': [
+                [[50.12, 1.2], [50.13, 1.2], [50.14, 1.21]],
+                [[-4.53, 55.72], [-3.88, 55.72], [-3.88, 55.96], [-4.53, 55.96]],
+            ],
+        },
+        status=BroadcastStatusType.BROADCASTING
+    )
+    event = create_broadcast_event(broadcast_message)
+    mock_create_broadcast = mocker.patch(
+        f'app.clients.cbc_proxy.CBCProxy{provider_capitalised}.create_and_send_broadcast',
+    )
+
+    send_broadcast_provider_message(provider=provider, broadcast_event_id=str(event.id))
+
+    mock_create_broadcast.assert_called_once_with(
+        identifier=mocker.ANY,
+        message_number=mocker.ANY,
+        headline='GOV.UK Notify Broadcast',
+        description='this is an emergency broadcast message',
+        areas=mocker.ANY,
+        sent=mocker.ANY,
+        expires=mocker.ANY,
+        channel="test",
+    )
+
+
 @freeze_time('2020-08-01 12:00')
 @pytest.mark.parametrize('provider,provider_capitalised', [
     ['ee', 'EE'],