retry sending broadcasts

Retry tasks if they fail to send a broadcast event. Note that each task
tries the regular proxy and the failover proxy for that provider. This
runs a bit differently than our other retries:

Retry with exponential backoff. Our other tasks retry with a fixed delay
of 5 minutes between tries. If we can't send a broadcast, we want to try
immediately. So instead, implement an exponential backoff (1, 2, 4, 8,
... seconds delay). We can't delay for longer than 310 seconds due to
visibility timeout settings in SQS, so cap the delay at that amount.

Normally we give up retrying after a set amount of retries (often 4
hours). As broadcast content is much more important than normal
notifications, we don't ever want to give up on sending them to phones...

...UNLESS WE DO!

Sometimes we do want to give up sending a broadcast though! Broadcasts
have an expiry time, when they stop showing up on peoples devices, so if
that has passed then we don't need to send the broadcast out.

Broadcast events can also be superceded by updates or cancels. Check
that the event is the most recent event for that broadcast message, if
not, give up, as we don't want to accidentally send out two conflicting
events for the same message.

app/celery/broadcast_message_tasks.py

@@ -1,10 +1,13 @@
 import uuid
+from datetime import datetime
 
 from flask import current_app
 from notifications_utils.statsd_decorators import statsd
 from sqlalchemy.schema import Sequence
+from celery.exceptions import MaxRetriesExceededError
 
 from app import cbc_proxy_client, db, notify_celery
+from app.clients.cbc_proxy import CBCProxyFatalException, CBCProxyRetryableException
 from app.config import QueueNames
 from app.models import BroadcastEventMessageType, BroadcastProvider
 from app.dao.broadcast_message_dao import dao_get_broadcast_event_by_id, create_broadcast_provider_message
@@ -12,6 +15,40 @@ from app.dao.broadcast_message_dao import dao_get_broadcast_event_by_id, create_
 from app.utils import format_sequential_number
 
 
+def get_retry_delay(retry_count):
+    """
+    Given a count of retries so far, return a delay for the next one.
+    `retry_count` should be 0 the first time a task fails.
+    """
+    # TODO: replace with celery's built in exponential backoff
+
+    # 2 to the power of x. 1, 2, 4, 8, 16, 32, ...
+    delay = 2**retry_count
+    # never wait longer than 5 minutes
+    return min(delay, 300)
+
+
+def check_provider_message_should_retry(broadcast_provider_message):
+    this_event = broadcast_provider_message.broadcast_event
+
+    if this_event.transmitted_finishes_at < datetime.utcnow():
+        print(this_event.transmitted_finishes_at, datetime.utcnow(),)
+        raise MaxRetriesExceededError(
+            f'Given up sending broadcast_event {this_event.id} ' +
+            f'to provider {broadcast_provider_message.provider}: ' +
+            f'The expiry time of {this_event.transmitted_finishes_at} has already passed'
+        )
+
+    newest_event = max(this_event.broadcast_message.events, key=lambda x: x.sent_at)
+
+    if this_event != newest_event:
+        raise MaxRetriesExceededError(
+            f'Given up sending broadcast_event {this_event.id} ' +
+            f'to provider {broadcast_provider_message.provider}: ' +
+            f'This event has been superceeded by {newest_event.message_type} broadcast_event {newest_event.id}'
+        )
+
+
 @notify_celery.task(name="send-broadcast-event")
 @statsd(namespace="tasks")
 def send_broadcast_event(broadcast_event_id):
@@ -27,9 +64,10 @@ def send_broadcast_event(broadcast_event_id):
         )
 
 
-@notify_celery.task(name="send-broadcast-provider-message")
+# max_retries=None: retry forever
+@notify_celery.task(bind=True, name="send-broadcast-provider-message", max_retries=None)
 @statsd(namespace="tasks")
-def send_broadcast_provider_message(broadcast_event_id, provider):
+def send_broadcast_provider_message(self, broadcast_event_id, provider):
     broadcast_event = dao_get_broadcast_event_by_id(broadcast_event_id)
 
     broadcast_provider_message = create_broadcast_provider_message(broadcast_event, provider)
@@ -54,6 +92,7 @@ def send_broadcast_provider_message(broadcast_event_id, provider):
 
     cbc_proxy_provider_client = cbc_proxy_client.get_proxy(provider)
 
+    try:
         if broadcast_event.message_type == BroadcastEventMessageType.ALERT:
             cbc_proxy_provider_client.create_and_send_broadcast(
                 identifier=str(broadcast_provider_message.id),
@@ -90,6 +129,16 @@ 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,
             )
+    except CBCProxyRetryableException as exc:
+        # this will raise MaxRetriesExceededError if we no longer want to retry
+        # (because the message has expired)
+        check_provider_message_should_retry(broadcast_provider_message)
+
+        self.retry(
+            exc=exc,
+            countdown=get_retry_delay(self.request.retries),
+            queue=QueueNames.BROADCASTS,
+        )
 
 
 @notify_celery.task(name='trigger-link-test')

app/clients/cbc_proxy.py

@@ -25,7 +25,11 @@ from app.utils import DATETIME_FORMAT, format_sequential_number
 #    the preceeding Alert message in the previous_provider_messages field
 
 
-class CBCProxyException(Exception):
+class CBCProxyFatalException(Exception):
+    pass
+
+
+class CBCProxyRetryableException(Exception):
     pass
 
 
@@ -115,7 +119,9 @@ class CBCProxyClientBase(ABC):
         if not result:
             failover_result = self._invoke_lambda(self.failover_lambda_name, payload)
             if not failover_result:
-                raise CBCProxyException(f'Lambda failed for both {self.lambda_name} and {self.failover_lambda_name}')
+                raise CBCProxyRetryableException(
+                    f'Lambda failed for both {self.lambda_name} and {self.failover_lambda_name}'
+                )
 
         return result
 

tests/app/celery/test_broadcast_message_tasks.py

@@ -1,7 +1,9 @@
 import uuid
+from datetime import datetime
 from unittest.mock import call, ANY
 
 from freezegun import freeze_time
+from celery.exceptions import MaxRetriesExceededError
 import pytest
 
 from app.models import (
@@ -12,7 +14,14 @@ from app.models import (
     ServiceBroadcastProviderRestriction,
     ServiceBroadcastSettings,
 )
-from app.celery.broadcast_message_tasks import send_broadcast_event, send_broadcast_provider_message, trigger_link_test
+from app.clients.cbc_proxy import CBCProxyRetryableException
+from app.celery.broadcast_message_tasks import (
+    check_provider_message_should_retry,
+    get_retry_delay,
+    send_broadcast_event,
+    send_broadcast_provider_message,
+    trigger_link_test,
+)
 
 from tests.app.db import (
     create_template,
@@ -415,14 +424,12 @@ def test_send_broadcast_provider_message_errors(mocker, sample_service, provider
 
     mock_create_broadcast = mocker.patch(
         f'app.clients.cbc_proxy.CBCProxy{provider_capitalised}.create_and_send_broadcast',
-        side_effect=Exception('oh no'),
+        side_effect=CBCProxyRetryableException('oh no'),
     )
+    mock_retry = mocker.patch('app.celery.broadcast_message_tasks.send_broadcast_provider_message.retry')
 
-    with pytest.raises(Exception) as ex:
     send_broadcast_provider_message(provider=provider, broadcast_event_id=str(event.id))
 
-    assert ex.match('oh no')
-
     mock_create_broadcast.assert_called_once_with(
         identifier=ANY,
         message_number=mocker.ANY,
@@ -439,6 +446,56 @@ def test_send_broadcast_provider_message_errors(mocker, sample_service, provider
         expires=event.transmitted_finishes_at_as_cap_datetime_string,
         channel="test"
     )
+    mock_retry.assert_called_once_with(
+        countdown=1,
+        exc=mock_create_broadcast.side_effect,
+        queue='broadcast-tasks'
+    )
+
+
+@pytest.mark.parametrize('num_retries, expected_countdown', [
+    (0, 1),
+    (5, 32),
+    (20, 300),
+])
+def test_send_broadcast_provider_message_delays_retry_exponentially(
+    mocker,
+    sample_service,
+    num_retries,
+    expected_countdown
+):
+    template = create_template(sample_service, BROADCAST_TYPE)
+
+    broadcast_message = create_broadcast_message(template,  status=BroadcastStatusType.BROADCASTING)
+    event = create_broadcast_event(broadcast_message)
+
+    mock_create_broadcast = mocker.patch(
+        'app.clients.cbc_proxy.CBCProxyEE.create_and_send_broadcast',
+        side_effect=CBCProxyRetryableException('oh no'),
+    )
+    mock_retry = mocker.patch('app.celery.broadcast_message_tasks.send_broadcast_provider_message.retry')
+
+    # patch celery request context as shown here: https://stackoverflow.com/a/59870468
+    mock_celery_task_request_context = mocker.patch("celery.app.task.Task.request")
+    mock_celery_task_request_context.retries = num_retries
+
+    send_broadcast_provider_message(provider='ee', broadcast_event_id=str(event.id))
+
+    mock_create_broadcast.assert_called_once_with(
+        identifier=ANY,
+        message_number=mocker.ANY,
+        headline="GOV.UK Notify Broadcast",
+        description='this is an emergency broadcast message',
+        areas=[],
+        sent=event.sent_at_as_cap_datetime_string,
+        expires=event.transmitted_finishes_at_as_cap_datetime_string,
+        channel='test',
+    )
+    mock_retry.assert_called_once_with(
+        countdown=expected_countdown,
+        exc=mock_create_broadcast.side_effect,
+        queue='broadcast-tasks'
+    )
 
 
 @pytest.mark.parametrize("provider,provider_capitalised", [
@@ -471,3 +528,90 @@ def test_trigger_link_tests_invokes_cbc_proxy_client(
         assert len(mock_send_link_test.mock_calls[0][1][1]) == 8
     else:
         assert not mock_send_link_test.mock_calls[0][1][1]
+
+
+@pytest.mark.parametrize('retry_count, expected_delay', [
+    (0, 1),
+    (1, 2),
+    (2, 4),
+    (8, 256),
+    (9, 300),
+    (10, 300),
+    (1000, 300),
+])
+def test_get_retry_delay_has_capped_backoff(retry_count, expected_delay):
+    assert get_retry_delay(retry_count) == expected_delay
+
+
+@freeze_time('2021-01-01 12:00')
+def test_check_provider_message_should_retry_doesnt_raise_if_event_hasnt_expired_yet(sample_template):
+    broadcast_message = create_broadcast_message(sample_template)
+    current_event = create_broadcast_event(
+        broadcast_message,
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 1, 12, 1),
+    )
+    provider_message = create_broadcast_provider_message(current_event, 'ee')
+
+    check_provider_message_should_retry(provider_message)
+
+
+@freeze_time('2021-01-01 12:00')
+def test_check_provider_message_should_retry_raises_if_event_has_expired(sample_template):
+    broadcast_message = create_broadcast_message(sample_template)
+    current_event = create_broadcast_event(
+        broadcast_message,
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 1, 11, 59),
+    )
+    provider_message = create_broadcast_provider_message(current_event, 'ee')
+
+    with pytest.raises(MaxRetriesExceededError) as exc:
+        check_provider_message_should_retry(provider_message)
+    assert 'The expiry time of 2021-01-01 11:59:00 has already passed' in str(exc.value)
+
+
+@freeze_time('2021-01-01 12:00')
+def test_check_provider_message_should_retry_raises_if_a_newer_event_exists(sample_template):
+    broadcast_message = create_broadcast_message(sample_template)
+    # event approved at midnight
+    past_event = create_broadcast_event(
+        broadcast_message,
+        message_type='alert',
+        sent_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 2, 0, 0),
+    )
+    # event updated at 5am (this is the event we're currently trying to send)
+    current_event = create_broadcast_event(
+        broadcast_message,
+        message_type='update',
+        sent_at=datetime(2021, 1, 1, 5, 0),
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 2, 0, 0),
+    )
+    # event updated at 7am
+    future_event = create_broadcast_event(
+        broadcast_message,
+        message_type='update',
+        sent_at=datetime(2021, 1, 1, 7, 0),
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 2, 0, 0),
+    )
+    # event cancelled at 10am
+    futurest_event = create_broadcast_event(
+        broadcast_message,
+        message_type='cancel',
+        sent_at=datetime(2021, 1, 1, 10, 0),
+        transmitted_starts_at=datetime(2021, 1, 1, 0, 0),
+        transmitted_finishes_at=datetime(2021, 1, 2, 0, 0),
+    )
+
+    provider_message = create_broadcast_provider_message(current_event, 'ee')
+
+    # even though the task is going on until midnight tomorrow, we shouldn't send the update now, because the cancel
+    # message will be in the pipeline somewhere.
+    with pytest.raises(MaxRetriesExceededError) as exc:
+        check_provider_message_should_retry(provider_message)
+
+    assert f'This event has been superceeded by cancel broadcast_event {futurest_event.id}' in str(exc.value)

tests/app/clients/test_cbc_proxy.py

@@ -7,7 +7,7 @@ from unittest.mock import Mock, call
 import pytest
 
 from app.clients.cbc_proxy import (
-    CBCProxyClient, CBCProxyException, CBCProxyEE, CBCProxyCanary, CBCProxyVodafone, CBCProxyThree, CBCProxyO2
+    CBCProxyClient, CBCProxyRetryableException, CBCProxyEE, CBCProxyCanary, CBCProxyVodafone, CBCProxyThree, CBCProxyO2
 )
 from app.utils import DATETIME_FORMAT
 
@@ -433,7 +433,7 @@ def test_cbc_proxy_create_and_send_tries_failover_lambda_on_invoke_error_and_rai
         'StatusCode': 400,
     }
 
-    with pytest.raises(CBCProxyException) as e:
+    with pytest.raises(CBCProxyRetryableException) as e:
         cbc_proxy.create_and_send_broadcast(
             identifier='my-identifier',
             message_number='0000007b',
@@ -482,7 +482,7 @@ def test_cbc_proxy_create_and_send_tries_failover_lambda_on_function_error_and_r
         }
     }
 
-    with pytest.raises(CBCProxyException) as e:
+    with pytest.raises(CBCProxyRetryableException) as e:
         cbc_proxy.create_and_send_broadcast(
             identifier='my-identifier',
             message_number='0000007b',

tests/app/db.py

@@ -1049,7 +1049,7 @@ def create_broadcast_message(
         starts_at=starts_at,
         finishes_at=finishes_at,
         created_by_id=created_by.id if created_by else service.created_by_id,
-        areas=areas or {},
+        areas=areas or {'areas': [], 'simple_polygons': []},
         content=content,
         stubbed=stubbed
     )
@@ -1077,7 +1077,7 @@ def create_broadcast_event(
         transmitted_areas=transmitted_areas or broadcast_message.areas,
         transmitted_sender=transmitted_sender or 'www.notifications.service.gov.uk',
         transmitted_starts_at=transmitted_starts_at,
-        transmitted_finishes_at=transmitted_finishes_at or datetime.utcnow(),
+        transmitted_finishes_at=transmitted_finishes_at or datetime.utcnow() + timedelta(hours=24),
     )
     db.session.add(b_e)
     db.session.commit()
@@ -1105,4 +1105,4 @@ def create_broadcast_provider_message(
             broadcast_provider_message_id=broadcast_provider_message_id)
         db.session.add(provider_message_number)
         db.session.commit()
-    return provider_message, provider_message_number
+    return provider_message