Fix retrying email notification task (#4772)

# What this PR does
Email tasks are failing and retrying when they use the fallback default
notification policy as it does not get saved in the DB. This PR uses the
same way as UserNotificationPolicyLogRecord to set that field to null to
avoid `ValueError("save() prohibited to prevent data loss due to unsaved
related object 'notification_policy'.").` when that is the case.

## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/email/models.py

@@ -1,12 +1,21 @@
-import logging
 import uuid
 
 from django.db import models
 
-logger = logging.getLogger(__name__)
+
+class EmailMessageQuerySet(models.QuerySet):
+    def create(self, **kwargs):
+        from apps.base.models.user_notification_policy_log_record import (
+            _check_if_notification_policy_is_transient_fallback,
+        )
+
+        _check_if_notification_policy_is_transient_fallback(kwargs)
+        return super().create(**kwargs)
 
 
 class EmailMessage(models.Model):
+    objects = EmailMessageQuerySet.as_manager()
+
     message_uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 
     exceeded_limit = models.BooleanField(null=True, default=None)

engine/apps/email/tasks.py

@@ -62,6 +62,11 @@ def _create_user_notification_policy_log_record(**kwargs):
             **kwargs, using_fallback_default_notification_policy_step=using_fallback_default_notification_policy_step
         )
 
+    def _create_email_message(**kwargs):
+        return EmailMessage.objects.create(
+            **kwargs, using_fallback_default_notification_policy_step=using_fallback_default_notification_policy_step
+        )
+
     # create an error log in case EMAIL_HOST is not specified
     if not live_settings.EMAIL_HOST:
         _create_user_notification_policy_log_record(
@@ -88,7 +93,7 @@ def _create_user_notification_policy_log_record(**kwargs):
             notification_channel=notification_policy.notify_by,
             notification_error_code=UserNotificationPolicyLogRecord.ERROR_NOTIFICATION_MAIL_LIMIT_EXCEEDED,
         )
-        EmailMessage.objects.create(
+        _create_email_message(
             represents_alert_group=alert_group,
             notification_policy=notification_policy,
             receiver=user,
@@ -115,7 +120,7 @@ def _create_user_notification_policy_log_record(**kwargs):
 
     try:
         send_mail(subject, message, from_email, recipient_list, html_message=html_message, connection=connection)
-        EmailMessage.objects.create(
+        _create_email_message(
             represents_alert_group=alert_group,
             notification_policy=notification_policy,
             receiver=user,

engine/apps/email/tests/test_notify_user.py

@@ -7,6 +7,7 @@
 
 from apps.base.models import UserNotificationPolicy, UserNotificationPolicyLogRecord
 from apps.email.alert_rendering import build_subject_and_message
+from apps.email.models import EmailMessage
 from apps.email.tasks import get_from_email, notify_user_async
 from apps.user_management.subscription_strategy.free_public_beta_subscription_strategy import (
     FreePublicBetaSubscriptionStrategy,
@@ -212,3 +213,34 @@ def test_subject_newlines_removed(
 
     subject, _ = build_subject_and_message(alert_group, 1)
     assert subject == "testnewlines"
+
+
+@pytest.mark.django_db
+def test_notify_user_fallback_default_policy(
+    settings,
+    make_organization,
+    make_user_for_organization,
+    make_token_for_organization,
+    make_alert_receive_channel,
+    make_alert_group,
+    make_alert,
+    make_user_notification_policy,
+):
+    settings.EMAIL_BACKEND = "django.core.mail.backends.locmem.EmailBackend"
+    settings.EMAIL_HOST = "test"
+
+    organization = make_organization()
+    user = make_user_for_organization(organization)
+
+    alert_receive_channel = make_alert_receive_channel(organization)
+    alert_group = make_alert_group(alert_receive_channel)
+
+    make_alert(alert_group=alert_group, raw_request_data=alert_receive_channel.config.example_payload)
+
+    notify_user_async(user.pk, alert_group.pk, None)
+    assert len(mail.outbox) == 1
+
+    log_record = UserNotificationPolicyLogRecord.objects.filter(author=user, alert_group=alert_group).first()
+    assert log_record.type == UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_SUCCESS
+
+    EmailMessage.objects.get(receiver=user, represents_alert_group=alert_group, notification_policy=None)