Adding @nested_transactional for transactions that require more than one

db update/insert.

Using a savepoint for the multiple transactions allows us to rollback if
there is an error when executing the second db transaction.
However, this does add a bit of complexity. Developers need to manage
the db session when calling multiple nested tranactions.

Unit tests have been added to test this functionality and some end to
end tests have been done to make sure all transactions are rollback if
there is an exception while executing the transaction.

app/service/rest.py

@@ -7,6 +7,7 @@ from notifications_utils.timezones import convert_utc_to_bst
 from sqlalchemy.exc import IntegrityError, SQLAlchemyError
 from sqlalchemy.orm.exc import NoResultFound
 
+from app import db
 from app.aws import s3
 from app.config import QueueNames
 from app.dao import fact_notification_status_dao, notifications_dao
@@ -254,18 +255,13 @@ def create_service():
     # unpack valid json into service object
     valid_service = Service.from_json(data)
 
-    dao_create_service(valid_service, user)
-
-    # Need to do the annual billing update in a separate transaction because the both the
-    # dao_add_service_to_organisation and set_default_free_allowance_for_service are wrapped in a transaction.
-    # Catch and report an error if the annual billing doesn't happen - but don't rollback the service update.
     try:
-        set_default_free_allowance_for_service(valid_service, year_start=None)
-    except SQLAlchemyError:
-        # No need to worry about key errors because service.organisation_type has a foreign key to organisation_types
-        current_app.logger.exception(
-            f"Exception caught when trying to insert annual billing creating a service {valid_service.id} "
-            f"for organisation_type {valid_service.organisation_type}")
+        dao_create_service(valid_service, user)
+        set_default_free_allowance_for_service(service=valid_service, year_start=None)
+        db.session.commit()
+    except SQLAlchemyError as e:
+        db.session.rollback()
+        raise e
 
     return jsonify(data=service_schema.dump(valid_service).data), 201