New strategy for transaction management.

Introduce a contextmanger function to handle exceptions and nested
transactions. Using the nested_transaction will start a
nested transaction with `db.session.begin_nested`, once the nested
transaction is complete the commit will happen.
`@transactional` has been updated to commit unless in a nested
transaction.

app/service/rest.py

@@ -4,10 +4,9 @@ from datetime import datetime
 from flask import Blueprint, current_app, jsonify, request
 from notifications_utils.letter_timings import letter_can_be_cancelled
 from notifications_utils.timezones import convert_utc_to_bst
-from sqlalchemy.exc import IntegrityError, SQLAlchemyError
+from sqlalchemy.exc import IntegrityError
 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
@@ -19,7 +18,7 @@ from app.dao.api_key_dao import (
     save_model_api_key,
 )
 from app.dao.broadcast_service_dao import set_broadcast_service_type
-from app.dao.dao_utils import dao_rollback
+from app.dao.dao_utils import dao_rollback, nested_transaction
 from app.dao.date_util import get_financial_year
 from app.dao.fact_notification_status_dao import (
     fetch_monthly_template_usage_for_service,
@@ -255,13 +254,9 @@ def create_service():
     # unpack valid json into service object
     valid_service = Service.from_json(data)
 
-    try:
+    with nested_transaction():
         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