async @after() handler crashes with TypeError

[OrangeTux]

Release v0.7.0 introduces a bug when executing @after() handler that are marked with the async keyword. Thanks @shirley-li-powerflex for reporting it.

Here a code sample demonstrating the problem:

import asyncio
import logging
from datetime import datetime

try:
    import websockets
except ModuleNotFoundError:
    print("This example relies on the 'websockets' package.")
    print("Please install it by running: ")
    print()
    print(" $ pip install websockets")
    import sys
    sys.exit(1)

from ocpp.routing import on, after
from ocpp.v16 import ChargePoint as cp
from ocpp.v16.enums import Action, RegistrationStatus
from ocpp.v16 import call_result

logging.basicConfig(level=logging.INFO)


class ChargePoint(cp):
    @on(Action.BootNotification)
    def on_boot_notitication(self, charge_point_vendor: str, charge_point_model: str, **kwargs):
        return call_result.BootNotificationPayload(
            current_time=datetime.utcnow().isoformat(),
            interval=10,
            status=RegistrationStatus.accepted
        )

    @after(Action.BootNotification)
    async def after_boot_notification(self, *args, **kwargs):
        pass

async def on_connect(websocket, path):
    """ For every new charge point that connects, create a ChargePoint instance
    and start listening for messages.

    """
    charge_point_id = path.strip('/')
    cp = ChargePoint(charge_point_id, websocket)

    await cp.start()


async def main():
    server = await websockets.serve(
        on_connect,
        '0.0.0.0',
        9000,
        subprotocols=['ocpp1.6']
    )

    await server.wait_closed()


if __name__ == '__main__':
    try:
        # asyncio.run() is used when running this example with Python 3.7 and
        # higher.
        asyncio.run(main())
    except AttributeError:
        # For Python 3.6 a bit more code is required to run the main() task on
        # an event loop.
        loop = asyncio.get_event_loop()
        loop.run_until_complete(main())
        loop.close()

When ran using v0.7.0 the following stracktrace is generated:

Traceback (most recent call last):
  File "/home/developer/projects/tmh-ocpp/.env/lib/python3.8/site-packages/websockets/server.py", line 191, in handler
    await self.ws_handler(self, path)
  File "examples/v16/central_system.py", line 44, in on_connect
    await cp.start()
  File "/home/developer/projects/tmh-ocpp/.env/lib/python3.8/site-packages/ocpp/charge_point.py", line 126, in start
    await self.route_message(message)
  File "/home/developer/projects/tmh-ocpp/.env/lib/python3.8/site-packages/ocpp/charge_point.py", line 144, in route_message
    await self._handle_call(msg)
  File "/home/developer/projects/tmh-ocpp/.env/lib/python3.8/site-packages/ocpp/charge_point.py", line 222, in _handle_call
    asyncio.ensure_future(response)
  File "/home/developer/.pyenv/versions/3.8.0/lib/python3.8/asyncio/tasks.py", line 673, in ensure_future
    raise TypeError('An asyncio.Future, a coroutine or an awaitable is '
TypeError: An asyncio.Future, a coroutine or an awaitable is required

Non-async @after() handlers are not affected.