Cannot start WSGI application using Dispatcher middleware to support both FastAPI and Flash app at the same time.

[nabheet]

Hi,

I am not sure if our use case is supported. We have FastAPI application that exposes our core API. I am hoping to add the RQ Dashboard to the same API but at /rq path. When I use the Dispatcher middleware, Hypercorn reports a timeout exception. However, if I try to only start the Flash app that is wrapped in AsyncioWSGIMiddleware, it starts up correctly.

install packages (we use pipenv, I am assuming you can replace it with pip easily)

pipenv install 'hypercorn[uvloop]==0.17.2' rq-dashboard

Here is the minimal code to reproduce the issue:

from hypercorn.middleware import (
    DispatcherMiddleware,
    AsyncioWSGIMiddleware,
)
from flask import Flask
import rq_dashboard


class RqSettings2:
    def __init__(self) -> None:
        self.RQ_DASHBOARD_REDIS_URL = "redis://redis/"


def setup_rq_dashboard() -> AsyncioWSGIMiddleware:
    app_rq_settings = RqSettings2()
    dashboard = Flask(__name__)
    dashboard.config.from_object(app_rq_settings)
    rq_dashboard.web.setup_rq_connection(dashboard)
    dashboard.register_blueprint(rq_dashboard.blueprint, url_prefix="/rq")
    return AsyncioWSGIMiddleware(dashboard)


app1 = setup_rq_dashboard()
app2 = DispatcherMiddleware(
    {
        "/rq": setup_rq_dashboard(),
        #"/": setup_fast_api() -> I am hoping to host both in the same instance 🤞 
    }
)

This works:

$ pipenv run hypercorn dash:app1
[2024-06-05 21:19:20 +0000] [751745] [INFO] Running on http://127.0.0.1:8000 (CTRL + C to quit)

This does not:

$ pipenv run hypercorn dash:app2
Process SpawnProcess-1:
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/asyncio/tasks.py", line 500, in wait_for
    return fut.result()
           ^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/asyncio/locks.py", line 213, in wait
    await fut
asyncio.exceptions.CancelledError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/workspaces/business-api/.venv/lib/python3.11/site-packages/hypercorn/asyncio/lifespan.py", line 84, in wait_for_startup
    await asyncio.wait_for(self.startup.wait(), timeout=self.config.startup_timeout)
  File "/usr/local/lib/python3.11/asyncio/tasks.py", line 502, in wait_for
    raise exceptions.TimeoutError() from exc
TimeoutError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/local/lib/python3.11/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/workspaces/business-api/.venv/lib/python3.11/site-packages/hypercorn/asyncio/run.py", line 196, in asyncio_worker
    _run(
  File "/workspaces/business-api/.venv/lib/python3.11/site-packages/hypercorn/asyncio/run.py", line 234, in _run
    runner.run(main(shutdown_trigger=shutdown_trigger))
  File "/usr/local/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/workspaces/business-api/.venv/lib/python3.11/site-packages/hypercorn/asyncio/run.py", line 84, in worker_serve
    await lifespan.wait_for_startup()
  File "/workspaces/business-api/.venv/lib/python3.11/site-packages/hypercorn/asyncio/lifespan.py", line 86, in wait_for_startup
    raise LifespanTimeoutError("startup") from error
hypercorn.utils.LifespanTimeoutError: Timeout whilst awaiting startup. Your application may not support the ASGI Lifespan protocol correctly, alternatively the startup_timeout configuration is incorrect.

Please advise on how best to proceed.

[npt]

I have a similar-looking issue: when I use DispatcherMiddleware to run a Quart app that uses before_serving (with only the one app in the routes), and the before_serving method throws an exception, the exception is logged, the server hangs (not responding to anything except SIGKILL) for several seconds, and then I get the same chain of CancelledError / TimeoutError / LifespanTimeoutError. When DispatcherMiddleware isn't involved, the program immediately terminates as expected.

Aside from the issue when a startup routine throws an exception, when I use DispatcherMiddleware wrapping the one app and exit the server with Ctrl-C, I receive this error:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File ".../__main__.py", line 69, in <module>
    asyncio.run(serve(dispatcher, config))
  File "/usr/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 664, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.12/site-packages/hypercorn/asyncio/__init__.py", line 44, in serve
    await worker_serve(
  File ".../.venv/lib/python3.12/site-packages/hypercorn/asyncio/run.py", line 181, in worker_serve
    await lifespan_task
  File ".../.venv/lib/python3.12/site-packages/hypercorn/asyncio/lifespan.py", line 55, in handle_lifespan
    await self.app(
  File ".../.venv/lib/python3.12/site-packages/hypercorn/app_wrappers.py", line 34, in __call__
    await self.app(scope, receive, send)
  File ".../src/connector/__main__.py", line 57, in __call__
    await self.app(scope, receive, send)
  File ".../.venv/lib/python3.12/site-packages/hypercorn/middleware/dispatcher.py", line 19, in __call__
    await self._handle_lifespan(scope, receive, send)
  File ".../.venv/lib/python3.12/site-packages/hypercorn/middleware/dispatcher.py", line 46, in _handle_lifespan
    async with TaskGroup(asyncio.get_event_loop()) as task_group:
  File ".../.venv/lib/python3.12/site-packages/hypercorn/asyncio/task_group.py", line 74, in __aexit__
    await self._task_group.__aexit__(exc_type, exc_value, tb)
  File "/usr/lib/python3.12/asyncio/taskgroups.py", line 136, in __aexit__
    raise propagate_cancellation_error
  File "/usr/lib/python3.12/asyncio/taskgroups.py", line 112, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError

No such error occurs when not using DispatcherMiddleware.