objects defining async __call__ not run/awaited when used in on_startup

Simple reproducer:

def test_async_startup_hangs():

    class Foo:
        async def __call__(self):
            raise Exception()

    app = Starlette(on_startup=[Foo()])

    with TestClient(app) as client:
        pass

No exception is raised when it should be. This, however, raises an exception as expected:

def test_async_startup_hangs():

    async def foo():
        raise Exception()

    app = Starlette(on_startup=[foo])

    with TestClient(app) as client:
        pass

[tomchristie]

TBH I’d be quite keen to moving Starlette towards always expecting an async style on endpoints, startup/shutdown functions, error handlers etc.

Users could still use sync style functions, but we’d require them to be explicitly wrapped in a threadpooling sync -> async decorator.

[cjw296]

Turns out this issue also affects partial:

https://stackoverflow.com/questions/52422860/partial-asynchronous-functions-are-not-detected-as-asynchronous

@tomchristie suggested Starlette should grow its own function to make these checks, once that’s happened, we should see about pushing it back into CPython core.