Fix loader functon for async methods

[j0k2r]

Working with Falcon ASGI Request, webargs has issue parsing "media" inputs, because of the loader_func is not waited.

The code base of iscoroutinefunction do not detect Falcon "media" loader as coroutine function.

def iscoroutinefunction(func):
    """Return True if func is a decorated coroutine function."""
    return (inspect.iscoroutinefunction(func) or
            getattr(func, '_is_coroutine', None) is _is_coroutine)

Getting loader_func output and checking if the result is a coroutine works better.

[sirosen]

Thanks for the contribution! I think this is correct for some cases, like req.media for Falcon, as you say, but not for others.

If you look at the FalconParser's support for other locations (form, json, etc), you'll see that it accesses properties like req.stream with no notion that they may be async awaitables. I think that any framework data which can become awaitable may need additional handling, on a case-by-case basis.

Since the change is definitely not harmful and works for some cases, I think we should merge and release it, and we can deal with any other async trickiness as the need arises. @lafrech, do you agree?

[lafrech]

@sirosen I have no idea TBH. My knowledge about async code is limited. I don't think I'll be able to provide an educated answer any soon so I'll follow you in this one rather than block this indefinitely.
Thanks.

[sirosen]

In that case, and having given myself a few more days to think about it, I'm going to merge this and write up a changelog entry for it. If I can get some time in my schedule to look into it, I might start investigating full support for async Falcon usage. If I can't, we can release with this fix on its own.

I'll run my changelog past you largely to knowledge-share how I understand this change.