axum-extra: implement FromRequest for Either*

[mladedav]

Motivation

Either can currently be used only for FromRequestParts extractor. We might want to support extraction of different kinds of bodies. Example of a user wanting something like this is #3315

Solution

Have Either buffer the whole body into Bytes and then try the extractors in turn.

[jplatte]

This seems like a little bit of a footgun potentially. Using Either<Json<T>, Form<T>> is going to buffer the request body regardless of the content type.

I'm also wondering whether / how to support Either<impl FromRequest, impl FromRequestParts> or Either<impl FromRequestParts, impl FromRequest>. I'm pretty sure those don't work with this PR as-is.

Any comments / thoughts?

[mladedav]

Yeah, it buffers the whole requests. Another option is to instead drive all the extractors at the same time. But I'd expect something like Either3<Json<T>, String, Bytes> to then actually have three copies of all the data in memory so I'd say one buffered reused copy would be preferable.

We might also consider not using Either for this at and make another more descriptive type specifically for this, something like BufferedEither/FromRequestBufferredEither?

As for supporting Either<impl FromRequest, impl FromRequestParts>, I've put together something working, but I originally thought we might not want that. But we can use it.

Then again we probably do want to have a speicifc type for this, because if we implement both FromRequestParts and FromReqeust for the same type, it has two different FromRequest implementations, one with ViaParts and the explicit one. So the way it's set up now, this would break people who have handlers where the last argument is Either* containing only impl FromRequestParts types.

So I guess another type for this would be the way to go? Or just not do this at all and let people clone/try extract requests explicitly?

[yanns]

I'm not sure about this.
For people who are aware of the downsides of buffering the body, this is a nice addition. 💯
I'm just afraid of people adopting this without conscientiously acknowledging the trade-offs. 🤷

A possible other approach is to make it easy for handlers to accept a buffered body, and make it easy to run extractors on it.