Rewrite a method for a specific route, before method routing #3016

edgarogh · 2024-11-04T22:15:40Z

edgarogh
Nov 4, 2024

Summary

Hello, I'm trying to implement a feature found in multiple web frameworks that allows POST forms to contain a _method field that overrides the real HTTP POST during method routing. This makes it easier to create actual REST forms, to e.g. delete a resource.

I have successfully written a middleware util_middleware::form_method_rewriter that passes the #[debug_middleware] test and should work in theory. Here's how I add it to my router:

        .route(
            "/channel/:channel",
            get(channel_details)
                .fallback_service(axum::middleware::from_fn(util_middleware::form_method_rewriter).layer(
                    post(channel_rename).delete(channel_delete),
                )),
        )

Here's the trait resolution error I get:

error[E0277]: the trait bound `axum::middleware::FromFn<fn(axum::http::Request<axum::body::Body>, Next) -> impl std::future::Future<Output = axum::http::Response<axum::body::Body>> {util_middleware::form_method_rewriter}, (), MethodRouter<AppState>, _>: Service<axum::http::Request<axum::body::Body>>` is not satisfied
   --> src/lib.rs:134:35
    |
134 |                   .fallback_service(axum::middleware::from_fn(util_middleware::form_method_rewriter).layer(
    |  __________________----------------_^
    | |                  |
    | |                  required by a bound introduced by this call
135 | |                     post(channel_rename).delete(channel_delete),
136 | |                 )),
    | |_________________^ the trait `Service<axum::http::Request<axum::body::Body>>` is not implemented for `FromFn<fn(Request<Body>, Next) -> impl Future<Output = Response<Body>> {form_method_rewriter}, (), ..., ...>`
    |
    = help: the following other types implement trait `Service<Request>`:
              axum::middleware::FromFn<F, S, I, (T1, T2)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7, T8)>
              axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7, T8, T9)>
            and 8 others
note: required by a bound in `MethodRouter::<S, E>::fallback_service`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.7.7/src/routing/method_routing.rs:871:12
    |
869 |     pub fn fallback_service<T>(mut self, svc: T) -> Self
    |            ---------------- required by a bound in this associated function
870 |     where
871 |         T: Service<Request, Error = E> + Clone + Send + 'static,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `MethodRouter::<S, E>::fallback_service`

I don't understand why FromFn is expected to implement Service. I don't fully grasp the tower vocabulary but I'd expect a middleware to specifically not be a Service but rather a Layer (until it wraps a service to create a new service). Where does this trait bound come from? The whole tower/axum API is confusing because it seems that multiple .layer methods have their argument and self in an inconsistant order (one being the service, the other the layer).

I have a similar from_fn middleware that wraps my entire router and I haven't had issues with it, and it has the exact same signature (a plain old fn(Request, Next) -> Response).

axum version

0.7.7

Answered by edgarogh

Nov 6, 2024

Solution:
I had a small misunderstanding, FromFn is a service and should definitely implement the Service trait. I was confusing it with FromFnLayer.
FromFn: Service wasn't satisfied because the service I was wrapping inside my layer (a MethodRouter) didn't actually implement Service for a reason that was clearly documented 🥴 (it had an explicit app state bound to it, which I needed to "erase" by providing mine explicitly).

Adding with_state at the end of my MethodRouter builder reset the explicit state to () which made it a valid service.

View full answer

edgarogh · 2024-11-06T01:30:13Z

edgarogh
Nov 6, 2024
Author

Solution:
I had a small misunderstanding, FromFn is a service and should definitely implement the Service trait. I was confusing it with FromFnLayer.
FromFn: Service wasn't satisfied because the service I was wrapping inside my layer (a MethodRouter) didn't actually implement Service for a reason that was clearly documented 🥴 (it had an explicit app state bound to it, which I needed to "erase" by providing mine explicitly).

Adding with_state at the end of my MethodRouter builder reset the explicit state to () which made it a valid service.

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Rewrite a method for a specific route, before method routing #3016

{{title}}

Replies: 1 comment

{{title}}

Select a reply

Rewrite a method for a specific route, before method routing #3016

edgarogh Nov 4, 2024

Summary

axum version

Replies: 1 comment

edgarogh Nov 6, 2024 Author

edgarogh
Nov 4, 2024

edgarogh
Nov 6, 2024
Author