Handling errors from middleware from_fn

[zkrzyzanowski]

Hi!
First off, thanks for this crate! This is my first experience with building a web app with rust and axum has been great so far!

I've got a a middleware that I'm using to protect a route from being accessed if the user making the request isn't authorized. I'm new to rust, so I'm using the middleware::from_fn function to just convert a function to middleware. This function returns a Result, so I want to be able to log errors if they occur. I also only want the /calculate route to be protected by that middleware, but want all routes to have a timeout of 20s.

 let app = Router::new()
        .route("/calculate", post(calculate_route::run))
        .route_layer(middleware::from_fn(move |req, next| {
            routes::protected(
                req,
                next,
                some_arg,
            )
        }))
        .route("/status", get(|| status()))
        .layer(
            ServiceBuilder::new()
                // this middleware goes above `TimeoutLayer` because it will receive
                // errors returned by `TimeoutLayer`
                .layer(HandleErrorLayer::new(|_: BoxError| async {
                    StatusCode::REQUEST_TIMEOUT
                }))
                .layer(TimeoutLayer::new(Duration::from_secs(20))),
        );

• Is this the correct ordering of layers so all have a 20s timeout but only /cacluate is protected by the protected middleware?
• How can I take the result returned from routes::protected and log an error?

I tried this:

 .route_layer(
            ServiceBuilder::new()
                .layer(HandleErrorLayer::new(|_: BoxError| async {
                    error!("some error");
                    StatusCode::INTERNAL_SERVER_ERROR
                }))
                .layer(middleware::from_fn(move |req, next| {
                    firebase::protected(
                        req,
                        next,
                        maybe_emulator_host.clone(),
                        firebase_project_id.clone(),
                    )
                })),
        )

but get the following compiler error and I'm not quite sure what to do

error[E0277]: the trait bound `HandleError<axum::middleware::FromFn<[closure@src/main.rs:44:44: 51:18], Route<_>>, [closure@src/main.rs:41:46: 43:18], _>: Service<Request<_>>` is not satisfied
   --> src/main.rs:39:10
    |
39  |         .route_layer(
    |          ^^^^^^^^^^^ the trait `Service<Request<_>>` is not implemented for `HandleError<axum::middleware::FromFn<[closure@src/main.rs:44:44: 51:18], Route<_>>, [closure@src/main.rs:41:46: 43:18], _>`
    |
    = help: the following implementations were found:
              <HandleError<S, F, ()> as Service<Request<ReqBody>>>
              <HandleError<S, F, (T1, T2)> as Service<Request<ReqBody>>>
              <HandleError<S, F, (T1, T2, T3)> as Service<Request<ReqBody>>>
              <HandleError<S, F, (T1, T2, T3, T4)> as Service<Request<ReqBody>>>
            and 13 others
note: required by a bound in `Router::<B>::route_layer`
   --> /Users/zachkrzyzanowski/.cargo/registry/src/github.com-1ecc6299db9ec823/axum-0.5.9/src/routing/mod.rs:335:21

[davidpdrsn]

Is this the correct ordering of layers so all have a 20s timeout but only /cacluate is protected by the protected middleware?

Yes, though I perfer .route("/calculate", post(...).layer(...)). I think thats more clear but it ultimately doesn't matter.

How can I take the result returned from routes::protected and log an error?

I think you're misunderstanding how axum thinks about errors. Returning 500 Internal Server (or some other status) from a handler is not an error. Thats just another response. Errors only occur when you apply fallible middleware like TimeoutLayer. Notice that it's error type isn't Infallible which axum requires.

I recommend you just log error directly in routes::protected. There you also have the full context to provide a rich error message. Otherwise you're essentially lossily converting the error into HTTP only to log it.

Specifically for timeouts I recommend https://docs.rs/tower-http/latest/tower_http/timeout/index.html as it doesn't require HandleErrorLayer.

[zkrzyzanowski]

Wow, thanks for the quick response! That explanation makes sense, I was hoping to be able to log errors in one place, but I think I see what you're saying about losing the context when returning the response.

Thanks again for a thorough explanation!

[davidpdrsn]

You can do this and log the error in the IntoResponse impl.