Async function error handling

[kellytk]

The documentation states:

You generally don’t have to think about errors if you’re only using async functions as handlers.

What's the recommended practice if you must? Consider a simple handler relying upon an Askama template, temporarily with incomplete error handling:

async fn handler() -> Html<String> {
    Html(MyAskamaTemplate{}.render().unwrap())
}

As this is a necessary resource for the server to operate, this appears to be a valid case of server error and not covered by request-oriented errors discussed in https://docs.rs/axum/0.3.2/axum/error_handling/index.html#axums-error-handling-model.

Is my understanding correct? If it is, how should async function handlers properly address server errors? Simply using a Result<Html<String>, E>?

[davidpdrsn]

You have to handle the error somehow and return an appropriate response to the client. You can return Result<T, E> if both types implement IntoResponse.

Note this still isn't an "error" in the tower::Service sense since a response is still generated and sent back to the client.

[davidpdrsn]

Use an extractor to get the state then pass that state into you IntoResponse type

[kellytk]

When the handler creates and returns the IntoResponse error enum, it should pass the shared state via the error enum variant?

enum AppError {
    A,
    B(State),
}

[davidpdrsn]

Something like this:

async fn handler(Extension(state): Extension<State>) -> Result<Html<&'static str>, Error> {
    Err(Error::Foo(state))
}

#[derive(Clone)]
struct State {}

enum Error {
    Foo(State),
}

impl IntoResponse for Error {
    type Body = Body;
    type BodyError = <Self::Body as axum::body::HttpBody>::Error;

    fn into_response(self) -> axum::http::Response<Self::Body> {
        todo!()
    }
}

[kellytk]

I have it compiling with a simple response, such as Response::new(Body::empty()).

In the contrived buildable example at https://github.com/kellytk/x-axum-response/blob/main/src/main.rs#L56 you'll see that the error response can be either Xhtml or plain text. How should that be correctly implemented?

[davidpdrsn]

You can build different responses in each branch of the if and if bodies are different types you can use box_body to make the types match.