Error handling model and utilities

# Table of contents

- [axum's error handling model](#axums-error-handling-model)
- [Routing to fallible services](#routing-to-fallible-services)
- [Applying fallible middleware](#applying-fallible-middleware)
- [Running extractors for error handling](#running-extractors-for-error-handling)

# axum's error handling model

axum is based on [`tower::Service`] which bundles errors through its associated
`Error` type. If you have a [`Service`] that produces an error and that error
makes it all the way up to hyper, the connection will be terminated _without_
sending a response. This is generally not desirable so axum makes sure you
always produce a response by relying on the type system.

axum does this by requiring all services have [`Infallible`] as their error
type. `Infallible` is the error type for errors that can never happen.

This means if you define a handler like:

```rust
use axum::http::StatusCode;

async fn handler() -> Result<String, StatusCode> {
    # todo!()
    // ...
}
```

While it looks like it might fail with a `StatusCode` this actually isn't an
"error". If this handler returns `Err(some_status_code)` that will still be
converted into a [`Response`] and sent back to the client. This is done
through `StatusCode`'s [`IntoResponse`] implementation.

It doesn't matter whether you return `Err(StatusCode::NOT_FOUND)` or
`Err(StatusCode::INTERNAL_SERVER_ERROR)`. These are not considered errors in
axum.

Instead of a direct `StatusCode`, it makes sense to use intermediate error type
that can ultimately be converted to `Response`. This allows using `?` operator
in handlers. See those examples:

* [`anyhow-error-response`][anyhow] for generic boxed errors
* [`error-handling`][error-handling] for application-specific detailed errors

[anyhow]: https://github.com/tokio-rs/axum/blob/main/examples/anyhow-error-response/src/main.rs
[error-handling]: https://github.com/tokio-rs/axum/blob/main/examples/error-handling/src/main.rs

This also applies to extractors. If an extractor doesn't match the request the
request will be rejected and a response will be returned without calling your
handler. See [`extract`](crate::extract) to learn more about handling extractor
failures.

# Routing to fallible services

You generally don't have to think about errors if you're only using async
functions as handlers. However if you're embedding general `Service`s or
applying middleware, which might produce errors you have to tell axum how to
convert those errors into responses.

```rust
use axum::{
    Router,
    body::Body,
    http::{Request, Response, StatusCode},
    error_handling::HandleError,
};

async fn thing_that_might_fail() -> Result<(), anyhow::Error> {
    # Ok(())
    // ...
}

// this service might fail with `anyhow::Error`
let some_fallible_service = tower::service_fn(|_req| async {
    thing_that_might_fail().await?;
    Ok::<_, anyhow::Error>(Response::new(Body::empty()))
});

let app = Router::new().route_service(
    "/",
    // we cannot route to `some_fallible_service` directly since it might fail.
    // we have to use `handle_error` which converts its errors into responses
    // and changes its error type from `anyhow::Error` to `Infallible`.
    HandleError::new(some_fallible_service, handle_anyhow_error),
);

// handle errors by converting them into something that implements
// `IntoResponse`
async fn handle_anyhow_error(err: anyhow::Error) -> (StatusCode, String) {
    (
        StatusCode::INTERNAL_SERVER_ERROR,
        format!("Something went wrong: {err}"),
    )
}
# let _: Router = app;
```

# Applying fallible middleware

Similarly axum requires you to handle errors from middleware. That is done with
[`HandleErrorLayer`]:

```rust
use axum::{
    Router,
    BoxError,
    routing::get,
    http::StatusCode,
    error_handling::HandleErrorLayer,
};
use std::time::Duration;
use tower::ServiceBuilder;

let app = Router::new()
    .route("/", get(|| async {}))
    .layer(
        ServiceBuilder::new()
            // `timeout` will produce an error if the handler takes
            // too long so we must handle those
            .layer(HandleErrorLayer::new(handle_timeout_error))
            .timeout(Duration::from_secs(30))
    );

async fn handle_timeout_error(err: BoxError) -> (StatusCode, String) {
    if err.is::<tower::timeout::error::Elapsed>() {
        (
            StatusCode::REQUEST_TIMEOUT,
            "Request took too long".to_string(),
        )
    } else {
        (
            StatusCode::INTERNAL_SERVER_ERROR,
            format!("Unhandled internal error: {err}"),
        )
    }
}
# let _: Router = app;
```

# Running extractors for error handling

`HandleErrorLayer` also supports running extractors:

```rust
use axum::{
    Router,
    BoxError,
    routing::get,
    http::{StatusCode, Method, Uri},
    error_handling::HandleErrorLayer,
};
use std::time::Duration;
use tower::ServiceBuilder;

let app = Router::new()
    .route("/", get(|| async {}))
    .layer(
        ServiceBuilder::new()
            // `timeout` will produce an error if the handler takes
            // too long so we must handle those
            .layer(HandleErrorLayer::new(handle_timeout_error))
            .timeout(Duration::from_secs(30))
    );

async fn handle_timeout_error(
    // `Method` and `Uri` are extractors so they can be used here
    method: Method,
    uri: Uri,
    // the last argument must be the error itself
    err: BoxError,
) -> (StatusCode, String) {
    (
        StatusCode::INTERNAL_SERVER_ERROR,
        format!("`{method} {uri}` failed with {err}"),
    )
}
# let _: Router = app;
```

[`tower::Service`]: `tower::Service`
[`Infallible`]: std::convert::Infallible
[`Response`]: crate::response::Response
[`IntoResponse`]: crate::response::IntoResponse
