fallback in nested router isn't used when other routes on the root router match

[syphar]

• I have looked for existing issues (including closed) about this

Bug Report

Version

├── axum v0.8.1
│   ├── axum-core v0.5.0

Platform

Darwin mondo-0984 23.5.0 Darwin Kernel Version 23.5.0: Wed May 1 20:12:58 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6000 arm64

Description

When trying to migrate docs.rs to axum 0.8 I stumbled onto an issue with nested routing and fallbacks. In the breaking changes from the changelog I didn't see anything obvious that I did wrong, and the documentation still states the behaviour that I expect.

I have this example code:

use axum::{
    http::{StatusCode, Uri},
    routing::get,
    Json, Router,
};

async fn fallback() -> (StatusCode, &'static str) {
    (StatusCode::NOT_FOUND, "Not Found Root")
}

async fn api_fallback() -> (StatusCode, Json<serde_json::Value>) {
    (
        StatusCode::NOT_FOUND,
        Json(serde_json::json!({ "status": "Not Found API" })),
    )
}

#[tokio::main]
async fn main() {
    let api_routes = Router::new()
        .route("/users", get(|| async {}))
        .fallback(api_fallback);

    let app = Router::new()
        .nest("/api", api_routes)
        .route(
            "/{name}/{version}",
            get(|uri: Uri| async move { uri.to_string() }),
        )
        .fallback(fallback);

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

Following the documentation about fallbacks and a nested router I would expect that a request to /api/something would lead to api_fallback being called.

But what happens is that the route behind /{name}/{version} is called.

In our case I have a nested router for static assets behind /-/static and want a custom static fallback if the first static routes don't match.

with axum ~0.7 we used get_service here, which is not allowed any more, which is why I wanted to migrate to fallback_service.

[jplatte]

This seems like a matchit thing.. I think.
Can you try (independently):

• using .nest_service for nesting the API router (requires calling .with_state on it first if you're using state in the API handlers)
• using .route("/{*rest}", any(api_fallback)) for the inner fallback

[syphar]

now that's interesting:

1. .nest_service: fixes the issue in the test-app.
2. .route(...): fixes the issue when requesting /api/doesnt_exist, but doesn't return the API 404 when requesting the /api root. Another router with / fixes this for /api, but not for /api/

[syphar]

changing .nest to .nest_service also fixes the issue in docs.rs

[liningpan]

I think I tracked the issue down to this function and line 541 in particular.

axum/axum/src/routing/path_router.rs

Lines 534 to 545 in a192480

	pub(crate) fn path_for_nested_route<'a>(prefix: &'a str, path: &'a str) -> Cow<'a, str> {
	debug_assert!(prefix.starts_with('/'));
	debug_assert!(path.starts_with('/'));
	if prefix.ends_with('/') {
	format!("{prefix}{}", path.trim_start_matches('/')).into()
	} else if path == "/" {
	prefix.into()
	} else {
	format!("{prefix}{path}").into()
	}
	}

It seems like when nesting / route in /api the intention is the match /api/, but the trailing slash is silently dropped, which make this issue really confusing to debug.

[mladedav]

@liningpan you are probably looking for #2659 which is a different issue. You might want to look there for more details.