Axum 0.7.5认证中间件编译失败问题求助

问题背景 #

使用Axum 0.7.5构建后端服务时，自定义的auth_middleware无法通过编译，初始报错提示tower_service::Service trait未实现，添加#[axum::debug_middleware]后暴露了更深层的Send安全问题。

初始编译错误 #

error[E0277]: the trait bound `axum::middleware::FromFn<fn(hyper::Request<axum::body::Body>, Next) -> impl Future<Output = Result<Response<axum::body::Body>, StatusCode>> {auth_middleware}, (), Route, _>: tower_service::Service<hyper::Request<axum::body::Body>>` is not satisfied
   --> src/routes/mod.rs:40:16
    |
40  |         .layer(from_fn(auth_middleware))
    |          ----- ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `tower_service::Service<hyper::Request<axum::body::Body>>` is not implemented for `axum::middleware::FromFn<fn(hyper::Request<axum::body::Body>, Next) -> impl Future<Output = Result<Response<axum::body::Body>, StatusCode>> {auth_middleware}, (), Route, _>`
    |          |
    |          required by a bound introduced by this call
    |
 = help: the following other types implement trait `tower_service::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 `Router::<S>::layer`
   --> /home/admin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.7.7/src/routing/mod.rs:279:21
    |
276 |     pub fn layer<L>(self, layer: L) -> Router<S>
    |            ----- required by a bound in this associated function
...
279 |         L::Service: Service<Request> + Clone + Send + 'static,
    |                     ^^^^^^^^^^^^^^^^ required by this bound in `Router::<S>::layer`

排查过程 #

为定位问题，给auth_middleware添加#[axum::debug_middleware]属性后，得到更明确的错误提示：

error: future cannot be sent between threads safely
  --> src/middleware/auth_middleware.rs:9:1
   |
9  | #[axum::debug_middleware]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `auth_middleware` is not `Send`
   |
 = help: the trait `Send` is not implemented for `dyn StdError`, which is required by `impl Future<Output = Result<Response<axum::body::Body>, StatusCode>>: Send`
note: future is not `Send` as this value is used across an await
  --> src/middleware/auth_middleware.rs:50:30
   |
41 |       match decode_token(
   |  ___________-
42 | |         token,
43 | |         true, // Indicates that we're validating an access token
44 | |         &state.access_token_secret,
45 | |         &state.refresh_token_secret,
46 | |     ) {
   | |_____- has type `Result<TokenContent, Box<dyn StdError>>` which is not `Send`
...
50 |               Ok(next.run(req).await)
   |                                ^^^^^ await occurs here, with the value maybe used later
note: required by a bound in `__axum_macros_check_auth_middleware_future::check`
  --> src/middleware/auth_middleware.rs:9:1
   |
9  | #[axum::debug_middleware]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `check`

解决方案 #

将decode_token函数的返回类型从Result<TokenContent, Box<dyn std::error::Error>>修改为：

Result<TokenContent, Box<dyn std::error::Error + Send + Sync>>

原因分析 #

Axum 0.7+版本的中间件要求其返回的Future必须实现Send trait，因为Axum服务运行在多线程环境中，需要在线程间安全传递Future。

原decode_token返回的Box<dyn Error>默认不包含Send和Sync约束，导致auth_middleware返回的Future因持有非Send类型的错误对象而无法实现Send，进而导致FromFn类型无法满足Service trait的约束，触发初始编译错误。添加Send + Sync约束后，错误对象可安全在线程间传递，Future满足Send要求，最终解决编译问题。

内容的提问来源于stack exchange，提问作者oas