如何在actix_web中间件中正确返回错误响应？

问题背景 #

开发基于actix-web的认证中间件时，认证通过的情况下能正常返回OK响应，但认证失败时尝试返回401错误响应却编译报错。以下是实现代码：

/// ResponseBody是自定义的错误/内容结构体
#[derive(Debug, Serialize, Deserialize)]
pub struct ResponseBody<T> {
   pub message: String,
   pub data: T,
}

impl<T> ResponseBody<T> {
   pub fn new(message: &str, data: T) -> ResponseBody<T> {
       ResponseBody {
           message: message.to_string(),
           data,
       }
   }
}

/// 中间件中的逻辑
if authenticate_pass {
        let fut = self.service.call(req);
        Box::pin(async move {
            let res = fut.await?;
            Ok(res)
        })
} else {
        Box::pin(async move {
            Ok(req.into_response(
                HttpResponse::Unauthorized()
                    .json(ResponseBody::new("Session ends. Please login again", ""))
                    .into_body(),
            ))
        })
}

编译时出现如下错误：

error[E0277]: the trait bound `actix_web::dev::Response<_>: std::convert::From<BoxBody>` is not satisfied
   --> src\auth.rs:103:21
    |
102 |                   Ok(req.into_response(
    |                          ------------- required by a bound introduced by this call
103 | /                     HttpResponse::Unauthorized()
104 | |                         .json(ResponseBody::new("Session ends. Please login again", ""))
105 | |                         .into_body(),
    | |____________________________________^ the trait `std::convert::From<BoxBody>` is not implemented for `actix_web::dev::Response<_>`
    |
 = help: the following other types implement trait `std::convert::From<T>`:
           <actix_web::dev::Response<&'static [u8]> as std::convert::From<&'static [u8]> >
           <actix_web::dev::Response<&'static str> as std::convert::From<&'static str> >
           <actix_web::dev::Response<B> as std::convert::From<HttpResponse<B>> >
           <actix_web::dev::Response<B> as std::convert::From<ServiceResponse<B>> >
           <actix_web::dev::Response<BoxBody> as std::convert::From<&actix_http::ws::HandshakeError>> >
           <actix_web::dev::Response<BoxBody> as std::convert::From<HttpResponseBuilder>> >
           <actix_web::dev::Response<BoxBody> as std::convert::From<Infallible>> >
           <actix_web::dev::Response<BoxBody> as std::convert::From<Result<I, E>> >
         and 13 others
 = note: required because of the requirements on the impl of `Into<actix_web::dev::Response<_>>` for `BoxBody`
note: required by a bound in `ServiceRequest::into_response`
   --> C:\Users\mdenn\.cargo\registry\src\github.com-1ecc6299db9ec823\actix-web-4.1.0\src\service.rs:144:32
    |
144 |     pub fn into_response<B, R: Into<Response<B>>>(self, res: R) -> ServiceResponse<B> {
    |                                ^^^^^^^^^^^^^^^^^ required by this bound in `ServiceRequest::into_response`

使用的依赖版本：

actix-web = "4.1.0"
futures = "0.3.24"
tokio = { version = "1.18.2", features = ["full"] }
serde = { version = "1.0.137", features = ["derive"] }

错误原因分析 #

错误的核心是错误地调用了.into_body()方法：

• HttpResponse::Unauthorized().json(...)返回的是一个完整的HttpResponse<BoxBody>实例，它本身已经实现了Into<Response<BoxBody>> trait，符合into_response方法的参数要求。
• 调用.into_body()后，把完整的HttpResponse转换成了BoxBody类型（仅响应体部分），而Response<_>并没有实现从BoxBody直接转换的trait，导致类型不匹配。

解决方案 #

直接去掉.into_body()调用，将完整的HttpResponse实例传给req.into_response()即可。

修正后的代码 #

/// ResponseBody是自定义的错误/内容结构体
#[derive(Debug, Serialize, Deserialize)]
pub struct ResponseBody<T> {
   pub message: String,
   pub data: T,
}

impl<T> ResponseBody<T> {
   pub fn new(message: &str, data: T) -> ResponseBody<T> {
       ResponseBody {
           message: message.to_string(),
           data,
       }
   }
}

/// 中间件中的逻辑
if authenticate_pass {
        let fut = self.service.call(req);
        Box::pin(async move {
            let res = fut.await?;
            Ok(res)
        })
} else {
        Box::pin(async move {
            Ok(req.into_response(
                HttpResponse::Unauthorized()
                    .json(ResponseBody::new("Session ends. Please login again", ""))
                // 移除.into_body()调用
            ))
        })
}

关于BoxBody的补充说明 #

BoxBody是actix-web中用于封装响应体的动态类型，它是Body trait的装箱实现，允许在运行时使用不同的响应体类型。在大多数场景下，你不需要直接操作BoxBody，HttpResponse构建器会自动处理响应体的封装，直接传递HttpResponse实例给相关方法即可满足类型要求。

内容的提问来源于stack exchange，提问作者DennyHiu