How to handle error from middleware

[SardineFish]

Problem

In the middleware service chain, we call next service with ServiceRequest and return the ServiceResponse from the next service.
But if next service return an error, how can we make it into a ServiceResponse

Background

In our project, we need to make our APIs always response in a certain JSON format.
But if we have error in middleware, we can only return it out and let actix-web response an Internal Serivce Error.
We cannot make a custom ServiceResponse because ServiceRequest is moved to next service and the inner HttpRequest cannot clone.

For example in official middleware ErrorHandler line 157

actix-web/actix-web/src/middleware/err_handlers.rs

Lines 148 to 177 in 6a5b370

	impl<Fut, B> Future for ErrorHandlersFuture<Fut, B>
	where
	Fut: Future<Output = Result<ServiceResponse<B>, Error>>,
	{
	type Output = Result<ServiceResponse<EitherBody<B>>, Error>;
	fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
	match self.as_mut().project() {
	ErrorHandlersProj::ServiceFuture { fut, handlers } => {
	let res = ready!(fut.poll(cx))?;
	match handlers.get(&res.status()) {
	Some(handler) => match handler(res)? {
	ErrorHandlerResponse::Response(res) => Poll::Ready(Ok(res)),
	ErrorHandlerResponse::Future(fut) => {
	self.as_mut()
	.set(ErrorHandlersFuture::ErrorHandlerFuture { fut });
	self.poll(cx)
	}
	},
	None => Poll::Ready(Ok(res.map_into_left_body())),
	}
	}
	ErrorHandlersProj::ErrorHandlerFuture { fut } => fut.as_mut().poll(cx),
	}
	}
	}

The error from next service is just returned to upper service.
Finally it will become a 500 response with default text body Internal Serivce Error.

[fakeshadow]

In general you don't.
Most of middlewares would never use error path and it's just a placeholder. You can handle error if you write multiple middlewares and nest them but don't expect other middlewares would follow the suite.(In that case you use Error(HttpRequest) and unlike you stated it's Clone https://docs.rs/actix-web/4.0.1/actix_web/struct.HttpRequest.html#impl-Clone )

[SardineFish]

But the Scope::wrap<M, B> restrict the middleware to implement trait
Transform< T::Service, ServiceRequest, Response = ServiceResponse<B>, Error = Error, InitError = (), > + 'static
The associated type Error is constraint to actix_web::Error so I cannot use my custom error type that wraps the HttpRequest

[fakeshadow]

https://docs.rs/actix-web/latest/actix_web/struct.Error.html#method.as_error.
actix_web::Error is a trait object for ResponseError. You can downcast the object to the real type impl ResponseError.