axum: allow body types other than axum::body::Body in Services passed to serve

[mladedav]

Motivation

A user in #3202 tried to use middlewares around the whole Router so that their layers run before routing happens. They ran into problems caused by their layers changing the Body type.

So far I've seen only the NormalizePathLayer used in this way which does not change the Body type so there were never issues. But I don't see a reason not to support this for other layers that do change the body, such as CompressionLayer.

Solution

This PR adds another generic parameter to Serve to keep track of the Response::Body type returned by the service provided.

There are no breaking changes as far as I can tell as inference knows the body type. This could break someone who for some reason names the type parameters explicitly. There is no rush with this and we may want to wait with this and release it in the next breaking release.

Alternatives

We may also choose not to support this. Users can just map the response body with .map_response_body(axum::body::Body::new) before callign serve. The price is just one box (and maybe a bit of ergonomics) as far as I can tell.

[yanns]

If we add some routes, which type will be the body? The axum body type? Or CustomBody?

[mladedav]

CustomBody. The layer returns a service which always returns that without even using the Router in this test.

[yanns]

Could we add an handle to check if this works? Or are our handlers tight to our body implementation?

[mladedav]

I can do that (in a few days).

The handler (and Router) would still return axum::body::Body, it's just the outer layer which changes that to something else.

[mladedav]

I've added a route there.

Note though that the whole Router is completely ignored by the outer layer. I could even write this instead and it would work the same way even without the router.

let app = tower::service_fn(|_| std::future::ready(Ok(Response::new(CustomBody))));

[yanns]

I see. What would be a better example?
A middleware changing the body type depending of the request?
(I have issues understanding the value)

[mladedav]

#3202 is good example I think. Doing stuff like compression. I just wanted to add a test that stuff like this compiles going forward. I don't think we can use a layer like that here though because we don't want to depend on these layers from here.

We can add a real example or extend the one showing middlewares wrapping the whole router (assuming we already have one).

[yanns]

There's this example I wrote: https://github.com/tokio-rs/axum/blob/main/examples/compression/src/main.rs
(but I guess there's no need to layer outside the router for this)
I'll approve this PR code-wise. Feel free to enhance it with examples / tests showing its value as I cannot find any myself.

[ttys3]

seems this PR is a reverse of #1751 (which is mainly to solve #1110 ) ?

[jplatte]

This is not reverting anything from #1751. That was about the router being generic over the body. axum::serve did not exist back then.

[jplatte]

Can you add a changelog entry?

[mladedav]

I've added the changelog entry.

Just to make sure, adding a generic parameter like this isn't considered breaking? I don't think this should break anyone calling serve(...) but if someone is doing thins like serve<L, M, S>(...) then that could be a problem so I guess we should stay on the safe side since there is a workaround for this.

I assume we're not merging breaking changes yet.