Implement a JSON tower layer

This will help requesting JSON APIs

crates/http/src/layers/json.rs

@@ -0,0 +1,116 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::{marker::PhantomData, task::Poll};
+
+use futures_util::future::BoxFuture;
+use http::{header::ACCEPT, HeaderValue, Request, Response};
+use http_body::Body;
+use serde::de::DeserializeOwned;
+use thiserror::Error;
+use tower::{Layer, Service};
+
+#[derive(Debug, Error)]
+pub enum Error<Service, Body> {
+    #[error("service")]
+    Service { inner: Service },
+    #[error("body")]
+    Body { inner: Body },
+    #[error("json")]
+    Json { inner: serde_json::Error },
+}
+
+impl<S, B> Error<S, B> {
+    fn service(source: S) -> Self {
+        Self::Service { inner: source }
+    }
+
+    fn body(source: B) -> Self {
+        Self::Body { inner: source }
+    }
+
+    fn json(source: serde_json::Error) -> Self {
+        Self::Json { inner: source }
+    }
+}
+
+pub struct Json<S, T> {
+    inner: S,
+    _t: PhantomData<T>,
+}
+
+impl<S, T> Json<S, T> {
+    pub const fn new(inner: S) -> Self {
+        Self {
+            inner,
+            _t: PhantomData,
+        }
+    }
+}
+
+impl<S, T, B, C> Service<Request<B>> for Json<S, T>
+where
+    S: Service<Request<B>, Response = Response<C>>,
+    S::Future: Send + 'static,
+    C: Body + Send + 'static,
+    C::Data: Send + 'static,
+    T: DeserializeOwned,
+{
+    type Error = Error<S::Error, C::Error>;
+    type Response = Response<T>;
+    type Future = BoxFuture<'static, Result<Self::Response, Self::Error>>;
+
+    fn poll_ready(&mut self, cx: &mut std::task::Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_ready(cx).map_err(Error::service)
+    }
+
+    fn call(&mut self, mut req: Request<B>) -> Self::Future {
+        req.headers_mut()
+            .insert(ACCEPT, HeaderValue::from_static("application/json"));
+
+        let fut = self.inner.call(req);
+
+        let fut = async {
+            let res = fut.await.map_err(Error::service)?;
+            let (parts, body) = res.into_parts();
+
+            futures_util::pin_mut!(body);
+            let bytes = hyper::body::to_bytes(&mut body)
+                .await
+                .map_err(Error::body)?;
+
+            let body = serde_json::from_slice(&bytes.to_vec()).map_err(Error::json)?;
+
+            let res = Response::from_parts(parts, body);
+            Ok(res)
+        };
+
+        Box::pin(fut)
+    }
+}
+
+#[derive(Default, Clone, Copy)]
+pub struct JsonResponseLayer<T, ReqBody>(PhantomData<(T, ReqBody)>);
+
+impl<ReqBody, ResBody, S, T> Layer<S> for JsonResponseLayer<T, ReqBody>
+where
+    S: Service<Request<ReqBody>, Response = Response<ResBody>>,
+    T: serde::de::DeserializeOwned,
+{
+    type Service = Json<S, T>;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        Json::new(inner)
+    }
+}