Better Tower layers

crates/http/src/layers/json_response.rs

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+// 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 bytes::Buf;
+use futures_util::FutureExt;
+use http::{header::ACCEPT, HeaderValue, Request, Response};
+use serde::de::DeserializeOwned;
+use thiserror::Error;
+use tower::{Layer, Service};
+
+#[derive(Debug, Error)]
+pub enum Error<Service> {
+    #[error(transparent)]
+    Service { inner: Service },
+
+    #[error("could not parse JSON payload")]
+    Json {
+        #[source]
+        inner: serde_json::Error,
+    },
+}
+
+impl<S> Error<S> {
+    fn service(source: S) -> Self {
+        Self::Service { inner: source }
+    }
+
+    fn json(source: serde_json::Error) -> Self {
+        Self::Json { inner: source }
+    }
+}
+
+#[derive(Clone)]
+pub struct JsonResponse<S, T> {
+    inner: S,
+    _t: PhantomData<T>,
+}
+
+impl<S, T> JsonResponse<S, T> {
+    pub const fn new(inner: S) -> Self {
+        Self {
+            inner,
+            _t: PhantomData,
+        }
+    }
+}
+
+impl<S, T, B, C> Service<Request<B>> for JsonResponse<S, T>
+where
+    S: Service<Request<B>, Response = Response<C>>,
+    S::Future: Send + 'static,
+    C: Buf,
+    T: DeserializeOwned,
+{
+    type Error = Error<S::Error>;
+    type Response = Response<T>;
+    type Future = futures_util::future::Map<
+        S::Future,
+        fn(Result<Response<C>, S::Error>) -> 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 request: Request<B>) -> Self::Future {
+        fn mapper<C, T, E>(res: Result<Response<C>, E>) -> Result<Response<T>, Error<E>>
+        where
+            C: Buf,
+            T: DeserializeOwned,
+        {
+            let response = res.map_err(Error::service)?;
+            let (parts, body) = response.into_parts();
+
+            let body = serde_json::from_reader(body.reader()).map_err(Error::json)?;
+
+            let res = Response::from_parts(parts, body);
+            Ok(res)
+        }
+
+        request
+            .headers_mut()
+            .insert(ACCEPT, HeaderValue::from_static("application/json"));
+
+        self.inner.call(request).map(mapper::<C, T, S::Error>)
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct JsonResponseLayer<T> {
+    _t: PhantomData<T>,
+}
+
+impl<T> Default for JsonResponseLayer<T> {
+    fn default() -> Self {
+        Self {
+            _t: PhantomData::default(),
+        }
+    }
+}
+
+impl<S, T> Layer<S> for JsonResponseLayer<T> {
+    type Service = JsonResponse<S, T>;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        JsonResponse::new(inner)
+    }
+}