Add example showing how to proxy reqwest::Response

We get this question from time to time in Discord so figured its worth
having an example for.

examples/reqwest-response/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "example-reqwest-response"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+reqwest = { version = "0.11", features = ["stream"] }
+tokio = { version = "1.0", features = ["full"] }
+tokio-stream = "0.1"
+tower-http = { version = "0.4", features = ["trace"] }
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }

examples/reqwest-response/src/main.rs

@@ -0,0 +1,79 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-reqwest-response
+//! ```
+
+use std::{convert::Infallible, time::Duration};
+
+use axum::{
+    body::{Body, Bytes},
+    extract::State,
+    response::{IntoResponse, Response},
+    routing::get,
+    Router,
+};
+use reqwest::{Client, StatusCode};
+use tokio_stream::StreamExt;
+use tower_http::trace::TraceLayer;
+use tracing::Span;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::registry()
+        .with(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| "example_reqwest_response=debug,tower_http=debug".into()),
+        )
+        .with(tracing_subscriber::fmt::layer())
+        .init();
+
+    let client = Client::new();
+
+    let app = Router::new()
+        .route("/", get(proxy_via_reqwest))
+        .route("/stream", get(stream_some_data))
+        // Add some logging so we can see the streams going through
+        .layer(TraceLayer::new_for_http().on_body_chunk(
+            |chunk: &Bytes, _latency: Duration, _span: &Span| {
+                tracing::debug!("streaming {} bytes", chunk.len());
+            },
+        ))
+        .with_state(client);
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:3000")
+        .await
+        .unwrap();
+    tracing::debug!("listening on {}", listener.local_addr().unwrap());
+    axum::serve(listener, app).await.unwrap();
+}
+
+async fn proxy_via_reqwest(State(client): State<Client>) -> Response {
+    let reqwest_response = match client.get("http://127.0.0.1:3000/stream").send().await {
+        Ok(res) => res,
+        Err(err) => {
+            tracing::error!(%err, "request failed");
+            return StatusCode::BAD_GATEWAY.into_response();
+        }
+    };
+
+    let mut response_builder = Response::builder().status(reqwest_response.status());
+
+    // This unwrap is fine because we haven't insert any headers yet so there can't be any invalid
+    // headers
+    *response_builder.headers_mut().unwrap() = reqwest_response.headers().clone();
+
+    response_builder
+        .body(Body::from_stream(reqwest_response.bytes_stream()))
+        // Same goes for this unwrap
+        .unwrap()
+}
+
+async fn stream_some_data() -> Body {
+    let stream = tokio_stream::iter(0..5)
+        .throttle(Duration::from_secs(1))
+        .map(|n| n.to_string())
+        .map(Ok::<_, Infallible>);
+    Body::from_stream(stream)
+}