fix(http1): only send 100 Continue if request body is polled

Before, if a client request included an `Expect: 100-continue` header,
the `100 Continue` response was sent immediately. However, this is
problematic if the service is going to reply with some 4xx status code
and reject the body.

This change delays the automatic sending of the `100 Continue` status
until the service has call `poll_data` on the request body once.

tests/server.rs

@@ -785,6 +785,57 @@ fn expect_continue_but_no_body_is_ignored() {
     assert_eq!(&resp[..expected.len()], expected);
 }
 
+#[tokio::test]
+async fn expect_continue_waits_for_body_poll() {
+    let _ = pretty_env_logger::try_init();
+    let mut listener = tcp_bind(&"127.0.0.1:0".parse().unwrap()).unwrap();
+    let addr = listener.local_addr().unwrap();
+
+    let child = thread::spawn(move || {
+        let mut tcp = connect(&addr);
+
+        tcp.write_all(
+            b"\
+            POST /foo HTTP/1.1\r\n\
+            Host: example.domain\r\n\
+            Expect: 100-continue\r\n\
+            Content-Length: 100\r\n\
+            Connection: Close\r\n\
+            \r\n\
+        ",
+        )
+        .expect("write");
+
+        let expected = "HTTP/1.1 400 Bad Request\r\n";
+        let mut resp = String::new();
+        tcp.read_to_string(&mut resp).expect("read");
+
+        assert_eq!(&resp[..expected.len()], expected);
+    });
+
+    let (socket, _) = listener.accept().await.expect("accept");
+
+    Http::new()
+        .serve_connection(
+            socket,
+            service_fn(|req| {
+                assert_eq!(req.headers()["expect"], "100-continue");
+                // But! We're never going to poll the body!
+                tokio::time::delay_for(Duration::from_millis(50)).map(move |_| {
+                    // Move and drop the req, so we don't auto-close
+                    drop(req);
+                    Response::builder()
+                        .status(StatusCode::BAD_REQUEST)
+                        .body(hyper::Body::empty())
+                })
+            }),
+        )
+        .await
+        .expect("serve_connection");
+
+    child.join().expect("client thread");
+}
+
 #[test]
 fn pipeline_disabled() {
     let server = serve();