add keep alive to h2 through ping pong (#2433)

actix-http/tests/test_h2_ping_pong.rs

@@ -0,0 +1,77 @@
+use std::io;
+
+use actix_http::{error::Error, HttpService, Response};
+use actix_server::Server;
+
+#[actix_rt::test]
+async fn h2_ping_pong() -> io::Result<()> {
+    let (tx, rx) = std::sync::mpsc::sync_channel(1);
+
+    let lst = std::net::TcpListener::bind("127.0.0.1:0")?;
+
+    let addr = lst.local_addr().unwrap();
+
+    let join = std::thread::spawn(move || {
+        actix_rt::System::new().block_on(async move {
+            let handle = Server::build()
+                .disable_signals()
+                .workers(1)
+                .listen("h2_ping_pong", lst, || {
+                    HttpService::build()
+                        .keep_alive(3)
+                        .h2(|_| async { Ok::<_, Error>(Response::ok()) })
+                        .tcp()
+                })?
+                .run();
+
+            tx.send(handle.clone()).unwrap();
+
+            handle.await
+        })
+    });
+
+    let handle = rx.recv().unwrap();
+
+    let (sync_tx, rx) = std::sync::mpsc::sync_channel(1);
+
+    // use a separate thread for h2 client so it can be blocked.
+    std::thread::spawn(move || {
+        tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+            .block_on(async move {
+                let stream = tokio::net::TcpStream::connect(addr).await.unwrap();
+
+                let (mut tx, conn) = h2::client::handshake(stream).await.unwrap();
+
+                tokio::spawn(async move { conn.await.unwrap() });
+
+                let (res, _) = tx.send_request(::http::Request::new(()), true).unwrap();
+                let res = res.await.unwrap();
+
+                assert_eq!(res.status().as_u16(), 200);
+
+                sync_tx.send(()).unwrap();
+
+                // intentionally block the client thread so it can not answer ping pong.
+                std::thread::sleep(std::time::Duration::from_secs(1000));
+            })
+    });
+
+    rx.recv().unwrap();
+
+    let now = std::time::Instant::now();
+
+    // stop server gracefully. this step would take up to 30 seconds.
+    handle.stop(true).await;
+
+    // join server thread. only when connection are all gone this step would finish.
+    join.join().unwrap()?;
+
+    // check the time used for join server thread so it's known that the server shutdown
+    // is from keep alive and not server graceful shutdown timeout.
+    assert!(now.elapsed() < std::time::Duration::from_secs(30));
+
+    Ok(())
+}