Test interaction between large datagram frames and ACKs

quinn-proto/src/tests/mod.rs

@@ -3074,3 +3074,32 @@ fn gso_truncation() {
         );
     }
 }
+
+/// Verify that a large application datagram is sent successfully when an ACK frame too large to fit
+/// alongside it is also queued, in exactly 2 UDP datagrams.
+#[test]
+fn large_datagram_with_acks() {
+    let _guard = subscribe();
+    let mut pair = Pair::default();
+    let (client_ch, server_ch) = pair.connect();
+
+    // Force the client to generate a large ACK frame by dropping several packets
+    for _ in 0..10 {
+        pair.server_conn_mut(server_ch).ping();
+        pair.drive_server();
+        pair.client.inbound.pop_back();
+        pair.server_conn_mut(server_ch).ping();
+        pair.drive_server();
+    }
+
+    let max_size = pair.client_datagrams(client_ch).max_size().unwrap();
+    let msg = Bytes::from(vec![0; max_size]);
+    pair.client_datagrams(client_ch)
+        .send(msg.clone(), true)
+        .unwrap();
+    let initial_datagrams = pair.client_conn_mut(client_ch).stats().udp_tx.datagrams;
+    pair.drive();
+    let final_datagrams = pair.client_conn_mut(client_ch).stats().udp_tx.datagrams;
+    assert_eq!(pair.server_datagrams(server_ch).recv().unwrap(), msg);
+    assert_eq!(final_datagrams - initial_datagrams, 2);
+}