Test voluntary acks when sending max-size datagrams

quinn-proto/src/tests/mod.rs

@@ -3103,3 +3103,33 @@ fn large_datagram_with_acks() {
     assert_eq!(pair.server_datagrams(server_ch).recv().unwrap(), msg);
     assert_eq!(final_datagrams - initial_datagrams, 2);
 }
+
+/// Verify that an ACK prompted by receipt of many non-ACK-eliciting packets is sent alongside
+/// outgoing application datagrams too large to coexist in the same packet with it.
+#[test]
+fn voluntary_ack_with_large_datagrams() {
+    let _guard = subscribe();
+    let mut pair = Pair::default();
+    let (client_ch, _) = pair.connect();
+
+    // Prompt many large ACKs from the server
+    let initial_datagrams = pair.client_conn_mut(client_ch).stats().udp_tx.datagrams;
+    // Send enough packets that we're confident some packet numbers will be skipped, ensuring that
+    // larger ACKs occur
+    const COUNT: usize = 256;
+    for _ in 0..COUNT {
+        let max_size = pair.client_datagrams(client_ch).max_size().unwrap();
+        pair.client_datagrams(client_ch)
+            .send(vec![0; max_size].into(), true)
+            .unwrap();
+        pair.drive();
+    }
+    let final_datagrams = pair.client_conn_mut(client_ch).stats().udp_tx.datagrams;
+    // Failure may indicate `max_size` is too small and ACKs are reliably being packed into the same
+    // datagram, which is reasonable behavior but makes this test ineffective.
+    assert_ne!(
+        final_datagrams - initial_datagrams,
+        COUNT as u64,
+        "client should have sent some ACK-only packets"
+    );
+}