fix(mmserver): use QUIC PING frames to keep clients alive

This fixes a bug where a client would be kicked before its own timeout
for a long-running operation like LaunchSession. By only sending
keepalives if the client has open streams, we enforce that clients are
still around, but let them time out if they're not doing anything.

Author: colinmarc

mm-server/src/server.rs

@@ -117,7 +117,6 @@ impl Server {
         };
 
         config.set_application_protos(&[protocol::ALPN_PROTOCOL_VERSION])?;
-        config.set_max_idle_timeout(10_000);
         config.set_initial_max_data(65536);
         config.set_initial_max_stream_data_bidi_remote(65536);
         config.set_initial_max_stream_data_bidi_local(65536);
@@ -127,6 +126,11 @@ impl Server {
         config.enable_dgram(true, 0, 1024 * 1024);
         config.enable_early_data();
 
+        // Set the idle timeout to 10s. If any streams are active, we send
+        // ack-eliciting frames so that we don't accidentally kill a client
+        // that's in the middle of something slow (like launching a session).
+        config.set_max_idle_timeout(10_000);
+
         // Storage for packets that would have blocked on sending.
         let outgoing_packets = VecDeque::new();
 
@@ -418,6 +422,13 @@ impl Server {
                 let span = trace_span!("gather_send", conn_id = ?client.conn_id);
                 let _guard = span.enter();
 
+                // Generate ack-eliciting keepalives for any clients with open
+                // streams. Clients with no open streams are allowed to time
+                // out.
+                if !client.in_flight.is_empty() {
+                    client.conn.send_ack_eliciting()?;
+                }
+
                 loop {
                     let start = off;
                     self.scratch.resize(off + MAX_QUIC_PACKET_SIZE, 0);