quic: fix stall datagrams, if no pending streams

src/quic/application.cc

@@ -540,7 +540,8 @@ void Session::Application::SendPendingData() {
     // We call Application::ResumeStream directly (not Session::ResumeStream)
     // to avoid creating a SendPendingDataScope — we're already inside
     // SendPendingData and re-entering would just hit nwrite=0 again.
-    if (nwrite == 0) {
+    if (nwrite == 0 &&
+        (stream_data.id >= 0 || !session_->HasPendingDatagrams())) {
       Debug(session_, "Congestion or not our turn to send");
       if (stream_data.id >= 0 && (stream_data.count > 0 || stream_data.fin)) {
         ResumeStream(stream_data.id);

src/quic/session.cc

@@ -2593,6 +2593,7 @@ datagram_id Session::SendDatagram(Store&& data) {
       return did;
     }
   }
+  Session::SendPendingDataScope send_scope(this);
 
   // Queue the datagram. It will be serialized into packets by
   // SendPendingData alongside stream data.

test/parallel/test-quic-session-preferred-address-ipv6.mjs

@@ -100,6 +100,9 @@ const clientSession = await connect(serverEndpoint.address, {
       family: 'ipv6',
     },
   },
+  maxDatagramSendAttempts: 100, // While the connection is restablished,
+  // all the acknowledgement packets of ngtcp2 are counted as send attempts
+  // so either this or a delay, or a change in ngtcp2 interfaces
 });
 await clientSession.opened;
 

test/parallel/test-quic-session-preferred-address.mjs

@@ -78,6 +78,9 @@ const clientSession = await connect(serverEndpoint.address, {
     strictEqual(oldRemote, null);
     strictEqual(preferred, true);
   }),
+  maxDatagramSendAttempts: 100, // While the connection is restablished,
+  // all the acknowledgement packets of ngtcp2 are counted as send attempts
+  // so either this or a delay, or a change in ngtcp2 interfaces
 });
 await clientSession.opened;