QUIC: optimized sending stream response. When a stream is created by client, it's often the case that nginx will send immediate response on that stream. An example is HTTP/3 request stream, which in most cases quickly replies with at least HTTP headers. QUIC stream init handlers are called from a posted event. Output QUIC frames are also sent to client from a posted event, called the push event. If the push event is posted before the stream init event, then output produced by stream may trigger sending an extra UDP datagram. To address this, push event is now re-posted when a new stream init event is posted. An example is handling 0-RTT packets. Client typically sends an init packet coalesced with a 0-RTT packet. Previously, nginx replied with a padded CRYPTO datagram, followed by a 1-RTT stream reply datagram. Now CRYPTO and STREAM packets are coalesced in one reply datagram, which saves bandwidth. Other examples include coalescing 1-RTT first stream response, and MAX_STREAMS/STREAM sent in response to ACK/STREAM.

--- a/src/event/quic/ngx_event_quic_streams.c
+++ b/src/event/quic/ngx_event_quic_streams.c
@@ -472,6 +472,17 @@ ngx_quic_get_stream(ngx_connection_t *c,
 
         if (qc->streams.initialized) {
             ngx_post_event(rev, &ngx_posted_events);
+
+            if (qc->push.posted) {
+                /*
+                 * The posted stream can produce output immediately.
+                 * By postponing the push event, we coalesce the stream
+                 * output with queued frames in one UDP datagram.
+                 */
+
+                ngx_delete_posted_event(&qc->push);
+                ngx_post_event(&qc->push, &ngx_posted_events);
+            }
         }
     }