[PATCH] QUIC: optimized sending stream response

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1677152799 -14400
#      Thu Feb 23 15:46:39 2023 +0400
# Branch quic
# Node ID 6d471753917c083b4044f73557f9af8d91c20d36
# Parent  3c33d39a51d334d99fcc7d2b45e8d8190c431492
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.

diff --git a/src/event/quic/ngx_event_quic_streams.c b/src/event/quic/ngx_event_quic_streams.c
--- 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) {
+                /*
+                 * It's highly likely the posted stream will 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);
+            }
         }
     }