[PATCH 1 of 4] QUIC: optimized ACK delay

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1690461437 -14400
#      Thu Jul 27 16:37:17 2023 +0400
# Node ID 2fd16fc76920ef0b8ea2fa64858934e38c4477c5
# Parent  904c99bede1770d92566b56939c5b6ec85f05b55
QUIC: optimized ACK delay.

Previously ACK was not generated if max_ack_delay was not yet expired and the
number of unacknowledged ack-eliciting packets was less than two, as allowed by
RFC 9000 13.2.1-13.2.2.  However this only makes sense to avoid sending ACK-only
packets, as explained by the RFC:

  On the other hand, reducing the frequency of packets that carry only
  acknowledgments reduces packet transmission and processing cost at both
  endpoints.

Now ACK is delayed only if output frame queue is empty.  Otherwise ACK is sent
immediately, which significantly improves QUIC performance with certain tests.

diff --git a/src/event/quic/ngx_event_quic_ack.c b/src/event/quic/ngx_event_quic_ack.c
--- a/src/event/quic/ngx_event_quic_ack.c
+++ b/src/event/quic/ngx_event_quic_ack.c
@@ -1171,7 +1171,8 @@ ngx_quic_generate_ack(ngx_connection_t *
         delay = ngx_current_msec - ctx->ack_delay_start;
         qc = ngx_quic_get_connection(c);
 
-        if (ctx->send_ack < NGX_QUIC_MAX_ACK_GAP
+        if (ngx_queue_empty(&ctx->frames)
+            && ctx->send_ack < NGX_QUIC_MAX_ACK_GAP
             && delay < qc->tp.max_ack_delay)
         {
             if (!qc->push.timer_set && !qc->closing) {