[PATCH 2 of 4] QUIC: always add ACK frame to the queue head

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1690461509 -14400
#      Thu Jul 27 16:38:29 2023 +0400
# Node ID 0d12ada84c168c62e9bae847af2725641da583d0
# Parent  2fd16fc76920ef0b8ea2fa64858934e38c4477c5
QUIC: always add ACK frame to the queue head.

Previously it was added to the tail as all other frames.  However, if the
amount of queued data is large, it could delay the delivery of ACK, which
could trigger frames retransmissions and slow down the connection.

diff --git a/src/event/quic/ngx_event_quic_output.c b/src/event/quic/ngx_event_quic_output.c
--- a/src/event/quic/ngx_event_quic_output.c
+++ b/src/event/quic/ngx_event_quic_output.c
@@ -1175,7 +1175,9 @@ ngx_quic_send_ack(ngx_connection_t *c, n
     frame->u.ack.range_count = ctx->nranges;
     frame->u.ack.first_range = ctx->first_range;
 
-    ngx_quic_queue_frame(qc, frame);
+    ngx_queue_insert_head(&ctx->frames, &frame->queue);
+
+    frame->len = ngx_quic_create_frame(NULL, frame);
 
     return NGX_OK;
 }