[QUIC] padding of Initial packets

[Vladimir Homutov]

# HG changeset patch
# User Vladimir Homutov <vl at nginx.com>
# Date 1643796973 -10800
#      Wed Feb 02 13:16:13 2022 +0300
# Branch quic
# Node ID fbfbcf66990e8964bcf308f3869f37d1a1acceeb
# Parent  8c6645ecaeb6cbf27976fd9035440bfcab943117
QUIC: fixed padding of initial packets in case of limited path.

Previously, non-padded initial packet could be sent as a result of the
following situation:

 - initial queue is not empty (so padding to 1200 is required)
 - handhsake queue is not empty (so padding is to be added after h/s packet)
 - path is limited

If serializing handshake packet would violate path limit, such packet was
omitted, and the non-padded initial packet was sent.

The fix is to avoid sending the packet at all in such case.  This follows the
original intention introduced in c5155a0cb12f.

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
@@ -158,7 +158,14 @@ ngx_quic_create_datagrams(ngx_connection
                   ? NGX_QUIC_MIN_INITIAL_SIZE - (p - dst) : 0;

             if (min > len) {
-                continue;
+                /* padding can't be applied - avoid sending the packet */
+
+                for (i = 0; i < NGX_QUIC_SEND_CTX_LAST; i++) {
+                    ctx = &qc->send_ctx[i];
+                    ngx_quic_revert_send(c, ctx, preserved_pnum[i]);
+                }
+
+                return NGX_OK;
             }

             n = ngx_quic_output_packet(c, ctx, p, len, min);