BUG/MINOR: quic: drop multiple Retry on same connection

Ensures that only a single Retry packet is handled by a QUIC haproxy
client per connection. This is mandated by RFC 9000. The first received
token should still be sufficient to validate the connection.

This change is applied directly in quic_rx_pkt_parse(). In case of
multiple Retry, packets are silently ignored, whether token is identical
or not.

This fix is particularly important to prevent a memory leak on several
elements, first <retry_token> member of quic_conn. This also concerns
elements from the TLS stack as initial encryption level would be
reinitialized needlessly.

Reported-by: Claude (ANT-2026-CJ4Z875H)

This must be backported up to 3.3.

Author: a-denoyelle

src/quic_rx.c

@@ -2062,6 +2062,19 @@ static int quic_rx_pkt_parse(struct quic_conn *qc, struct quic_rx_packet *pkt,
 				goto drop;
 			}
 
+			/* RFC 9000 17.2.5.2. Handling a Retry Packet
+			 *
+			 * A client MUST accept and process at most one Retry packet for each
+			 * connection attempt. After the client has received and processed an
+			 * Initial or Retry packet from the server, it MUST discard any
+			 * subsequent Retry packets that it receives.
+			 */
+			if (qc->retry_token) {
+				TRACE_PROTO("Drop duplicate Retry packet",
+				            QUIC_EV_CONN_LPKT, NULL, NULL, NULL, pkt->version);
+				goto drop_silent;
+			}
+
 			if (!quic_retry_packet_check(qc, pkt, beg, end, pos, &qc->retry_token_len))
 				/* TODO: should close the connection? */
 				goto drop;