MINOR: quic-be: helper quic_reuse_srv_params() function to reuse server params (0-RTT)

Implement quic_reuse_srv_params() whose role it to reuse the ALPN used
from a first connection to a QUIC backend alongside its transport parameters
from a null terminated string <alpn> and a quic_early_transport_params struct
passed as parameters.

include/haproxy/quic_conn.h

@@ -82,6 +82,9 @@ void qc_idle_timer_rearm(struct quic_conn *qc, int read, int arm_ack);
 void qc_check_close_on_released_mux(struct quic_conn *qc);
 int quic_stateless_reset_token_cpy(unsigned char *pos, size_t len,
                                    const unsigned char *salt, size_t saltlen);
+int quic_reuse_srv_params(struct quic_conn *qc,
+                          const unsigned char *alpn,
+                          const struct quic_early_transport_params *etps);
 
 /* Free the CIDs attached to <conn> QUIC connection. */
 static inline void free_quic_conn_cids(struct quic_conn *conn)

src/quic_conn.c

@@ -285,6 +285,30 @@ int quic_set_app_ops(struct quic_conn *qc, const unsigned char *alpn, size_t alp
 	return 1;
 }
 
+/* Try to reuse <alpn> ALPN and <etps> early transport parameters.
+ * Return 1 if succeeded, 0 if not.
+ */
+int quic_reuse_srv_params(struct quic_conn *qc,
+                          const unsigned char *alpn,
+                          const struct quic_early_transport_params *etps)
+{
+	int ret = 0;
+
+	TRACE_ENTER(QUIC_EV_CONN_NEW, qc);
+
+	if (!alpn || !quic_set_app_ops(qc, alpn, strlen((char *)alpn)))
+		goto err;
+
+	qc_early_transport_params_reuse(qc, &qc->tx.params, etps);
+	ret = 1;
+ leave:
+	TRACE_LEAVE(QUIC_EV_CONN_NEW, qc);
+	return ret;
+ err:
+	TRACE_DEVEL("leaving on error", QUIC_EV_CONN_NEW, qc);
+	goto leave;
+}
+
 /* Schedule a CONNECTION_CLOSE emission on <qc> if the MUX has been released
  * and all STREAM data are acknowledged. The MUX is responsible to have set
  * <qc.err> before as it is reused for the CONNECTION_CLOSE frame.