QUIC CHANNEL: Optimise key update using ACKs

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from #21029)

include/internal/quic_txp.h

@@ -162,6 +162,13 @@ void ossl_quic_tx_packetiser_schedule_handshake_done(OSSL_QUIC_TX_PACKETISER *tx
 void ossl_quic_tx_packetiser_schedule_ack_eliciting(OSSL_QUIC_TX_PACKETISER *txp,
                                                     uint32_t pn_space);
 
+/*
+ * Asks the TXP to ensure an ACK is put in the next packet in the given PN
+ * space.
+ */
+void ossl_quic_tx_packetiser_schedule_ack(OSSL_QUIC_TX_PACKETISER *txp,
+                                          uint32_t pn_space);
+
 /*
  * Schedules a connection close. *f and f->reason are copied. This operation is
  * irreversible and causes all further packets generated by the TXP to contain a

ssl/quic/quic_channel.c

@@ -701,6 +701,23 @@ static void rxku_detected(QUIC_PN pn, void *arg)
     if (decision == DECISION_SOLICITED_TXKU)
         /* NOT gated by usual txku_allowed() */
         ch_trigger_txku(ch);
+
+    /*
+     * Ordinarily, we only generate ACK when some ACK-eliciting frame has been
+     * received. In some cases, this may not occur for a long time, for example
+     * if transmission of application data is going in only one direction and
+     * nothing else is happening with the connection. However, since the peer
+     * cannot initiate a subsequent (spontaneous) TXKU until its prior
+     * (spontaneous or solicited) TXKU has completed - meaning that that prior
+     * TXKU's trigger packet (or subsequent packet) has been acknowledged, this
+     * can lead to very long times before a TXKU is considered 'completed'.
+     * Optimise this by forcing ACK generation after triggering TXKU.
+     * (Basically, we consider a RXKU event something that is 'ACK-eliciting',
+     * which it more or less should be; it is necessarily separate from ordinary
+     * processing of ACK-eliciting frames as key update is not indicated via a
+     * frame.)
+     */
+    ossl_quic_tx_packetiser_schedule_ack(ch->txp, QUIC_PN_SPACE_APP);
 }
 
 /* Called per tick to handle RXKU timer events. */

ssl/quic/quic_txp.c

@@ -519,6 +519,12 @@ void ossl_quic_tx_packetiser_schedule_ack_eliciting(OSSL_QUIC_TX_PACKETISER *txp
     txp->force_ack_eliciting |= (1UL << pn_space);
 }
 
+void ossl_quic_tx_packetiser_schedule_ack(OSSL_QUIC_TX_PACKETISER *txp,
+                                          uint32_t pn_space)
+{
+    txp->want_ack |= (1UL << pn_space);
+}
+
 #define TXP_ERR_INTERNAL     0  /* Internal (e.g. alloc) error */
 #define TXP_ERR_SUCCESS      1  /* Success */
 #define TXP_ERR_SPACE        2  /* Not enough room for another packet */