QUIC TXP: Allow callbacks on ACK transmission

Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NPaul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21029)

include/internal/quic_txp.h

@@ -184,6 +184,17 @@ void ossl_quic_tx_packetiser_set_msg_callback_arg(OSSL_QUIC_TX_PACKETISER *txp,
  */
 QUIC_PN ossl_quic_tx_packetiser_get_next_pn(OSSL_QUIC_TX_PACKETISER *txp,
                                             uint32_t pn_space);
+
+/*
+ * Sets a callback which is called whenever TXP sends an ACK frame. The callee
+ * must not modify the ACK frame data. Can be used to snoop on PNs being ACKed.
+ */
+void ossl_quic_tx_packetiser_set_ack_tx_cb(OSSL_QUIC_TX_PACKETISER *txp,
+                                           void (*cb)(const OSSL_QUIC_FRAME_ACK *ack,
+                                                      uint32_t pn_space,
+                                                      void *arg),
+                                           void *cb_arg);
+
 # endif
 
 #endif

ssl/quic/quic_txp.c

@@ -75,6 +75,11 @@ struct ossl_quic_tx_packetiser_st {
     void *msg_callback_arg;
     SSL *msg_callback_ssl;
 
+    /* Callbacks. */
+    void            (*ack_tx_cb)(const OSSL_QUIC_FRAME_ACK *ack,
+                                 uint32_t pn_space,
+                                 void *arg);
+    void            *ack_tx_cb_arg;
 };
 
 /*
@@ -474,6 +479,16 @@ int ossl_quic_tx_packetiser_set_peer(OSSL_QUIC_TX_PACKETISER *txp,
     return 1;
 }
 
+void ossl_quic_tx_packetiser_set_ack_tx_cb(OSSL_QUIC_TX_PACKETISER *txp,
+                                           void (*cb)(const OSSL_QUIC_FRAME_ACK *ack,
+                                                      uint32_t pn_space,
+                                                      void *arg),
+                                           void *cb_arg)
+{
+    txp->ack_tx_cb      = cb;
+    txp->ack_tx_cb_arg  = cb_arg;
+}
+
 int ossl_quic_tx_packetiser_discard_enc_level(OSSL_QUIC_TX_PACKETISER *txp,
                                               uint32_t enc_level)
 {
@@ -1247,6 +1262,9 @@ static int txp_generate_pre_token(OSSL_QUIC_TX_PACKETISER *txp,
 
             if (ack->num_ack_ranges > 0)
                 tpkt->ackm_pkt.largest_acked = ack->ack_ranges[0].end;
+
+            if (txp->ack_tx_cb != NULL)
+                txp->ack_tx_cb(&ack2, pn_space, txp->ack_tx_cb_arg);
         } else {
             tx_helper_rollback(h);
         }