QUIC ACKM: Allow largest acked PN to be queried

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_ackm.h

@@ -263,6 +263,9 @@ int ossl_ackm_mark_packet_pseudo_lost(OSSL_ACKM *ackm,
  */
 OSSL_TIME ossl_ackm_get_pto_duration(OSSL_ACKM *ackm);
 
+/* Returns the largest acked PN in the given PN space. */
+QUIC_PN ossl_ackm_get_largest_acked(OSSL_ACKM *ackm, int pkt_space);
+
 # endif
 
 #endif

ssl/quic/quic_ackm.c

@@ -1677,3 +1677,8 @@ OSSL_TIME ossl_ackm_get_pto_duration(OSSL_ACKM *ackm)
 
     return duration;
 }
+
+QUIC_PN ossl_ackm_get_largest_acked(OSSL_ACKM *ackm, int pkt_space)
+{
+    return ackm->largest_acked_pkt[pkt_space];
+}