cache: fix TTL overflow in packet due to min_ttl

- `min_ttl()` enforces packet being alive longer than original TTL
  of some records; but
- the packet is copied to cache as it was.

Resolution: just serve packet the same but with those record's TTLs
remaining at zero.

diff --git a/lib/cache/entry_pkt.c b/lib/cache/entry_pkt.c
index ad7bfea70434ef6746ac84f1b9da181e3c14ea47..9f061e77321e43edbe8e1cfed61b64b2c10d0087 100644 (file)
--- a/lib/cache/entry_pkt.c
+++ b/lib/cache/entry_pkt.c
@@ -195,7 +195,7 @@ int answer_from_pkt(kr_layer_t *ctx, knot_pkt_t *pkt, uint16_t type,
                pkt->rr[i].additional = rr_rank;
        }
 
-       /* Adjust TTL in records.  We know that no RR has expired yet. */
+       /* Adjust TTL in each record. */
        const uint32_t drift = eh->ttl - new_ttl;
        for (knot_section_t i = KNOT_ANSWER; i <= KNOT_ADDITIONAL; ++i) {
                const knot_pktsection_t *sec = knot_pkt_section(pkt, i);
@@ -203,7 +203,17 @@ int answer_from_pkt(kr_layer_t *ctx, knot_pkt_t *pkt, uint16_t type,
                        const knot_rrset_t *rr = knot_pkt_rr(sec, k);
                        knot_rdata_t *rd = rr->rrs.data;
                        for (uint16_t i = 0; i < rr->rrs.rr_count; ++i) {
-                               knot_rdata_set_ttl(rd, knot_rdata_ttl(rd) - drift);
+                               /* We need to be careful:
+                                * due to enforcing minimum TTL on packet,
+                                * some records may be below that value.
+                                * We keep those records at TTL 0. */
+                               uint32_t ttl = knot_rdata_ttl(rd);
+                               if (drift <= ttl) {
+                                       ttl -= drift;
+                               } else {
+                                       ttl = 0;
+                               }
+                               knot_rdata_set_ttl(rd, ttl);
                                rd = kr_rdataset_next(rd);
                        }
                }