Allow for timeouts when scheduling zone expire.

Increase the expire time for zones with very short expiry. Without
this a zone may expire if there is a timeout, and once it has expired
it won't be served until it is refreshed. Sometimes, customers load
zones with SOA values that we have little control over.

Signed-off-by: Jan Kadlec <jan.kadlec@nic.cz>

src/knot/server/zones.c

@@ -252,6 +252,9 @@ int zones_refresh_ev(event_t *e)
 	/* Schedule EXPIRE timer on first attempt. */
 	if (!zd->xfr_in.expire) {
 		uint32_t expire_tmr = zones_jitter(zones_soa_expire(zone));
+		// Allow for timeouts.  Otherwise zones with very short
+		// expiry may expire before the timeout is reached.
+		expire_tmr += 2 * (conf()->max_conn_idle * 1000);
 		zd->xfr_in.expire = evsched_schedule_cb(
 					      e->parent,
 					      zones_expire_ev,