layer/validate: disable explicit RRSIG query if RRSIG for DS is missed

Relates to #372 (closed) - forwarding mode.

Previously if an misconfigured upstream returned DS without RRSIG the kresd started a loop to ask for RRSIGs.

Even if RRSIG for DS exists, upstream will not return it because DS is parent-side RR so this query is meaningless.

lib/layer/validate.c

@@ -482,6 +482,12 @@ static int rrsig_not_found(kr_layer_t *ctx, const knot_rrset_t *rr)
 	struct kr_request *req = ctx->req;
 	struct kr_query *qry = req->current_query;
 
+	/* Parent-side record, so don't ask for RRSIG.
+	 * We won't receive it anyway. */
+	if (qry->stype == KNOT_RRTYPE_DS) {
+		return KR_STATE_FAIL;
+	}
+
 	struct kr_zonecut *cut = &qry->zone_cut;
 	const knot_dname_t *cut_name_start = qry->zone_cut.name;
 	bool use_cut = true;