lib/nsrep: lower bound on rtt, optional throttling

when a NS is in the TIMEOUT, it can’t be autoselected by default but only probed after a successful dice roll - however in some scenarios we’d like to probe timeouting servers more thoroughly 

lib/nsrep.c

@@ -21,6 +21,7 @@
 #include <libknot/internal/sockaddr.h>
 
 #include "lib/nsrep.h"
+#include "lib/rplan.h"
 #include "lib/defines.h"
 #include "lib/generic/pack.h"
 
@@ -97,7 +98,7 @@ static int eval_nsrep(const char *k, void *v, void *baton)
 	 * The fastest NS is preferred by workers until it is depleted (timeouts or degrades),
 	 * at the same time long distance scouts probe other sources (low probability).
 	 * Servers on TIMEOUT (depleted) can be probed by the dice roll only */
-	if (score < ns->score && score < KR_NS_TIMEOUT) {
+	if (score < ns->score && (ns->flags & QUERY_NO_THROTTLE || score < KR_NS_TIMEOUT)) {
 		update_nsrep(ns, (const knot_dname_t *)k, addr, score);
 	} else {
 		/* With 5% chance, probe server with a probability given by its RTT / MAX_RTT */
@@ -135,6 +136,9 @@ int kr_nsrep_update(struct kr_nsrep *ns, unsigned score, kr_nsrep_lru_t *repcach
 	if (score > KR_NS_MAX_SCORE) {
 		score = KR_NS_MAX_SCORE;
 	}
+	if (score <= KR_NS_UNKNOWN) {
+		score = KR_NS_UNKNOWN + 1;
+	}
 	/* Set initial value or smooth over last two measurements */
 	if (*cur != 0) {
 		*cur = (*cur + score) / 2;

lib/nsrep.h

@@ -46,6 +46,7 @@ typedef lru_hash(unsigned) kr_nsrep_lru_t;
 struct kr_nsrep
 {
 	unsigned score;                  /**< Server score */
+	unsigned flags;                  /**< Server flags */
 	const knot_dname_t *name;        /**< Server name */
 	kr_nsrep_lru_t *repcache;        /**< Reputation cache pointer */
 	union {

lib/resolve.c

@@ -431,6 +431,11 @@ int kr_resolve_produce(struct kr_request *request, struct sockaddr **dst, int *t
 ns_election:
 	/* Elect best nameserver candidate */
 	assert(++ns_election_iter < KR_ITER_LIMIT);
+	/* Set slow NS throttling mode */
+	qry->ns.flags = 0;
+	if (qry->flags & QUERY_NO_THROTTLE) {
+		qry->ns.flags = QUERY_NO_THROTTLE;
+	}
 	kr_nsrep_elect(&qry->ns, &qry->zone_cut.nsset, request->ctx->nsrep);
 	if (qry->ns.score > KR_NS_MAX_SCORE) {
 		DEBUG_MSG("=> no valid NS left\n");

lib/rplan.h

@@ -29,13 +29,14 @@
 /** Query flags */
 enum kr_query_flag {
 	QUERY_NO_MINIMIZE = 1 << 0, /**< Don't minimize QNAME. */
-	QUERY_TCP         = 1 << 1, /**< Use TCP for this query. */
-	QUERY_RESOLVED    = 1 << 2, /**< Query is resolved. */
-	QUERY_AWAIT_IPV4  = 1 << 3, /**< Query is waiting for A address. */
-	QUERY_AWAIT_IPV6  = 1 << 4, /**< Query is waiting for AAAA address. */
-	QUERY_AWAIT_CUT   = 1 << 5, /**< Query is waiting for zone cut lookup */
-	QUERY_SAFEMODE    = 1 << 6, /**< Don't use fancy stuff (EDNS...) */
-	QUERY_CACHED      = 1 << 7  /**< Query response is cached. */
+	QUERY_NO_THROTTLE = 1 << 1, /**< No query/slow NS throttling. */
+	QUERY_TCP         = 1 << 2, /**< Use TCP for this query. */
+	QUERY_RESOLVED    = 1 << 3, /**< Query is resolved. */
+	QUERY_AWAIT_IPV4  = 1 << 4, /**< Query is waiting for A address. */
+	QUERY_AWAIT_IPV6  = 1 << 5, /**< Query is waiting for AAAA address. */
+	QUERY_AWAIT_CUT   = 1 << 6, /**< Query is waiting for zone cut lookup */
+	QUERY_SAFEMODE    = 1 << 7, /**< Don't use fancy stuff (EDNS...) */
+	QUERY_CACHED      = 1 << 8  /**< Query response is cached. */
 };
 
 /**