From 9e53506240fa49c3dcf2351a14f8bf401d97d53a Mon Sep 17 00:00:00 2001
From: George Thessalonikefs <george@nlnetlabs.nl>
Date: Tue, 4 Oct 2022 10:26:33 +0200
Subject: [PATCH] - Introduce leniency for target discovery when under load.

---
 iterator/iterator.c | 43 +++++++++++++++++++++++++++++++++++++------
 iterator/iterator.h |  2 +-
 2 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/iterator/iterator.c b/iterator/iterator.c
index e1c556b39..7c5a0fc43 100644
--- a/iterator/iterator.c
+++ b/iterator/iterator.c
@@ -2264,6 +2264,7 @@ processQueryTargets(struct module_qstate* qstate, struct iter_qstate* iq,
 	size_t qout_orig_len = 0;
 	int sq_check_ratelimit = 1;
 	int sq_was_ratelimited = 0;
+	int can_do_promisc = 0;
 
 	/* NOTE: a request will encounter this state for each target it
 	 * needs to send a query to. That is, at least one per referral,
@@ -2591,12 +2592,12 @@ processQueryTargets(struct module_qstate* qstate, struct iter_qstate* iq,
 	if(iq->depth < ie->max_dependency_depth
 		&& iq->num_target_queries == 0
 		&& (!iq->target_count || iq->target_count[TARGET_COUNT_NX]==0)
-		&& iq->sent_count < TARGET_FETCH_STOP
-		/* if the mesh query list is full, then do not waste cpu
-		 * and sockets to fetch promiscuous targets. They can be
-		 * looked up when needed. */
-		&& !mesh_jostle_exceeded(qstate->env->mesh)
-		) {
+		&& iq->sent_count < TARGET_FETCH_STOP) {
+		can_do_promisc = 1;
+	}
+	/* if the mesh query list is full, then do not waste cpu and sockets to
+	 * fetch promiscuous targets. They can be looked up when needed. */
+	if(can_do_promisc && !mesh_jostle_exceeded(qstate->env->mesh)) {
 		tf_policy = ie->target_fetch_policy[iq->depth];
 	}
 
@@ -2768,6 +2769,36 @@ processQueryTargets(struct module_qstate* qstate, struct iter_qstate* iq,
 		return 0;
 	}
 
+	/* We have a target. We could have created promiscuous target
+	 * queries but we are currently under pressure (mesh_jostle_exceeded).
+	 * If we are configured to allow promiscuous target queries and haven't
+	 * gone out to the network for a target query for this delegation, then
+	 * it is possible to slip in a promiscuous one with a 1/10 chance. */
+	if(can_do_promisc && tf_policy == 0 && iq->depth == 0
+		&& ie->target_fetch_policy[iq->depth] != 0
+		&& iq->dp_target_count == 0
+		&& !ub_random_max(qstate->env->rnd, 10)) {
+		int extra = 0;
+		verbose(VERB_ALGO, "available target exists in cache but "
+			"attempt to get extra 1 target");
+		(void)query_for_targets(qstate, iq, ie, id, 1, &extra);
+		/* errors ignored, these targets are not strictly necessary for
+		* this result, we do not have to reply with SERVFAIL */
+		if(extra > 0) {
+			iq->num_target_queries += extra;
+			target_count_increase(iq, extra);
+			check_waiting_queries(iq, qstate, id);
+			/* undo qname minimise step because we'll get back here
+			 * to do it again */
+			if(qout_orig && iq->minimise_count > 0) {
+				iq->minimise_count--;
+				iq->qinfo_out.qname = qout_orig;
+				iq->qinfo_out.qname_len = qout_orig_len;
+			}
+			return 0;
+		}
+	}
+
 	/* Do not check ratelimit for forwarding queries or if we already got a
 	 * pass. */
 	sq_check_ratelimit = (!(iq->chase_flags & BIT_RD) && !iq->ratelimit_ok);
diff --git a/iterator/iterator.h b/iterator/iterator.h
index b71b7fe99..e35718cf3 100644
--- a/iterator/iterator.h
+++ b/iterator/iterator.h
@@ -324,7 +324,7 @@ struct iter_qstate {
 	/** the number of times this query has been restarted. */
 	int query_restart_count;
 
-	/** the number of times this query as followed a referral. */
+	/** the number of times this query has followed a referral. */
 	int referral_count;
 
 	/** number of queries fired off */