engine: throttle outbound queries only when busy

resolver will always attempt to contact upstreams
known to be bad if it's not busy.
this fixes a problem on low-volume resolvers
where a short connection outage could make
resolvers deny resolving queries even after the
connection is restored

daemon/engine.h

@@ -29,6 +29,9 @@
 #ifndef RECVMMSG_BATCH
 #define RECVMMSG_BATCH 4
 #endif
+#ifndef QUERY_RATE_THRESHOLD
+#define QUERY_RATE_THRESHOLD (2 * MP_FREELIST_SIZE) /**< Nr of parallel queries considered as high rate */
+#endif
 
 /*
  * @internal These are forward decls to allow building modules with engine but without Lua.

daemon/worker.c

@@ -287,6 +287,10 @@ static struct qr_task *qr_task_create(struct worker_ctx *worker, uv_handle_t *ha
 	kr_resolve_begin(&task->req, &engine->resolver, answer);
 	worker->stats.concurrent += 1;
 	worker->stats.queries += 1;
+	/* Throttle outbound queries only when high pressure */
+	if (worker->stats.concurrent < QUERY_RATE_THRESHOLD) {
+		task->req.options |= QUERY_NO_THROTTLE;
+	}
 	return task;
 }