chg: usr: Exempt prefetches from the fetches-per-zone and fetches-per-server quotas

Fetches generated automatically as a result of 'prefetch' are now
exempt from the 'fetches-per-zone' and 'fetches-per-server' quotas.
This should help in maintaining the cache from which query responses
can be given.

Closes #4219

Merge branch '4219-exempt-good-queries-from-fetch-limits' into 'main'

Closes #4219

See merge request isc-projects/bind9!9095

doc/arm/reference.rst

@@ -3798,6 +3798,11 @@ system.
    is sent to that domain, it is recreated with the counters set
    to zero.)
 
+   .. note::
+
+       Fetches generated automatically in the result of :any:`prefetch` are
+       exempt from this quota.
+
 .. namedconf:statement:: fetches-per-server
    :tags: server, query
    :short: Sets the maximum number of simultaneous iterative queries allowed to be sent by a server to an upstream name server before the server blocks additional queries.
@@ -3829,6 +3834,11 @@ system.
    increased. The :any:`fetch-quota-params` options can be used to adjust
    the parameters for this calculation.
 
+   .. note::
+
+       Fetches generated automatically in the result of :any:`prefetch` are
+       exempt from this quota, but they are included in the quota calculations.
+
 .. namedconf:statement:: fetch-quota-params
    :tags: server, query
    :short: Sets the parameters for dynamic resizing of the :any:`fetches-per-server` quota in response to detected congestion.

lib/dns/adb.c

@@ -1478,7 +1478,9 @@ copy_namehook_lists(dns_adb_t *adb, dns_adbfind_t *find, dns_adbname_t *name) {
 			dns_adbaddrinfo_t *addrinfo = NULL;
 			entry = namehook->entry;
 
-			if (adbentry_overquota(entry)) {
+			if ((find->options & DNS_ADBFIND_QUOTAEXEMPT) == 0 &&
+			    adbentry_overquota(entry))
+			{
 				find->options |= DNS_ADBFIND_OVERQUOTA;
 				goto nextv4;
 			}
@@ -1500,7 +1502,9 @@ copy_namehook_lists(dns_adb_t *adb, dns_adbfind_t *find, dns_adbname_t *name) {
 			dns_adbaddrinfo_t *addrinfo = NULL;
 			entry = namehook->entry;
 
-			if (adbentry_overquota(entry)) {
+			if ((find->options & DNS_ADBFIND_QUOTAEXEMPT) == 0 &&
+			    adbentry_overquota(entry))
+			{
 				find->options |= DNS_ADBFIND_OVERQUOTA;
 				goto nextv6;
 			}

lib/dns/include/dns/adb.h

@@ -179,6 +179,10 @@ struct dns_adbfind {
  *	This is useful for reestablishing glue that has expired.
  */
 #define DNS_ADBFIND_STARTATZONE 0x00000020
+/*%
+ *	Fetches will be exempted from the quota.
+ */
+#define DNS_ADBFIND_QUOTAEXEMPT 0x00000040
 /*%
  *      The server's fetch quota is exceeded; it will be treated as
  *      lame for this query.

lib/dns/resolver.c

@@ -3257,6 +3257,13 @@ findname(fetchctx_t *fctx, const dns_name_t *name, in_port_t port,
 		options |= DNS_ADBFIND_STARTATZONE;
 	}
 
+	/*
+	 * Exempt prefetches from ADB quota.
+	 */
+	if ((fctx->options & DNS_FETCHOPT_PREFETCH) != 0) {
+		options |= DNS_ADBFIND_QUOTAEXEMPT;
+	}
+
 	/*
 	 * See what we know about this address.
 	 */
@@ -4661,13 +4668,18 @@ fctx_create(dns_resolver_t *res, isc_loop_t *loop, const dns_name_t *name,
 	}
 
 	/*
-	 * Are there too many simultaneous queries for this domain?
+	 * Exempt prefetch queries from the fetches-per-zone quota check
 	 */
-	result = fcount_incr(fctx, false);
+	if ((fctx->options & DNS_FETCHOPT_PREFETCH) == 0) {
-	if (result != ISC_R_SUCCESS) {
+		/*
-		result = fctx->res->quotaresp[dns_quotatype_zone];
+		 * Are there too many simultaneous queries for this domain?
-		inc_stats(res, dns_resstatscounter_zonequota);
+		 */
-		goto cleanup_nameservers;
+		result = fcount_incr(fctx, false);
+		if (result != ISC_R_SUCCESS) {
+			result = fctx->res->quotaresp[dns_quotatype_zone];
+			inc_stats(res, dns_resstatscounter_zonequota);
+			goto cleanup_nameservers;
+		}
 	}
 
 	log_ns_ttl(fctx, "fctx_create");