Exempt prefetches from the fetches-per-server quota

Give prefetches a free pass through the quota so that the cache
entries for popular zones could be updated successfully even if the
quota for is already reached.

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.
 	 */