fast resolver: omit if only one resolver is present

resolver/fast.go

@@ -16,7 +16,7 @@ type FastResolver struct {
 	upstreams []LookupNetIPer
 }
 
-func FastFromURLs(urls ...string) (*FastResolver, error) {
+func FastFromURLs(urls ...string) (LookupNetIPer, error) {
 	resolvers := make([]LookupNetIPer, 0, len(urls))
 	for i, u := range urls {
 		res, err := FromURL(u)
@@ -25,6 +25,9 @@ func FastFromURLs(urls ...string) (*FastResolver, error) {
 		}
 		resolvers = append(resolvers, res)
 	}
+	if len(resolvers) == 1 {
+		return resolvers[0], nil
+	}
 	return NewFastResolver(resolvers...), nil
 }