core: export ClientWrite (#849)

Make ClientWrite available for middleware to use.

core/dnsserver/server.go

@@ -205,7 +205,7 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		if h, ok := s.zones[string(b[:l])]; ok {
 			if r.Question[0].Qtype != dns.TypeDS {
 				rcode, _ := h.middlewareChain.ServeDNS(ctx, w, r)
-				if rcodeNoClientWrite(rcode) {
+				if !middleware.ClientWrite(rcode) {
 					DefaultErrorFunc(w, r, rcode)
 				}
 				return
@@ -226,7 +226,7 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	if dshandler != nil {
 		// DS request, and we found a zone, use the handler for the query
 		rcode, _ := dshandler.middlewareChain.ServeDNS(ctx, w, r)
-		if rcodeNoClientWrite(rcode) {
+		if !middleware.ClientWrite(rcode) {
 			DefaultErrorFunc(w, r, rcode)
 		}
 		return
@@ -235,7 +235,7 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	// Wildcard match, if we have found nothing try the root zone as a last resort.
 	if h, ok := s.zones["."]; ok {
 		rcode, _ := h.middlewareChain.ServeDNS(ctx, w, r)
-		if rcodeNoClientWrite(rcode) {
+		if !middleware.ClientWrite(rcode) {
 			DefaultErrorFunc(w, r, rcode)
 		}
 		return
@@ -282,20 +282,6 @@ func DefaultErrorFunc(w dns.ResponseWriter, r *dns.Msg, rc int) {
 	w.WriteMsg(answer)
 }
 
-func rcodeNoClientWrite(rcode int) bool {
-	switch rcode {
-	case dns.RcodeServerFailure:
-		fallthrough
-	case dns.RcodeRefused:
-		fallthrough
-	case dns.RcodeFormatError:
-		fallthrough
-	case dns.RcodeNotImplemented:
-		return true
-	}
-	return false
-}
-
 const (
 	tcp = 0
 	udp = 1

middleware/middleware.go

@@ -82,5 +82,21 @@ func NextOrFailure(name string, next Handler, ctx context.Context, w dns.Respons
 	return dns.RcodeServerFailure, Error(name, errors.New("no next middleware found"))
 }
 
+// ClientWrite returns true if the response has been written to the client.
+// Each middleware to adhire to this protocol.
+func ClientWrite(rcode int) bool {
+	switch rcode {
+	case dns.RcodeServerFailure:
+		fallthrough
+	case dns.RcodeRefused:
+		fallthrough
+	case dns.RcodeFormatError:
+		fallthrough
+	case dns.RcodeNotImplemented:
+		return false
+	}
+	return true
+}
+
 // Namespace is the namespace used for the metrics.
 const Namespace = "coredns"