Move nonwriter to mw/pkg/nonwriter (#948)

Make it its own package as shared between autopath and federation.

Fixes #933

middleware/federation/federation.go

@@ -17,6 +17,7 @@ import (
 	"github.com/coredns/coredns/middleware"
 	"github.com/coredns/coredns/middleware/etcd/msg"
 	"github.com/coredns/coredns/middleware/pkg/dnsutil"
+	"github.com/coredns/coredns/middleware/pkg/nonwriter"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -67,7 +68,7 @@ func (f *Federation) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 
 	// Start the next middleware, but with a nowriter, capture the result, if NXDOMAIN
 	// perform federation, otherwise just write the result.
-	nw := NewNonWriter(w)
+	nw := nonwriter.New(w)
 	ret, err := middleware.NextOrFailure(f.Name(), f.Next, ctx, nw, r)
 
 	if !middleware.ClientWrite(ret) {

middleware/federation/nonwriter.go

@@ -1,22 +0,0 @@
-package federation
-
-import (
-	"github.com/miekg/dns"
-)
-
-// NonWriter is a type of ResponseWriter that captures the message, but never writes to the client.
-type NonWriter struct {
-	dns.ResponseWriter
-	Msg *dns.Msg
-}
-
-// NewNonWriter makes and returns a new NonWriter.
-func NewNonWriter(w dns.ResponseWriter) *NonWriter { return &NonWriter{ResponseWriter: w} }
-
-// WriteMsg records the message, but doesn't write it itself.
-func (r *NonWriter) WriteMsg(res *dns.Msg) error {
-	r.Msg = res
-	return nil
-}
-
-func (r *NonWriter) Write(buf []byte) (int, error) { return len(buf), nil }