Explicitly panic after (*response).Close (#769)

* Explicitly panic after (*response).Close

* Prefix panics with package name

* Harden TestResponseAfterClose by comparing panic message

server.go

@@ -753,24 +753,33 @@ func (w *response) Write(m []byte) (int, error) {
 
 		n, err := io.Copy(w.tcp, bytes.NewReader(m))
 		return int(n), err
+	default:
+		panic("dns: Write called after Close")
 	}
-	panic("not reached")
 }
 
 // LocalAddr implements the ResponseWriter.LocalAddr method.
 func (w *response) LocalAddr() net.Addr {
-	if w.tcp != nil {
+	switch {
+	case w.udp != nil:
+		return w.udp.LocalAddr()
+	case w.tcp != nil:
 		return w.tcp.LocalAddr()
+	default:
+		panic("dns: LocalAddr called after Close")
 	}
-	return w.udp.LocalAddr()
 }
 
 // RemoteAddr implements the ResponseWriter.RemoteAddr method.
 func (w *response) RemoteAddr() net.Addr {
-	if w.tcp != nil {
+	switch {
+	case w.udpSession != nil:
+		return w.udpSession.RemoteAddr()
+	case w.tcp != nil:
 		return w.tcp.RemoteAddr()
+	default:
+		panic("dns: RemoteAddr called after Close")
 	}
-	return w.udpSession.RemoteAddr()
 }
 
 // TsigStatus implements the ResponseWriter.TsigStatus method.

server_test.go

@@ -990,6 +990,35 @@ func TestServerRoundtripTsig(t *testing.T) {
 	}
 }
 
+func TestResponseAfterClose(t *testing.T) {
+	testPanic := func(name string, fn func()) {
+		defer func() {
+			expect := fmt.Sprintf("dns: %s called after Close", name)
+			if err := recover(); err == nil {
+				t.Errorf("expected panic from %s after Close", name)
+			} else if err != expect {
+				t.Errorf("expected explicit panic from %s after Close, expected %q, got %q", name, expect, err)
+			}
+		}()
+		fn()
+	}
+
+	rw := &response{
+		tcp:        nil, // Close sets tcp to nil
+		udp:        nil,
+		udpSession: nil,
+	}
+	testPanic("Write", func() {
+		rw.Write(make([]byte, 2))
+	})
+	testPanic("LocalAddr", func() {
+		rw.LocalAddr()
+	})
+	testPanic("RemoteAddr", func() {
+		rw.RemoteAddr()
+	})
+}
+
 type ExampleFrameLengthWriter struct {
 	Writer
 }