loopd: add option to configure CORS origin

loopd/config.go

@@ -35,6 +35,7 @@ type config struct {
 	TLSPathSwapSrv string `long:"tlspathswapserver" description:"Path to swap server tls certificate. Only needed if the swap server uses a self-signed certificate."`
 	RPCListen      string `long:"rpclisten" description:"Address to listen on for gRPC clients"`
 	RESTListen     string `long:"restlisten" description:"Address to listen on for REST clients"`
+	CORSOrigin     string `long:"corsorigin" description:"The value to send in the Access-Control-Allow-Origin header. Header will be omitted if empty."`
 
 	LogDir         string `long:"logdir" description:"Directory to log output."`
 	MaxLogFiles    int    `long:"maxlogfiles" description:"Maximum logfiles to keep (0 for no rotation)"`

loopd/daemon.go

@@ -119,6 +119,10 @@ func daemon(config *config, lisCfg *listenerCfg) error {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	mux := proxy.NewServeMux(customMarshalerOption)
+	var restHandler http.Handler = mux
+	if config.CORSOrigin != "" {
+		restHandler = allowCORS(restHandler, config.CORSOrigin)
+	}
 	proxyOpts := []grpc.DialOption{
 		grpc.WithInsecure(),
 		grpc.WithDefaultCallOptions(maxMsgRecvSize),
@@ -141,7 +145,7 @@ func daemon(config *config, lisCfg *listenerCfg) error {
 		log.Infof("Starting REST proxy listener")
 
 		defer restListener.Close()
-		proxy := &http.Server{Handler: mux}
+		proxy := &http.Server{Handler: restHandler}
 
 		go func() {
 			err := proxy.Serve(restListener)
@@ -225,3 +229,12 @@ func daemon(config *config, lisCfg *listenerCfg) error {
 
 	return nil
 }
+
+// allowCORS wraps the given http.Handler with a function that adds the
+// Access-Control-Allow-Origin header to the response.
+func allowCORS(handler http.Handler, origin string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", origin)
+		handler.ServeHTTP(w, r)
+	})
+}