cmd/noxy: add CORS headers to all endpoints

the simplest approach at the moment: allow requests from all origins,
only GET and OPTIONS methods.

allowed headers are '*'. this prevents clients from providing
credentials via cookies which isn't used anyway. nuff cookies.

cmd/noxy/main.go

@@ -130,7 +130,7 @@ func main() {
 
 	log.Printf("listening on %s", *listenAddr)
 	log.Printf("known relays: %s", strings.Join(knownRelays, ", "))
-	http.ListenAndServe(*listenAddr, logHandler(mux))
+	http.ListenAndServe(*listenAddr, logHandler(cors(mux)))
 }
 
 // handles requests to /
@@ -198,6 +198,21 @@ func writeError(w http.ResponseWriter, err error) {
 	fmt.Fprint(w, err.Error())
 }
 
+func cors(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+		w.Header().Set("Access-Control-Allow-Headers", "*") // nb: wildcard prevents authentication
+		w.Header().Set("Access-Control-Allow-Methods", "GET, OPTIONS")
+		if r.Method == "OPTIONS" {
+			w.Header().Set("Access-Control-Max-Age", "2592000") // valid for 30 days
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+		w.Header().Set("Access-Control-Expose-Headers", "*");
+		h.ServeHTTP(w, r)
+	})
+}
+
 func logHandler(h http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		log.Printf("%s %s", r.Method, r.RequestURI)