add mutex lock

birdtown-visit-counter.go

@@ -55,7 +55,9 @@ func (h *visitsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if r.Method == http.MethodGet {
+		h.store.Lock()
 		jsonBytes, err := json.Marshal(h.store.m)
+		h.store.Unlock()
 		if err != nil {
 			w.WriteHeader(http.StatusInternalServerError)
 			w.Write([]byte("cannot serialize"))