use read locks wherever we can

main.go

@@ -112,12 +112,12 @@ func updateROMCache() {
 		}
 
 		// skip already cached files
-		romCache.Lock()
+		romCache.RLock()
 		if _, ok := romCache.Cached[v.Name()]; ok {
-			romCache.Unlock()
+			romCache.RUnlock()
 			continue
 		}
-		romCache.Unlock()
+		romCache.RUnlock()
 
 		wg.Add(1)
 		go func(v fs.DirEntry) {
@@ -188,9 +188,9 @@ func lineageOSROMListHandler(w http.ResponseWriter, r *http.Request) {
 		}
 	}()
 
-	romCache.Lock()
+	romCache.RLock()
 	httpResponseJSON := &HTTPResponseJSON{Response: romCache.ROMs}
-	romCache.Unlock()
+	romCache.RUnlock()
 
 	b, err := json.Marshal(httpResponseJSON)
 	if err != nil {