Check some fs operation to async

backend/util-server.ts

@@ -5,6 +5,7 @@ import { log } from "./log";
 import { ERROR_TYPE_VALIDATION } from "./util-common";
 import { R } from "redbean-node";
 import { verifyPassword } from "./password-hash";
+import fs from "fs";
 
 export interface JWTDecoded {
     username : string;
@@ -82,3 +83,9 @@ export async function doubleCheckPassword(socket : DockgeSocket, currentPassword
 
     return user;
 }
+
+export function fileExists(file : string) {
+    return fs.promises.access(file, fs.constants.F_OK)
+        .then(() => true)
+        .catch(() => false);
+}