Vercel + Firebase AuthenticationでSafari対応しようとしたら4度くらい罠にハマった

import { readFileSync, writeFileSync } from "fs";

type HandleValue =
  | "rewrite"
  | "filesystem" // check matches after the filesystem misses
  | "resource"
  | "miss" // check matches after every filesystem miss
  | "hit"
  | "error"; //  check matches after error (500, 404, etc.)

type Handler = {
  handle: HandleValue;
  src?: string;
  dest?: string;
  status?: number;
};

type Source = {
  src: string;
  dest?: string;
  headers?: Record<string, string>;
  methods?: string[];
  continue?: boolean;
  caseSensitive?: boolean;
  check?: boolean;
  status?: number;
  middlewareRawSrc?: string[];
  middlewarePath?: string;
};

type Route = Source | Handler;

type Config = {
  version: 3;
  routes?: Route[];
};

const configJson = JSON.parse(
  readFileSync(".vercel/output/config.json", "utf8"),
) as Config;

const routes = configJson.routes ?? [];

const source: Source = {
  src: "/__/auth/(.*)",
  dest: "https://<projectId>.firebaseapp.com/__/auth/$1",
};

routes.unshift(source);

writeFileSync(
  ".vercel/output/config.json",
  JSON.stringify(configJson, null, 2),
);

const productionDomain =
  import.meta.env.VITE_PRODUCTION_DOMAIN ??
  import.meta.env.VITE_VERCEL_BRANCH_URL;

const prodFirebaseConfig = {
  apiKey: "...",
  authDomain: productionDomain,
  projectId: "...",
  storageBucket: "...",
  messagingSenderId: "...",
  appId: "...",
};

const devFirebaseConfig = {
  apiKey: "...",
  authDomain: "...",
  projectId: "...",
  storageBucket: "...",
  messagingSenderId: "...",
  appId: "...",
};

export const useGoogleAuth =
  import.meta.env.VITE_VERCEL_ENV === "production" ||
  import.meta.env.VITE_ENABLE_GOOGLE_AUTH === "true";

export const firebaseConfig = useGoogleAuth
  ? prodFirebaseConfig
  : devFirebaseConfig;

export const MyPhotoURL = "...";