feat: add support for app url variables (#915)

* feat: add support for app url variables

* fix: test not working

* fix: format issue

packages/common/src/url.ts

@@ -1,3 +1,5 @@
+import type { ReadonlyHeaders } from "next/dist/server/web/spec-extension/adapters/headers";
+
 export const appendPath = (url: URL | string, path: string) => {
   const newUrl = new URL(url);
   newUrl.pathname = removeTrailingSlash(newUrl.pathname) + path;
@@ -7,3 +9,16 @@ export const appendPath = (url: URL | string, path: string) => {
 const removeTrailingSlash = (path: string) => {
   return path.at(-1) === "/" ? path.substring(0, path.length - 1) : path;
 };
+
+export const extractBaseUrlFromHeaders = (headers: ReadonlyHeaders): `${string}://${string}` => {
+  let protocol = headers.get("x-forwarded-proto") ?? "http";
+
+  // @see https://support.glitch.com/t/x-forwarded-proto-contains-multiple-protocols/17219
+  if (protocol.includes(",")) {
+    protocol = protocol.includes("https") ? "https" : "http";
+  }
+
+  const host = headers.get("x-forwarded-host") ?? headers.get("host");
+
+  return `${protocol}://${host}`;
+};