[fix] Fix Apple login callbacks not redirecting back to the App for Expo

apps/nextjs/src/app/api/auth/[...nextauth]/route.ts

@@ -23,9 +23,46 @@ function rewriteRequestUrlInDevelopment(req: NextRequest) {
   return new NextRequest(newURL, req);
 }
 
-export const POST = async (_req: NextRequest) => {
+async function handleExpoSigninCallback(req: NextRequest, redirectURL: string) {
+  cookies().delete(EXPO_COOKIE_NAME);
+
+  // Run original handler, then extract the session token from the response
+  // Send it back via a query param in the Expo deep link. The Expo app
+  // will then get that and set it in the session storage.
+  const authResponse = await handlers.POST(req);
+  const setCookie = authResponse.headers
+    .getSetCookie()
+    .find((cookie) => AUTH_COOKIE_PATTERN.test(cookie));
+  const match = setCookie?.match(AUTH_COOKIE_PATTERN)?.[1];
+
+  if (!match)
+    throw new Error(
+      "Unable to find session cookie: " +
+        JSON.stringify(authResponse.headers.getSetCookie()),
+    );
+
+  const url = new URL(redirectURL);
+  url.searchParams.set("session_token", match);
+
+  return NextResponse.redirect(url);
+}
+
+export const POST = async (
+  _req: NextRequest,
+  props: { params: { nextauth: string[] } },
+) => {
   // First step must be to correct the request URL.
   const req = rewriteRequestUrlInDevelopment(_req);
+
+  const nextauthAction = props.params.nextauth[0];
+  const isExpoCallback = cookies().get(EXPO_COOKIE_NAME);
+
+  // callback handler required separately in the POST handler
+  // since Apple sends a POST request instead of a GET
+  if (nextauthAction === "callback" && !!isExpoCallback) {
+    return handleExpoSigninCallback(req, isExpoCallback.value);
+  }
+
   return handlers.POST(req);
 };
 
@@ -52,26 +89,7 @@ export const GET = async (
   }
 
   if (nextauthAction === "callback" && !!isExpoCallback) {
-    cookies().delete(EXPO_COOKIE_NAME);
-
-    // Run original handler, then extract the session token from the response
-    // Send it back via a query param in the Expo deep link. The Expo app
-    // will then get that and set it in the session storage.
-    const authResponse = await handlers.GET(req);
-    const setCookie = authResponse.headers
-      .getSetCookie()
-      .find((cookie) => AUTH_COOKIE_PATTERN.test(cookie));
-    const match = setCookie?.match(AUTH_COOKIE_PATTERN)?.[1];
-
-    if (!match)
-      throw new Error(
-        "Unable to find session cookie: " +
-          JSON.stringify(authResponse.headers.getSetCookie()),
-      );
-
-    const url = new URL(isExpoCallback.value);
-    url.searchParams.set("session_token", match);
-    return NextResponse.redirect(url);
+    return handleExpoSigninCallback(req, isExpoCallback.value);
   }
 
   // Every other request just calls the default handler