fix: add Vary header to cache interceptor (#916)

* fix: add Vary header to cache interceptor

* review

* more comment

* more comment

* Update packages/open-next/src/core/routing/cacheInterceptor.ts

Co-authored-by: khuezy <[email protected]>

* next pr

* add Next-Url header

---------

Co-authored-by: khuezy <[email protected]>

Author: sommeeeer

.changeset/large-ligers-decide.md

@@ -0,0 +1,5 @@
+---
+"@opennextjs/aws": patch
+---
+
+fix: add vary header to cache interceptor

packages/open-next/src/core/routing/cacheInterceptor.ts

@@ -13,6 +13,18 @@ import { generateMessageGroupId } from "./queue";
 const CACHE_ONE_YEAR = 60 * 60 * 24 * 365;
 const CACHE_ONE_MONTH = 60 * 60 * 24 * 30;
 
+/*
+ * We use this header to prevent Firefox (and possibly some CDNs) from incorrectly reusing the RSC responses during caching.
+ * This can especially happen when there's a redirect in the middleware as the `_rsc` query parameter is not visible there.
+ * So it will get dropped during the redirect, which results in the RSC response being cached instead of the actual HTML on the path `/`.
+ * This value can be found in the routes manifest, under `rsc.varyHeader`.
+ * They recompute it here in Next:
+ * https://github.com/vercel/next.js/blob/c5bf5bb4c8b01b1befbbfa7ad97a97476ee9d0d7/packages/next/src/server/base-server.ts#L2011
+ * Also see this PR: https://github.com/vercel/next.js/pull/79426
+ */
+const VARY_HEADER =
+  "RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Router-Segment-Prefetch, Next-Url";
+
 async function computeCacheControl(
   path: string,
   body: string,
@@ -127,6 +139,7 @@ async function generateResult(
       ...cacheControl,
       "content-type": type,
       ...cachedValue.meta?.headers,
+      vary: VARY_HEADER,
     },
   };
 }