fix(cache): preserve legacy io probe parity

Author: james-elicx

packages/vinext/src/server/app-rsc-handler.ts

@@ -2012,7 +2012,10 @@ export function createAppRscHandler<TRoute extends AppRscHandlerRoute>(
             ? markUnverifiedInterceptionResponseUncacheable(response)
             : response;
         },
-        { route: () => new URL(request.url).pathname },
+        {
+          cacheComponents: options.createPprFallbackShells !== undefined,
+          route: () => new URL(request.url).pathname,
+        },
       ),
     );
     let response: Response;

packages/vinext/src/server/prerender-work-unit-setup.ts

@@ -17,9 +17,12 @@ import { NO_STORE_CACHE_CONTROL } from "./cache-control.js";
 
 export function runWithPrerenderWorkUnit(
   fn: () => Promise<Response>,
-  options?: { route?: string | (() => string) },
+  options?: { cacheComponents?: boolean; route?: string | (() => string) },
 ): Promise<Response> {
   if (process.env.VINEXT_PRERENDER === "1" || isRouteCacheabilityProbe()) {
+    if (options?.cacheComponents !== true) {
+      return workUnitAsyncStorage.run({ type: "prerender-legacy" }, fn);
+    }
     return runWithPrerenderWorkUnitOwner(fn, options);
   }
   return fn();

tests/e2e/ppr-impact-demo/cacheability-probe.spec.ts

@@ -142,6 +142,17 @@ test("classifies completed App Page renders inside workerd", async ({ request })
     state: "static-candidate",
     version: 1,
   });
+
+  // Ported from Next.js: packages/next/src/server/request/io.ts
+  // https://github.com/vercel/next.js/blob/canary/packages/next/src/server/request/io.ts
+  const legacyIoProbe = await request.get("/cacheability/explicit-io", { headers });
+  await expect(legacyIoProbe.json()).resolves.toMatchObject({
+    kind: "app-page",
+    pattern: "/cacheability/explicit-io",
+    state: "static-candidate",
+    status: 200,
+    version: 1,
+  });
 });
 
 test("rejects forged probes without exposing capability headers to user code", async ({

tests/fixtures/ppr-impact-demo/app/cacheability/explicit-io/page.tsx

@@ -0,0 +1,13 @@
+// `io()` is implemented by vinext for Next.js parity but is not declared by
+// the stable Next.js types installed in this fixture yet.
+// @ts-expect-error -- experimental Next.js Cache Components API
+import { io } from "next/cache";
+
+export const revalidate = 60;
+
+export default async function ExplicitIoPage() {
+  // Next.js resolves io() immediately in a prerender-legacy work unit. It only
+  // suspends and makes the route dynamic when Cache Components are enabled.
+  await io();
+  return <p id="cacheability-result">legacy io completed</p>;
+}