fix(cache): expose build phase during staged discovery

Author: james-elicx

packages/vinext/src/server/server-globals.ts

@@ -62,7 +62,10 @@ export function installServerGlobals(): void {
       get() {
         const context = getRequestExecutionContext();
         const state = context ? Reflect.get(context, CACHEABILITY_REQUEST_STATE) : undefined;
-        if (Reflect.get(state ?? {}, "mode") === "probe") {
+        if (
+          context?.isPrerenderPathDiscovery === true ||
+          Reflect.get(state ?? {}, "mode") === "probe"
+        ) {
           return PHASE_PRODUCTION_BUILD;
         }
         return Reflect.get(process.env, "NEXT_PHASE");

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

@@ -14,6 +14,15 @@ function prerenderSecret(): string {
 test("classifies completed App Page renders inside workerd", async ({ request }) => {
   const headers = { [probeHeader]: "1", [secretHeader]: prerenderSecret() };
 
+  // Next.js evaluates generateStaticParams during phase-production-build:
+  // https://github.com/vercel/next.js/blob/canary/packages/next/src/build/index.ts
+  const discovery = await request.get(
+    "/__vinext/prerender/static-params?pattern=%2Fcacheability%2Fprerender-phase%2F%3Aslug",
+    { headers: { [secretHeader]: prerenderSecret() } },
+  );
+  expect(discovery.ok()).toBe(true);
+  await expect(discovery.json()).resolves.toEqual([{ slug: "known" }]);
+
   const staticProbe = await request.get("/cacheability/static", { headers });
   expect(staticProbe.ok()).toBe(true);
   await expect(staticProbe.json()).resolves.toMatchObject({

tests/fixtures/ppr-impact-demo/app/cacheability/prerender-phase/[slug]/page.tsx

@@ -3,7 +3,7 @@ import { headers } from "next/headers";
 export const revalidate = 3;
 
 export function generateStaticParams() {
-  return [{ slug: "known" }];
+  return process.env.NEXT_PHASE === "phase-production-build" ? [{ slug: "known" }] : [];
 }
 
 export default async function PrerenderPhasePage({