fix(pages-router): await Promise-shaped gssp props

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

@@ -530,7 +530,14 @@ export function createSSRHandler(
             return;
           }
           if (result && "props" in result) {
-            pageProps = result.props;
+            // Next.js explicitly supports a Promise value for `props`. Await
+            // it before serialising; otherwise pageProps would be a Promise
+            // and the rendered page would receive empty props. See
+            // packages/next/src/server/render.tsx (deferredContent).
+            pageProps =
+              result.props && typeof (result.props as { then?: unknown }).then === "function"
+                ? await (result.props as Promise<Record<string, unknown>>)
+                : result.props;
           }
           if (result && "redirect" in result) {
             const { redirect } = result;

packages/vinext/src/server/pages-page-data.ts

@@ -388,7 +388,15 @@ export async function resolvePagesPageData(
     }
 
     if (result?.props) {
-      pageProps = result.props;
+      // Next.js explicitly supports a Promise value for `props`. Await it
+      // before serialising; otherwise pageProps would be a Promise and the
+      // rendered page would receive empty props. See
+      // packages/next/src/server/render.tsx (deferredContent).
+      const rawProps = result.props as unknown;
+      pageProps =
+        rawProps && typeof (rawProps as { then?: unknown }).then === "function"
+          ? await (rawProps as Promise<Record<string, unknown>>)
+          : (rawProps as Record<string, unknown>);
     }
 
     if (result?.redirect) {

tests/fixtures/pages-basic/pages/ssr-promise-props.tsx

@@ -0,0 +1,22 @@
+interface SSRPromisePropsPageProps {
+  hello: string;
+  count: number;
+}
+
+export default function SSRPromisePropsPage({ hello, count }: SSRPromisePropsPageProps) {
+  return (
+    <div>
+      <h1>SSR Promise Props</h1>
+      <p data-testid="hello">{hello}</p>
+      <p data-testid="count">count: {count}</p>
+    </div>
+  );
+}
+
+export function getServerSideProps() {
+  // Next.js explicitly supports a Promise value for `props`. vinext must
+  // await it before serialising — otherwise pageProps is empty.
+  return {
+    props: Promise.resolve({ hello: "world", count: 42 }),
+  };
+}

tests/pages-router.test.ts

@@ -415,6 +415,24 @@ describe("Pages Router integration", () => {
     expect(html).toContain("Rendered at:");
   });
 
+  // Regression test for #1459: Next.js explicitly supports a Promise value
+  // for `getServerSideProps` `props`. vinext must `await` the value before
+  // serialising — otherwise pageProps end up as a Promise and the rendered
+  // page shows empty values.
+  it("awaits Promise-shaped getServerSideProps props", async () => {
+    const res = await fetch(`${baseUrl}/ssr-promise-props`);
+    expect(res.status).toBe(200);
+
+    const html = await res.text();
+    expect(html).toContain("SSR Promise Props");
+    expect(html).toContain("world");
+    // React SSR inserts a `<!-- -->` comment between text and expressions.
+    expect(html).toMatch(/count:\s*(<!--\s*-->)?\s*42/);
+    // The serialized __NEXT_DATA__ payload must contain the resolved values
+    // (not an empty pageProps object).
+    expect(html).toMatch(/"pageProps":\s*\{[^}]*"hello":\s*"world"/);
+  });
+
   // Regression test for #1354: when a page declares `getServerSideProps` as
   // a local `const` and exports it via `export { getServerSideProps }`, the
   // client-bundle transform must strip the export specifier without
@@ -3692,6 +3710,20 @@ describe("Production server middleware (Pages Router)", () => {
     expect(await res.json()).toEqual({ ok: true, source: "gssp-res-end" });
   });
 
+  // Regression test for #1459: Next.js supports a Promise value for `props`
+  // returned from getServerSideProps. The prod worker entry must await it
+  // before serialising into __NEXT_DATA__ / pageProps.
+  it("awaits Promise-shaped getServerSideProps props in production", async () => {
+    const res = await fetch(`${prodUrl}/ssr-promise-props`);
+    expect(res.status).toBe(200);
+    const html = await res.text();
+    expect(html).toContain("SSR Promise Props");
+    expect(html).toContain("world");
+    // React SSR inserts a `<!-- -->` comment between text and expressions.
+    expect(html).toMatch(/count:\s*(<!--\s*-->)?\s*42/);
+    expect(html).toMatch(/"pageProps":\s*\{[^}]*"hello":\s*"world"/);
+  });
+
   it("returns 400 for malformed percent-encoded path (not crash)", async () => {
     const res = await fetch(`${prodUrl}/%E0%A4%A`);
     expect(res.status).toBe(400);