perf(nitro): respond with early hints in node-based environments (#7893)

packages/nuxt/src/core/runtime/nitro/renderer.ts

@@ -1,4 +1,4 @@
-import { createRenderer } from 'vue-bundle-renderer/runtime'
+import { createRenderer, renderResourceHeaders } from 'vue-bundle-renderer/runtime'
 import type { RenderResponse } from 'nitropack'
 import type { Manifest } from 'vite'
 import { appendHeader, getQuery } from 'h3'
@@ -100,7 +100,10 @@ const getSPARenderer = lazyCachedFunction(async () => {
     return Promise.resolve(result)
   }
 
-  return { renderToString }
+  return {
+    rendererContext: renderer.rendererContext,
+    renderToString
+  }
 })
 
 const PAYLOAD_CACHE = (process.env.NUXT_PAYLOAD_EXTRACTION && process.env.prerender) ? new Map() : null // TODO: Use LRU cache
@@ -150,6 +153,14 @@ export default defineRenderHandler(async (event) => {
 
   // Render app
   const renderer = (process.env.NUXT_NO_SSR || ssrContext.noSSR) ? await getSPARenderer() : await getSSRRenderer()
+
+  // Render 103 Early Hints
+  if (!isRenderingPayload && !process.env.prerender && event.res.socket) {
+    const { link } = renderResourceHeaders({}, renderer.rendererContext)
+    // TODO: use https://github.com/nodejs/node/pull/44180 when we drop support for node 16
+    event.res.socket!.write(`HTTP/1.1 103 Early Hints\r\nLink: ${link}\r\n\r\n`, 'utf-8')
+  }
+
   const _rendered = await renderer.renderToString(ssrContext).catch((err) => {
     if (!ssrError) {
       // Use explicitly thrown error in preference to subsequent rendering errors