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Allow SSR to finish microtasky work before flushing #86311

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Merged
lubieowoce merged 2 commits into from
Nov 20, 2025
Merged

Allow SSR to finish microtasky work before flushing #86311

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4350ec8
let fizz flush microtasks before flowing
lubieowoce Nov 19, 2025
4c84937
test: update test that expected tag to be in body
lubieowoce Nov 19, 2025
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12 changes: 10 additions & 2 deletions packages/next/src/server/stream-utils/node-web-streams-helper.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,11 @@ import type { ReactDOMServerReadableStream } from 'react-dom/server'
import { getTracer } from '../lib/trace/tracer'
import { AppRenderSpan } from '../lib/trace/constants'
import { DetachedPromise } from '../../lib/detached-promise'
import { scheduleImmediate, atLeastOneTask } from '../../lib/scheduler'
import {
scheduleImmediate,
atLeastOneTask,
waitAtLeastOneReactRenderTask,
} from '../../lib/scheduler'
import { ENCODED_TAGS } from './encoded-tags'
import {
indexOfUint8Array,
Expand Down Expand Up @@ -797,9 +801,13 @@ export async function continueFizzStream(
// Suffix itself might contain close tags at the end, so we need to split it.
const suffixUnclosed = suffix ? suffix.split(CLOSE_TAG, 1)[0] : null

// If we're generating static HTML we need to wait for it to resolve before continuing.
if (isStaticGeneration) {
// If we're generating static HTML we need to wait for it to resolve before continuing.
await renderStream.allReady
} else {
// Otherwise, we want to make sure Fizz is done with all microtasky work
// before we start pulling the stream and cause a flush.
await waitAtLeastOneReactRenderTask()
}

return chainTransformers(renderStream, [
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3 changes: 2 additions & 1 deletion test/e2e/app-dir/metadata-icons/app/custom-icon/delay-icons/page.tsx
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Expand Up @@ -18,8 +18,9 @@ export default async function Page() {

export async function generateMetadata() {
await connection()
// Delay until after the shell is flushed so that the tags end up in the body.
await new Promise((resolve) => setTimeout(resolve, 10))
return {
// This long text description will lead to the metadata being inserted after the head tag.
description: 'long text description'.repeat(1000),
icons: {
icon: `/heart.png`,
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