From 6d561d96bb107a019b9dc96ce4aa9e54fb3c2c5a Mon Sep 17 00:00:00 2001
From: Josh Story <story@hey.com>
Date: Tue, 20 Aug 2024 19:49:37 -0700
Subject: [PATCH] [Fizz] use microtasks rather than tasks when scheduling work
 while prerendering

Similar to https://github.com/facebook/react/pull/30768 we want to schedule work during prerendering in microtasks both for the root task and pings. We continue to schedule flushes as Tasks to allow as much work to be batched up as possible.
---
 packages/react-server/src/ReactFizzServer.js | 27 +++++++++++++++-----
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/packages/react-server/src/ReactFizzServer.js b/packages/react-server/src/ReactFizzServer.js
index daea492db5b8e..7ccbd65d16b74 100644
--- a/packages/react-server/src/ReactFizzServer.js
+++ b/packages/react-server/src/ReactFizzServer.js
@@ -38,6 +38,7 @@ import {describeObjectForErrorMessage} from 'shared/ReactSerializationErrors';
 
 import {
   scheduleWork,
+  scheduleMicrotask,
   beginWriting,
   writeChunk,
   writeChunkAndReturn,
@@ -669,7 +670,11 @@ function pingTask(request: Request, task: Task): void {
   pingedTasks.push(task);
   if (request.pingedTasks.length === 1) {
     request.flushScheduled = request.destination !== null;
-    scheduleWork(() => performWork(request));
+    if (request.trackedPostpones !== null) {
+      scheduleMicrotask(() => performWork(request));
+    } else {
+      scheduleWork(() => performWork(request));
+    }
   }
 }
 
@@ -4893,12 +4898,22 @@ function flushCompletedQueues(
 
 export function startWork(request: Request): void {
   request.flushScheduled = request.destination !== null;
-  if (supportsRequestStorage) {
-    scheduleWork(() => requestStorage.run(request, performWork, request));
+  if (request.trackedPostpones !== null) {
+    // When prerendering we use microtasks for pinging work
+    if (supportsRequestStorage) {
+      scheduleMicrotask(() =>
+        requestStorage.run(request, performWork, request),
+      );
+    } else {
+      scheduleMicrotask(() => performWork(request));
+    }
   } else {
-    scheduleWork(() => performWork(request));
-  }
-  if (request.trackedPostpones === null) {
+    // When rendering/resuming we use regular tasks and we also emit early preloads
+    if (supportsRequestStorage) {
+      scheduleWork(() => requestStorage.run(request, performWork, request));
+    } else {
+      scheduleWork(() => performWork(request));
+    }
     // this is either a regular render or a resume. For regular render we want
     // to call emitEarlyPreloads after the first performWork because we want
     // are responding to a live request and need to balance sending something early