[Fizz] use microtasks rather than tasks when scheduling work while pr…

…erendering (#30770)

Similar to #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

@@ -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