Repository: facebook/react
Pull request: [Fizz] hoistables should never flush before the preamble

Hoistables should never flush before the preamble however there is a surprisingly easy way to trigger this to happen by suspending in the shell of the app. This change modifies the flushing behavior to not emit any hoistables before the preamble has written. It accomplishes this by aborting the flush early if there are any pending root tasks remaining. It's unfortunate we need this extra condition but it's essential that we don't emit anything before the preamble and at the moment I don't see a way to do that without introducing a new condition.

There is a test that began to fail with this update. It turns out that in node the root can be blocked during a resume even for a component inside a Suspense boundary if that boundary was part of the prerender. This means that with the current heuristic in this PR boundaries cannot be flushed during resume until the root is unblocked. This is not ideal but this is already how Edge works because the root blocks the stream in that case. This just makes Node deopt in a similar way to edge. We should improve this but we ought to do so in a way that works for edge too and it needs to be more comprehensive.

Changed files:
filename: packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js
patch: @@ -6926,7 +6926,14 @@ describe('ReactDOMFizzServer', () => {
     expect(getVisibleChildren(container)).toEqual(
       <div>
         {'Loading1'}
-        {'Hello'}
+        {/*
+          This used to show "Hello" in this slot because the boundary was able to be flushed
+          early but we now prevent flushing while pendingRootTasks is not zero. This is how Edge
+          would work anyway because you don't get the stream until the root is unblocked on a resume
+          so Node now aligns with edge bevavior
+          {'Hello'}
+        */}
+        {'Loading2'}
         {'Loading3'}
       </div>,
     );

filename: packages/react-dom/src/__tests__/ReactDOMFloat-test.js
patch: @@ -5034,6 +5034,51 @@ body {
     );
   });
 
+  it('should never flush hoistables before the preamble', async () => {
+    let resolve;
+    const promise = new Promise(res => {
+      resolve = res;
+    });
+
+    function App() {
+      ReactDOM.preinit('foo', {as: 'script'});
+      React.use(promise);
+      return (
+        <html>
+          <body>hello</body>
+        </html>
+      );
+    }
+
+    await act(() => {
+      renderToPipeableStream(<App />).pipe(writable);
+    });
+
+    // we assert the default JSDOM still in tact
+    expect(getMeaningfulChildren(document)).toEqual(
+      <html>
+        <head />
+        <body>
+          <div id="container" />
+        </body>
+      </html>,
+    );
+
+    await act(() => {
+      resolve();
+    });
+
+    // we assert the DOM was replaced entirely because we streamed an opening html tag
+    expect(getMeaningfulChildren(document)).toEqual(
+      <html>
+        <head>
+          <script async="" src="foo" />
+        </head>
+        <body>hello</body>
+      </html>,
+    );
+  });
+
   describe('ReactDOM.prefetchDNS(href)', () => {
     it('creates a dns-prefetch resource when called', async () => {
       function App({url}) {

filename: packages/react-server/src/ReactFizzServer.js
patch: @@ -4057,22 +4057,25 @@ function flushCompletedQueues(
     // that item fully and then yield. At that point we remove the already completed
     // items up until the point we completed them.
 
+    if (request.pendingRootTasks > 0) {
+      // When there are pending root tasks we don't want to flush anything
+      return;
+    }
+
     let i;
     const completedRootSegment = request.completedRootSegment;
     if (completedRootSegment !== null) {
       if (completedRootSegment.status === POSTPONED) {
         // We postponed the root, so we write nothing.
         return;
-      } else if (request.pendingRootTasks === 0) {
-        flushPreamble(request, destination, completedRootSegment);
-        flushSegment(request, destination, completedRootSegment, null);
-        request.completedRootSegment = null;
-        writeCompletedRoot(destination, request.renderState);
-      } else {
-        // We haven't flushed the root yet so we don't need to check any other branches further down
-        return;
       }
+
+      flushPreamble(request, destination, completedRootSegment);
+      flushSegment(request, destination, completedRootSegment, null);
+      request.completedRootSegment = null;
+      writeCompletedRoot(destination, request.renderState);
     }
+
     writeHoistables(destination, request.resumableState, request.renderState);
     // We emit client rendering instructions for already emitted boundaries first.
     // This is so that we can signal to the client to start client rendering them as