[Flight] Properly close stream when no chunks need to be written after prerender (facebook#33982)

There is an edge case when prerendering where if you have nothing to
write you can end up in a state where the prerender is in status closed
before you can provide a destination. In this case the destination is
never closed becuase it assumes it already would have been.

This condition can happen now because of the introduction of the deubg
stream. Before this a request would never entere closed status if there
was no active destination. When a destination was added it would perform
a flush and possibly close the stream. Now, it is possible to flush
without a destination because you might have debug chunks to stream and
you can end up closing the stream independent of an active destination.

There are a number of ways we can solve this but the one that seems to
adhere best to the original design is to only set the status to CLOSED
when a destination is active. This means that if you don't have an
active destination when the pendingChunks count hits zero it will not
enter CLOSED status until you startFlowing.

Author: gnoff

packages/react-server-dom-webpack/src/__tests__/ReactFlightDOMNode-test.js

@@ -863,4 +863,46 @@ describe('ReactFlightDOMNode', () => {
       expect(ownerStack).toBeNull();
     }
   });
+
+  // @gate experimental
+  // @gate enableHalt
+  it('can handle an empty prelude when prerendering', async () => {
+    function App() {
+      return null;
+    }
+
+    const serverAbortController = new AbortController();
+    serverAbortController.abort();
+    const errors = [];
+    const {pendingResult} = await serverAct(async () => {
+      // destructure trick to avoid the act scope from awaiting the returned value
+      return {
+        pendingResult: ReactServerDOMStaticServer.unstable_prerender(
+          ReactServer.createElement(App, null),
+          webpackMap,
+          {
+            signal: serverAbortController.signal,
+            onError(error) {
+              errors.push(error);
+            },
+          },
+        ),
+      };
+    });
+
+    expect(errors).toEqual([]);
+
+    const {prelude} = await pendingResult;
+
+    const reader = prelude.getReader();
+    while (true) {
+      const {done} = await reader.read();
+      if (done) {
+        break;
+      }
+    }
+
+    // We don't really have an assertion other than to make sure
+    // the stream doesn't hang.
+  });
 });

packages/react-server/src/ReactFlightServer.js

@@ -5762,6 +5762,7 @@ function flushCompletedChunks(request: Request): void {
           // TODO: If this destination is not currently flowing we'll not close it when it resumes flowing.
           // We should keep a separate status for this.
           if (request.destination !== null) {
+            request.status = CLOSED;
             close(request.destination);
             request.destination = null;
           }
@@ -5779,8 +5780,8 @@ function flushCompletedChunks(request: Request): void {
       );
       request.cacheController.abort(abortReason);
     }
-    request.status = CLOSED;
     if (request.destination !== null) {
+      request.status = CLOSED;
       close(request.destination);
       request.destination = null;
     }