[Flight] Refactor the Render Loop to Behave More Like Fizz (facebook#…

…28065)

This refactors the Flight render loop to behave more like Fizz with
similar naming conventions. So it's easier to apply similar techniques
across both. This is not necessarily better/faster - at least not yet.

This doesn't yet implement serialization by writing segments to chunks
but we probably should do that since the built-in parts that
`JSON.stringify` gets us isn't really much anymore (except serializing
strings). When we switch to that it probably makes sense for the whole
thing to be recursive.

Right now it's not technically fully recursive because each recursive
render returns the next JSON value to encode. So it's kind of like a
trampoline. This means we can't have many contextual things on the
stack. It needs to use the Server Context `__POP` trick. However, it
does work for things that are contextual only for one sequence of server
component abstractions in a row. Since those are now recursive.

An interesting observation here is that `renderModel` means that
anything can suspend while still serializing the outer siblings.
Typically only Lazy or Components would suspend but in principle a Proxy
can suspend/postpone too and now that is left serialized by reference to
a future value. It's only if the thing that we rendered was something
that can reduce to Lazy e.g. an Element that we can serialize it as a
lazy.

Similarly to how Suspense boundaries in Fizz can catch errors, anything
that can be reduced to Lazy can also catch an error rather than bubbling
it. It only errors when the Lazy resolves. Unlike Suspense boundaries
though, those things don't render anything so they're otherwise going to
use the destructive form. To ensure that throwing in an Element can
reuse the current task, this must be handled by `renderModel`, not for
example `renderElement`.

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

@@ -242,6 +242,20 @@ describe('ReactFlightDOMEdge', () => {
     expect(result).toEqual(resolvedChildren);
   });
 
+  it('should execute repeated server components in a compact form', async () => {
+    async function ServerComponent({recurse}) {
+      if (recurse > 0) {
+        return <ServerComponent recurse={recurse - 1} />;
+      }
+      return <div>Fin</div>;
+    }
+    const stream = ReactServerDOMServer.renderToReadableStream(
+      <ServerComponent recurse={20} />,
+    );
+    const serializedContent = await readResult(stream);
+    expect(serializedContent.length).toBeLessThan(150);
+  });
+
   // @gate enableBinaryFlight
   it('should be able to serialize any kind of typed array', async () => {
     const buffer = new Uint8Array([

packages/react-server/src/ReactFizzServer.js

@@ -2201,8 +2201,12 @@ function renderNodeDestructive(
   task.node = node;
   task.childIndex = childIndex;
 
+  if (node === null) {
+    return;
+  }
+
   // Handle object types
-  if (typeof node === 'object' && node !== null) {
+  if (typeof node === 'object') {
     switch ((node: any).$$typeof) {
       case REACT_ELEMENT_TYPE: {
         const element: any = node;