[Flight] Unify plain Server Component and forwardRef under one function (facebook#28261)

This used to be trivial but it's no longer trivial.

In Fizz and Fiber this is split into renderWithHooks and
finishFunctionComponent since they also support indeterminate
components.

Interestingly thanks to this unification we always call functions with
an arity of 2 which is a bit weird - with the second argument being
undefined in everything except forwardRef and legacy context consumers.

This makes Flight makes the same thing but we could also call it with an
arity of 1.

Since Flight errors early if you try to pass it a ref, and there's no
legacy context, the second arg is always undefined.

The practical change in this PR is that returning a Promise from a
forwardRef now turns it into a lazy. We previously didn't support async
forwardRef since it wasn't supported on the client. However, since
eventually this will be supported by child-as-a-promise it seems fine to
support it.

Author: sebmarkbage

packages/react-server/src/ReactFlightServer.js

@@ -478,6 +478,58 @@ function createLazyWrapperAroundWakeable(wakeable: Wakeable) {
   return lazyType;
 }
 
+function renderFunctionComponent<Props>(
+  request: Request,
+  task: Task,
+  key: null | string,
+  Component: (p: Props, arg: void) => any,
+  props: Props,
+): ReactJSONValue {
+  // Reset the task's thenable state before continuing, so that if a later
+  // component suspends we can reuse the same task object. If the same
+  // component suspends again, the thenable state will be restored.
+  const prevThenableState = task.thenableState;
+  task.thenableState = null;
+
+  prepareToUseHooksForComponent(prevThenableState);
+  // The secondArg is always undefined in Server Components since refs error early.
+  const secondArg = undefined;
+  let result = Component(props, secondArg);
+  if (
+    typeof result === 'object' &&
+    result !== null &&
+    typeof result.then === 'function'
+  ) {
+    // When the return value is in children position we can resolve it immediately,
+    // to its value without a wrapper if it's synchronously available.
+    const thenable: Thenable<any> = result;
+    if (thenable.status === 'fulfilled') {
+      return thenable.value;
+    }
+    // TODO: Once we accept Promises as children on the client, we can just return
+    // the thenable here.
+    result = createLazyWrapperAroundWakeable(result);
+  }
+  // Track this element's key on the Server Component on the keyPath context..
+  const prevKeyPath = task.keyPath;
+  const prevImplicitSlot = task.implicitSlot;
+  if (key !== null) {
+    // Append the key to the path. Technically a null key should really add the child
+    // index. We don't do that to hold the payload small and implementation simple.
+    task.keyPath = prevKeyPath === null ? key : prevKeyPath + ',' + key;
+  } else if (prevKeyPath === null) {
+    // This sequence of Server Components has no keys. This means that it was rendered
+    // in a slot that needs to assign an implicit key. Even if children below have
+    // explicit keys, they should not be used for the outer most key since it might
+    // collide with other slots in that set.
+    task.implicitSlot = true;
+  }
+  const json = renderModelDestructive(request, task, emptyRoot, '', result);
+  task.keyPath = prevKeyPath;
+  task.implicitSlot = prevImplicitSlot;
+  return json;
+}
+
 function renderFragment(
   request: Request,
   task: Task,
@@ -581,48 +633,7 @@ function renderElement(
       return renderClientElement(task, type, key, props);
     }
     // This is a server-side component.
-
-    // Reset the task's thenable state before continuing, so that if a later
-    // component suspends we can reuse the same task object. If the same
-    // component suspends again, the thenable state will be restored.
-    const prevThenableState = task.thenableState;
-    task.thenableState = null;
-
-    prepareToUseHooksForComponent(prevThenableState);
-    let result = type(props);
-    if (
-      typeof result === 'object' &&
-      result !== null &&
-      typeof result.then === 'function'
-    ) {
-      // When the return value is in children position we can resolve it immediately,
-      // to its value without a wrapper if it's synchronously available.
-      const thenable: Thenable<any> = result;
-      if (thenable.status === 'fulfilled') {
-        return thenable.value;
-      }
-      // TODO: Once we accept Promises as children on the client, we can just return
-      // the thenable here.
-      result = createLazyWrapperAroundWakeable(result);
-    }
-    // Track this element's key on the Server Component on the keyPath context..
-    const prevKeyPath = task.keyPath;
-    const prevImplicitSlot = task.implicitSlot;
-    if (key !== null) {
-      // Append the key to the path. Technically a null key should really add the child
-      // index. We don't do that to hold the payload small and implementation simple.
-      task.keyPath = prevKeyPath === null ? key : prevKeyPath + ',' + key;
-    } else if (prevKeyPath === null) {
-      // This sequence of Server Components has no keys. This means that it was rendered
-      // in a slot that needs to assign an implicit key. Even if children below have
-      // explicit keys, they should not be used for the outer most key since it might
-      // collide with other slots in that set.
-      task.implicitSlot = true;
-    }
-    const json = renderModelDestructive(request, task, emptyRoot, '', result);
-    task.keyPath = prevKeyPath;
-    task.implicitSlot = prevImplicitSlot;
-    return json;
+    return renderFunctionComponent(request, task, key, type, props);
   } else if (typeof type === 'string') {
     // This is a host element. E.g. HTML.
     return renderClientElement(task, type, key, props);
@@ -660,39 +671,7 @@ function renderElement(
         return renderElement(request, task, wrappedType, key, ref, props);
       }
       case REACT_FORWARD_REF_TYPE: {
-        const render = type.render;
-
-        // Reset the task's thenable state before continuing, so that if a later
-        // component suspends we can reuse the same task object. If the same
-        // component suspends again, the thenable state will be restored.
-        const prevThenableState = task.thenableState;
-        task.thenableState = null;
-
-        prepareToUseHooksForComponent(prevThenableState);
-        const result = render(props, undefined);
-        const prevKeyPath = task.keyPath;
-        const prevImplicitSlot = task.implicitSlot;
-        if (key !== null) {
-          // Append the key to the path. Technically a null key should really add the child
-          // index. We don't do that to hold the payload small and implementation simple.
-          task.keyPath = prevKeyPath === null ? key : prevKeyPath + ',' + key;
-        } else if (prevKeyPath === null) {
-          // This sequence of Server Components has no keys. This means that it was rendered
-          // in a slot that needs to assign an implicit key. Even if children below have
-          // explicit keys, they should not be used for the outer most key since it might
-          // collide with other slots in that set.
-          task.implicitSlot = true;
-        }
-        const json = renderModelDestructive(
-          request,
-          task,
-          emptyRoot,
-          '',
-          result,
-        );
-        task.keyPath = prevKeyPath;
-        task.implicitSlot = prevImplicitSlot;
-        return json;
+        return renderFunctionComponent(request, task, key, type.render, props);
       }
       case REACT_MEMO_TYPE: {
         return renderElement(request, task, type.type, key, ref, props);