Basic version of resuming for top-level prerender

Implements a limited version of resuming that only applies to the root,
to account for the pathological case where a prerendered root must be
completely restarted before it can commit. This won't be necessary
once we implement resuming for real.

src/renderers/dom/shared/__tests__/ReactDOMRoot-test.js

@@ -78,4 +78,24 @@ describe('ReactDOMRoot', () => {
  work.commit();
  expect(container.textContent).toEqual('Hi');
  });
+
+ it("does not restart a blocked root that wasn't updated", () => {
+ let ops = [];
+ function Foo(props) {
+ ops.push('Foo');
+ return props.children;
+ }
+ const root = ReactDOM.createRoot(container);
+ const work = root.prerender(<Foo>Hi</Foo>);
+ expect(ops).toEqual(['Foo']);
+ // Hasn't updated yet
+ expect(container.textContent).toEqual('');
+
+ ops = [];
+
+ // Flush work. Shouldn't re-render Foo.
+ work.commit();
+ expect(ops).toEqual([]);
+ expect(container.textContent).toEqual('Hi');
+ });
 });

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -319,6 +319,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  }
 
  function updateHostRoot(current, workInProgress, renderExpirationTime) {
+ const root: FiberRoot = workInProgress.stateNode;
  pushHostRootContext(workInProgress);
  const updateQueue = workInProgress.updateQueue;
  if (updateQueue !== null) {
@@ -331,14 +332,23 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  null,
  renderExpirationTime,
  );
+ memoizeState(workInProgress, state);
+ if (root.completedAt === renderExpirationTime) {
+ // The root is already complete. Bail out and commit.
+ // TODO: This is a limited version of resuming that only applies to
+ // the root, to account for the pathological case where a completed
+ // root must be completely restarted before it can commit. Once we
+ // implement resuming for real, this special branch shouldn't
+ // be neccessary.
+ return null;
+ }
  if (prevState === state) {
  // If the state is the same as before, that's a bailout because we had
  // no work that expires at this time.
  resetHydrationState();
  return bailoutOnAlreadyFinishedWork(current, workInProgress);
  }
  const element = state.element;
- const root: FiberRoot = workInProgress.stateNode;
  if (
  (current === null || current.child === null) &&
  root.hydrate &&
@@ -370,7 +380,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  resetHydrationState();
  reconcileChildren(current, workInProgress, element);
  }
- memoizeState(workInProgress, state);
  return workInProgress.child;
  }
  resetHydrationState();

src/renderers/shared/fiber/ReactFiberRoot.js

@@ -26,6 +26,8 @@ export type FiberRoot = {
  // Determines if this root was blocked from committing.
  isBlocked: boolean,
  // The time at which this root completed.
+ // TODO: Remove once we add back resuming.
+ completedAt: ExpirationTime,
  forceExpire: ExpirationTime,
  // The work schedule is a linked list.
  nextScheduledRoot: FiberRoot | null,
@@ -48,6 +50,7 @@ exports.createFiberRoot = function(
  containerInfo: containerInfo,
  isScheduled: false,
  isBlocked: false,
+ completedAt: NoWork,
  forceExpire: NoWork,
  nextScheduledRoot: null,
  context: null,

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -314,10 +314,26 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  // unfortunately this is it.
  resetContextStack();
 
- nextUnitOfWork = createWorkInProgress(
- earliestExpirationRoot.current,
- earliestExpirationTime,
- );
+ if (earliestExpirationRoot.completedAt === nextRenderExpirationTime) {
+ // If the root is already complete, reuse the existing work-in-progress.
+ // TODO: This is a limited version of resuming that only applies to
+ // the root, to account for the pathological case where a completed
+ // root must be completely restarted before it can commit. Once we
+ // implement resuming for real, this special branch shouldn't
+ // be neccessary.
+ nextUnitOfWork = earliestExpirationRoot.current.alternate;
+ invariant(
+ nextUnitOfWork !== null,
+ 'Expected a completed root to have a work-in-progress. This error ' +
+ 'is likely caused by a bug in React. Please file an issue.',
+ );
+ } else {
+ nextUnitOfWork = createWorkInProgress(
+ earliestExpirationRoot.current,
+ earliestExpirationTime,
+ );
+ }
+
  if (earliestExpirationRoot !== nextRenderedTree) {
  // We've switched trees. Reset the nested update counter.
  nestedUpdateCount = 0;
@@ -462,6 +478,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  'related to the return field. This error is likely caused by a bug ' +
  'in React. Please file an issue.',
  );
+ root.completedAt = NoWork;
 
  if (nextRenderExpirationTime <= mostRecentCurrentTime) {
  // Keep track of the number of iterations to prevent an infinite
@@ -718,6 +735,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  } else {
  // We've reached the root.
  const root: FiberRoot = workInProgress.stateNode;
+ root.completedAt = nextRenderExpirationTime;
  if (nextCommitIsBlocked) {
  root.isBlocked = true;
  } else {