Clear finished discrete updates during commit phase (facebook#18515)

* Reproduce a bug where `flushDiscreteUpdates` causes fallback never to be committed

* Ping suspended level when canceling its timer

Make sure the suspended level is marked as pinged so that we return back
to it later, in case the render we're about to start gets aborted.
Generally we only reach this path via a ping, but we shouldn't assume
that will always be the case.

* Clear finished discrete updates during commit phase

If a root is finished at a priority lower than that of the latest pending discrete
updates on it, these updates must have been finished so we can clear them now.
Otherwise, a later call of `flushDiscreteUpdates` would start a new empty render
pass which may cause a scheduled timeout to be cancelled.

* Add TODO

Happened to find this while writing a test. A JSX element comparison
failed because one of them elements had a functional component as an
owner, which should ever happen.

I'll add a regression test later.

Co-authored-by: Andrew Clark <git@andrewclark.io>

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -1168,6 +1168,19 @@ function prepareFreshStack(root, expirationTime) {
     cancelTimeout(timeoutHandle);
   }
 
+  // Check if there's a suspended level at lower priority.
+  const lastSuspendedTime = root.lastSuspendedTime;
+  if (lastSuspendedTime !== NoWork && lastSuspendedTime < expirationTime) {
+    const lastPingedTime = root.lastPingedTime;
+    // Make sure the suspended level is marked as pinged so that we return back
+    // to it later, in case the render we're about to start gets aborted.
+    // Generally we only reach this path via a ping, but we shouldn't assume
+    // that will always be the case.
+    if (lastPingedTime === NoWork || lastPingedTime > lastSuspendedTime) {
+      root.lastPingedTime = lastSuspendedTime;
+    }
+  }
+
   if (workInProgress !== null) {
     let interruptedWork = workInProgress.return;
     while (interruptedWork !== null) {
@@ -1202,6 +1215,9 @@ function handleError(root, thrownValue) {
       resetContextDependencies();
       resetHooksAfterThrow();
       resetCurrentDebugFiberInDEV();
+      // TODO: I found and added this missing line while investigating a
+      // separate issue. Write a regression test using string refs.
+      ReactCurrentOwner.current = null;
 
       if (workInProgress === null || workInProgress.return === null) {
         // Expected to be working on a non-root fiber. This is a fatal error
@@ -1769,6 +1785,19 @@ function commitRootImpl(root, renderPriorityLevel) {
     remainingExpirationTimeBeforeCommit,
   );
 
+  // Clear already finished discrete updates in case that a later call of
+  // `flushDiscreteUpdates` starts a useless render pass which may cancels
+  // a scheduled timeout.
+  if (rootsWithPendingDiscreteUpdates !== null) {
+    const lastDiscreteTime = rootsWithPendingDiscreteUpdates.get(root);
+    if (
+      lastDiscreteTime !== undefined &&
+      remainingExpirationTimeBeforeCommit < lastDiscreteTime
+    ) {
+      rootsWithPendingDiscreteUpdates.delete(root);
+    }
+  }
+
   if (root === workInProgressRoot) {
     // We can reset these now that they are finished.
     workInProgressRoot = null;

packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.internal.js

@@ -3597,4 +3597,52 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       </>,
     );
   });
+
+  it('regression: empty render at high priority causes update to be dropped', async () => {
+    // Reproduces a bug where flushDiscreteUpdates starts a new (empty) render
+    // pass which cancels a scheduled timeout and causes the fallback never to
+    // be committed.
+    function App({text, shouldSuspend}) {
+      return (
+        <>
+          <Text text={text} />
+          <Suspense fallback={<Text text="Loading..." />}>
+            {shouldSuspend && <AsyncText text="B" />}
+          </Suspense>
+        </>
+      );
+    }
+
+    const root = ReactNoop.createRoot();
+    ReactNoop.discreteUpdates(() => {
+      // High pri
+      root.render(<App text="A" />);
+    });
+    // Low pri
+    root.render(<App text="A" shouldSuspend={true} />);
+
+    expect(Scheduler).toFlushAndYield([
+      // Render the high pri update
+      'A',
+      // Render the low pri update
+      'A',
+      'Suspend! [B]',
+      'Loading...',
+    ]);
+    expect(root).toMatchRenderedOutput(<span prop="A" />);
+
+    // Triggers erstwhile bug where flushDiscreteUpdates caused an empty render
+    // at a previously committed level
+    ReactNoop.flushDiscreteUpdates();
+
+    // Commit the placeholder
+    Scheduler.unstable_advanceTime(2000);
+    await advanceTimers(2000);
+    expect(root).toMatchRenderedOutput(
+      <>
+        <span prop="A" />
+        <span prop="Loading..." />
+      </>,
+    );
+  });
 });