Reactの内部構造を見ていくよ(スケジュールフェーズ編) scheduleUpdateOnFiber以降

export function scheduleUpdateOnFiber(
  root: FiberRoot,
  fiber: Fiber,
  lane: Lane,
) {
  if (__DEV__) {
    if (isRunningInsertionEffect) {
      console.error('useInsertionEffect must not schedule updates.');
    }
  }

  if (__DEV__) {
    if (isFlushingPassiveEffects) {
      didScheduleUpdateDuringPassiveEffects = true;
    }
  }

  // Check if the work loop is currently suspended and waiting for data to
  // finish loading.
  if (
    // Suspended render phase
    (root === workInProgressRoot &&
      workInProgressSuspendedReason === SuspendedOnData) ||
    // Suspended commit phase
    root.cancelPendingCommit !== null
  ) {
    // The incoming update might unblock the current render. Interrupt the
    // current attempt and restart from the top.
    prepareFreshStack(root, NoLanes);
    const didAttemptEntireTree = false;
    markRootSuspended(
      root,
      workInProgressRootRenderLanes,
      workInProgressDeferredLane,
      didAttemptEntireTree,
    );
  }

  // Mark that the root has a pending update.
  markRootUpdated(root, lane);

  if (
    (executionContext & RenderContext) !== NoLanes &&
    root === workInProgressRoot
  ) {
    // This update was dispatched during the render phase. This is a mistake
    // if the update originates from user space (with the exception of local
    // hook updates, which are handled differently and don't reach this
    // function), but there are some internal React features that use this as
    // an implementation detail, like selective hydration.
    warnAboutRenderPhaseUpdatesInDEV(fiber);

    // Track lanes that were updated during the render phase
    workInProgressRootRenderPhaseUpdatedLanes = mergeLanes(
      workInProgressRootRenderPhaseUpdatedLanes,
      lane,
    );
  } else {
    // This is a normal update, scheduled from outside the render phase. For
    // example, during an input event.
    if (enableUpdaterTracking) {
      if (isDevToolsPresent) {
        addFiberToLanesMap(root, fiber, lane);
      }
    }

    warnIfUpdatesNotWrappedWithActDEV(fiber);

    if (enableTransitionTracing) {
      const transition = ReactSharedInternals.T;
      if (transition !== null && transition.name != null) {
        if (transition.startTime === -1) {
          transition.startTime = now();
        }

        // $FlowFixMe[prop-missing]: The BatchConfigTransition and Transition types are incompatible but was previously untyped and thus uncaught
        // $FlowFixMe[incompatible-call]: "
        addTransitionToLanesMap(root, transition, lane);
      }
    }

    if (root === workInProgressRoot) {
      // Received an update to a tree that's in the middle of rendering. Mark
      // that there was an interleaved update work on this root.
      if ((executionContext & RenderContext) === NoContext) {
        workInProgressRootInterleavedUpdatedLanes = mergeLanes(
          workInProgressRootInterleavedUpdatedLanes,
          lane,
        );
      }
      if (workInProgressRootExitStatus === RootSuspendedWithDelay) {
        // The root already suspended with a delay, which means this render
        // definitely won't finish. Since we have a new update, let's mark it as
        // suspended now, right before marking the incoming update. This has the
        // effect of interrupting the current render and switching to the update.
        // TODO: Make sure this doesn't override pings that happen while we've
        // already started rendering.
        const didAttemptEntireTree = false;
        markRootSuspended(
          root,
          workInProgressRootRenderLanes,
          workInProgressDeferredLane,
          didAttemptEntireTree,
        );
      }
    }

    ensureRootIsScheduled(root);
    if (
      lane === SyncLane &&
      executionContext === NoContext &&
      !disableLegacyMode &&
      (fiber.mode & ConcurrentMode) === NoMode
    ) {
      if (__DEV__ && ReactSharedInternals.isBatchingLegacy) {
        // Treat `act` as if it's inside `batchedUpdates`, even in legacy mode.
      } else {
        // Flush the synchronous work now, unless we're already working or inside
        // a batch. This is intentionally inside scheduleUpdateOnFiber instead of
        // scheduleCallbackForFiber to preserve the ability to schedule a callback
        // without immediately flushing it. We only do this for user-initiated
        // updates, to preserve historical behavior of legacy mode.
        resetRenderTimer();
        flushSyncWorkOnLegacyRootsOnly();
      }
    }
  }
}

function markRootSuspended(
  root: FiberRoot,
  suspendedLanes: Lanes,
  spawnedLane: Lane,
  didAttemptEntireTree: boolean,
) {
  // When suspending, we should always exclude lanes that were pinged or (more
  // rarely, since we try to avoid it) updated during the render phase.
  suspendedLanes = removeLanes(suspendedLanes, workInProgressRootPingedLanes);
  suspendedLanes = removeLanes(
    suspendedLanes,
    workInProgressRootInterleavedUpdatedLanes,
  );
  _markRootSuspended(root, suspendedLanes, spawnedLane, didAttemptEntireTree);
}

export function markRootUpdated(root: FiberRoot, updateLane: Lane) {
  root.pendingLanes |= updateLane;

  // If there are any suspended transitions, it's possible this new update
  // could unblock them. Clear the suspended lanes so that we can try rendering
  // them again.
  //
  // TODO: We really only need to unsuspend only lanes that are in the
  // `subtreeLanes` of the updated fiber, or the update lanes of the return
  // path. This would exclude suspended updates in an unrelated sibling tree,
  // since there's no way for this update to unblock it.
  //
  // We don't do this if the incoming update is idle, because we never process
  // idle updates until after all the regular updates have finished; there's no
  // way it could unblock a transition.
  if (updateLane !== IdleLane) {
    root.suspendedLanes = NoLanes;
    root.pingedLanes = NoLanes;
    root.warmLanes = NoLanes;
  }
}

const SyncLane = 0b0000000000000000000000000000001;
const InputContinuousLane = 0b0000000000000000000000000000100;

// `|`でレーンをマージ
// `SyncLane`と`InputContinuousLane`を持つビットマスクになる
const merged = SyncLane | InputContinuousLane; 
console.log(merged); // 0b0000000000000000000000000000101

// `&`でビットマスクが対象のレーンを含むか確認できる
console.log((merged & InputContinuousLane) != 0); // true

export function ensureRootIsScheduled(root: FiberRoot): void {
  // This function is called whenever a root receives an update. It does two
  // things 1) it ensures the root is in the root schedule, and 2) it ensures
  // there's a pending microtask to process the root schedule.
  //
  // Most of the actual scheduling logic does not happen until
  // `scheduleTaskForRootDuringMicrotask` runs.

  // Add the root to the schedule
  if (root === lastScheduledRoot || root.next !== null) {
    // Fast path. This root is already scheduled.
  } else {
    if (lastScheduledRoot === null) {
      firstScheduledRoot = lastScheduledRoot = root;
    } else {
      lastScheduledRoot.next = root;
      lastScheduledRoot = root;
    }
  }

  // Any time a root received an update, we set this to true until the next time
  // we process the schedule. If it's false, then we can quickly exit flushSync
  // without consulting the schedule.
  mightHavePendingSyncWork = true;

  // At the end of the current event, go through each of the roots and ensure
  // there's a task scheduled for each one at the correct priority.
  if (__DEV__ && ReactSharedInternals.actQueue !== null) {
    // We're inside an `act` scope.
    if (!didScheduleMicrotask_act) {
      didScheduleMicrotask_act = true;
      scheduleImmediateTask(processRootScheduleInMicrotask);
    }
  } else {
    if (!didScheduleMicrotask) {
      didScheduleMicrotask = true;
      scheduleImmediateTask(processRootScheduleInMicrotask);
    }
  }

  if (!enableDeferRootSchedulingToMicrotask) {
    // While this flag is disabled, we schedule the render task immediately
    // instead of waiting a microtask.
    // TODO: We need to land enableDeferRootSchedulingToMicrotask ASAP to
    // unblock additional features we have planned.
    scheduleTaskForRootDuringMicrotask(root, now());
  }

  if (
    __DEV__ &&
    !disableLegacyMode &&
    ReactSharedInternals.isBatchingLegacy &&
    root.tag === LegacyRoot
  ) {
    // Special `act` case: Record whenever a legacy update is scheduled.
    ReactSharedInternals.didScheduleLegacyUpdate = true;
  }
}

function scheduleTaskForRootDuringMicrotask(
  root: FiberRoot,
  currentTime: number,
): Lane {
  // This function is always called inside a microtask, or at the very end of a
  // rendering task right before we yield to the main thread. It should never be
  // called synchronously.
  //
  // TODO: Unless enableDeferRootSchedulingToMicrotask is off. We need to land
  // that ASAP to unblock additional features we have planned.
  //
  // This function also never performs React work synchronously; it should
  // only schedule work to be performed later, in a separate task or microtask.

  // Check if any lanes are being starved by other work. If so, mark them as
  // expired so we know to work on those next.
  markStarvedLanesAsExpired(root, currentTime);

  // Determine the next lanes to work on, and their priority.
  const workInProgressRoot = getWorkInProgressRoot();
  const workInProgressRootRenderLanes = getWorkInProgressRootRenderLanes();
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );

  const existingCallbackNode = root.callbackNode;
  if (
    // Check if there's nothing to work on
    nextLanes === NoLanes ||
    // If this root is currently suspended and waiting for data to resolve, don't
    // schedule a task to render it. We'll either wait for a ping, or wait to
    // receive an update.
    //
    // Suspended render phase
    (root === workInProgressRoot && isWorkLoopSuspendedOnData()) ||
    // Suspended commit phase
    root.cancelPendingCommit !== null
  ) {
    // Fast path: There's nothing to work on.
    if (existingCallbackNode !== null) {
      cancelCallback(existingCallbackNode);
    }
    root.callbackNode = null;
    root.callbackPriority = NoLane;
    return NoLane;
  }

  // Schedule a new callback in the host environment.
  if (
    includesSyncLane(nextLanes) &&
    // If we're prerendering, then we should use the concurrent work loop
    // even if the lanes are synchronous, so that prerendering never blocks
    // the main thread.
    !(enableSiblingPrerendering && checkIfRootIsPrerendering(root, nextLanes))
  ) {
    // Synchronous work is always flushed at the end of the microtask, so we
    // don't need to schedule an additional task.
    if (existingCallbackNode !== null) {
      cancelCallback(existingCallbackNode);
    }
    root.callbackPriority = SyncLane;
    root.callbackNode = null;
    return SyncLane;
  } else {
    // We use the highest priority lane to represent the priority of the callback.
    const existingCallbackPriority = root.callbackPriority;
    const newCallbackPriority = getHighestPriorityLane(nextLanes);

    if (
      newCallbackPriority === existingCallbackPriority &&
      // Special case related to `act`. If the currently scheduled task is a
      // Scheduler task, rather than an `act` task, cancel it and re-schedule
      // on the `act` queue.
      !(
        __DEV__ &&
        ReactSharedInternals.actQueue !== null &&
        existingCallbackNode !== fakeActCallbackNode
      )
    ) {
      // The priority hasn't changed. We can reuse the existing task.
      return newCallbackPriority;
    } else {
      // Cancel the existing callback. We'll schedule a new one below.
      cancelCallback(existingCallbackNode);
    }

    let schedulerPriorityLevel;
    switch (lanesToEventPriority(nextLanes)) {
      // Scheduler does have an "ImmediatePriority", but now that we use
      // microtasks for sync work we no longer use that. Any sync work that
      // reaches this path is meant to be time sliced.
      case DiscreteEventPriority:
      case ContinuousEventPriority:
        schedulerPriorityLevel = UserBlockingSchedulerPriority;
        break;
      case DefaultEventPriority:
        schedulerPriorityLevel = NormalSchedulerPriority;
        break;
      case IdleEventPriority:
        schedulerPriorityLevel = IdleSchedulerPriority;
        break;
      default:
        schedulerPriorityLevel = NormalSchedulerPriority;
        break;
    }

    const newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performWorkOnRootViaSchedulerTask.bind(null, root),
    );

    root.callbackPriority = newCallbackPriority;
    root.callbackNode = newCallbackNode;
    return newCallbackPriority;
  }
}

function scheduleCallback(
  priorityLevel: PriorityLevel,
  callback: RenderTaskFn,
) {
  if (__DEV__ && ReactSharedInternals.actQueue !== null) {
    // Special case: We're inside an `act` scope (a testing utility).
    // Instead of scheduling work in the host environment, add it to a
    // fake internal queue that's managed by the `act` implementation.
    ReactSharedInternals.actQueue.push(callback);
    return fakeActCallbackNode;
  } else {
    return Scheduler_scheduleCallback(priorityLevel, callback);
  }
}

export const scheduleCallback = Scheduler.unstable_scheduleCallback;

function unstable_scheduleCallback(
  priorityLevel: PriorityLevel,
  callback: Callback,
  options?: {delay: number},
): Task {
  var currentTime = getCurrentTime();

  var startTime;
  if (typeof options === 'object' && options !== null) {
    var delay = options.delay;
    if (typeof delay === 'number' && delay > 0) {
      startTime = currentTime + delay;
    } else {
      startTime = currentTime;
    }
  } else {
    startTime = currentTime;
  }

  var timeout;
  switch (priorityLevel) {
    case ImmediatePriority:
      // Times out immediately
      timeout = -1;
      break;
    case UserBlockingPriority:
      // Eventually times out
      timeout = userBlockingPriorityTimeout;
      break;
    case IdlePriority:
      // Never times out
      timeout = maxSigned31BitInt;
      break;
    case LowPriority:
      // Eventually times out
      timeout = lowPriorityTimeout;
      break;
    case NormalPriority:
    default:
      // Eventually times out
      timeout = normalPriorityTimeout;
      break;
  }

  var expirationTime = startTime + timeout;

  var newTask: Task = {
    id: taskIdCounter++,
    callback,
    priorityLevel,
    startTime,
    expirationTime,
    sortIndex: -1,
  };
  if (enableProfiling) {
    newTask.isQueued = false;
  }

  if (startTime > currentTime) {
    // This is a delayed task.
    newTask.sortIndex = startTime;
    push(timerQueue, newTask);
    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {
      // All tasks are delayed, and this is the task with the earliest delay.
      if (isHostTimeoutScheduled) {
        // Cancel an existing timeout.
        cancelHostTimeout();
      } else {
        isHostTimeoutScheduled = true;
      }
      // Schedule a timeout.
      requestHostTimeout(handleTimeout, startTime - currentTime);
    }
  } else {
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);
    if (enableProfiling) {
      markTaskStart(newTask, currentTime);
      newTask.isQueued = true;
    }
    // Schedule a host callback, if needed. If we're already performing work,
    // wait until the next time we yield.
    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback();
    }
  }

  return newTask;
}

    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback();
    }

function requestHostCallback() {
  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;
    schedulePerformWorkUntilDeadline();
  }
}

let schedulePerformWorkUntilDeadline;
if (typeof localSetImmediate === 'function') {
  // Node.js and old IE.
  // There's a few reasons for why we prefer setImmediate.
  //
  // Unlike MessageChannel, it doesn't prevent a Node.js process from exiting.
  // (Even though this is a DOM fork of the Scheduler, you could get here
  // with a mix of Node.js 15+, which has a MessageChannel, and jsdom.)
  // https://github.com/facebook/react/issues/20756
  //
  // But also, it runs earlier which is the semantic we want.
  // If other browsers ever implement it, it's better to use it.
  // Although both of these would be inferior to native scheduling.
  schedulePerformWorkUntilDeadline = () => {
    localSetImmediate(performWorkUntilDeadline);
  };
} else if (typeof MessageChannel !== 'undefined') {
  // DOM and Worker environments.
  // We prefer MessageChannel because of the 4ms setTimeout clamping.
  const channel = new MessageChannel();
  const port = channel.port2;
  channel.port1.onmessage = performWorkUntilDeadline;
  schedulePerformWorkUntilDeadline = () => {
    port.postMessage(null);
  };
} else {
  // We should only fallback here in non-browser environments.
  schedulePerformWorkUntilDeadline = () => {
    // $FlowFixMe[not-a-function] nullable value
    localSetTimeout(performWorkUntilDeadline, 0);
  };
}

const performWorkUntilDeadline = () => {
  if (isMessageLoopRunning) {
    const currentTime = getCurrentTime();
    // Keep track of the start time so we can measure how long the main thread
    // has been blocked.
    startTime = currentTime;

    // If a scheduler task throws, exit the current browser task so the
    // error can be observed.
    //
    // Intentionally not using a try-catch, since that makes some debugging
    // techniques harder. Instead, if `flushWork` errors, then `hasMoreWork` will
    // remain true, and we'll continue the work loop.
    let hasMoreWork = true;
    try {
      hasMoreWork = flushWork(currentTime);
    } finally {
      if (hasMoreWork) {
        // If there's more work, schedule the next message event at the end
        // of the preceding one.
        schedulePerformWorkUntilDeadline();
      } else {
        isMessageLoopRunning = false;
      }
    }
  }
};

function flushWork(initialTime: number) {
  if (enableProfiling) {
    markSchedulerUnsuspended(initialTime);
  }

  // We'll need a host callback the next time work is scheduled.
  isHostCallbackScheduled = false;
  if (isHostTimeoutScheduled) {
    // We scheduled a timeout but it's no longer needed. Cancel it.
    isHostTimeoutScheduled = false;
    cancelHostTimeout();
  }

  isPerformingWork = true;
  const previousPriorityLevel = currentPriorityLevel;
  try {
    if (enableProfiling) {
      try {
        return workLoop(initialTime);
      } catch (error) {
        if (currentTask !== null) {
          const currentTime = getCurrentTime();
          // $FlowFixMe[incompatible-call] found when upgrading Flow
          markTaskErrored(currentTask, currentTime);
          // $FlowFixMe[incompatible-use] found when upgrading Flow
          currentTask.isQueued = false;
        }
        throw error;
      }
    } else {
      // No catch in prod code path.
      return workLoop(initialTime);
    }
  } finally {
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;
    if (enableProfiling) {
      const currentTime = getCurrentTime();
      markSchedulerSuspended(currentTime);
    }
  }
}

function workLoop(initialTime: number) {
  let currentTime = initialTime;
  advanceTimers(currentTime);
  currentTask = peek(taskQueue);
  while (
    currentTask !== null &&
    !(enableSchedulerDebugging && isSchedulerPaused)
  ) {
    if (currentTask.expirationTime > currentTime && shouldYieldToHost()) {
      // This currentTask hasn't expired, and we've reached the deadline.
      break;
    }
    // $FlowFixMe[incompatible-use] found when upgrading Flow
    const callback = currentTask.callback;
    if (typeof callback === 'function') {
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      currentTask.callback = null;
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      currentPriorityLevel = currentTask.priorityLevel;
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      if (enableProfiling) {
        // $FlowFixMe[incompatible-call] found when upgrading Flow
        markTaskRun(currentTask, currentTime);
      }
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();
      if (typeof continuationCallback === 'function') {
        // If a continuation is returned, immediately yield to the main thread
        // regardless of how much time is left in the current time slice.
        // $FlowFixMe[incompatible-use] found when upgrading Flow
        currentTask.callback = continuationCallback;
        if (enableProfiling) {
          // $FlowFixMe[incompatible-call] found when upgrading Flow
          markTaskYield(currentTask, currentTime);
        }
        advanceTimers(currentTime);
        return true;
      } else {
        if (enableProfiling) {
          // $FlowFixMe[incompatible-call] found when upgrading Flow
          markTaskCompleted(currentTask, currentTime);
          // $FlowFixMe[incompatible-use] found when upgrading Flow
          currentTask.isQueued = false;
        }
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
        advanceTimers(currentTime);
      }
    } else {
      pop(taskQueue);
    }
    currentTask = peek(taskQueue);
  }
  // Return whether there's additional work
  if (currentTask !== null) {
    return true;
  } else {
    const firstTimer = peek(timerQueue);
    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    return false;
  }
}

var newTask = {
  id: taskIdCounter++,
  callback,
  priorityLevel,
  startTime,
  expirationTime,
  sortIndex: -1,
};

  if (startTime > currentTime) {
    // This is a delayed task.
    newTask.sortIndex = startTime;
    push(timerQueue, newTask);
(省略)
    }
  } else {
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);
(省略)

   // $FlowFixMe[incompatible-use] found when upgrading Flow
   const callback = currentTask.callback;
   if (typeof callback === 'function') {
  (省略)
     const continuationCallback = callback(didUserCallbackTimeout);
...

    const newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performWorkOnRootViaSchedulerTask.bind(null, root),
    );

function performWorkOnRootViaSchedulerTask(
  root: FiberRoot,
  didTimeout: boolean,
): RenderTaskFn | null {
  // This is the entry point for concurrent tasks scheduled via Scheduler (and
  // postTask, in the future).

  if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
    resetNestedUpdateFlag();
  }

  // Flush any pending passive effects before deciding which lanes to work on,
  // in case they schedule additional work.
  const originalCallbackNode = root.callbackNode;
  const didFlushPassiveEffects = flushPassiveEffects();
  if (didFlushPassiveEffects) {
    // Something in the passive effect phase may have canceled the current task.
    // Check if the task node for this root was changed.
    if (root.callbackNode !== originalCallbackNode) {
      // The current task was canceled. Exit. We don't need to call
      // `ensureRootIsScheduled` because the check above implies either that
      // there's a new task, or that there's no remaining work on this root.
      return null;
    } else {
      // Current task was not canceled. Continue.
    }
  }

  // Determine the next lanes to work on, using the fields stored on the root.
  // TODO: We already called getNextLanes when we scheduled the callback; we
  // should be able to avoid calling it again by stashing the result on the
  // root object. However, because we always schedule the callback during
  // a microtask (scheduleTaskForRootDuringMicrotask), it's possible that
  // an update was scheduled earlier during this same browser task (and
  // therefore before the microtasks have run). That's because Scheduler batches
  // together multiple callbacks into a single browser macrotask, without
  // yielding to microtasks in between. We should probably change this to align
  // with the postTask behavior (and literally use postTask when
  // it's available).
  const workInProgressRoot = getWorkInProgressRoot();
  const workInProgressRootRenderLanes = getWorkInProgressRootRenderLanes();
  const lanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  if (lanes === NoLanes) {
    // No more work on this root.
    return null;
  }

  // Enter the work loop.
  // TODO: We only check `didTimeout` defensively, to account for a Scheduler
  // bug we're still investigating. Once the bug in Scheduler is fixed,
  // we can remove this, since we track expiration ourselves.
  const forceSync = !disableSchedulerTimeoutInWorkLoop && didTimeout;
  performWorkOnRoot(root, lanes, forceSync);

  // The work loop yielded, but there may or may not be work left at the current
  // priority. Need to determine whether we need to schedule a continuation.
  // Usually `scheduleTaskForRootDuringMicrotask` only runs inside a microtask;
  // however, since most of the logic for determining if we need a continuation
  // versus a new task is the same, we cheat a bit and call it here. This is
  // only safe to do because we know we're at the end of the browser task.
  // So although it's not an actual microtask, it might as well be.
  scheduleTaskForRootDuringMicrotask(root, now());
  if (root.callbackNode === originalCallbackNode) {
    // The task node scheduled for this root is the same one that's
    // currently executed. Need to return a continuation.
    return performWorkOnRootViaSchedulerTask.bind(null, root);
  }
  return null;
}

export function performWorkOnRoot(
  root: FiberRoot,
  lanes: Lanes,
  forceSync: boolean,
): void {
  if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {
    throw new Error('Should not already be working.');
  }

  // We disable time-slicing in some cases: if the work has been CPU-bound
  // for too long ("expired" work, to prevent starvation), or we're in
  // sync-updates-by-default mode.
  const shouldTimeSlice =
    (!forceSync &&
      !includesBlockingLane(lanes) &&
      !includesExpiredLane(root, lanes)) ||
    // If we're prerendering, then we should use the concurrent work loop
    // even if the lanes are synchronous, so that prerendering never blocks
    // the main thread.
    // TODO: We should consider doing this whenever a sync lane is suspended,
    // even for regular pings.
    (enableSiblingPrerendering && checkIfRootIsPrerendering(root, lanes));

  let exitStatus = shouldTimeSlice
    ? renderRootConcurrent(root, lanes)
    : renderRootSync(root, lanes, true);

  do {
    let renderWasConcurrent = shouldTimeSlice;
    if (exitStatus === RootInProgress) {
      // Render phase is still in progress.
      if (
        enableSiblingPrerendering &&
        workInProgressRootIsPrerendering &&
        !shouldTimeSlice
      ) {
        // We're in prerendering mode, but time slicing is not enabled. This
        // happens when something suspends during a synchronous update. Exit the
        // the work loop. When we resume, we'll use the concurrent work loop so
        // that prerendering is non-blocking.
        //
        // Mark the root as suspended. Usually we do this at the end of the
        // render phase, but we do it here so that we resume in
        // prerendering mode.
        // TODO: Consider always calling markRootSuspended immediately.
        // Needs to be *after* we attach a ping listener, though.
        const didAttemptEntireTree = false;
        markRootSuspended(root, lanes, NoLane, didAttemptEntireTree);
      }
      break;
    } else if (exitStatus === RootDidNotComplete) {
      // The render unwound without completing the tree. This happens in special
      // cases where need to exit the current render without producing a
      // consistent tree or committing.
      const didAttemptEntireTree = !workInProgressRootDidSkipSuspendedSiblings;
      markRootSuspended(root, lanes, NoLane, didAttemptEntireTree);
    } else {
      // The render completed.

      // Check if this render may have yielded to a concurrent event, and if so,
      // confirm that any newly rendered stores are consistent.
      // TODO: It's possible that even a concurrent render may never have yielded
      // to the main thread, if it was fast enough, or if it expired. We could
      // skip the consistency check in that case, too.
      const finishedWork: Fiber = (root.current.alternate: any);
      if (
        renderWasConcurrent &&
        !isRenderConsistentWithExternalStores(finishedWork)
      ) {
        // A store was mutated in an interleaved event. Render again,
        // synchronously, to block further mutations.
        exitStatus = renderRootSync(root, lanes, false);
        // We assume the tree is now consistent because we didn't yield to any
        // concurrent events.
        renderWasConcurrent = false;
        // Need to check the exit status again.
        continue;
      }

      // Check if something threw
      if (
        (disableLegacyMode || root.tag !== LegacyRoot) &&
        exitStatus === RootErrored
      ) {
        const lanesThatJustErrored = lanes;
        const errorRetryLanes = getLanesToRetrySynchronouslyOnError(
          root,
          lanesThatJustErrored,
        );
        if (errorRetryLanes !== NoLanes) {
          lanes = errorRetryLanes;
          exitStatus = recoverFromConcurrentError(
            root,
            lanesThatJustErrored,
            errorRetryLanes,
          );
          renderWasConcurrent = false;
          // Need to check the exit status again.
          if (exitStatus !== RootErrored) {
            // The root did not error this time. Restart the exit algorithm
            // from the beginning.
            // TODO: Refactor the exit algorithm to be less confusing. Maybe
            // more branches + recursion instead of a loop. I think the only
            // thing that causes it to be a loop is the RootDidNotComplete
            // check. If that's true, then we don't need a loop/recursion
            // at all.
            continue;
          } else {
            // The root errored yet again. Proceed to commit the tree.
          }
        }
      }
      if (exitStatus === RootFatalErrored) {
        prepareFreshStack(root, NoLanes);
        // Since this is a fatal error, we're going to pretend we attempted
        // the entire tree, to avoid scheduling a prerender.
        const didAttemptEntireTree = true;
        markRootSuspended(root, lanes, NoLane, didAttemptEntireTree);
        break;
      }

      // We now have a consistent tree. The next step is either to commit it,
      // or, if something suspended, wait to commit it after a timeout.
      finishConcurrentRender(root, exitStatus, finishedWork, lanes);
    }
    break;
  } while (true);

  ensureRootIsScheduled(root);
}

  let exitStatus = shouldTimeSlice
    ? renderRootConcurrent(root, lanes)
    : renderRootSync(root, lanes, true);

const localSetImmediate =
  typeof setImmediate !== 'undefined' ? setImmediate : null; // IE and Node.js + jsdom

const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;