From 08986601546f69ba2c3107b472862cf2cb30d2ce Mon Sep 17 00:00:00 2001
From: Ricky <rickhanlonii@gmail.com>
Date: Tue, 10 Nov 2020 12:11:46 -0500
Subject: [PATCH] Add version of scheduler that only swaps MessageChannel for
 postTask (#20206)

* Fork SchedulerDOM to SchedulerPostTaskOnly

* Swap in postTask for MessageChannel

* Add SchedulerPostTaskOnly-test.js

* Update getCurrentTime

* Gate tests to source

* Prettier
---
 .../__tests__/SchedulerPostTaskOnly-test.js   | 275 ++++++++
 .../src/forks/SchedulerPostTaskOnly.js        | 592 ++++++++++++++++++
 packages/scheduler/unstable_post_task_only.js |  10 +
 scripts/rollup/bundles.js                     |  15 +
 4 files changed, 892 insertions(+)
 create mode 100644 packages/scheduler/src/__tests__/SchedulerPostTaskOnly-test.js
 create mode 100644 packages/scheduler/src/forks/SchedulerPostTaskOnly.js
 create mode 100644 packages/scheduler/unstable_post_task_only.js

diff --git a/packages/scheduler/src/__tests__/SchedulerPostTaskOnly-test.js b/packages/scheduler/src/__tests__/SchedulerPostTaskOnly-test.js
new file mode 100644
index 0000000000000..bc25f3dbb148e
--- /dev/null
+++ b/packages/scheduler/src/__tests__/SchedulerPostTaskOnly-test.js
@@ -0,0 +1,275 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ * @jest-environment node
+ */
+
+/* eslint-disable no-for-of-loops/no-for-of-loops */
+
+'use strict';
+
+let Scheduler;
+let runtime;
+let performance;
+let cancelCallback;
+let scheduleCallback;
+let NormalPriority;
+
+describe('SchedulerPostTaskOnly', () => {
+  beforeEach(() => {
+    if (!process.env.IS_BUILD) {
+      jest.resetModules();
+
+      // Un-mock scheduler
+      jest.mock('scheduler', () =>
+        require.requireActual('scheduler/unstable_post_task_only'),
+      );
+
+      runtime = installMockBrowserRuntime();
+      performance = window.performance;
+      Scheduler = require('scheduler');
+      cancelCallback = Scheduler.unstable_cancelCallback;
+      scheduleCallback = Scheduler.unstable_scheduleCallback;
+      NormalPriority = Scheduler.unstable_NormalPriority;
+    }
+  });
+
+  afterEach(() => {
+    if (!process.env.IS_BUILD) {
+      if (!runtime.isLogEmpty()) {
+        throw Error('Test exited without clearing log.');
+      }
+    }
+  });
+
+  function installMockBrowserRuntime() {
+    let hasPendingTask = false;
+    let timerIDCounter = 0;
+    let eventLog = [];
+
+    // Mock window functions
+    const window = {};
+    global.window = window;
+
+    // TODO: Scheduler no longer requires these methods to be polyfilled. But
+    // maybe we want to continue warning if they don't exist, to preserve the
+    // option to rely on it in the future?
+    window.requestAnimationFrame = window.cancelAnimationFrame = () => {};
+
+    let currentTime = 0;
+    window.performance = {
+      now() {
+        return currentTime;
+      },
+    };
+
+    window.setTimeout = (cb, delay) => {
+      const id = timerIDCounter++;
+      log(`Set Timer`);
+      // TODO
+      return id;
+    };
+    window.clearTimeout = id => {
+      // TODO
+    };
+
+    // Mock browser scheduler.
+    const scheduler = {};
+    global.scheduler = scheduler;
+
+    let nextTask;
+    scheduler.postTask = function(callback) {
+      if (hasPendingTask) {
+        throw Error('Task already scheduled');
+      }
+      log('Post Task');
+      hasPendingTask = true;
+      nextTask = callback;
+    };
+
+    function ensureLogIsEmpty() {
+      if (eventLog.length !== 0) {
+        throw Error('Log is not empty. Call assertLog before continuing.');
+      }
+    }
+
+    function advanceTime(ms) {
+      currentTime += ms;
+    }
+
+    function fireNextTask() {
+      ensureLogIsEmpty();
+      if (!hasPendingTask) {
+        throw Error('No task was scheduled');
+      }
+      hasPendingTask = false;
+
+      log('Task Event');
+
+      // If there's a continuation, it will call postTask again
+      // which will set nextTask. That means we need to clear
+      // nextTask before the invocation, otherwise we would
+      // delete the continuation task.
+      const task = nextTask;
+      nextTask = null;
+      task();
+    }
+
+    function log(val) {
+      eventLog.push(val);
+    }
+
+    function isLogEmpty() {
+      return eventLog.length === 0;
+    }
+
+    function assertLog(expected) {
+      const actual = eventLog;
+      eventLog = [];
+      expect(actual).toEqual(expected);
+    }
+
+    return {
+      advanceTime,
+      fireNextTask,
+      log,
+      isLogEmpty,
+      assertLog,
+    };
+  }
+
+  // @gate source
+  it('task that finishes before deadline', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Task']);
+  });
+
+  // @gate source
+  it('task with continuation', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+      while (!Scheduler.unstable_shouldYield()) {
+        runtime.advanceTime(1);
+      }
+      runtime.log(`Yield at ${performance.now()}ms`);
+      return () => {
+        runtime.log('Continuation');
+      };
+    });
+    runtime.assertLog(['Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Task', 'Yield at 5ms', 'Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Continuation']);
+  });
+
+  // @gate source
+  it('multiple tasks', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'A', 'B']);
+  });
+
+  // @gate source
+  it('multiple tasks with a yield in between', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+      runtime.advanceTime(4999);
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog([
+      'Task Event',
+      'A',
+      // Ran out of time. Post a continuation event.
+      'Post Task',
+    ]);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+
+  // @gate source
+  it('cancels tasks', () => {
+    const task = scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+    });
+    runtime.assertLog(['Post Task']);
+    cancelCallback(task);
+    runtime.assertLog([]);
+  });
+
+  // @gate source
+  it('throws when a task errors then continues in a new event', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Oops!');
+      throw Error('Oops!');
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Yay');
+    });
+    runtime.assertLog(['Post Task']);
+
+    expect(() => runtime.fireNextTask()).toThrow('Oops!');
+    runtime.assertLog(['Task Event', 'Oops!', 'Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Yay']);
+  });
+
+  // @gate source
+  it('schedule new task after queue has emptied', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'A']);
+
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+
+  // @gate source
+  it('schedule new task after a cancellation', () => {
+    const handle = scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+
+    runtime.assertLog(['Post Task']);
+    cancelCallback(handle);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event']);
+
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+});
diff --git a/packages/scheduler/src/forks/SchedulerPostTaskOnly.js b/packages/scheduler/src/forks/SchedulerPostTaskOnly.js
new file mode 100644
index 0000000000000..ba081d3236e5b
--- /dev/null
+++ b/packages/scheduler/src/forks/SchedulerPostTaskOnly.js
@@ -0,0 +1,592 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+/* eslint-disable no-var */
+
+import {
+  enableSchedulerDebugging,
+  enableProfiling,
+} from '../SchedulerFeatureFlags';
+
+import {push, pop, peek} from '../SchedulerMinHeap';
+
+// TODO: Use symbols?
+import {
+  ImmediatePriority,
+  UserBlockingPriority,
+  NormalPriority,
+  LowPriority,
+  IdlePriority,
+} from '../SchedulerPriorities';
+import {
+  sharedProfilingBuffer,
+  markTaskRun,
+  markTaskYield,
+  markTaskCompleted,
+  markTaskCanceled,
+  markTaskErrored,
+  markSchedulerSuspended,
+  markSchedulerUnsuspended,
+  markTaskStart,
+  stopLoggingProfilingEvents,
+  startLoggingProfilingEvents,
+} from '../SchedulerProfiling';
+
+import {enableIsInputPending} from '../SchedulerFeatureFlags';
+
+const perf = window.performance;
+
+function getCurrentTime() {
+  return perf.now();
+}
+
+// Max 31 bit integer. The max integer size in V8 for 32-bit systems.
+// Math.pow(2, 30) - 1
+// 0b111111111111111111111111111111
+var maxSigned31BitInt = 1073741823;
+
+// Times out immediately
+var IMMEDIATE_PRIORITY_TIMEOUT = -1;
+// Eventually times out
+var USER_BLOCKING_PRIORITY_TIMEOUT = 250;
+var NORMAL_PRIORITY_TIMEOUT = 5000;
+var LOW_PRIORITY_TIMEOUT = 10000;
+// Never times out
+var IDLE_PRIORITY_TIMEOUT = maxSigned31BitInt;
+
+// Tasks are stored on a min heap
+var taskQueue = [];
+var timerQueue = [];
+
+// Incrementing id counter. Used to maintain insertion order.
+var taskIdCounter = 1;
+
+// Pausing the scheduler is useful for debugging.
+var isSchedulerPaused = false;
+
+var currentTask = null;
+var currentPriorityLevel = NormalPriority;
+
+// This is set while performing work, to prevent re-entrancy.
+var isPerformingWork = false;
+
+var isHostCallbackScheduled = false;
+var isHostTimeoutScheduled = false;
+
+// Capture local references to native APIs, in case a polyfill overrides them.
+const setTimeout = window.setTimeout;
+const clearTimeout = window.clearTimeout;
+
+if (typeof console !== 'undefined') {
+  // TODO: Scheduler no longer requires these methods to be polyfilled. But
+  // maybe we want to continue warning if they don't exist, to preserve the
+  // option to rely on it in the future?
+  const requestAnimationFrame = window.requestAnimationFrame;
+  const cancelAnimationFrame = window.cancelAnimationFrame;
+
+  if (typeof requestAnimationFrame !== 'function') {
+    // Using console['error'] to evade Babel and ESLint
+    console['error'](
+      "This browser doesn't support requestAnimationFrame. " +
+        'Make sure that you load a ' +
+        'polyfill in older browsers. https://reactjs.org/link/react-polyfills',
+    );
+  }
+  if (typeof cancelAnimationFrame !== 'function') {
+    // Using console['error'] to evade Babel and ESLint
+    console['error'](
+      "This browser doesn't support cancelAnimationFrame. " +
+        'Make sure that you load a ' +
+        'polyfill in older browsers. https://reactjs.org/link/react-polyfills',
+    );
+  }
+}
+
+function advanceTimers(currentTime) {
+  // Check for tasks that are no longer delayed and add them to the queue.
+  let timer = peek(timerQueue);
+  while (timer !== null) {
+    if (timer.callback === null) {
+      // Timer was cancelled.
+      pop(timerQueue);
+    } else if (timer.startTime <= currentTime) {
+      // Timer fired. Transfer to the task queue.
+      pop(timerQueue);
+      timer.sortIndex = timer.expirationTime;
+      push(taskQueue, timer);
+      if (enableProfiling) {
+        markTaskStart(timer, currentTime);
+        timer.isQueued = true;
+      }
+    } else {
+      // Remaining timers are pending.
+      return;
+    }
+    timer = peek(timerQueue);
+  }
+}
+
+function handleTimeout(currentTime) {
+  isHostTimeoutScheduled = false;
+  advanceTimers(currentTime);
+
+  if (!isHostCallbackScheduled) {
+    if (peek(taskQueue) !== null) {
+      isHostCallbackScheduled = true;
+      requestHostCallback(flushWork);
+    } else {
+      const firstTimer = peek(timerQueue);
+      if (firstTimer !== null) {
+        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
+      }
+    }
+  }
+}
+
+function flushWork(hasTimeRemaining, initialTime) {
+  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(hasTimeRemaining, initialTime);
+      } catch (error) {
+        if (currentTask !== null) {
+          const currentTime = getCurrentTime();
+          markTaskErrored(currentTask, currentTime);
+          currentTask.isQueued = false;
+        }
+        throw error;
+      }
+    } else {
+      // No catch in prod code path.
+      return workLoop(hasTimeRemaining, initialTime);
+    }
+  } finally {
+    currentTask = null;
+    currentPriorityLevel = previousPriorityLevel;
+    isPerformingWork = false;
+    if (enableProfiling) {
+      const currentTime = getCurrentTime();
+      markSchedulerSuspended(currentTime);
+    }
+  }
+}
+
+function workLoop(hasTimeRemaining, initialTime) {
+  let currentTime = initialTime;
+  advanceTimers(currentTime);
+  currentTask = peek(taskQueue);
+  while (
+    currentTask !== null &&
+    !(enableSchedulerDebugging && isSchedulerPaused)
+  ) {
+    if (
+      currentTask.expirationTime > currentTime &&
+      (!hasTimeRemaining || shouldYieldToHost())
+    ) {
+      // This currentTask hasn't expired, and we've reached the deadline.
+      break;
+    }
+    const callback = currentTask.callback;
+    if (typeof callback === 'function') {
+      currentTask.callback = null;
+      currentPriorityLevel = currentTask.priorityLevel;
+      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
+      markTaskRun(currentTask, currentTime);
+      const continuationCallback = callback(didUserCallbackTimeout);
+      currentTime = getCurrentTime();
+      if (typeof continuationCallback === 'function') {
+        currentTask.callback = continuationCallback;
+        markTaskYield(currentTask, currentTime);
+      } else {
+        if (enableProfiling) {
+          markTaskCompleted(currentTask, currentTime);
+          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;
+  }
+}
+
+function unstable_runWithPriority(priorityLevel, eventHandler) {
+  switch (priorityLevel) {
+    case ImmediatePriority:
+    case UserBlockingPriority:
+    case NormalPriority:
+    case LowPriority:
+    case IdlePriority:
+      break;
+    default:
+      priorityLevel = NormalPriority;
+  }
+
+  var previousPriorityLevel = currentPriorityLevel;
+  currentPriorityLevel = priorityLevel;
+
+  try {
+    return eventHandler();
+  } finally {
+    currentPriorityLevel = previousPriorityLevel;
+  }
+}
+
+function unstable_next(eventHandler) {
+  var priorityLevel;
+  switch (currentPriorityLevel) {
+    case ImmediatePriority:
+    case UserBlockingPriority:
+    case NormalPriority:
+      // Shift down to normal priority
+      priorityLevel = NormalPriority;
+      break;
+    default:
+      // Anything lower than normal priority should remain at the current level.
+      priorityLevel = currentPriorityLevel;
+      break;
+  }
+
+  var previousPriorityLevel = currentPriorityLevel;
+  currentPriorityLevel = priorityLevel;
+
+  try {
+    return eventHandler();
+  } finally {
+    currentPriorityLevel = previousPriorityLevel;
+  }
+}
+
+function unstable_wrapCallback(callback) {
+  var parentPriorityLevel = currentPriorityLevel;
+  return function() {
+    // This is a fork of runWithPriority, inlined for performance.
+    var previousPriorityLevel = currentPriorityLevel;
+    currentPriorityLevel = parentPriorityLevel;
+
+    try {
+      return callback.apply(this, arguments);
+    } finally {
+      currentPriorityLevel = previousPriorityLevel;
+    }
+  };
+}
+
+function unstable_scheduleCallback(priorityLevel, callback, options) {
+  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:
+      timeout = IMMEDIATE_PRIORITY_TIMEOUT;
+      break;
+    case UserBlockingPriority:
+      timeout = USER_BLOCKING_PRIORITY_TIMEOUT;
+      break;
+    case IdlePriority:
+      timeout = IDLE_PRIORITY_TIMEOUT;
+      break;
+    case LowPriority:
+      timeout = LOW_PRIORITY_TIMEOUT;
+      break;
+    case NormalPriority:
+    default:
+      timeout = NORMAL_PRIORITY_TIMEOUT;
+      break;
+  }
+
+  var expirationTime = startTime + timeout;
+
+  var newTask = {
+    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(flushWork);
+    }
+  }
+
+  return newTask;
+}
+
+function unstable_pauseExecution() {
+  isSchedulerPaused = true;
+}
+
+function unstable_continueExecution() {
+  isSchedulerPaused = false;
+  if (!isHostCallbackScheduled && !isPerformingWork) {
+    isHostCallbackScheduled = true;
+    requestHostCallback(flushWork);
+  }
+}
+
+function unstable_getFirstCallbackNode() {
+  return peek(taskQueue);
+}
+
+function unstable_cancelCallback(task) {
+  if (enableProfiling) {
+    if (task.isQueued) {
+      const currentTime = getCurrentTime();
+      markTaskCanceled(task, currentTime);
+      task.isQueued = false;
+    }
+  }
+
+  // Null out the callback to indicate the task has been canceled. (Can't
+  // remove from the queue because you can't remove arbitrary nodes from an
+  // array based heap, only the first one.)
+  task.callback = null;
+}
+
+function unstable_getCurrentPriorityLevel() {
+  return currentPriorityLevel;
+}
+
+let isTaskLoopRunning = false;
+let scheduledHostCallback = null;
+let taskTimeoutID = -1;
+
+// Scheduler periodically yields in case there is other work on the main
+// thread, like user events. By default, it yields multiple times per frame.
+// It does not attempt to align with frame boundaries, since most tasks don't
+// need to be frame aligned; for those that do, use requestAnimationFrame.
+let yieldInterval = 5;
+let deadline = 0;
+
+// TODO: Make this configurable
+// TODO: Adjust this based on priority?
+const maxYieldInterval = 300;
+let needsPaint = false;
+
+function shouldYieldToHost() {
+  if (
+    enableIsInputPending &&
+    navigator !== undefined &&
+    navigator.scheduling !== undefined &&
+    navigator.scheduling.isInputPending !== undefined
+  ) {
+    const scheduling = navigator.scheduling;
+    const currentTime = getCurrentTime();
+    if (currentTime >= deadline) {
+      // There's no time left. We may want to yield control of the main
+      // thread, so the browser can perform high priority tasks. The main ones
+      // are painting and user input. If there's a pending paint or a pending
+      // input, then we should yield. But if there's neither, then we can
+      // yield less often while remaining responsive. We'll eventually yield
+      // regardless, since there could be a pending paint that wasn't
+      // accompanied by a call to `requestPaint`, or other main thread tasks
+      // like network events.
+      if (needsPaint || scheduling.isInputPending()) {
+        // There is either a pending paint or a pending input.
+        return true;
+      }
+      // There's no pending input. Only yield if we've reached the max
+      // yield interval.
+      return currentTime >= maxYieldInterval;
+    } else {
+      // There's still time left in the frame.
+      return false;
+    }
+  } else {
+    // `isInputPending` is not available. Since we have no way of knowing if
+    // there's pending input, always yield at the end of the frame.
+    return getCurrentTime() >= deadline;
+  }
+}
+
+function requestPaint() {
+  if (
+    enableIsInputPending &&
+    navigator !== undefined &&
+    navigator.scheduling !== undefined &&
+    navigator.scheduling.isInputPending !== undefined
+  ) {
+    needsPaint = true;
+  }
+
+  // Since we yield every frame regardless, `requestPaint` has no effect.
+}
+
+function forceFrameRate(fps) {
+  if (fps < 0 || fps > 125) {
+    // Using console['error'] to evade Babel and ESLint
+    console['error'](
+      'forceFrameRate takes a positive int between 0 and 125, ' +
+        'forcing frame rates higher than 125 fps is not supported',
+    );
+    return;
+  }
+  if (fps > 0) {
+    yieldInterval = Math.floor(1000 / fps);
+  } else {
+    // reset the framerate
+    yieldInterval = 5;
+  }
+}
+
+const performWorkUntilDeadline = () => {
+  if (scheduledHostCallback !== null) {
+    const currentTime = getCurrentTime();
+    // Yield after `yieldInterval` ms, regardless of where we are in the vsync
+    // cycle. This means there's always time remaining at the beginning of
+    // the message event.
+    deadline = currentTime + yieldInterval;
+    const hasTimeRemaining = true;
+    try {
+      const hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);
+      if (!hasMoreWork) {
+        isTaskLoopRunning = false;
+        scheduledHostCallback = null;
+      } else {
+        // If there's more work, schedule the next message event at the end
+        // of the preceding one.
+        postTask(performWorkUntilDeadline);
+      }
+    } catch (error) {
+      // If a scheduler task throws, exit the current browser task so the
+      // error can be observed.
+      postTask(performWorkUntilDeadline);
+      throw error;
+    }
+  } else {
+    isTaskLoopRunning = false;
+  }
+  // Yielding to the browser will give it a chance to paint, so we can
+  // reset this.
+  needsPaint = false;
+};
+
+function postTask(callback) {
+  // Use experimental Chrome Scheduler postTask API.
+  global.scheduler.postTask(callback);
+}
+
+function requestHostCallback(callback) {
+  scheduledHostCallback = callback;
+  if (!isTaskLoopRunning) {
+    isTaskLoopRunning = true;
+    postTask(performWorkUntilDeadline);
+  }
+}
+
+function requestHostTimeout(callback, ms) {
+  taskTimeoutID = setTimeout(() => {
+    callback(getCurrentTime());
+  }, ms);
+}
+
+function cancelHostTimeout() {
+  clearTimeout(taskTimeoutID);
+  taskTimeoutID = -1;
+}
+
+const unstable_requestPaint = requestPaint;
+
+export {
+  ImmediatePriority as unstable_ImmediatePriority,
+  UserBlockingPriority as unstable_UserBlockingPriority,
+  NormalPriority as unstable_NormalPriority,
+  IdlePriority as unstable_IdlePriority,
+  LowPriority as unstable_LowPriority,
+  unstable_runWithPriority,
+  unstable_next,
+  unstable_scheduleCallback,
+  unstable_cancelCallback,
+  unstable_wrapCallback,
+  unstable_getCurrentPriorityLevel,
+  shouldYieldToHost as unstable_shouldYield,
+  unstable_requestPaint,
+  unstable_continueExecution,
+  unstable_pauseExecution,
+  unstable_getFirstCallbackNode,
+  getCurrentTime as unstable_now,
+  forceFrameRate as unstable_forceFrameRate,
+};
+
+export const unstable_Profiling = enableProfiling
+  ? {
+      startLoggingProfilingEvents,
+      stopLoggingProfilingEvents,
+      sharedProfilingBuffer,
+    }
+  : null;
diff --git a/packages/scheduler/unstable_post_task_only.js b/packages/scheduler/unstable_post_task_only.js
new file mode 100644
index 0000000000000..d6e22c230162c
--- /dev/null
+++ b/packages/scheduler/unstable_post_task_only.js
@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+export * from './src/forks/SchedulerPostTaskOnly';
diff --git a/scripts/rollup/bundles.js b/scripts/rollup/bundles.js
index cd873e7df7fff..51d5c495a23ca 100644
--- a/scripts/rollup/bundles.js
+++ b/scripts/rollup/bundles.js
@@ -659,6 +659,21 @@ const bundles = [
     externals: [],
   },
 
+  /******* React Scheduler Post Task Only (experimental) *******/
+  {
+    bundleTypes: [
+      NODE_DEV,
+      NODE_PROD,
+      FB_WWW_DEV,
+      FB_WWW_PROD,
+      FB_WWW_PROFILING,
+    ],
+    moduleType: ISOMORPHIC,
+    entry: 'scheduler/unstable_post_task_only',
+    global: 'SchedulerPostTaskOnly',
+    externals: [],
+  },
+
   /******* React Scheduler No DOM (experimental) *******/
   {
     bundleTypes: [