timers.js

// Copyright Joyent, Inc. and other Node contributors.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to permit
// persons to whom the Software is furnished to do so, subject to the
// following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
// USE OR OTHER DEALINGS IN THE SOFTWARE.

'use strict';

const async_wrap = process.binding('async_wrap');
const TimerWrap = process.binding('timer_wrap').Timer;
const L = require('internal/linkedlist');
const internalUtil = require('internal/util');
const { createPromise, promiseResolve } = process.binding('util');
const assert = require('assert');
const util = require('util');
const debug = util.debuglog('timer');
const kOnTimeout = TimerWrap.kOnTimeout | 0;
// Two arrays that share state between C++ and JS.
const { async_hook_fields, async_id_fields } = async_wrap;
const {
  initTriggerId,
  // The needed emit*() functions.
  emitInit,
  emitBefore,
  emitAfter,
  emitDestroy
} = require('async_hooks');
// Grab the constants necessary for working with internal arrays.
const { kInit, kDestroy, kAsyncIdCounter } = async_wrap.constants;
// Symbols for storing async id state.
const async_id_symbol = Symbol('asyncId');
const trigger_async_id_symbol = Symbol('triggerAsyncId');

// Timeout values > TIMEOUT_MAX are set to 1.
const TIMEOUT_MAX = 2147483647; // 2^31-1


// HOW and WHY the timers implementation works the way it does.
//
// Timers are crucial to Node.js. Internally, any TCP I/O connection creates a
// timer so that we can time out of connections. Additionally, many user
// libraries and applications also use timers. As such there may be a
// significantly large amount of timeouts scheduled at any given time.
// Therefore, it is very important that the timers implementation is performant
// and efficient.
//
// Note: It is suggested you first read though the lib/internal/linkedlist.js
// linked list implementation, since timers depend on it extensively. It can be
// somewhat counter-intuitive at first, as it is not actually a class. Instead,
// it is a set of helpers that operate on an existing object.
//
// In order to be as performant as possible, the architecture and data
// structures are designed so that they are optimized to handle the following
// use cases as efficiently as possible:

// - Adding a new timer. (insert)
// - Removing an existing timer. (remove)
// - Handling a timer timing out. (timeout)
//
// Whenever possible, the implementation tries to make the complexity of these
// operations as close to constant-time as possible.
// (So that performance is not impacted by the number of scheduled timers.)
//
// Object maps are kept which contain linked lists keyed by their duration in
// milliseconds.
// The linked lists within also have some meta-properties, one of which is a
// TimerWrap C++ handle, which makes the call after the duration to process the
// list it is attached to.
//
//
// ╔════ > Object Map
// ║
// ╠══
// ║ refedLists: { '40': { }, '320': { etc } } (keys of millisecond duration)
// ╚══          ┌─────────┘
//              │
// ╔══          │
// ║ TimersList { _idleNext: { }, _idlePrev: (self), _timer: (TimerWrap) }
// ║         ┌────────────────┘
// ║    ╔══  │                              ^
// ║    ║    { _idleNext: { },  _idlePrev: { }, _onTimeout: (callback) }
// ║    ║      ┌───────────┘
// ║    ║      │                                  ^
// ║    ║      { _idleNext: { etc },  _idlePrev: { }, _onTimeout: (callback) }
// ╠══  ╠══
// ║    ║
// ║    ╚════ >  Actual JavaScript timeouts
// ║
// ╚════ > Linked List
//
//
// With this, virtually constant-time insertion (append), removal, and timeout
// is possible in the JavaScript layer. Any one list of timers is able to be
// sorted by just appending to it because all timers within share the same
// duration. Therefore, any timer added later will always have been scheduled to
// timeout later, thus only needing to be appended.
// Removal from an object-property linked list is also virtually constant-time
// as can be seen in the lib/internal/linkedlist.js implementation.
// Timeouts only need to process any timers currently due to expire, which will
// always be at the beginning of the list for reasons stated above. Any timers
// after the first one encountered that does not yet need to timeout will also
// always be due to timeout at a later time.
//
// Less-than constant time operations are thus contained in two places:
// TimerWrap's backing libuv timers implementation (a performant heap-based
// queue), and the object map lookup of a specific list by the duration of
// timers within (or creation of a new list).
// However, these operations combined have shown to be trivial in comparison to
// other alternative timers architectures.


// Object maps containing linked lists of timers, keyed and sorted by their
// duration in milliseconds.
//
// The difference between these two objects is that the former contains timers
// that will keep the process open if they are the only thing left, while the
// latter will not.
//
// - key = time in milliseconds
// - value = linked list
const refedLists = Object.create(null);
const unrefedLists = Object.create(null);


// Schedule or re-schedule a timer.
// The item must have been enroll()'d first.
const active = exports.active = function(item) {
  insert(item, false);
};

// Internal APIs that need timeouts should use `_unrefActive()` instead of
// `active()` so that they do not unnecessarily keep the process open.
exports._unrefActive = function(item) {
  insert(item, true);
};


// The underlying logic for scheduling or re-scheduling a timer.
//
// Appends a timer onto the end of an existing timers list, or creates a new
// TimerWrap backed list if one does not already exist for the specified timeout
// duration.
function insert(item, unrefed) {
  const msecs = item._idleTimeout;
  if (msecs < 0 || msecs === undefined) return;

  item._idleStart = TimerWrap.now();

  const lists = unrefed === true ? unrefedLists : refedLists;

  // Use an existing list if there is one, otherwise we need to make a new one.
  var list = lists[msecs];
  if (!list) {
    debug('no %d list was found in insert, creating a new one', msecs);
    lists[msecs] = list = createTimersList(msecs, unrefed);
  }

  if (!item[async_id_symbol] || item._destroyed) {
    item._destroyed = false;
    item[async_id_symbol] = ++async_id_fields[kAsyncIdCounter];
    item[trigger_async_id_symbol] = initTriggerId();
    if (async_hook_fields[kInit] > 0)
      emitInit(
        item[async_id_symbol], 'Timeout', item[trigger_async_id_symbol], item
      );
  }

  L.append(list, item);
  assert(!L.isEmpty(list)); // list is not empty
}

function createTimersList(msecs, unrefed) {
  // Make a new linked list of timers, and create a TimerWrap to schedule
  // processing for the list.
  const list = new TimersList(msecs, unrefed);
  L.init(list);
  list._timer._list = list;

  if (unrefed === true) list._timer.unref();
  list._timer.start(msecs);

  list._timer[kOnTimeout] = listOnTimeout;

  return list;
}

function TimersList(msecs, unrefed) {
  this._idleNext = null; // Create the list with the linkedlist properties to
  this._idlePrev = null; // prevent any unnecessary hidden class changes.
  this._timer = new TimerWrap();
  this._unrefed = unrefed;
  this.msecs = msecs;
  this.nextTick = false;
}

function listOnTimeout() {
  var list = this._list;
  var msecs = list.msecs;

  if (list.nextTick) {
    list.nextTick = false;
    process.nextTick(listOnTimeoutNT, list);
    return;
  }

  debug('timeout callback %d', msecs);

  var now = TimerWrap.now();
  debug('now: %d', now);

  var diff, timer;
  while (timer = L.peek(list)) {
    diff = now - timer._idleStart;

    // Check if this loop iteration is too early for the next timer.
    // This happens if there are more timers scheduled for later in the list.
    if (diff < msecs) {
      var timeRemaining = msecs - (TimerWrap.now() - timer._idleStart);
      if (timeRemaining < 0) {
        timeRemaining = 1;
      }
      this.start(timeRemaining);
      debug('%d list wait because diff is %d', msecs, diff);
      return;
    }

    // The actual logic for when a timeout happens.

    L.remove(timer);
    assert(timer !== L.peek(list));

    if (!timer._onTimeout) {
      if (async_hook_fields[kDestroy] > 0 && !timer._destroyed &&
            typeof timer[async_id_symbol] === 'number') {
        emitDestroy(timer[async_id_symbol]);
        timer._destroyed = true;
      }
      continue;
    }

    var domain = timer.domain;
    if (domain) {

      // If the timer callback throws and the
      // domain or uncaughtException handler ignore the exception,
      // other timers that expire on this tick should still run.
      //
      // https://github.com/nodejs/node-v0.x-archive/issues/2631
      if (domain._disposed)
        continue;

      domain.enter();
    }

    tryOnTimeout(timer, list);

    if (domain)
      domain.exit();
  }

  // If `L.peek(list)` returned nothing, the list was either empty or we have
  // called all of the timer timeouts.
  // As such, we can remove the list and clean up the TimerWrap C++ handle.
  debug('%d list empty', msecs);
  assert(L.isEmpty(list));

  // Either refedLists[msecs] or unrefedLists[msecs] may have been removed and
  // recreated since the reference to `list` was created. Make sure they're
  // the same instance of the list before destroying.
  if (list._unrefed === true && list === unrefedLists[msecs]) {
    delete unrefedLists[msecs];
  } else if (list === refedLists[msecs]) {
    delete refedLists[msecs];
  }

  // Do not close the underlying handle if its ownership has changed
  // (e.g it was unrefed in its callback).
  if (this.owner)
    return;

  this.close();
}


// An optimization so that the try/finally only de-optimizes (since at least v8
// 4.7) what is in this smaller function.
function tryOnTimeout(timer, list) {
  timer._called = true;
  const timerAsyncId = (typeof timer[async_id_symbol] === 'number') ?
    timer[async_id_symbol] : null;
  var threw = true;
  if (timerAsyncId !== null)
    emitBefore(timerAsyncId, timer[trigger_async_id_symbol]);
  try {
    ontimeout(timer);
    threw = false;
  } finally {
    if (timerAsyncId !== null) {
      if (!threw)
        emitAfter(timerAsyncId);
      if (!timer._repeat && async_hook_fields[kDestroy] > 0 &&
          !timer._destroyed) {
        emitDestroy(timerAsyncId);
        timer._destroyed = true;
      }
    }

    if (!threw) return;

    // Postpone all later list events to next tick. We need to do this
    // so that the events are called in the order they were created.
    const lists = list._unrefed === true ? unrefedLists : refedLists;
    for (var key in lists) {
      if (key > list.msecs) {
        lists[key].nextTick = true;
      }
    }
    // We need to continue processing after domain error handling
    // is complete, but not by using whatever domain was left over
    // when the timeout threw its exception.
    const domain = process.domain;
    process.domain = null;
    // If we threw, we need to process the rest of the list in nextTick.
    process.nextTick(listOnTimeoutNT, list);
    process.domain = domain;
  }
}


function listOnTimeoutNT(list) {
  list._timer[kOnTimeout]();
}


// A convenience function for re-using TimerWrap handles more easily.
//
// This mostly exists to fix https://github.com/nodejs/node/issues/1264.
// Handles in libuv take at least one `uv_run` to be registered as unreferenced.
// Re-using an existing handle allows us to skip that, so that a second `uv_run`
// will return no active handles, even when running `setTimeout(fn).unref()`.
function reuse(item) {
  L.remove(item);

  var list = refedLists[item._idleTimeout];
  // if empty - reuse the watcher
  if (list && L.isEmpty(list)) {
    debug('reuse hit');
    list._timer.stop();
    delete refedLists[item._idleTimeout];
    return list._timer;
  }

  return null;
}


// Remove a timer. Cancels the timeout and resets the relevant timer properties.
const unenroll = exports.unenroll = function(item) {
  // Fewer checks may be possible, but these cover everything.
  if (async_hook_fields[kDestroy] > 0 &&
      item &&
      typeof item[async_id_symbol] === 'number' &&
      !item._destroyed) {
    emitDestroy(item[async_id_symbol]);
    item._destroyed = true;
  }

  var handle = reuse(item);
  if (handle) {
    debug('unenroll: list empty');
    handle.close();
  }
  // if active is called later, then we want to make sure not to insert again
  item._idleTimeout = -1;
};


// Make a regular object able to act as a timer by setting some properties.
// This function does not start the timer, see `active()`.
// Using existing objects as timers slightly reduces object overhead.
exports.enroll = function(item, msecs) {
  if (typeof msecs !== 'number') {
    throw new TypeError('"msecs" argument must be a number');
  }

  if (msecs < 0 || !isFinite(msecs)) {
    throw new RangeError('"msecs" argument must be ' +
                         'a non-negative finite number');
  }

  // if this item was already in a list somewhere
  // then we should unenroll it from that
  if (item._idleNext) unenroll(item);

  // Ensure that msecs fits into signed int32
  if (msecs > TIMEOUT_MAX) {
    msecs = TIMEOUT_MAX;
  }

  item._idleTimeout = msecs;
  L.init(item);
};


/*
 * DOM-style timers
 */


function setTimeout(callback, after, arg1, arg2, arg3) {
  if (typeof callback !== 'function') {
    throw new TypeError('"callback" argument must be a function');
  }

  var len = arguments.length;
  var args;
  if (len === 3) {
    args = [arg1];
  } else if (len === 4) {
    args = [arg1, arg2];
  } else if (len > 4) {
    args = [arg1, arg2, arg3];
    for (var i = 5; i < len; i++)
      // extend array dynamically, makes .apply run much faster in v6.0.0
      args[i - 2] = arguments[i];
  }

  return createSingleTimeout(callback, after, args);
}

setTimeout[internalUtil.promisify.custom] = function(after, value) {
  const promise = createPromise();
  createSingleTimeout(promise, after, [value]);
  return promise;
};

exports.setTimeout = setTimeout;

function createSingleTimeout(callback, after, args) {
  after *= 1; // coalesce to number or NaN
  if (!(after >= 1 && after <= TIMEOUT_MAX))
    after = 1; // schedule on next tick, follows browser behavior

  var timer = new Timeout(after, callback, args);
  if (process.domain)
    timer.domain = process.domain;

  active(timer);

  return timer;
}


function ontimeout(timer) {
  var args = timer._timerArgs;
  var callback = timer._onTimeout;
  if (typeof callback !== 'function')
    return promiseResolve(callback, args[0]);
  if (!args)
    timer._onTimeout();
  else {
    switch (args.length) {
      case 1:
        timer._onTimeout(args[0]);
        break;
      case 2:
        timer._onTimeout(args[0], args[1]);
        break;
      case 3:
        timer._onTimeout(args[0], args[1], args[2]);
        break;
      default:
        Function.prototype.apply.call(callback, timer, args);
    }
  }
  if (timer._repeat)
    rearm(timer);
}


function rearm(timer) {
  // // Do not re-arm unenroll'd or closed timers.
  if (timer._idleTimeout === -1) return;

  // If timer is unref'd (or was - it's permanently removed from the list.)
  if (timer._handle && timer instanceof Timeout) {
    timer._handle.start(timer._repeat);
  } else {
    timer._idleTimeout = timer._repeat;
    active(timer);
  }
}


const clearTimeout = exports.clearTimeout = function(timer) {
  if (timer && (timer[kOnTimeout] || timer._onTimeout)) {
    timer[kOnTimeout] = timer._onTimeout = null;
    if (timer instanceof Timeout) {
      timer.close(); // for after === 0
    } else {
      unenroll(timer);
    }
  }
};


exports.setInterval = function(callback, repeat, arg1, arg2, arg3) {
  if (typeof callback !== 'function') {
    throw new TypeError('"callback" argument must be a function');
  }

  var len = arguments.length;
  var args;
  if (len === 3) {
    args = [arg1];
  } else if (len === 4) {
    args = [arg1, arg2];
  } else if (len > 4) {
    args = [arg1, arg2, arg3];
    for (var i = 5; i < len; i++)
      // extend array dynamically, makes .apply run much faster in v6.0.0
      args[i - 2] = arguments[i];
  }

  return createRepeatTimeout(callback, repeat, args);
};

function createRepeatTimeout(callback, repeat, args) {
  repeat *= 1; // coalesce to number or NaN
  if (!(repeat >= 1 && repeat <= TIMEOUT_MAX))
    repeat = 1; // schedule on next tick, follows browser behavior

  var timer = new Timeout(repeat, callback, args);
  timer._repeat = repeat;
  if (process.domain)
    timer.domain = process.domain;

  active(timer);

  return timer;
}

exports.clearInterval = function(timer) {
  if (timer && timer._repeat) {
    timer._repeat = null;
    clearTimeout(timer);
  }
};


function Timeout(after, callback, args) {
  this._called = false;
  this._idleTimeout = after;
  this._idlePrev = this;
  this._idleNext = this;
  this._idleStart = null;
  this._onTimeout = callback;
  this._timerArgs = args;
  this._repeat = null;
  this._destroyed = false;
  this[async_id_symbol] = ++async_id_fields[kAsyncIdCounter];
  this[trigger_async_id_symbol] = initTriggerId();
  if (async_hook_fields[kInit] > 0)
    emitInit(
      this[async_id_symbol], 'Timeout', this[trigger_async_id_symbol], this
    );
}


function unrefdHandle() {
  // Don't attempt to call the callback if it is not a function.
  if (typeof this.owner._onTimeout === 'function') {
    ontimeout(this.owner);
  }

  // Make sure we clean up if the callback is no longer a function
  // even if the timer is an interval.
  if (!this.owner._repeat ||
      typeof this.owner._onTimeout !== 'function') {
    this.owner.close();
  }
}


Timeout.prototype.unref = function() {
  if (this._handle) {
    this._handle.unref();
  } else if (typeof this._onTimeout === 'function') {
    var now = TimerWrap.now();
    if (!this._idleStart) this._idleStart = now;
    var delay = this._idleStart + this._idleTimeout - now;
    if (delay < 0) delay = 0;

    // Prevent running cb again when unref() is called during the same cb
    if (this._called && !this._repeat) {
      unenroll(this);
      return;
    }

    var handle = reuse(this);

    this._handle = handle || new TimerWrap();
    this._handle.owner = this;
    this._handle[kOnTimeout] = unrefdHandle;
    this._handle.start(delay);
    this._handle.domain = this.domain;
    this._handle.unref();
  }
  return this;
};

Timeout.prototype.ref = function() {
  if (this._handle)
    this._handle.ref();
  return this;
};

Timeout.prototype.close = function() {
  this._onTimeout = null;
  if (this._handle) {
    // Fewer checks may be possible, but these cover everything.
    if (async_hook_fields[kDestroy] > 0 &&
        this &&
        typeof this[async_id_symbol] === 'number' &&
        !this._destroyed) {
      emitDestroy(this[async_id_symbol]);
      this._destroyed = true;
    }

    this._idleTimeout = -1;
    this._handle[kOnTimeout] = null;
    this._handle.close();
  } else {
    unenroll(this);
  }
  return this;
};


// A linked list for storing `setImmediate()` requests
function ImmediateList() {
  this.head = null;
  this.tail = null;
}

// Appends an item to the end of the linked list, adjusting the current tail's
// previous and next pointers where applicable
ImmediateList.prototype.append = function(item) {
  if (this.tail) {
    this.tail._idleNext = item;
    item._idlePrev = this.tail;
  } else {
    this.head = item;
  }
  this.tail = item;
};

// Removes an item from the linked list, adjusting the pointers of adjacent
// items and the linked list's head or tail pointers as necessary
ImmediateList.prototype.remove = function(item) {
  if (item._idleNext) {
    item._idleNext._idlePrev = item._idlePrev;
  }

  if (item._idlePrev) {
    item._idlePrev._idleNext = item._idleNext;
  }

  if (item === this.head)
    this.head = item._idleNext;
  if (item === this.tail)
    this.tail = item._idlePrev;

  item._idleNext = null;
  item._idlePrev = null;
};

// Create a single linked list instance only once at startup
var immediateQueue = new ImmediateList();


function processImmediate() {
  var immediate = immediateQueue.head;
  var tail = immediateQueue.tail;
  var domain;

  // Clear the linked list early in case new `setImmediate()` calls occur while
  // immediate callbacks are executed
  immediateQueue.head = immediateQueue.tail = null;

  while (immediate) {
    domain = immediate.domain;

    if (!immediate._onImmediate) {
      immediate = immediate._idleNext;
      continue;
    }

    if (domain)
      domain.enter();

    immediate._callback = immediate._onImmediate;

    // Save next in case `clearImmediate(immediate)` is called from callback
    var next = immediate._idleNext;

    tryOnImmediate(immediate, tail);

    if (domain)
      domain.exit();

    // If `clearImmediate(immediate)` wasn't called from the callback, use the
    // `immediate`'s next item
    if (immediate._idleNext)
      immediate = immediate._idleNext;
    else
      immediate = next;
  }

  // Only round-trip to C++ land if we have to. Calling clearImmediate() on an
  // immediate that's in |queue| is okay. Worst case is we make a superfluous
  // call to NeedImmediateCallbackSetter().
  if (!immediateQueue.head) {
    process._needImmediateCallback = false;
  }
}


// An optimization so that the try/finally only de-optimizes (since at least v8
// 4.7) what is in this smaller function.
function tryOnImmediate(immediate, oldTail) {
  var threw = true;
  emitBefore(immediate[async_id_symbol], immediate[trigger_async_id_symbol]);
  try {
    // make the actual call outside the try/finally to allow it to be optimized
    runCallback(immediate);
    threw = false;
  } finally {
    // clearImmediate checks _callback === null for kDestroy hooks.
    immediate._callback = null;
    if (!threw)
      emitAfter(immediate[async_id_symbol]);
    if (async_hook_fields[kDestroy] > 0 && !immediate._destroyed) {
      emitDestroy(immediate[async_id_symbol]);
      immediate._destroyed = true;
    }

    if (threw && immediate._idleNext) {
      // Handle any remaining on next tick, assuming we're still alive to do so.
      const curHead = immediateQueue.head;
      const next = immediate._idleNext;
      if (curHead) {
        curHead._idlePrev = oldTail;
        oldTail._idleNext = curHead;
        next._idlePrev = null;
        immediateQueue.head = next;
      } else {
        immediateQueue.head = next;
        immediateQueue.tail = oldTail;
      }
      process.nextTick(processImmediate);
    }
  }
}

function runCallback(timer) {
  const argv = timer._argv;
  const argc = argv ? argv.length : 0;
  if (typeof timer._callback !== 'function')
    return promiseResolve(timer._callback, argv[0]);
  switch (argc) {
    // fast-path callbacks with 0-3 arguments
    case 0:
      return timer._callback();
    case 1:
      return timer._callback(argv[0]);
    case 2:
      return timer._callback(argv[0], argv[1]);
    case 3:
      return timer._callback(argv[0], argv[1], argv[2]);
    // more than 3 arguments run slower with .apply
    default:
      return Function.prototype.apply.call(timer._callback, timer, argv);
  }
}


function Immediate() {
  // assigning the callback here can cause optimize/deoptimize thrashing
  // so have caller annotate the object (node v6.0.0, v8 5.0.71.35)
  this._idleNext = null;
  this._idlePrev = null;
  this._callback = null;
  this._argv = null;
  this._onImmediate = null;
  this._destroyed = false;
  this.domain = process.domain;
  this[async_id_symbol] = ++async_id_fields[kAsyncIdCounter];
  this[trigger_async_id_symbol] = initTriggerId();
  if (async_hook_fields[kInit] > 0)
    emitInit(
      this[async_id_symbol], 'Immediate', this[trigger_async_id_symbol], this
    );
}

function setImmediate(callback, arg1, arg2, arg3) {
  if (typeof callback !== 'function') {
    throw new TypeError('"callback" argument must be a function');
  }

  var i, args;

  switch (arguments.length) {
    // fast cases
    case 1:
      break;
    case 2:
      args = [arg1];
      break;
    case 3:
      args = [arg1, arg2];
      break;
    default:
      args = [arg1, arg2, arg3];
      for (i = 4; i < arguments.length; i++)
        // extend array dynamically, makes .apply run much faster in v6.0.0
        args[i - 1] = arguments[i];
      break;
  }
  return createImmediate(args, callback);
}

setImmediate[internalUtil.promisify.custom] = function(value) {
  const promise = createPromise();
  createImmediate([value], promise);
  return promise;
};

exports.setImmediate = setImmediate;

function createImmediate(args, callback) {
  // declaring it `const immediate` causes v6.0.0 to deoptimize this function
  var immediate = new Immediate();
  immediate._callback = callback;
  immediate._argv = args;
  immediate._onImmediate = callback;

  if (!process._needImmediateCallback) {
    process._needImmediateCallback = true;
    process._immediateCallback = processImmediate;
  }

  immediateQueue.append(immediate);

  return immediate;
}


exports.clearImmediate = function(immediate) {
  if (!immediate) return;

  if (async_hook_fields[kDestroy] > 0 &&
      immediate._callback !== null &&
      !immediate._destroyed) {
    emitDestroy(immediate[async_id_symbol]);
    immediate._destroyed = true;
  }

  immediate._onImmediate = null;

  immediateQueue.remove(immediate);

  if (!immediateQueue.head) {
    process._needImmediateCallback = false;
  }
};