node/lib/internal/process/next_tick.js

'use strict';

exports.setup = setupNextTick;
// Will be overwritten when setupNextTick() is called.
exports.nextTick = null;

function setupNextTick() {
  const async_wrap = process.binding('async_wrap');
  const async_hooks = require('internal/async_hooks');
  const promises = require('internal/process/promises');
  const errors = require('internal/errors');
  const emitPendingUnhandledRejections = promises.setup(scheduleMicrotasks);
  const getDefaultTriggerAsyncId = async_hooks.getDefaultTriggerAsyncId;
  // Two arrays that share state between C++ and JS.
  const { async_hook_fields, async_id_fields } = async_wrap;
  // Used to change the state of the async id stack.
  const { emitInit, emitBefore, emitAfter, emitDestroy } = async_hooks;
  // Grab the constants necessary for working with internal arrays.
  const { kInit, kDestroy, kAsyncIdCounter } = async_wrap.constants;
  const { async_id_symbol, trigger_async_id_symbol } = async_wrap;

  // tickInfo is used so that the C++ code in src/node.cc can
  // have easy access to our nextTick state, and avoid unnecessary
  // calls into JS land.
  // runMicrotasks is used to run V8's micro task queue.
  const [
    tickInfo,
    runMicrotasks
  ] = process._setupNextTick(_tickCallback);

  // *Must* match Environment::TickInfo::Fields in src/env.h.
  const kScheduled = 0;

  const nextTickQueue = {
    head: null,
    tail: null,
    push(data) {
      const entry = { data, next: null };
      if (this.tail !== null) {
        this.tail.next = entry;
      } else {
        this.head = entry;
        tickInfo[kScheduled] = 1;
      }
      this.tail = entry;
    },
    shift() {
      if (this.head === null)
        return;
      const ret = this.head.data;
      if (this.head === this.tail) {
        this.head = this.tail = null;
        tickInfo[kScheduled] = 0;
      } else {
        this.head = this.head.next;
      }
      return ret;
    }
  };

  var microtasksScheduled = false;

  process.nextTick = nextTick;
  // Needs to be accessible from beyond this scope.
  process._tickCallback = _tickCallback;

  // Set the nextTick() function for internal usage.
  exports.nextTick = internalNextTick;

  const microTasksTickObject = {
    callback: runMicrotasksCallback,
    args: undefined,
    [async_id_symbol]: 0,
    [trigger_async_id_symbol]: 0
  };
  function scheduleMicrotasks() {
    if (microtasksScheduled)
      return;

    // For the moment all microtasks come from the void until the PromiseHook
    // API is implemented.
    nextTickQueue.push(microTasksTickObject);
    microtasksScheduled = true;
  }

  function runMicrotasksCallback() {
    microtasksScheduled = false;
    runMicrotasks();

    if (nextTickQueue.head !== null || emitPendingUnhandledRejections())
      scheduleMicrotasks();
  }

  function _tickCallback() {
    let tock;
    do {
      while (tock = nextTickQueue.shift()) {
        const asyncId = tock[async_id_symbol];
        emitBefore(asyncId, tock[trigger_async_id_symbol]);
        // emitDestroy() places the async_id_symbol into an asynchronous queue
        // that calls the destroy callback in the future. It's called before
        // calling tock.callback so destroy will be called even if the callback
        // throws an exception that is handled by 'uncaughtException' or a
        // domain.
        // TODO(trevnorris): This is a bit of a hack. It relies on the fact
        // that nextTick() doesn't allow the event loop to proceed, but if
        // any async hooks are enabled during the callback's execution then
        // this tock's after hook will be called, but not its destroy hook.
        if (async_hook_fields[kDestroy] > 0)
          emitDestroy(asyncId);

        const callback = tock.callback;
        if (tock.args === undefined)
          callback();
        else
          Reflect.apply(callback, undefined, tock.args);

        emitAfter(asyncId);
      }
      runMicrotasks();
      emitPendingUnhandledRejections();
    } while (nextTickQueue.head !== null);
  }

  class TickObject {
    constructor(callback, args, triggerAsyncId) {
      // this must be set to null first to avoid function tracking
      // on the hidden class, revisit in V8 versions after 6.2
      this.callback = null;
      this.callback = callback;
      this.args = args;

      const asyncId = ++async_id_fields[kAsyncIdCounter];
      this[async_id_symbol] = asyncId;
      this[trigger_async_id_symbol] = triggerAsyncId;

      if (async_hook_fields[kInit] > 0) {
        emitInit(asyncId,
                 'TickObject',
                 triggerAsyncId,
                 this);
      }
    }
  }

  // `nextTick()` will not enqueue any callback when the process is about to
  // exit since the callback would not have a chance to be executed.
  function nextTick(callback) {
    if (typeof callback !== 'function')
      throw new errors.TypeError('ERR_INVALID_CALLBACK');

    if (process._exiting)
      return;

    var args;
    switch (arguments.length) {
      case 1: break;
      case 2: args = [arguments[1]]; break;
      case 3: args = [arguments[1], arguments[2]]; break;
      case 4: args = [arguments[1], arguments[2], arguments[3]]; break;
      default:
        args = new Array(arguments.length - 1);
        for (var i = 1; i < arguments.length; i++)
          args[i - 1] = arguments[i];
    }

    nextTickQueue.push(new TickObject(callback, args,
                                      getDefaultTriggerAsyncId()));
  }

  // `internalNextTick()` will not enqueue any callback when the process is
  // about to exit since the callback would not have a chance to be executed.
  function internalNextTick(triggerAsyncId, callback) {
    if (typeof callback !== 'function')
      throw new errors.TypeError('ERR_INVALID_CALLBACK');
    // CHECK(Number.isSafeInteger(triggerAsyncId) || triggerAsyncId === null)
    // CHECK(triggerAsyncId > 0 || triggerAsyncId === null)

    if (process._exiting)
      return;

    var args;
    switch (arguments.length) {
      case 2: break;
      case 3: args = [arguments[2]]; break;
      case 4: args = [arguments[2], arguments[3]]; break;
      case 5: args = [arguments[2], arguments[3], arguments[4]]; break;
      default:
        args = new Array(arguments.length - 2);
        for (var i = 2; i < arguments.length; i++)
          args[i - 2] = arguments[i];
    }

    if (triggerAsyncId === null)
      triggerAsyncId = getDefaultTriggerAsyncId();
    nextTickQueue.push(new TickObject(callback, args, triggerAsyncId));
  }
}