fcoin/lib/utils/lock.js

/*!
 * lock.js - lock and queue for bcoin
 * Copyright (c) 2014-2015, Fedor Indutny (MIT License)
 * Copyright (c) 2014-2017, Christopher Jeffrey (MIT License).
 * https://github.com/bcoin-org/bcoin
 */

'use strict';

const assert = require('assert');

/**
 * Represents a mutex lock for locking asynchronous object methods.
 * @alias module:utils.Lock
 * @constructor
 * @param {Boolean?} named - Whether to
 * maintain a map of queued jobs by job name.
 */

function Lock(named) {
  if (!(this instanceof Lock))
    return Lock.create(named);

  this.named = named === true;

  this.jobs = [];
  this.busy = false;
  this.destroyed = false;

  this.map = Object.create(null);
  this.current = null;

  this.unlocker = this.unlock.bind(this);
}

/**
 * Create a closure scoped lock.
 * @param {Boolean?} named
 * @returns {Function} Lock method.
 */

Lock.create = function create(named) {
  let lock = new Lock(named);
  return function _lock(arg1, arg2) {
    return lock.lock(arg1, arg2);
  };
};

/**
 * Test whether the lock has a pending
 * job or a job in progress (by name).
 * @param {String} name
 * @returns {Boolean}
 */

Lock.prototype.has = function has(name) {
  assert(this.named, 'Must use named jobs.');

  if (this.current === name)
    return true;

  return this.map[name] > 0;
};

/**
 * Test whether the lock has
 * a pending job by name.
 * @param {String} name
 * @returns {Boolean}
 */

Lock.prototype.hasPending = function hasPending(name) {
  assert(this.named, 'Must use named jobs.');
  return this.map[name] > 0;
};

/**
 * Lock the parent object and all its methods
 * which use the lock. Begin to queue calls.
 * @param {String?} name - Job name.
 * @param {Boolean?} force - Bypass the lock.
 * @returns {Promise} - Returns {Function}, must be
 * called once the method finishes executing in order
 * to resolve the queue.
 */

Lock.prototype.lock = function lock(arg1, arg2) {
  let name, force;

  if (this.named) {
    name = arg1 || null;
    force = arg2;
  } else {
    name = null;
    force = arg1;
  }

  if (this.destroyed)
    return Promise.reject(new Error('Lock is destroyed.'));

  if (force) {
    assert(this.busy);
    return Promise.resolve(nop);
  }

  if (this.busy) {
    if (name) {
      if (!this.map[name])
        this.map[name] = 0;
      this.map[name]++;
    }
    return new Promise((resolve, reject) => {
      this.jobs.push(new Job(resolve, reject, name));
    });
  }

  this.busy = true;
  this.current = name;

  return Promise.resolve(this.unlocker);
};

/**
 * The actual unlock callback.
 * @private
 */

Lock.prototype.unlock = function unlock() {
  let job;

  assert(this.destroyed || this.busy);

  this.busy = false;
  this.current = null;

  if (this.jobs.length === 0)
    return;

  assert(!this.destroyed);

  job = this.jobs.shift();

  if (job.name) {
    assert(this.map[job.name] > 0);
    if (--this.map[job.name] === 0)
      delete this.map[job.name];
  }

  this.busy = true;
  this.current = job.name;

  job.resolve(this.unlocker);
};

/**
 * Destroy the lock. Purge all pending calls.
 */

Lock.prototype.destroy = function destroy() {
  let jobs;

  assert(!this.destroyed, 'Lock is already destroyed.');

  this.destroyed = true;

  jobs = this.jobs.slice();

  this.busy = false;
  this.jobs.length = 0;
  this.map = Object.create(null);
  this.current = null;

  for (let job of jobs)
    job.reject(new Error('Lock was destroyed.'));
};

/**
 * Lock Job
 * @constructor
 * @ignore
 * @param {Function} resolve
 * @param {Function} reject
 * @param {String?} name
 */

function Job(resolve, reject, name) {
  this.resolve = resolve;
  this.reject = reject;
  this.name = name || null;
}

/*
 * Helpers
 */

function nop() {}

/*
 * Expose
 */

module.exports = Lock;