mixpanel-browser/src/shared-lock.js

The official Mixpanel JavaScript browser client library

import { Promise } from './promise-polyfill';
import { console_with_prefix, localStorageSupported, _ } from './utils'; // eslint-disable-line camelcase
import { window } from './window';

var logger = console_with_prefix('lock');

/**
 * SharedLock: a mutex built on HTML5 localStorage, to ensure that only one browser
 * window/tab at a time will be able to access shared resources.
 *
 * Based on the Alur and Taubenfeld fast lock
 * (http://www.cs.rochester.edu/research/synchronization/pseudocode/fastlock.html)
 * with an added timeout to ensure there will be eventual progress in the event
 * that a window is closed in the middle of the callback.
 *
 * Implementation based on the original version by David Wolever (https://github.com/wolever)
 * at https://gist.github.com/wolever/5fd7573d1ef6166e8f8c4af286a69432.
 *
 * @example
 * const myLock = new SharedLock('some-key');
 * myLock.withLock(function() {
 *   console.log('I hold the mutex!');
 * });
 *
 * @constructor
 */
var SharedLock = function(key, options) {
    options = options || {};

    this.storageKey = key;
    this.storage = options.storage || window.localStorage;
    this.pollIntervalMS = options.pollIntervalMS || 100;
    this.timeoutMS = options.timeoutMS || 2000;

    // dependency-inject promise implementation for testing purposes
    this.promiseImpl = options.promiseImpl || Promise;
};

// pass in a specific pid to test contention scenarios; otherwise
// it is chosen randomly for each acquisition attempt
SharedLock.prototype.withLock = function(lockedCB, pid) {
    var Promise = this.promiseImpl;
    return new Promise(_.bind(function (resolve, reject) {
        var i = pid || (new Date().getTime() + '|' + Math.random());
        var startTime = new Date().getTime();
        var key = this.storageKey;
        var pollIntervalMS = this.pollIntervalMS;
        var timeoutMS = this.timeoutMS;
        var storage = this.storage;

        var keyX = key + ':X';
        var keyY = key + ':Y';
        var keyZ = key + ':Z';

        var delay = function(cb) {
            if (new Date().getTime() - startTime > timeoutMS) {
                logger.error('Timeout waiting for mutex on ' + key + '; clearing lock. [' + i + ']');
                storage.removeItem(keyZ);
                storage.removeItem(keyY);
                loop();
                return;
            }
            setTimeout(function() {
                try {
                    cb();
                } catch(err) {
                    reject(err);
                }
            }, pollIntervalMS * (Math.random() + 0.1));
        };

        var waitFor = function(predicate, cb) {
            if (predicate()) {
                cb();
            } else {
                delay(function() {
                    waitFor(predicate, cb);
                });
            }
        };

        var getSetY = function() {
            var valY = storage.getItem(keyY);
            if (valY && valY !== i) { // if Y == i then this process already has the lock (useful for test cases)
                return false;
            } else {
                storage.setItem(keyY, i);
                if (storage.getItem(keyY) === i) {
                    return true;
                } else {
                    if (!localStorageSupported(storage, true)) {
                        reject(new Error('localStorage support dropped while acquiring lock'));
                    }
                    return false;
                }
            }
        };

        var loop = function() {
            storage.setItem(keyX, i);

            waitFor(getSetY, function() {
                if (storage.getItem(keyX) === i) {
                    criticalSection();
                    return;
                }

                delay(function() {
                    if (storage.getItem(keyY) !== i) {
                        loop();
                        return;
                    }
                    waitFor(function() {
                        return !storage.getItem(keyZ);
                    }, criticalSection);
                });
            });
        };

        var criticalSection = function() {
            storage.setItem(keyZ, '1');
            var removeLock = function () {
                storage.removeItem(keyZ);
                if (storage.getItem(keyY) === i) {
                    storage.removeItem(keyY);
                }
                if (storage.getItem(keyX) === i) {
                    storage.removeItem(keyX);
                }
            };

            lockedCB()
                .then(function (ret) {
                    removeLock();
                    resolve(ret);
                })
                .catch(function (err) {
                    removeLock();
                    reject(err);
                });
        };

        try {
            if (localStorageSupported(storage, true)) {
                loop();
            } else {
                throw new Error('localStorage support check failed');
            }
        } catch(err) {
            reject(err);
        }
    }, this));
};

export { SharedLock };