summaryrefslogtreecommitdiffstats
path: root/toolkit/modules/DeferredTask.jsm
blob: f13c71f53c655b4eb3ea25831d2f97e35398996b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
/* vim: set ts=2 et sw=2 tw=80 filetype=javascript: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

this.EXPORTED_SYMBOLS = [
  "DeferredTask",
];

/**
 * Sets up a function or an asynchronous task whose execution can be triggered
 * after a defined delay.  Multiple attempts to run the task before the delay
 * has passed are coalesced.  The task cannot be re-entered while running, but
 * can be executed again after a previous run finished.
 *
 * A common use case occurs when a data structure should be saved into a file
 * every time the data changes, using asynchronous calls, and multiple changes
 * to the data may happen within a short time:
 *
 *   let saveDeferredTask = new DeferredTask(function* () {
 *     yield OS.File.writeAtomic(...);
 *     // Any uncaught exception will be reported.
 *   }, 2000);
 *
 *   // The task is ready, but will not be executed until requested.
 *
 * The "arm" method can be used to start the internal timer that will result in
 * the eventual execution of the task.  Multiple attempts to arm the timer don't
 * introduce further delays:
 *
 *   saveDeferredTask.arm();
 *
 *   // The task will be executed in 2 seconds from now.
 *
 *   yield waitOneSecond();
 *   saveDeferredTask.arm();
 *
 *   // The task will be executed in 1 second from now.
 *
 * The timer can be disarmed to reset the delay, or just to cancel execution:
 *
 *   saveDeferredTask.disarm();
 *   saveDeferredTask.arm();
 *
 *   // The task will be executed in 2 seconds from now.
 *
 * When the internal timer fires and the execution of the task starts, the task
 * cannot be canceled anymore.  It is however possible to arm the timer again
 * during the execution of the task, in which case the task will need to finish
 * before the timer is started again, thus guaranteeing a time of inactivity
 * between executions that is at least equal to the provided delay.
 *
 * The "finalize" method can be used to ensure that the task terminates
 * properly.  The promise it returns is resolved only after the last execution
 * of the task is finished.  To guarantee that the task is executed for the
 * last time, the method prevents any attempt to arm the timer again.
 *
 * If the timer is already armed when the "finalize" method is called, then the
 * task is executed immediately.  If the task was already running at this point,
 * then one last execution from start to finish will happen again, immediately
 * after the current execution terminates.  If the timer is not armed, the
 * "finalize" method only ensures that any running task terminates.
 *
 * For example, during shutdown, you may want to ensure that any pending write
 * is processed, using the latest version of the data if the timer is armed:
 *
 *   AsyncShutdown.profileBeforeChange.addBlocker(
 *     "Example service: shutting down",
 *     () => saveDeferredTask.finalize()
 *   );
 *
 * Instead, if you are going to delete the saved data from disk anyways, you
 * might as well prevent any pending write from starting, while still ensuring
 * that any write that is currently in progress terminates, so that the file is
 * not in use anymore:
 *
 *   saveDeferredTask.disarm();
 *   saveDeferredTask.finalize().then(() => OS.File.remove(...))
 *                              .then(null, Components.utils.reportError);
 */

// Globals

const { classes: Cc, interfaces: Ci, utils: Cu, results: Cr } = Components;

Cu.import("resource://gre/modules/XPCOMUtils.jsm");

XPCOMUtils.defineLazyModuleGetter(this, "Promise",
                                  "resource://gre/modules/Promise.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "Task",
                                  "resource://gre/modules/Task.jsm");

const Timer = Components.Constructor("@mozilla.org/timer;1", "nsITimer",
                                     "initWithCallback");

// DeferredTask

/**
 * Sets up a task whose execution can be triggered after a delay.
 *
 * @param aTaskFn
 *        Function or generator function to execute.  This argument is passed to
 *        the "Task.spawn" method every time the task should be executed.  This
 *        task is never re-entered while running.
 * @param aDelayMs
 *        Time between executions, in milliseconds.  Multiple attempts to run
 *        the task before the delay has passed are coalesced.  This time of
 *        inactivity is guaranteed to pass between multiple executions of the
 *        task, except on finalization, when the task may restart immediately
 *        after the previous execution finished.
 */
this.DeferredTask = function (aTaskFn, aDelayMs) {
  this._taskFn = aTaskFn;
  this._delayMs = aDelayMs;
}

this.DeferredTask.prototype = {
  /**
   * Function or generator function to execute.
   */
  _taskFn: null,

  /**
   * Time between executions, in milliseconds.
   */
  _delayMs: null,

  /**
   * Indicates whether the task is currently requested to start again later,
   * regardless of whether it is currently running.
   */
  get isArmed() {
    return this._armed;
  },
  _armed: false,

  /**
   * Indicates whether the task is currently running.  This is always true when
   * read from code inside the task function, but can also be true when read
   * from external code, in case the task is an asynchronous generator function.
   */
  get isRunning() {
    return !!this._runningPromise;
  },

  /**
   * Promise resolved when the current execution of the task terminates, or null
   * if the task is not currently running.
   */
  _runningPromise: null,

  /**
   * nsITimer used for triggering the task after a delay, or null in case the
   * task is running or there is no task scheduled for execution.
   */
  _timer: null,

  /**
   * Actually starts the timer with the delay specified on construction.
   */
  _startTimer: function ()
  {
    this._timer = new Timer(this._timerCallback.bind(this), this._delayMs,
                            Ci.nsITimer.TYPE_ONE_SHOT);
  },

  /**
   * Requests the execution of the task after the delay specified on
   * construction.  Multiple calls don't introduce further delays.  If the task
   * is running, the delay will start when the current execution finishes.
   *
   * The task will always be executed on a different tick of the event loop,
   * even if the delay specified on construction is zero.  Multiple "arm" calls
   * within the same tick of the event loop are guaranteed to result in a single
   * execution of the task.
   *
   * @note By design, this method doesn't provide a way for the caller to detect
   *       when the next execution terminates, or collect a result.  In fact,
   *       doing that would often result in duplicate processing or logging.  If
   *       a special operation or error logging is needed on completion, it can
   *       be better handled from within the task itself, for example using a
   *       try/catch/finally clause in the task.  The "finalize" method can be
   *       used in the common case of waiting for completion on shutdown.
   */
  arm: function ()
  {
    if (this._finalized) {
      throw new Error("Unable to arm timer, the object has been finalized.");
    }

    this._armed = true;

    // In case the timer callback is running, do not create the timer now,
    // because this will be handled by the timer callback itself.  Also, the
    // timer is not restarted in case it is already running.
    if (!this._runningPromise && !this._timer) {
      this._startTimer();
    }
  },

  /**
   * Cancels any request for a delayed the execution of the task, though the
   * task itself cannot be canceled in case it is already running.
   *
   * This method stops any currently running timer, thus the delay will restart
   * from its original value in case the "arm" method is called again.
   */
  disarm: function () {
    this._armed = false;
    if (this._timer) {
      // Calling the "cancel" method and discarding the timer reference makes
      // sure that the timer callback will not be called later, even if the
      // timer thread has already posted the timer event on the main thread.
      this._timer.cancel();
      this._timer = null;
    }
  },

  /**
   * Ensures that any pending task is executed from start to finish, while
   * preventing any attempt to arm the timer again.
   *
   * - If the task is running and the timer is armed, then one last execution
   *   from start to finish will happen again, immediately after the current
   *   execution terminates, then the returned promise will be resolved.
   * - If the task is running and the timer is not armed, the returned promise
   *   will be resolved when the current execution terminates.
   * - If the task is not running and the timer is armed, then the task is
   *   started immediately, and the returned promise resolves when the new
   *   execution terminates.
   * - If the task is not running and the timer is not armed, the method returns
   *   a resolved promise.
   *
   * @return {Promise}
   * @resolves After the last execution of the task is finished.
   * @rejects Never.
   */
  finalize: function () {
    if (this._finalized) {
      throw new Error("The object has been already finalized.");
    }
    this._finalized = true;

    // If the timer is armed, it means that the task is not running but it is
    // scheduled for execution.  Cancel the timer and run the task immediately.
    if (this._timer) {
      this.disarm();
      this._timerCallback();
    }

    // Wait for the operation to be completed, or resolve immediately.
    if (this._runningPromise) {
      return this._runningPromise;
    }
    return Promise.resolve();
  },
  _finalized: false,

  /**
   * Timer callback used to run the delayed task.
   */
  _timerCallback: function ()
  {
    let runningDeferred = Promise.defer();

    // All these state changes must occur at the same time directly inside the
    // timer callback, to prevent race conditions and to ensure that all the
    // methods behave consistently even if called from inside the task.  This
    // means that the assignment of "this._runningPromise" must complete before
    // the task gets a chance to start.
    this._timer = null;
    this._armed = false;
    this._runningPromise = runningDeferred.promise;

    runningDeferred.resolve(Task.spawn(function* () {
      // Execute the provided function asynchronously.
      yield Task.spawn(this._taskFn).then(null, Cu.reportError);

      // Now that the task has finished, we check the state of the object to
      // determine if we should restart the task again.
      if (this._armed) {
        if (!this._finalized) {
          this._startTimer();
        } else {
          // Execute the task again immediately, for the last time.  The isArmed
          // property should return false while the task is running, and should
          // remain false after the last execution terminates.
          this._armed = false;
          yield Task.spawn(this._taskFn).then(null, Cu.reportError);
        }
      }

      // Indicate that the execution of the task has finished.  This happens
      // synchronously with the previous state changes in the function.
      this._runningPromise = null;
    }.bind(this)).then(null, Cu.reportError));
  },
};