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authorMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
committerMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
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+/* -*- 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));
+ },
+};