import FIREFOX_52_6_0esr_RELEASE from mozilla-esr52 hg repo

toolkit/modules/DeferredTask.jsm

@@ -0,0 +1,301 @@
+/* -*- 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));
+  },
+};