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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";
var 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(async function() {
* await 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.
*
* await 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
ChromeUtils.defineModuleGetter(
this,
"PromiseUtils",
);
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 to execute. If the function returns a promise, the task is
* not considered complete until that promise resolves. 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.
* @param aIdleTimeoutMs
* The maximum time to wait for an idle slot on the main thread after
* aDelayMs have elapsed. If omitted, waits indefinitely for an idle
* callback.
*/
var DeferredTask = function(aTaskFn, aDelayMs, aIdleTimeoutMs) {
this._taskFn = aTaskFn;
this._delayMs = aDelayMs;
this._timeoutMs = aIdleTimeoutMs;
};
DeferredTask.prototype = {
/**
* 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 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() {
let callback, timer;
if (this._timeoutMs === 0) {
callback = () => this._timerCallback();
} else {
callback = () => {
this._startIdleDispatch(() => {
// _timer could have changed by now:
// - to null if disarm() or finalize() has been called.
// - to a new nsITimer if disarm() was called, followed by arm().
// In either case, don't invoke _timerCallback any more.
if (this._timer === timer) {
this._timerCallback();
}
}, this._timeoutMs);
};
}
timer = new Timer(callback, this._delayMs, Ci.nsITimer.TYPE_ONE_SHOT);
this._timer = timer;
},
/**
* Dispatches idle task. Can be overridden for testing by test_DeferredTask.
*/
_startIdleDispatch(callback, timeout) {
ChromeUtils.idleDispatch(callback, { timeout });
},
/**
* 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() {
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() {
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() {
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,
// so we don't risk blocking async shutdown longer than necessary.
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() {
let runningDeferred = PromiseUtils.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(
(async () => {
// Execute the provided function asynchronously.
await this._runTask();
// 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;
await this._runTask();
}
}
// Indicate that the execution of the task has finished. This happens
// synchronously with the previous state changes in the function.
this._runningPromise = null;
})().catch(Cu.reportError)
);
},
/**
* Executes the associated task and catches exceptions.
*/
async _runTask() {
try {
await this._taskFn();
} catch (ex) {
Cu.reportError(ex);
}
},
};