gecko/toolkit/modules/Task.jsm
2016-02-03 17:27:36 -08:00

527 lines
17 KiB
JavaScript

/* -*- 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 = [
"Task"
];
/**
* This module implements a subset of "Task.js" <http://taskjs.org/>.
*
* Paraphrasing from the Task.js site, tasks make sequential, asynchronous
* operations simple, using the power of JavaScript's "yield" operator.
*
* Tasks are built upon generator functions and promises, documented here:
*
* <https://developer.mozilla.org/en/JavaScript/Guide/Iterators_and_Generators>
* <http://wiki.commonjs.org/wiki/Promises/A>
*
* The "Task.spawn" function takes a generator function and starts running it as
* a task. Every time the task yields a promise, it waits until the promise is
* fulfilled. "Task.spawn" returns a promise that is resolved when the task
* completes successfully, or is rejected if an exception occurs.
*
* -----------------------------------------------------------------------------
*
* Cu.import("resource://gre/modules/Task.jsm");
*
* Task.spawn(function* () {
*
* // This is our task. Let's create a promise object, wait on it and capture
* // its resolution value.
* let myPromise = getPromiseResolvedOnTimeoutWithValue(1000, "Value");
* let result = yield myPromise;
*
* // This part is executed only after the promise above is fulfilled (after
* // one second, in this imaginary example). We can easily loop while
* // calling asynchronous functions, and wait multiple times.
* for (let i = 0; i < 3; i++) {
* result += yield getPromiseResolvedOnTimeoutWithValue(50, "!");
* }
*
* return "Resolution result for the task: " + result;
* }).then(function (result) {
*
* // result == "Resolution result for the task: Value!!!"
*
* // The result is undefined if no value was returned.
*
* }, function (exception) {
*
* // Failure! We can inspect or report the exception.
*
* });
*
* -----------------------------------------------------------------------------
*
* This module implements only the "Task.js" interfaces described above, with no
* additional features to control the task externally, or do custom scheduling.
* It also provides the following extensions that simplify task usage in the
* most common cases:
*
* - The "Task.spawn" function also accepts an iterator returned by a generator
* function, in addition to a generator function. This way, you can call into
* the generator function with the parameters you want, and with "this" bound
* to the correct value. Also, "this" is never bound to the task object when
* "Task.spawn" calls the generator function.
*
* - In addition to a promise object, a task can yield the iterator returned by
* a generator function. The iterator is turned into a task automatically.
* This reduces the syntax overhead of calling "Task.spawn" explicitly when
* you want to recurse into other task functions.
*
* - The "Task.spawn" function also accepts a primitive value, or a function
* returning a primitive value, and treats the value as the result of the
* task. This makes it possible to call an externally provided function and
* spawn a task from it, regardless of whether it is an asynchronous generator
* or a synchronous function. This comes in handy when iterating over
* function lists where some items have been converted to tasks and some not.
*/
////////////////////////////////////////////////////////////////////////////////
//// Globals
const Cc = Components.classes;
const Ci = Components.interfaces;
const Cu = Components.utils;
const Cr = Components.results;
// For now, we're worried about add-ons using Tasks with CPOWs, so we'll
// permit them in this scope, but this support will go away soon.
Cu.permitCPOWsInScope(this);
Cu.import("resource://gre/modules/Promise.jsm");
// The following error types are considered programmer errors, which should be
// reported (possibly redundantly) so as to let programmers fix their code.
const ERRORS_TO_REPORT = ["EvalError", "RangeError", "ReferenceError", "TypeError"];
/**
* The Task currently being executed
*/
var gCurrentTask = null;
/**
* If `true`, capture stacks whenever entering a Task and rewrite the
* stack any exception thrown through a Task.
*/
var gMaintainStack = false;
/**
* Iterate through the lines of a string.
*
* @return Iterator<string>
*/
function* linesOf(string) {
let reLine = /([^\r\n])+/g;
let match;
while ((match = reLine.exec(string))) {
yield [match[0], match.index];
}
}
/**
* Detect whether a value is a generator.
*
* @param aValue
* The value to identify.
* @return A boolean indicating whether the value is a generator.
*/
function isGenerator(aValue) {
return Object.prototype.toString.call(aValue) == "[object Generator]";
}
////////////////////////////////////////////////////////////////////////////////
//// Task
/**
* This object provides the public module functions.
*/
this.Task = {
/**
* Creates and starts a new task.
*
* @param aTask
* - If you specify a generator function, it is called with no
* arguments to retrieve the associated iterator. The generator
* function is a task, that is can yield promise objects to wait
* upon.
* - If you specify the iterator returned by a generator function you
* called, the generator function is also executed as a task. This
* allows you to call the function with arguments.
* - If you specify a function that is not a generator, it is called
* with no arguments, and its return value is used to resolve the
* returned promise.
* - If you specify anything else, you get a promise that is already
* resolved with the specified value.
*
* @return A promise object where you can register completion callbacks to be
* called when the task terminates.
*/
spawn: function Task_spawn(aTask) {
return createAsyncFunction(aTask).call(undefined);
},
/**
* Create and return an 'async function' that starts a new task.
*
* This is similar to 'spawn' except that it doesn't immediately start
* the task, it binds the task to the async function's 'this' object and
* arguments, and it requires the task to be a function.
*
* It simplifies the common pattern of implementing a method via a task,
* like this simple object with a 'greet' method that has a 'name' parameter
* and spawns a task to send a greeting and return its reply:
*
* let greeter = {
* message: "Hello, NAME!",
* greet: function(name) {
* return Task.spawn((function* () {
* return yield sendGreeting(this.message.replace(/NAME/, name));
* }).bind(this);
* })
* };
*
* With Task.async, the method can be declared succinctly:
*
* let greeter = {
* message: "Hello, NAME!",
* greet: Task.async(function* (name) {
* return yield sendGreeting(this.message.replace(/NAME/, name));
* })
* };
*
* While maintaining identical semantics:
*
* greeter.greet("Mitchell").then((reply) => { ... }); // behaves the same
*
* @param aTask
* The task function to start.
*
* @return A function that starts the task function and returns its promise.
*/
async: function Task_async(aTask) {
if (typeof(aTask) != "function") {
throw new TypeError("aTask argument must be a function");
}
return createAsyncFunction(aTask);
},
/**
* Constructs a special exception that, when thrown inside a legacy generator
* function (non-star generator), allows the associated task to be resolved
* with a specific value.
*
* Example: throw new Task.Result("Value");
*/
Result: function Task_Result(aValue) {
this.value = aValue;
}
};
function createAsyncFunction(aTask) {
let asyncFunction = function () {
let result = aTask;
if (aTask && typeof(aTask) == "function") {
if (aTask.isAsyncFunction) {
throw new TypeError(
"Cannot use an async function in place of a promise. " +
"You should either invoke the async function first " +
"or use 'Task.spawn' instead of 'Task.async' to start " +
"the Task and return its promise.");
}
try {
// Let's call into the function ourselves.
result = aTask.apply(this, arguments);
} catch (ex) {
if (ex instanceof Task.Result) {
return Promise.resolve(ex.value);
}
return Promise.reject(ex);
}
}
if (isGenerator(result)) {
// This is an iterator resulting from calling a generator function.
return new TaskImpl(result).deferred.promise;
}
// Just propagate the given value to the caller as a resolved promise.
return Promise.resolve(result);
};
asyncFunction.isAsyncFunction = true;
return asyncFunction;
}
////////////////////////////////////////////////////////////////////////////////
//// TaskImpl
/**
* Executes the specified iterator as a task, and gives access to the promise
* that is fulfilled when the task terminates.
*/
function TaskImpl(iterator) {
if (gMaintainStack) {
this._stack = (new Error()).stack;
}
this.deferred = Promise.defer();
this._iterator = iterator;
this._isStarGenerator = !("send" in iterator);
this._run(true);
}
TaskImpl.prototype = {
/**
* Includes the promise object where task completion callbacks are registered,
* and methods to resolve or reject the promise at task completion.
*/
deferred: null,
/**
* The iterator returned by the generator function associated with this task.
*/
_iterator: null,
/**
* Whether this Task is using a star generator.
*/
_isStarGenerator: false,
/**
* Main execution routine, that calls into the generator function.
*
* @param aSendResolved
* If true, indicates that we should continue into the generator
* function regularly (if we were waiting on a promise, it was
* resolved). If true, indicates that we should cause an exception to
* be thrown into the generator function (if we were waiting on a
* promise, it was rejected).
* @param aSendValue
* Resolution result or rejection exception, if any.
*/
_run: function TaskImpl_run(aSendResolved, aSendValue) {
try {
gCurrentTask = this;
if (this._isStarGenerator) {
try {
let result = aSendResolved ? this._iterator.next(aSendValue)
: this._iterator.throw(aSendValue);
if (result.done) {
// The generator function returned.
this.deferred.resolve(result.value);
} else {
// The generator function yielded.
this._handleResultValue(result.value);
}
} catch (ex) {
// The generator function failed with an uncaught exception.
this._handleException(ex);
}
} else {
try {
let yielded = aSendResolved ? this._iterator.send(aSendValue)
: this._iterator.throw(aSendValue);
this._handleResultValue(yielded);
} catch (ex) {
if (ex instanceof Task.Result) {
// The generator function threw the special exception that allows it to
// return a specific value on resolution.
this.deferred.resolve(ex.value);
} else if (ex instanceof StopIteration) {
// The generator function terminated with no specific result.
this.deferred.resolve(undefined);
} else {
// The generator function failed with an uncaught exception.
this._handleException(ex);
}
}
}
} finally {
//
// At this stage, the Task may have finished executing, or have
// walked through a `yield` or passed control to a sub-Task.
// Regardless, if we still own `gCurrentTask`, reset it. If we
// have not finished execution of this Task, re-entering `_run`
// will set `gCurrentTask` to `this` as needed.
//
// We just need to be careful here in case we hit the following
// pattern:
//
// Task.spawn(foo);
// Task.spawn(bar);
//
// Here, `foo` and `bar` may be interleaved, so when we finish
// executing `foo`, `gCurrentTask` may actually either `foo` or
// `bar`. If `gCurrentTask` has already been set to `bar`, leave
// it be and it will be reset to `null` once `bar` is complete.
//
if (gCurrentTask == this) {
gCurrentTask = null;
}
}
},
/**
* Handle a value yielded by a generator.
*
* @param aValue
* The yielded value to handle.
*/
_handleResultValue: function TaskImpl_handleResultValue(aValue) {
// If our task yielded an iterator resulting from calling another
// generator function, automatically spawn a task from it, effectively
// turning it into a promise that is fulfilled on task completion.
if (isGenerator(aValue)) {
aValue = Task.spawn(aValue);
}
if (aValue && typeof(aValue.then) == "function") {
// We have a promise object now. When fulfilled, call again into this
// function to continue the task, with either a resolution or rejection
// condition.
aValue.then(this._run.bind(this, true),
this._run.bind(this, false));
} else {
// If our task yielded a value that is not a promise, just continue and
// pass it directly as the result of the yield statement.
this._run(true, aValue);
}
},
/**
* Handle an uncaught exception thrown from a generator.
*
* @param aException
* The uncaught exception to handle.
*/
_handleException: function TaskImpl_handleException(aException) {
gCurrentTask = this;
if (aException && typeof aException == "object" && "stack" in aException) {
let stack = aException.stack;
if (gMaintainStack &&
aException._capturedTaskStack != this._stack &&
typeof stack == "string") {
// Rewrite the stack for more readability.
let bottomStack = this._stack;
let topStack = stack;
stack = Task.Debugging.generateReadableStack(stack);
aException.stack = stack;
// If aException is reinjected in the same task and rethrown,
// we don't want to perform the rewrite again.
aException._capturedTaskStack = bottomStack;
} else if (!stack) {
stack = "Not available";
}
if ("name" in aException &&
ERRORS_TO_REPORT.indexOf(aException.name) != -1) {
// We suspect that the exception is a programmer error, so we now
// display it using dump(). Note that we do not use Cu.reportError as
// we assume that this is a programming error, so we do not want end
// users to see it. Also, if the programmer handles errors correctly,
// they will either treat the error or log them somewhere.
dump("*************************\n");
dump("A coding exception was thrown and uncaught in a Task.\n\n");
dump("Full message: " + aException + "\n");
dump("Full stack: " + aException.stack + "\n");
dump("*************************\n");
}
}
this.deferred.reject(aException);
},
get callerStack() {
// Cut `this._stack` at the last line of the first block that
// contains Task.jsm, keep the tail.
for (let [line, index] of linesOf(this._stack || "")) {
if (line.indexOf("/Task.jsm:") == -1) {
return this._stack.substring(index);
}
}
return "";
}
};
Task.Debugging = {
/**
* Control stack rewriting.
*
* If `true`, any exception thrown from a Task will be rewritten to
* provide a human-readable stack trace. Otherwise, stack traces will
* be left unchanged.
*
* There is a (small but existing) runtime cost associated to stack
* rewriting, so you should probably not activate this in production
* code.
*
* @type {bool}
*/
get maintainStack() {
return gMaintainStack;
},
set maintainStack(x) {
if (!x) {
gCurrentTask = null;
}
return gMaintainStack = x;
},
/**
* Generate a human-readable stack for an error raised in
* a Task.
*
* @param {string} topStack The stack provided by the error.
* @param {string=} prefix Optionally, a prefix for each line.
*/
generateReadableStack: function(topStack, prefix = "") {
if (!gCurrentTask) {
return topStack;
}
// Cut `topStack` at the first line that contains Task.jsm, keep the head.
let lines = [];
for (let [line] of linesOf(topStack)) {
if (line.indexOf("/Task.jsm:") != -1) {
break;
}
lines.push(prefix + line);
}
if (!prefix) {
lines.push(gCurrentTask.callerStack);
} else {
for (let [line] of linesOf(gCurrentTask.callerStack)) {
lines.push(prefix + line);
}
}
return lines.join("\n");
}
};