The Tor Browser: toolkit/modules/Promise-backend.js@129ffea94266 (annotated)

toolkit/modules/Promise-backend.js@129ffea94266 (annotated)

toolkit/modules/Promise-backend.js

Sat, 03 Jan 2015 20:18:00 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Sat, 03 Jan 2015 20:18:00 +0100
branch: TOR_BUG_3246
changeset 7: 129ffea94266
permissions: -rwxr-xr-x

Conditionally enable double key logic according to:
private browsing mode or privacy.thirdparty.isolate preference and
implement in GetCookieStringCommon and FindCookie where it counts...
With some reservations of how to convince FindCookie users to test
condition and pass a nullptr when disabling double key logic.

 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 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 implementation file is imported by the Promise.jsm module, and as a
  * special case by the debugger server.  To support chrome debugging, the
  * debugger server needs to have all its code in one global, so it must use
  * loadSubScript directly.
  *
  * In the general case, this script should be used by importing Promise.jsm:
  *
  * Components.utils.import("resource://gre/modules/Promise.jsm");
  *
  * More documentation can be found in the Promise.jsm module.
  */
 ////////////////////////////////////////////////////////////////////////////////
 //// Globals
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 const STATUS_PENDING = 0;
 const STATUS_RESOLVED = 1;
 const STATUS_REJECTED = 2;
 // These "private names" allow some properties of the Promise object to be
 // accessed only by this module, while still being visible on the object
 // manually when using a debugger.  They don't strictly guarantee that the
 // properties are inaccessible by other code, but provide enough protection to
 // avoid using them by mistake.
 const salt = Math.floor(Math.random() * 100);
 const Name = (n) => "{private:" + n + ":" + salt + "}";
 const N_STATUS = Name("status");
 const N_VALUE = Name("value");
 const N_HANDLERS = Name("handlers");
 const N_WITNESS = Name("witness");
 /////// Warn-upon-finalization mechanism
 //
 // One of the difficult problems with promises is locating uncaught
 // rejections. We adopt the following strategy: if a promise is rejected
 // at the time of its garbage-collection *and* if the promise is at the
 // end of a promise chain (i.e. |thatPromise.then| has never been
 // called), then we print a warning.
 //
 //  let deferred = Promise.defer();
 //  let p = deferred.promise.then();
 //  deferred.reject(new Error("I am un uncaught error"));
 //  deferred = null;
 //  p = null;
 //
 // In this snippet, since |deferred.promise| is not the last in the
 // chain, no error will be reported for that promise. However, since
 // |p| is the last promise in the chain, the error will be reported
 // for |p|.
 //
 // Note that this may, in some cases, cause an error to be reported more
 // than once. For instance, consider:
 //
 //   let deferred = Promise.defer();
 //   let p1 = deferred.promise.then();
 //   let p2 = deferred.promise.then();
 //   deferred.reject(new Error("I am an uncaught error"));
 //   p1 = p2 = deferred = null;
 //
 // In this snippet, the error is reported both by p1 and by p2.
 //
 XPCOMUtils.defineLazyServiceGetter(this, "FinalizationWitnessService",
                                    "@mozilla.org/toolkit/finalizationwitness;1",
                                    "nsIFinalizationWitnessService");
 let PendingErrors = {
   // An internal counter, used to generate unique id.
   _counter: 0,
   // Functions registered to be notified when a pending error
   // is reported as uncaught.
   _observers: new Set(),
   _map: new Map(),
   /**
    * Initialize PendingErrors
    */
   init: function() {
     Services.obs.addObserver(function observe(aSubject, aTopic, aValue) {
       PendingErrors.report(aValue);
     }, "promise-finalization-witness", false);
   },
   /**
    * Register an error as tracked.
    *
    * @return The unique identifier of the error.
    */
   register: function(error) {
     let id = "pending-error-" + (this._counter++);
     //
     // At this stage, ideally, we would like to store the error itself
     // and delay any treatment until we are certain that we will need
     // to report that error. However, in the (unlikely but possible)
     // case the error holds a reference to the promise itself, doing so
     // would prevent the promise from being garbabe-collected, which
     // would both cause a memory leak and ensure that we cannot report
     // the uncaught error.
     //
     // To avoid this situation, we rather extract relevant data from
     // the error and further normalize it to strings.
     //
     let value = {
       date: new Date(),
       message: "" + error,
       fileName: null,
       stack: null,
       lineNumber: null
     };
     try { // Defend against non-enumerable values
       if (error && error instanceof Ci.nsIException) {
         // nsIException does things a little differently.
         try {
           // For starters |.toString()| does not only contain the message, but
           // also the top stack frame, and we don't really want that.
           value.message = error.message;
         } catch (ex) {
           // Ignore field
         }
         try {
           // All lowercase filename. ;)
           value.fileName = error.filename;
         } catch (ex) {
           // Ignore field
         }
         try {
           value.lineNumber = error.lineNumber;
         } catch (ex) {
           // Ignore field
         }
       } else if (typeof error == "object" && error) {
         for (let k of ["fileName", "stack", "lineNumber"]) {
           try { // Defend against fallible getters and string conversions
             let v = error[k];
             value[k] = v ? ("" + v) : null;
           } catch (ex) {
             // Ignore field
           }
         }
       }
       if (!value.stack) {
         // |error| is not an Error (or Error-alike). Try to figure out the stack.
         let stack = null;
         if (error && error.location &&
             error.location instanceof Ci.nsIStackFrame) {
           // nsIException has full stack frames in the |.location| member.
           stack = error.location;
         } else {
           // Components.stack to the rescue!
           stack  = Components.stack;
           // Remove those top frames that refer to Promise.jsm.
           while (stack) {
             if (!stack.filename.endsWith("/Promise.jsm")) {
               break;
             }
             stack = stack.caller;
           }
         }
         if (stack) {
           let frames = [];
           while (stack) {
             frames.push(stack);
             stack = stack.caller;
           }
           value.stack = frames.join("\n");
         }
       }
     } catch (ex) {
       // Ignore value
     }
     this._map.set(id, value);
     return id;
   },
   /**
    * Notify all observers that a pending error is now uncaught.
    *
    * @param id The identifier of the pending error, as returned by
    * |register|.
    */
   report: function(id) {
     let value = this._map.get(id);
     if (!value) {
       return; // The error has already been reported
     }
     this._map.delete(id);
     for (let obs of this._observers.values()) {
       obs(value);
     }
   },
   /**
    * Mark all pending errors are uncaught, notify the observers.
    */
   flush: function() {
     // Since we are going to modify the map while walking it,
     // let's copying the keys first.
     let keys = [key for (key of this._map.keys())];
     for (let key of keys) {
       this.report(key);
     }
   },
   /**
    * Stop tracking an error, as this error has been caught,
    * eventually.
    */
   unregister: function(id) {
     this._map.delete(id);
   },
   /**
    * Add an observer notified when an error is reported as uncaught.
    *
    * @param {function} observer A function notified when an error is
    * reported as uncaught. Its arguments are
    *   {message, date, fileName, stack, lineNumber}
    * All arguments are optional.
    */
   addObserver: function(observer) {
     this._observers.add(observer);
   },
   /**
    * Remove an observer added with addObserver
    */
   removeObserver: function(observer) {
     this._observers.delete(observer);
   },
   /**
    * Remove all the observers added with addObserver
    */
   removeAllObservers: function() {
     this._observers.clear();
   }
 };
 PendingErrors.init();
 // Default mechanism for displaying errors
 PendingErrors.addObserver(function(details) {
   let error = Cc['@mozilla.org/scripterror;1'].createInstance(Ci.nsIScriptError);
   if (!error || !Services.console) {
     // Too late during shutdown to use the nsIConsole
     dump("*************************\n");
     dump("A promise chain failed to handle a rejection\n\n");
     dump("On: " + details.date + "\n");
     dump("Full message: " + details.message + "\n");
     dump("See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise\n");
     dump("Full stack: " + (details.stack||"not available") + "\n");
     dump("*************************\n");
     return;
   }
   let message = details.message;
   if (details.stack) {
     message += "\nFull Stack: " + details.stack;
   }
   error.init(
              /*message*/"A promise chain failed to handle a rejection.\n\n" +
              "Date: " + details.date + "\nFull Message: " + details.message,
              /*sourceName*/ details.fileName,
              /*sourceLine*/ details.lineNumber?("" + details.lineNumber):0,
              /*lineNumber*/ details.lineNumber || 0,
              /*columnNumber*/ 0,
              /*flags*/ Ci.nsIScriptError.errorFlag,
              /*category*/ "chrome javascript");
   Services.console.logMessage(error);
 });
 ///////// Additional warnings for developers
 //
 // 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"];
 ////////////////////////////////////////////////////////////////////////////////
 //// Promise
 /**
  * The Promise constructor. Creates a new promise given an executor callback.
  * The executor callback is called with the resolve and reject handlers.
  *
  * @param aExecutor
  *        The callback that will be called with resolve and reject.
  */
 this.Promise = function Promise(aExecutor)
 {
   if (typeof(aExecutor) != "function") {
     throw new TypeError("Promise constructor must be called with an executor.");
   }
   /*
    * Internal status of the promise.  This can be equal to STATUS_PENDING,
    * STATUS_RESOLVED, or STATUS_REJECTED.
    */
   Object.defineProperty(this, N_STATUS, { value: STATUS_PENDING,
                                           writable: true });
   /*
    * When the N_STATUS property is STATUS_RESOLVED, this contains the final
    * resolution value, that cannot be a promise, because resolving with a
    * promise will cause its state to be eventually propagated instead.  When the
    * N_STATUS property is STATUS_REJECTED, this contains the final rejection
    * reason, that could be a promise, even if this is uncommon.
    */
   Object.defineProperty(this, N_VALUE, { writable: true });
   /*
    * Array of Handler objects registered by the "then" method, and not processed
    * yet.  Handlers are removed when the promise is resolved or rejected.
    */
   Object.defineProperty(this, N_HANDLERS, { value: [] });
   /**
    * When the N_STATUS property is STATUS_REJECTED and until there is
    * a rejection callback, this contains an array
    * - {string} id An id for use with |PendingErrors|;
    * - {FinalizationWitness} witness A witness broadcasting |id| on
    *   notification "promise-finalization-witness".
    */
   Object.defineProperty(this, N_WITNESS, { writable: true });
   Object.seal(this);
   let resolve = PromiseWalker.completePromise
                              .bind(PromiseWalker, this, STATUS_RESOLVED);
   let reject = PromiseWalker.completePromise
                             .bind(PromiseWalker, this, STATUS_REJECTED);
   try {
     aExecutor.call(undefined, resolve, reject);
   } catch (ex) {
     reject(ex);
   }
 }
 /**
  * Calls one of the provided functions as soon as this promise is either
  * resolved or rejected.  A new promise is returned, whose state evolves
  * depending on this promise and the provided callback functions.
  *
  * The appropriate callback is always invoked after this method returns, even
  * if this promise is already resolved or rejected.  You can also call the
  * "then" method multiple times on the same promise, and the callbacks will be
  * invoked in the same order as they were registered.
  *
  * @param aOnResolve
  *        If the promise is resolved, this function is invoked with the
  *        resolution value of the promise as its only argument, and the
  *        outcome of the function determines the state of the new promise
  *        returned by the "then" method.  In case this parameter is not a
  *        function (usually "null"), the new promise returned by the "then"
  *        method is resolved with the same value as the original promise.
  *
  * @param aOnReject
  *        If the promise is rejected, this function is invoked with the
  *        rejection reason of the promise as its only argument, and the
  *        outcome of the function determines the state of the new promise
  *        returned by the "then" method.  In case this parameter is not a
  *        function (usually left "undefined"), the new promise returned by the
  *        "then" method is rejected with the same reason as the original
  *        promise.
  *
  * @return A new promise that is initially pending, then assumes a state that
  *         depends on the outcome of the invoked callback function:
  *          - If the callback returns a value that is not a promise, including
  *            "undefined", the new promise is resolved with this resolution
  *            value, even if the original promise was rejected.
  *          - If the callback throws an exception, the new promise is rejected
  *            with the exception as the rejection reason, even if the original
  *            promise was resolved.
  *          - If the callback returns a promise, the new promise will
  *            eventually assume the same state as the returned promise.
  *
  * @note If the aOnResolve callback throws an exception, the aOnReject
  *       callback is not invoked.  You can register a rejection callback on
  *       the returned promise instead, to process any exception occurred in
  *       either of the callbacks registered on this promise.
  */
 Promise.prototype.then = function (aOnResolve, aOnReject)
 {
   let handler = new Handler(this, aOnResolve, aOnReject);
   this[N_HANDLERS].push(handler);
   // Ensure the handler is scheduled for processing if this promise is already
   // resolved or rejected.
   if (this[N_STATUS] != STATUS_PENDING) {
     // This promise is not the last in the chain anymore. Remove any watchdog.
     if (this[N_WITNESS] != null) {
       let [id, witness] = this[N_WITNESS];
       this[N_WITNESS] = null;
       witness.forget();
       PendingErrors.unregister(id);
     }
     PromiseWalker.schedulePromise(this);
   }
   return handler.nextPromise;
 };
 /**
  * Invokes `promise.then` with undefined for the resolve handler and a given
  * reject handler.
  *
  * @param aOnReject
  *        The rejection handler.
  *
  * @return A new pending promise returned.
  *
  * @see Promise.prototype.then
  */
 Promise.prototype.catch = function (aOnReject)
 {
   return this.then(undefined, aOnReject);
 };
 /**
  * Creates a new pending promise and provides methods to resolve or reject it.
  *
  * @return A new object, containing the new promise in the "promise" property,
  *         and the methods to change its state in the "resolve" and "reject"
  *         properties.  See the Deferred documentation for details.
  */
 Promise.defer = function ()
 {
   return new Deferred();
 };
 /**
  * Creates a new promise resolved with the specified value, or propagates the
  * state of an existing promise.
  *
  * @param aValue
  *        If this value is not a promise, including "undefined", it becomes
  *        the resolution value of the returned promise.  If this value is a
  *        promise, then the returned promise will eventually assume the same
  *        state as the provided promise.
  *
  * @return A promise that can be pending, resolved, or rejected.
  */
 Promise.resolve = function (aValue)
 {
   if (aValue && typeof(aValue) == "function" && aValue.isAsyncFunction) {
     throw new TypeError(
       "Cannot resolve a promise with an async function. " +
       "You should either invoke the async function first " +
       "or use 'Task.spawn' instead of 'Task.async' to start " +
       "the Task and return its promise.");
   }
   if (aValue instanceof Promise) {
     return aValue;
   }
   return new Promise((aResolve) => aResolve(aValue));
 };
 /**
  * Creates a new promise rejected with the specified reason.
  *
  * @param aReason
  *        The rejection reason for the returned promise.  Although the reason
  *        can be "undefined", it is generally an Error object, like in
  *        exception handling.
  *
  * @return A rejected promise.
  *
  * @note The aReason argument should not be a promise.  Using a rejected
  *       promise for the value of aReason would make the rejection reason
  *       equal to the rejected promise itself, and not its rejection reason.
  */
 Promise.reject = function (aReason)
 {
   return new Promise((_, aReject) => aReject(aReason));
 };
 /**
  * Returns a promise that is resolved or rejected when all values are
  * resolved or any is rejected.
  *
  * @param aValues
  *        Iterable of promises that may be pending, resolved, or rejected. When
  *        all are resolved or any is rejected, the returned promise will be
  *        resolved or rejected as well.
  *
  * @return A new promise that is fulfilled when all values are resolved or
  *         that is rejected when any of the values are rejected. Its
  *         resolution value will be an array of all resolved values in the
  *         given order, or undefined if aValues is an empty array. The reject
  *         reason will be forwarded from the first promise in the list of
  *         given promises to be rejected.
  */
 Promise.all = function (aValues)
 {
   if (aValues == null || typeof(aValues["@@iterator"]) != "function") {
     throw new Error("Promise.all() expects an iterable.");
   }
   return new Promise((resolve, reject) => {
     let values = Array.isArray(aValues) ? aValues : [...aValues];
     let countdown = values.length;
     let resolutionValues = new Array(countdown);
     if (!countdown) {
       resolve(resolutionValues);
       return;
     }
     function checkForCompletion(aValue, aIndex) {
       resolutionValues[aIndex] = aValue;
       if (--countdown === 0) {
         resolve(resolutionValues);
       }
     }
     for (let i = 0; i < values.length; i++) {
       let index = i;
       let value = values[i];
       let resolver = val => checkForCompletion(val, index);
       if (value && typeof(value.then) == "function") {
         value.then(resolver, reject);
       } else {
         // Given value is not a promise, forward it as a resolution value.
         resolver(value);
       }
     }
   });
 };
 /**
  * Returns a promise that is resolved or rejected when the first value is
  * resolved or rejected, taking on the value or reason of that promise.
  *
  * @param aValues
  *        Iterable of values or promises that may be pending, resolved, or
  *        rejected. When any is resolved or rejected, the returned promise will
  *        be resolved or rejected as to the given value or reason.
  *
  * @return A new promise that is fulfilled when any values are resolved or
  *         rejected. Its resolution value will be forwarded from the resolution
  *         value or rejection reason.
  */
 Promise.race = function (aValues)
 {
   if (aValues == null || typeof(aValues["@@iterator"]) != "function") {
     throw new Error("Promise.race() expects an iterable.");
   }
   return new Promise((resolve, reject) => {
     for (let value of aValues) {
       Promise.resolve(value).then(resolve, reject);
     }
   });
 };
 Promise.Debugging = {
   /**
    * Add an observer notified when an error is reported as uncaught.
    *
    * @param {function} observer A function notified when an error is
    * reported as uncaught. Its arguments are
    *   {message, date, fileName, stack, lineNumber}
    * All arguments are optional.
    */
   addUncaughtErrorObserver: function(observer) {
     PendingErrors.addObserver(observer);
   },
   /**
    * Remove an observer added with addUncaughtErrorObserver
    *
    * @param {function} An observer registered with
    * addUncaughtErrorObserver.
    */
   removeUncaughtErrorObserver: function(observer) {
     PendingErrors.removeObserver(observer);
   },
   /**
    * Remove all the observers added with addUncaughtErrorObserver
    */
   clearUncaughtErrorObservers: function() {
     PendingErrors.removeAllObservers();
   },
   /**
    * Force all pending errors to be reported immediately as uncaught.
    * Note that this may cause some false positives.
    */
   flushUncaughtErrors: function() {
     PendingErrors.flush();
   },
 };
 Object.freeze(Promise.Debugging);
 Object.freeze(Promise);
 ////////////////////////////////////////////////////////////////////////////////
 //// PromiseWalker
 /**
  * This singleton object invokes the handlers registered on resolved and
  * rejected promises, ensuring that processing is not recursive and is done in
  * the same order as registration occurred on each promise.
  *
  * There is no guarantee on the order of execution of handlers registered on
  * different promises.
  */
 this.PromiseWalker = {
   /**
    * Singleton array of all the unprocessed handlers currently registered on
    * resolved or rejected promises.  Handlers are removed from the array as soon
    * as they are processed.
    */
   handlers: [],
   /**
    * Called when a promise needs to change state to be resolved or rejected.
    *
    * @param aPromise
    *        Promise that needs to change state.  If this is already resolved or
    *        rejected, this method has no effect.
    * @param aStatus
    *        New desired status, either STATUS_RESOLVED or STATUS_REJECTED.
    * @param aValue
    *        Associated resolution value or rejection reason.
    */
   completePromise: function (aPromise, aStatus, aValue)
   {
     // Do nothing if the promise is already resolved or rejected.
     if (aPromise[N_STATUS] != STATUS_PENDING) {
       return;
     }
     // Resolving with another promise will cause this promise to eventually
     // assume the state of the provided promise.
     if (aStatus == STATUS_RESOLVED && aValue &&
         typeof(aValue.then) == "function") {
       aValue.then(this.completePromise.bind(this, aPromise, STATUS_RESOLVED),
                   this.completePromise.bind(this, aPromise, STATUS_REJECTED));
       return;
     }
     // Change the promise status and schedule our handlers for processing.
     aPromise[N_STATUS] = aStatus;
     aPromise[N_VALUE] = aValue;
     if (aPromise[N_HANDLERS].length > 0) {
       this.schedulePromise(aPromise);
     } else if (aStatus == STATUS_REJECTED) {
       // This is a rejection and the promise is the last in the chain.
       // For the time being we therefore have an uncaught error.
       let id = PendingErrors.register(aValue);
       let witness =
           FinalizationWitnessService.make("promise-finalization-witness", id);
       aPromise[N_WITNESS] = [id, witness];
     }
   },
   /**
    * Sets up the PromiseWalker loop to start on the next tick of the event loop
    */
   scheduleWalkerLoop: function()
   {
     this.walkerLoopScheduled = true;
     Services.tm.currentThread.dispatch(this.walkerLoop,
                                        Ci.nsIThread.DISPATCH_NORMAL);
   },
   /**
    * Schedules the resolution or rejection handlers registered on the provided
    * promise for processing.
    *
    * @param aPromise
    *        Resolved or rejected promise whose handlers should be processed.  It
    *        is expected that this promise has at least one handler to process.
    */
   schedulePromise: function (aPromise)
   {
     // Migrate the handlers from the provided promise to the global list.
     for (let handler of aPromise[N_HANDLERS]) {
       this.handlers.push(handler);
     }
     aPromise[N_HANDLERS].length = 0;
     // Schedule the walker loop on the next tick of the event loop.
     if (!this.walkerLoopScheduled) {
       this.scheduleWalkerLoop();
     }
   },
   /**
    * Indicates whether the walker loop is currently scheduled for execution on
    * the next tick of the event loop.
    */
   walkerLoopScheduled: false,
   /**
    * Processes all the known handlers during this tick of the event loop.  This
    * eager processing is done to avoid unnecessarily exiting and re-entering the
    * JavaScript context for each handler on a resolved or rejected promise.
    *
    * This function is called with "this" bound to the PromiseWalker object.
    */
   walkerLoop: function ()
   {
     // If there is more than one handler waiting, reschedule the walker loop
     // immediately.  Otherwise, use walkerLoopScheduled to tell schedulePromise()
     // to reschedule the loop if it adds more handlers to the queue.  This makes
     // this walker resilient to the case where one handler does not return, but
     // starts a nested event loop.  In that case, the newly scheduled walker will
     // take over.  In the common case, the newly scheduled walker will be invoked
     // after this one has returned, with no actual handler to process.  This
     // small overhead is required to make nested event loops work correctly, but
     // occurs at most once per resolution chain, thus having only a minor
     // impact on overall performance.
     if (this.handlers.length > 1) {
       this.scheduleWalkerLoop();
     } else {
       this.walkerLoopScheduled = false;
     }
     // Process all the known handlers eagerly.
     while (this.handlers.length > 0) {
       this.handlers.shift().process();
     }
   },
 };
 // Bind the function to the singleton once.
 PromiseWalker.walkerLoop = PromiseWalker.walkerLoop.bind(PromiseWalker);
 ////////////////////////////////////////////////////////////////////////////////
 //// Deferred
 /**
  * Returned by "Promise.defer" to provide a new promise along with methods to
  * change its state.
  */
 function Deferred()
 {
   this.promise = new Promise((aResolve, aReject) => {
     this.resolve = aResolve;
     this.reject = aReject;
   });
   Object.freeze(this);
 }
 Deferred.prototype = {
   /**
    * A newly created promise, initially in the pending state.
    */
   promise: null,
   /**
    * Resolves the associated promise with the specified value, or propagates the
    * state of an existing promise.  If the associated promise has already been
    * resolved or rejected, this method does nothing.
    *
    * This function is bound to its associated promise when "Promise.defer" is
    * called, and can be called with any value of "this".
    *
    * @param aValue
    *        If this value is not a promise, including "undefined", it becomes
    *        the resolution value of the associated promise.  If this value is a
    *        promise, then the associated promise will eventually assume the same
    *        state as the provided promise.
    *
    * @note Calling this method with a pending promise as the aValue argument,
    *       and then calling it again with another value before the promise is
    *       resolved or rejected, has unspecified behavior and should be avoided.
    */
   resolve: null,
   /**
    * Rejects the associated promise with the specified reason.  If the promise
    * has already been resolved or rejected, this method does nothing.
    *
    * This function is bound to its associated promise when "Promise.defer" is
    * called, and can be called with any value of "this".
    *
    * @param aReason
    *        The rejection reason for the associated promise.  Although the
    *        reason can be "undefined", it is generally an Error object, like in
    *        exception handling.
    *
    * @note The aReason argument should not generally be a promise.  In fact,
    *       using a rejected promise for the value of aReason would make the
    *       rejection reason equal to the rejected promise itself, not to the
    *       rejection reason of the rejected promise.
    */
   reject: null,
 };
 ////////////////////////////////////////////////////////////////////////////////
 //// Handler
 /**
  * Handler registered on a promise by the "then" function.
  */
 function Handler(aThisPromise, aOnResolve, aOnReject)
 {
   this.thisPromise = aThisPromise;
   this.onResolve = aOnResolve;
   this.onReject = aOnReject;
   this.nextPromise = new Promise(() => {});
 }
 Handler.prototype = {
   /**
    * Promise on which the "then" method was called.
    */
   thisPromise: null,
   /**
    * Unmodified resolution handler provided to the "then" method.
    */
   onResolve: null,
   /**
    * Unmodified rejection handler provided to the "then" method.
    */
   onReject: null,
   /**
    * New promise that will be returned by the "then" method.
    */
   nextPromise: null,
   /**
    * Called after thisPromise is resolved or rejected, invokes the appropriate
    * callback and propagates the result to nextPromise.
    */
   process: function()
   {
     // The state of this promise is propagated unless a handler is defined.
     let nextStatus = this.thisPromise[N_STATUS];
     let nextValue = this.thisPromise[N_VALUE];
     try {
       // If a handler is defined for either resolution or rejection, invoke it
       // to determine the state of the next promise, that will be resolved with
       // the returned value, that can also be another promise.
       if (nextStatus == STATUS_RESOLVED) {
         if (typeof(this.onResolve) == "function") {
           nextValue = this.onResolve.call(undefined, nextValue);
         }
       } else if (typeof(this.onReject) == "function") {
         nextValue = this.onReject.call(undefined, nextValue);
         nextStatus = STATUS_RESOLVED;
       }
     } catch (ex) {
       // An exception has occurred in the handler.
       if (ex && typeof ex == "object" && "name" in ex &&
           ERRORS_TO_REPORT.indexOf(ex.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 in a Promise " +
              ((nextStatus == STATUS_RESOLVED) ? "resolution":"rejection") +
              " callback.\n\n");
         dump("Full message: " + ex + "\n");
         dump("See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise\n");
         dump("Full stack: " + (("stack" in ex)?ex.stack:"not available") + "\n");
         dump("*************************\n");
       }
       // Additionally, reject the next promise.
       nextStatus = STATUS_REJECTED;
       nextValue = ex;
     }
     // Propagate the newly determined state to the next promise.
     PromiseWalker.completePromise(this.nextPromise, nextStatus, nextValue);
   },
 };