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 /* -*- 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);

   },

 };