v8/src/promise.js

// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

var $promiseCreate;
var $promiseResolve;
var $promiseReject;
var $promiseChain;
var $promiseCatch;
var $promiseThen;
var $promiseHasUserDefinedRejectHandler;
var $promiseStatus;
var $promiseValue;

(function(global, utils) {

"use strict";

%CheckIsBootstrapping();

// -------------------------------------------------------------------
// Imports

var InternalArray = utils.InternalArray;

// -------------------------------------------------------------------

// Status values: 0 = pending, +1 = resolved, -1 = rejected
var promiseStatus = GLOBAL_PRIVATE("Promise#status");
var promiseValue = GLOBAL_PRIVATE("Promise#value");
var promiseOnResolve = GLOBAL_PRIVATE("Promise#onResolve");
var promiseOnReject = GLOBAL_PRIVATE("Promise#onReject");
var promiseRaw = GLOBAL_PRIVATE("Promise#raw");
var promiseHasHandler = %PromiseHasHandlerSymbol();
var lastMicrotaskId = 0;

var GlobalPromise = function Promise(resolver) {
  if (resolver === promiseRaw) return;
  if (!%_IsConstructCall()) throw MakeTypeError(kNotAPromise, this);
  if (!IS_SPEC_FUNCTION(resolver))
    throw MakeTypeError(kResolverNotAFunction, resolver);
  var promise = PromiseInit(this);
  try {
    %DebugPushPromise(promise, Promise);
    resolver(function(x) { PromiseResolve(promise, x) },
             function(r) { PromiseReject(promise, r) });
  } catch (e) {
    PromiseReject(promise, e);
  } finally {
    %DebugPopPromise();
  }
}

// Core functionality.

function PromiseSet(promise, status, value, onResolve, onReject) {
  SET_PRIVATE(promise, promiseStatus, status);
  SET_PRIVATE(promise, promiseValue, value);
  SET_PRIVATE(promise, promiseOnResolve, onResolve);
  SET_PRIVATE(promise, promiseOnReject, onReject);
  if (DEBUG_IS_ACTIVE) {
    %DebugPromiseEvent({ promise: promise, status: status, value: value });
  }
  return promise;
}

function PromiseCreateAndSet(status, value) {
  var promise = new GlobalPromise(promiseRaw);
  // If debug is active, notify about the newly created promise first.
  if (DEBUG_IS_ACTIVE) PromiseSet(promise, 0, UNDEFINED);
  return PromiseSet(promise, status, value);
}

function PromiseInit(promise) {
  return PromiseSet(
      promise, 0, UNDEFINED, new InternalArray, new InternalArray)
}

function PromiseDone(promise, status, value, promiseQueue) {
  if (GET_PRIVATE(promise, promiseStatus) === 0) {
    var tasks = GET_PRIVATE(promise, promiseQueue);
    if (tasks.length) PromiseEnqueue(value, tasks, status);
    PromiseSet(promise, status, value);
  }
}

function PromiseCoerce(constructor, x) {
  if (!IsPromise(x) && IS_SPEC_OBJECT(x)) {
    var then;
    try {
      then = x.then;
    } catch(r) {
      return %_CallFunction(constructor, r, PromiseRejected);
    }
    if (IS_SPEC_FUNCTION(then)) {
      var deferred = %_CallFunction(constructor, PromiseDeferred);
      try {
        %_CallFunction(x, deferred.resolve, deferred.reject, then);
      } catch(r) {
        deferred.reject(r);
      }
      return deferred.promise;
    }
  }
  return x;
}

function PromiseHandle(value, handler, deferred) {
  try {
    %DebugPushPromise(deferred.promise, PromiseHandle);
    DEBUG_PREPARE_STEP_IN_IF_STEPPING(handler);
    var result = handler(value);
    if (result === deferred.promise)
      throw MakeTypeError(kPromiseCyclic, result);
    else if (IsPromise(result))
      %_CallFunction(result, deferred.resolve, deferred.reject, PromiseChain);
    else
      deferred.resolve(result);
  } catch (exception) {
    try { deferred.reject(exception); } catch (e) { }
  } finally {
    %DebugPopPromise();
  }
}

function PromiseEnqueue(value, tasks, status) {
  var id, name, instrumenting = DEBUG_IS_ACTIVE;
  %EnqueueMicrotask(function() {
    if (instrumenting) {
      %DebugAsyncTaskEvent({ type: "willHandle", id: id, name: name });
    }
    for (var i = 0; i < tasks.length; i += 2) {
      PromiseHandle(value, tasks[i], tasks[i + 1])
    }
    if (instrumenting) {
      %DebugAsyncTaskEvent({ type: "didHandle", id: id, name: name });
    }
  });
  if (instrumenting) {
    id = ++lastMicrotaskId;
    name = status > 0 ? "Promise.resolve" : "Promise.reject";
    %DebugAsyncTaskEvent({ type: "enqueue", id: id, name: name });
  }
}

function PromiseIdResolveHandler(x) { return x }
function PromiseIdRejectHandler(r) { throw r }

function PromiseNopResolver() {}

// -------------------------------------------------------------------
// Define exported functions.

// For bootstrapper.

function IsPromise(x) {
  return IS_SPEC_OBJECT(x) && HAS_DEFINED_PRIVATE(x, promiseStatus);
}

function PromiseCreate() {
  return new GlobalPromise(PromiseNopResolver)
}

function PromiseResolve(promise, x) {
  PromiseDone(promise, +1, x, promiseOnResolve)
}

function PromiseReject(promise, r) {
  // Check promise status to confirm that this reject has an effect.
  // Call runtime for callbacks to the debugger or for unhandled reject.
  if (GET_PRIVATE(promise, promiseStatus) == 0) {
    var debug_is_active = DEBUG_IS_ACTIVE;
    if (debug_is_active || !HAS_DEFINED_PRIVATE(promise, promiseHasHandler)) {
      %PromiseRejectEvent(promise, r, debug_is_active);
    }
  }
  PromiseDone(promise, -1, r, promiseOnReject)
}

// Convenience.

function PromiseDeferred() {
  if (this === GlobalPromise) {
    // Optimized case, avoid extra closure.
    var promise = PromiseInit(new GlobalPromise(promiseRaw));
    return {
      promise: promise,
      resolve: function(x) { PromiseResolve(promise, x) },
      reject: function(r) { PromiseReject(promise, r) }
    };
  } else {
    var result = {};
    result.promise = new this(function(resolve, reject) {
      result.resolve = resolve;
      result.reject = reject;
    })
    return result;
  }
}

function PromiseResolved(x) {
  if (this === GlobalPromise) {
    // Optimized case, avoid extra closure.
    return PromiseCreateAndSet(+1, x);
  } else {
    return new this(function(resolve, reject) { resolve(x) });
  }
}

function PromiseRejected(r) {
  var promise;
  if (this === GlobalPromise) {
    // Optimized case, avoid extra closure.
    promise = PromiseCreateAndSet(-1, r);
    // The debug event for this would always be an uncaught promise reject,
    // which is usually simply noise. Do not trigger that debug event.
    %PromiseRejectEvent(promise, r, false);
  } else {
    promise = new this(function(resolve, reject) { reject(r) });
  }
  return promise;
}

// Simple chaining.

function PromiseChain(onResolve, onReject) {  // a.k.a. flatMap
  onResolve = IS_UNDEFINED(onResolve) ? PromiseIdResolveHandler : onResolve;
  onReject = IS_UNDEFINED(onReject) ? PromiseIdRejectHandler : onReject;
  var deferred = %_CallFunction(this.constructor, PromiseDeferred);
  switch (GET_PRIVATE(this, promiseStatus)) {
    case UNDEFINED:
      throw MakeTypeError(kNotAPromise, this);
    case 0:  // Pending
      GET_PRIVATE(this, promiseOnResolve).push(onResolve, deferred);
      GET_PRIVATE(this, promiseOnReject).push(onReject, deferred);
      break;
    case +1:  // Resolved
      PromiseEnqueue(GET_PRIVATE(this, promiseValue),
                     [onResolve, deferred],
                     +1);
      break;
    case -1:  // Rejected
      if (!HAS_DEFINED_PRIVATE(this, promiseHasHandler)) {
        // Promise has already been rejected, but had no handler.
        // Revoke previously triggered reject event.
        %PromiseRevokeReject(this);
      }
      PromiseEnqueue(GET_PRIVATE(this, promiseValue),
                     [onReject, deferred],
                     -1);
      break;
  }
  // Mark this promise as having handler.
  SET_PRIVATE(this, promiseHasHandler, true);
  if (DEBUG_IS_ACTIVE) {
    %DebugPromiseEvent({ promise: deferred.promise, parentPromise: this });
  }
  return deferred.promise;
}

function PromiseCatch(onReject) {
  return this.then(UNDEFINED, onReject);
}

// Multi-unwrapped chaining with thenable coercion.

function PromiseThen(onResolve, onReject) {
  onResolve = IS_SPEC_FUNCTION(onResolve) ? onResolve
                                          : PromiseIdResolveHandler;
  onReject = IS_SPEC_FUNCTION(onReject) ? onReject
                                        : PromiseIdRejectHandler;
  var that = this;
  var constructor = this.constructor;
  return %_CallFunction(
    this,
    function(x) {
      x = PromiseCoerce(constructor, x);
      if (x === that) {
        DEBUG_PREPARE_STEP_IN_IF_STEPPING(onReject);
        return onReject(MakeTypeError(kPromiseCyclic, x));
      } else if (IsPromise(x)) {
        return x.then(onResolve, onReject);
      } else {
        DEBUG_PREPARE_STEP_IN_IF_STEPPING(onResolve);
        return onResolve(x);
      }
    },
    onReject,
    PromiseChain
  );
}

// Combinators.

function PromiseCast(x) {
  // TODO(rossberg): cannot do better until we support @@create.
  return IsPromise(x) ? x : new this(function(resolve) { resolve(x) });
}

function PromiseAll(iterable) {
  var deferred = %_CallFunction(this, PromiseDeferred);
  var resolutions = [];
  try {
    var count = 0;
    var i = 0;
    for (var value of iterable) {
      this.resolve(value).then(
          // Nested scope to get closure over current i.
          // TODO(arv): Use an inner let binding once available.
          (function(i) {
            return function(x) {
              resolutions[i] = x;
              if (--count === 0) deferred.resolve(resolutions);
            }
          })(i),
          function(r) { deferred.reject(r); });
      ++i;
      ++count;
    }

    if (count === 0) {
      deferred.resolve(resolutions);
    }

  } catch (e) {
    deferred.reject(e)
  }
  return deferred.promise;
}

function PromiseRace(iterable) {
  var deferred = %_CallFunction(this, PromiseDeferred);
  try {
    for (var value of iterable) {
      this.resolve(value).then(
          function(x) { deferred.resolve(x) },
          function(r) { deferred.reject(r) });
    }
  } catch (e) {
    deferred.reject(e)
  }
  return deferred.promise;
}


// Utility for debugger

function PromiseHasUserDefinedRejectHandlerRecursive(promise) {
  var queue = GET_PRIVATE(promise, promiseOnReject);
  if (IS_UNDEFINED(queue)) return false;
  for (var i = 0; i < queue.length; i += 2) {
    if (queue[i] != PromiseIdRejectHandler) return true;
    if (PromiseHasUserDefinedRejectHandlerRecursive(queue[i + 1].promise)) {
      return true;
    }
  }
  return false;
}

// Return whether the promise will be handled by a user-defined reject
// handler somewhere down the promise chain. For this, we do a depth-first
// search for a reject handler that's not the default PromiseIdRejectHandler.
function PromiseHasUserDefinedRejectHandler() {
  return PromiseHasUserDefinedRejectHandlerRecursive(this);
};

// -------------------------------------------------------------------
// Install exported functions.

%AddNamedProperty(global, 'Promise', GlobalPromise, DONT_ENUM);
%AddNamedProperty(GlobalPromise.prototype, symbolToStringTag, "Promise",
                  DONT_ENUM | READ_ONLY);

utils.InstallFunctions(GlobalPromise, DONT_ENUM, [
  "defer", PromiseDeferred,
  "accept", PromiseResolved,
  "reject", PromiseRejected,
  "all", PromiseAll,
  "race", PromiseRace,
  "resolve", PromiseCast
]);

utils.InstallFunctions(GlobalPromise.prototype, DONT_ENUM, [
  "chain", PromiseChain,
  "then", PromiseThen,
  "catch", PromiseCatch
]);

$promiseCreate = PromiseCreate;
$promiseResolve = PromiseResolve;
$promiseReject = PromiseReject;
$promiseChain = PromiseChain;
$promiseCatch = PromiseCatch;
$promiseThen = PromiseThen;
$promiseHasUserDefinedRejectHandler = PromiseHasUserDefinedRejectHandler;
$promiseStatus = promiseStatus;
$promiseValue = promiseValue;

})