Use a _FutureListener separate from the _Future to hold listeners.

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index f3cd5e41c6b9b8ee4e6e69f0d14e1753b6652519..a0cfeb16efe9b05e4778085a0988824e46168236 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -57,21 +57,82 @@ class _SyncCompleter<T> extends _Completer<T> {
   }
 }
 
-class _Future<T> implements Future<T> {
-  // State of the future. The state determines the interpretation of the
-  // [resultOrListeners] field.
-  // TODO(lrn): rename field since it can also contain a chained future.
+class _FutureListener {
+  static const int MASK_VALUE = 1;
+  static const int MASK_ERROR = 2;
+  static const int MASK_TEST_ERROR = 4;
+  static const int MASK_WHENCOMPLETE = 8;
+  static const int STATE_CHAIN = 0;
+  static const int STATE_THEN = MASK_VALUE;
+  static const int STATE_THEN_ONERROR = MASK_VALUE | MASK_ERROR;
+  static const int STATE_CATCHERROR = MASK_ERROR;
+  static const int STATE_CATCHERROR_TEST = MASK_ERROR | MASK_TEST_ERROR;
+  static const int STATE_WHENCOMPLETE = MASK_WHENCOMPLETE;
+  // Listeners on the same future are linked through this link.
+  _FutureListener _nextListener = null;
+  // The future to complete when this listener is activated.
+  final _Future result;
+  // Which fields means what.
+  final int state;
+  // Used for then/whenDone callback and error test
+  final Function callback;
+  // Used for error callbacks.
+  final Function errorCallback;
+
+  _FutureListener.then(this.result,
+                       _FutureOnValue onValue, Function errorCallback)
+      : callback = onValue,
+        errorCallback = errorCallback,
+        state = (errorCallback == null) ? STATE_THEN : STATE_THEN_ONERROR;
+
+  _FutureListener.catchError(this.result,
+                             this.errorCallback, _FutureErrorTest test)
+      : callback = test,
+        state = (test == null) ? STATE_CATCHERROR : STATE_CATCHERROR_TEST;
+
+  _FutureListener.whenComplete(this.result, _FutureAction onComplete)
+      : callback = onComplete,
+        errorCallback = null,
+        state = STATE_WHENCOMPLETE;
+
+  _FutureListener.chain(this.result)
+      : callback = null,
+        errorCallback = null,
+        state = STATE_CHAIN;
+
+  Zone get _zone => result._zone;
+
+  bool get handlesValue => (state & MASK_VALUE != 0);
+  bool get handlesError => (state & MASK_ERROR != 0);
+  bool get hasErrorTest => (state == STATE_CATCHERROR_TEST);
+  bool get handlesComplete => (state == STATE_WHENCOMPLETE);
+
+  _FutureOnValue get _onValue {
+    assert(handlesValue);
+    return callback;
+  }
+  Function get _onError => errorCallback;
+  _FutureErrorTest get _errorTest {
+    assert(hasErrorTest);
+    return callback;
+  }
+  _FutureAction get _whenCompleteAction {
+    assert(handlesComplete);
+    return callback;
+  }
+}
 
+class _Future<T> implements Future<T> {
   /// Initial state, waiting for a result. In this state, the
   /// [resultOrListeners] field holds a single-linked list of
-  /// [FutureListener] listeners.
+  /// [_FutureListener] listeners.
   static const int _INCOMPLETE = 0;
   /// Pending completion. Set when completed using [_asyncComplete] or
   /// [_asyncCompleteError]. It is an error to try to complete it again.
+  /// [resultOrListeners] holds listeners.
   static const int _PENDING_COMPLETE = 1;
   /// The future has been chained to another future. The result of that
   /// other future becomes the result of this future as well.
-  /// In this state, no callback should be executed anymore.
   // TODO(floitsch): we don't really need a special "_CHAINED" state. We could
   // just use the PENDING_COMPLETE state instead.
   static const int _CHAINED = 2;
@@ -83,23 +144,14 @@ class _Future<T> implements Future<T> {
   /** Whether the future is complete, and as what. */
   int _state = _INCOMPLETE;
 
-  final Zone _zone;
-
-  bool get _mayComplete => _state == _INCOMPLETE;
-  bool get _isChained => _state == _CHAINED;
-  bool get _isComplete => _state >= _VALUE;
-  bool get _hasValue => _state == _VALUE;
-  bool get _hasError => _state == _ERROR;
-
-  set _isChained(bool value) {
-    if (value) {
-      assert(!_isComplete);
-      _state = _CHAINED;
-    } else {
-      assert(_isChained);
-      _state = _INCOMPLETE;
-    }
-  }
+  /**
+   * Zone that the future was completed from.
+   * This is the zone that an error result belongs to.
+   *
+   * Until the future is completed, the field may hold the zone that
+   * listener callbacks used to create this future should be run in.
+   */
+  final Zone _zone = Zone.current;
 
   /**
    * Either the result, a list of listeners or another future.
@@ -116,95 +168,68 @@ class _Future<T> implements Future<T> {
    * will complete with the same result.
    * All listeners are forwarded to the other future.
    *
-   * The cases are disjoint (incomplete and unchained, incomplete and
-   * chained, or completed with value or error), so the field only needs to hold
+   * The cases are disjoint - incomplete and unchained ([_INCOMPLETE]),
+   * incomplete and chained ([_CHAINED]), or completed with value or error
+   * ([_VALUE] or [_ERROR]) - so the field only needs to hold
    * one value at a time.
    */
   var _resultOrListeners;
 
-  /**
-   * A [_Future] implements a linked list. If a future has more than one
-   * listener the [_nextListener] field of the first listener points to the
-   * remaining listeners.
-   */
-  // TODO(floitsch): since single listeners are the common case we should
-  // use a bit to indicate that the _resultOrListeners contains a container.
-  _Future _nextListener;
-
-  // TODO(floitsch): we only need two closure fields to store the callbacks.
-  // If we store the type of a closure in the state field (where there are
-  // still bits left), we can just store two closures instead of using 4
-  // fields of which 2 are always null.
-  _FutureOnValue _onValueCallback;
-  _FutureErrorTest _errorTestCallback;
-  Function _onErrorCallback;
-  _FutureAction _whenCompleteActionCallback;
-
-  _FutureOnValue get _onValue => _isChained ? null : _onValueCallback;
-  _FutureErrorTest get _errorTest => _isChained ? null : _errorTestCallback;
-  Function get _onError => _isChained ? null : _onErrorCallback;
-  _FutureAction get _whenCompleteAction
-      => _isChained ? null : _whenCompleteActionCallback;
-
-  _Future()
-      : _zone = Zone.current,
-        _onValueCallback = null, _errorTestCallback = null,
-        _onErrorCallback = null, _whenCompleteActionCallback = null;
+  _Future();
 
   /// Valid types for value: `T` or `Future<T>`.
-  _Future.immediate(value)
-        : _zone = Zone.current,
-          _onValueCallback = null, _errorTestCallback = null,
-          _onErrorCallback = null, _whenCompleteActionCallback = null {
+  _Future.immediate(value) {
     _asyncComplete(value);
   }
 
-  _Future.immediateError(var error, [StackTrace stackTrace])
-      : _zone = Zone.current,
-        _onValueCallback = null, _errorTestCallback = null,
-        _onErrorCallback = null, _whenCompleteActionCallback = null {
+  _Future.immediateError(var error, [StackTrace stackTrace]) {
     _asyncCompleteError(error, stackTrace);
   }
 
-  _Future._then(onValueCallback(value), Function onErrorCallback)
-      : _zone = Zone.current,
-        _onValueCallback = Zone.current.registerUnaryCallback(onValueCallback),
-        _onErrorCallback = _registerErrorHandler(onErrorCallback, Zone.current),
-        _errorTestCallback = null,
-        _whenCompleteActionCallback = null;
-
-  _Future._catchError(Function onErrorCallback, bool errorTestCallback(e))
-    : _zone = Zone.current,
-      _onErrorCallback = _registerErrorHandler(onErrorCallback, Zone.current),
-      _errorTestCallback =
-          Zone.current.registerUnaryCallback(errorTestCallback),
-      _onValueCallback = null,
-      _whenCompleteActionCallback = null;
-
-  _Future._whenComplete(whenCompleteActionCallback())
-      : _zone = Zone.current,
-        _whenCompleteActionCallback =
-            Zone.current.registerCallback(whenCompleteActionCallback),
-        _onValueCallback = null,
-        _errorTestCallback = null,
-        _onErrorCallback = null;
+  bool get _mayComplete => _state == _INCOMPLETE;
+  bool get _isChained => _state == _CHAINED;
+  bool get _isComplete => _state >= _VALUE;
+  bool get _hasValue => _state == _VALUE;
+  bool get _hasError => _state == _ERROR;
+
+  set _isChained(bool value) {
+    if (value) {
+      assert(!_isComplete);
+      _state = _CHAINED;
+    } else {
+      assert(_isChained);
+      _state = _INCOMPLETE;
+    }
+  }
 
   Future then(f(T value), { Function onError }) {
-    _Future result;
-    result = new _Future._then(f, onError);
-    _addListener(result);
+    _Future result = new _Future();
+    if (!identical(result._zone, _ROOT_ZONE)) {
+      f = result._zone.registerUnaryCallback(f);
+      if (onError != null) {
+        onError = _registerErrorHandler(onError, result._zone);
+      }
+    }
+    _addListener(new _FutureListener.then(result, f, onError));
     return result;
   }
 
   Future catchError(Function onError, { bool test(error) }) {
-    _Future result = new _Future._catchError(onError, test);
-    _addListener(result);
+    _Future result = new _Future();
+    if (!identical(result._zone, _ROOT_ZONE)) {
+      onError = _registerErrorHandler(onError, result._zone);
+      if (test != null) test = result._zone.registerUnaryCallback(test);
+    }
+    _addListener(new _FutureListener.catchError(result, onError, test));
     return result;
   }
 
   Future<T> whenComplete(action()) {
-    _Future result = new _Future<T>._whenComplete(action);
-    _addListener(result);
+    _Future result = new _Future<T>();
+    if (!identical(result._zone, _ROOT_ZONE)) {
+      action = result._zone.registerCallback(action);
+    }
+    _addListener(new _FutureListener.whenComplete(result, action));
     return result;
   }
 
@@ -231,13 +256,17 @@ class _Future<T> implements Future<T> {
     _resultOrListeners = value;
   }
 
-  void _setError(Object error, StackTrace stackTrace) {
+  void _setErrorObject(AsyncError error) {
     assert(!_isComplete);  // But may have a completion pending.
     _state = _ERROR;
-    _resultOrListeners = new AsyncError(error, stackTrace);
+    _resultOrListeners = error;
   }
 
-  void _addListener(_Future listener) {
+  void _setError(Object error, StackTrace stackTrace) {
+    _setErrorObject(new AsyncError(error, stackTrace));
+  }
+
+  void _addListener(_FutureListener listener) {
     assert(listener._nextListener == null);
     if (_isComplete) {
       // Handle late listeners asynchronously.
@@ -250,15 +279,15 @@ class _Future<T> implements Future<T> {
     }
   }
 
-  _Future _removeListeners() {
+  _FutureListener _removeListeners() {
     // Reverse listeners before returning them, so the resulting list is in
     // subscription order.
     assert(!_isComplete);
-    _Future current = _resultOrListeners;
+    _FutureListener current = _resultOrListeners;
     _resultOrListeners = null;
-    _Future prev = null;
+    _FutureListener prev = null;
     while (current != null) {
-      _Future next = current._nextListener;
+      _FutureListener next = current._nextListener;
       current._nextListener = prev;
       prev = current;
       current = next;
@@ -297,21 +326,16 @@ class _Future<T> implements Future<T> {
 
     // Mark the target as chained (and as such half-completed).
     target._isChained = true;
-    _Future internalFuture = source;
-    if (internalFuture._isComplete) {
-      _propagateToListeners(internalFuture, target);
+    _FutureListener listener = new _FutureListener.chain(target);
+    if (source._isComplete) {
+      _propagateToListeners(source, listener);
     } else {
-      internalFuture._addListener(target);
+      source._addListener(listener);
     }
   }
 
   void _complete(value) {
     assert(!_isComplete);
-    assert(_onValue == null);
-    assert(_onError == null);
-    assert(_whenCompleteAction == null);
-    assert(_errorTest == null);
-
     if (value is Future) {
       if (value is _Future) {
         _chainCoreFuture(value, this);
@@ -319,7 +343,7 @@ class _Future<T> implements Future<T> {
         _chainForeignFuture(value, this);
       }
     } else {
-      _Future listeners = _removeListeners();
+      _FutureListener listeners = _removeListeners();
       _setValue(value);
       _propagateToListeners(this, listeners);
     }
@@ -327,35 +351,23 @@ class _Future<T> implements Future<T> {
 
   void _completeWithValue(value) {
     assert(!_isComplete);
-    assert(_onValue == null);
-    assert(_onError == null);
-    assert(_whenCompleteAction == null);
-    assert(_errorTest == null);
     assert(value is! Future);
 
-    _Future listeners = _removeListeners();
+    _FutureListener listeners = _removeListeners();
     _setValue(value);
     _propagateToListeners(this, listeners);
   }
 
   void _completeError(error, [StackTrace stackTrace]) {
     assert(!_isComplete);
-    assert(_onValue == null);
-    assert(_onError == null);
-    assert(_whenCompleteAction == null);
-    assert(_errorTest == null);
 
-    _Future listeners = _removeListeners();
+    _FutureListener listeners = _removeListeners();
     _setError(error, stackTrace);
     _propagateToListeners(this, listeners);
   }
 
   void _asyncComplete(value) {
     assert(!_isComplete);
-    assert(_onValue == null);
-    assert(_onError == null);
-    assert(_whenCompleteAction == null);
-    assert(_errorTest == null);
     // Two corner cases if the value is a future:
     //   1. the future is already completed and an error.
     //   2. the future is not yet completed but might become an error.
@@ -403,10 +415,6 @@ class _Future<T> implements Future<T> {
 
   void _asyncCompleteError(error, StackTrace stackTrace) {
     assert(!_isComplete);
-    assert(_onValue == null);
-    assert(_onError == null);
-    assert(_whenCompleteAction == null);
-    assert(_errorTest == null);
 
     _markPendingCompletion();
     _zone.scheduleMicrotask(() {
@@ -415,53 +423,36 @@ class _Future<T> implements Future<T> {
   }
 
   /**
-   * Propagates the value/error of [source] to its [listeners].
-   *
-   * Unlinks all listeners and propagates the source to each listener
-   * separately.
-   */
-  static void _propagateMultipleListeners(_Future source, _Future listeners) {
-    assert(listeners != null);
-    assert(listeners._nextListener != null);
-    do {
-      _Future listener = listeners;
-      listeners = listener._nextListener;
-      listener._nextListener = null;
-      _propagateToListeners(source, listener);
-    } while (listeners != null);
-  }
-
-  /**
    * Propagates the value/error of [source] to its [listeners], executing the
    * listeners' callbacks.
-   *
-   * If [runCallback] is true (which should be the default) it executes
-   * the registered action of listeners. If it is `false` then the callback is
-   * skipped. This is used to complete futures with chained futures.
    */
-  static void _propagateToListeners(_Future source, _Future listeners) {
+  static void _propagateToListeners(_Future source, _FutureListener listeners) {
     while (true) {
-      if (!source._isComplete) return;  // Chained future.
+      assert(source._isComplete);
       bool hasError = source._hasError;
-      if (hasError && listeners == null) {
-        AsyncError asyncError = source._error;
-        source._zone.handleUncaughtError(
-            asyncError.error, asyncError.stackTrace);
+      if (listeners == null) {
+        if (hasError) {
+          AsyncError asyncError = source._error;
+          source._zone.handleUncaughtError(
+              asyncError.error, asyncError.stackTrace);
+        }
         return;
       }
-      if (listeners == null) return;
-      _Future listener = listeners;
-      if (listener._nextListener != null) {
-        // Usually futures only have one listener. If they have several, we
-        // handle them specially.
-        _propagateMultipleListeners(source, listeners);
-        return;
+      // Usually futures only have one listener. If they have several, we
+      // call handle them separately in recursive calls, continuing
+      // here only when there is only one listener left.
+      while (listeners._nextListener != null) {
+        _FutureListener listener = listeners;
+        listeners = listener._nextListener;
+        listener._nextListener = null;
+        _propagateToListeners(source, listener);
       }
+      _FutureListener listener = listeners;
       // Do the actual propagation.
       // Set initial state of listenerHasValue and listenerValueOrError. These
       // variables are updated, with the outcome of potential callbacks.
       bool listenerHasValue = true;
-      final sourceValue = source._hasValue ? source._value : null;
+      final sourceValue = hasError ? null : source._value;
       var listenerValueOrError = sourceValue;
       // Set to true if a whenComplete needs to wait for a future.
       // The whenComplete action will resume the propagation by itself.
@@ -472,9 +463,7 @@ class _Future<T> implements Future<T> {
       // Only if we either have an error or callbacks, go into this, somewhat
       // expensive, branch. Here we'll enter/leave the zone. Many futures
       // doesn't have callbacks, so this is a significant optimization.
-      if (hasError ||
-          listener._onValue != null ||
-          listener._whenCompleteAction != null) {
+      if (hasError || (listener.handlesValue || listener.handlesComplete)) {
         Zone zone = listener._zone;
         if (hasError && !source._zone.inSameErrorZone(zone)) {
           // Don't cross zone boundaries with errors.
@@ -503,13 +492,12 @@ class _Future<T> implements Future<T> {
 
         void handleError() {
           AsyncError asyncError = source._error;
-          _FutureErrorTest test = listener._errorTest;
           bool matchesTest = true;
-          if (test != null) {
+          if (listener.hasErrorTest) {
+            _FutureErrorTest test = listener._errorTest;
             try {
               matchesTest = zone.runUnary(test, asyncError.error);
             } catch (e, s) {
-              // TODO(ajohnsen): Should we suport rethrow for test throws?
               listenerValueOrError = identical(asyncError.error, e) ?
                   asyncError : new AsyncError(e, s);
               listenerHasValue = false;
@@ -552,14 +540,14 @@ class _Future<T> implements Future<T> {
               listenerValueOrError = new AsyncError(e, s);
             }
             listenerHasValue = false;
+            return;
           }
           if (completeResult is Future) {
-            listener._isChained = true;
+            _Future result = listener.result;
+            result._isChained = true;
             isPropagationAborted = true;
             completeResult.then((ignored) {
-              // Try again. Since the future is marked as chained it won't run
-              // the whenComplete again.
-              _propagateToListeners(source, listener);
+              _propagateToListeners(source, new _FutureListener.chain(result));
             }, onError: (error, [stackTrace]) {
               // When there is an error, we have to make the error the new
               // result of the current listener.
@@ -568,27 +556,24 @@ class _Future<T> implements Future<T> {
                 completeResult = new _Future();
                 completeResult._setError(error, stackTrace);
               }
-              _propagateToListeners(completeResult, listener);
+              _propagateToListeners(completeResult,
+                                    new _FutureListener.chain(result));
             });
           }
         }
 
         if (!hasError) {
-          if (listener._onValue != null) {
+          if (listener.handlesValue) {
             listenerHasValue = handleValueCallback();
           }
         } else {
           handleError();
         }
-        if (listener._whenCompleteAction != null) {
+        if (listener.handlesComplete) {
           handleWhenCompleteCallback();
         }
         // If we changed zone, oldZone will not be null.
         if (oldZone != null) Zone._leave(oldZone);
-        listener._onValueCallback = null;
-        listener._errorTestCallback = null;
-        listener._onErrorCallback = null;
-        listener._whenCompleteActionCallback = null;
 
         if (isPropagationAborted) return;
         // If the listener's value is a future we need to chain it. Note that
@@ -600,32 +585,33 @@ class _Future<T> implements Future<T> {
           Future chainSource = listenerValueOrError;
           // Shortcut if the chain-source is already completed. Just continue
           // the loop.
+          _Future result = listener.result;
           if (chainSource is _Future) {
             if (chainSource._isComplete) {
               // propagate the value (simulating a tail call).
-              listener._isChained = true;
+              result._isChained = true;
               source = chainSource;
-              listeners = listener;
+              listeners = new _FutureListener.chain(result);
               continue;
             } else {
-              _chainCoreFuture(chainSource, listener);
+              _chainCoreFuture(chainSource, result);
             }
           } else {
-            _chainForeignFuture(chainSource, listener);
+            _chainForeignFuture(chainSource, result);
           }
           return;
         }
       }
+      _Future result = listener.result;
+      listeners = result._removeListeners();
       if (listenerHasValue) {
-        listeners = listener._removeListeners();
-        listener._setValue(listenerValueOrError);
+        result._setValue(listenerValueOrError);
       } else {
-        listeners = listener._removeListeners();
         AsyncError asyncError = listenerValueOrError;
-        listener._setError(asyncError.error, asyncError.stackTrace);
+        result._setErrorObject(asyncError);
       }
       // Prepare for next round.
-      source = listener;
+      source = result;
     }
   }