Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index e31d5e8f350d5ec04f298fd33d9d5658ce8cf8e5..ecd8d8f677f6b481fc6ca1b586bab96a51542974 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -10,6 +10,7 @@ import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'declaring_transform.dart';
+import 'declaring_transformer.dart';
 import 'errors.dart';
 import 'lazy_transformer.dart';
 import 'log.dart';
@@ -44,7 +45,7 @@ class TransformNode {
   StreamSubscription<AssetNode> _phaseSubscription;
 
   /// Whether [this] is dirty and still has more processing to do.
-  bool get isDirty => !_state.isDone;
+  bool get isDirty => _state != _State.NOT_PRIMARY && _state != _State.APPLIED;
 
   /// Whether [transformer] is lazy and this transform has yet to be forced.
   bool _isLazy;
@@ -55,6 +56,8 @@ class TransformNode {
   /// The controllers for the asset nodes emitted by this node.
   final _outputControllers = new Map<AssetId, AssetNodeController>();
 
+  /// The ids of inputs the transformer tried and failed to read last time it
+  /// ran.
   final _missingInputs = new Set<AssetId>();
 
   /// The controller that's used to pass [primary] through [this] if it's not
@@ -89,8 +92,11 @@ class TransformNode {
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
+  /// A controller for log entries emitted by this node.
+  final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
+
   /// The current state of [this].
-  var _state = _TransformNodeState.PROCESSING;
+  var _state = _State.COMPUTING_IS_PRIMARY;
 
   /// Whether [this] has been marked as removed.
   bool get _isRemoved => _onAssetController.isClosed;
@@ -99,26 +105,34 @@ class TransformNode {
   /// consumes the primary input.
   ///
   /// Defaults to `false`. This is not meaningful unless [_state] is
-  /// [_TransformNodeState.APPLIED].
+  /// [_State.APPLIED].
   bool _consumePrimary = false;
 
+  /// The set of output ids that [transformer] declared it would emit.
+  ///
+  /// This is only non-null if [transformer] is a [DeclaringTransformer] and its
+  /// [declareOutputs] has been run successfully.
+  Set<AssetId> _declaredOutputs;
+
   TransformNode(this.phase, Transformer transformer, this.primary,
       this._location)
       : transformer = transformer,
         _isLazy = transformer is LazyTransformer {
+    _onLogPool.add(_onLogController.stream);
+
     _primarySubscription = primary.onStateChange.listen((state) {
       if (state.isRemoved) {
         remove();
       } else {
-        _dirty(primaryChanged: true);
+        _dirty();
       }
     });
 
     _phaseSubscription = phase.previous.onAsset.listen((node) {
-      if (_missingInputs.contains(node.id)) _dirty(primaryChanged: false);
+      if (_missingInputs.contains(node.id)) _dirty();
     });
 
-    _process();
+    _isPrimary();
   }
 
   /// The [TransformInfo] describing this node.
@@ -134,6 +148,7 @@ class TransformNode {
   /// from the primary input, but it's possible for a transform to no longer be
   /// valid even if its primary input still exists.
   void remove() {
+    _onLogController.close();
     _onAssetController.close();
     _onDoneController.close();
     _primarySubscription.cancel();
@@ -153,49 +168,39 @@ class TransformNode {
     // transform's outputs have gone unused, we switch it back to lazy mode.
     if (!_isLazy) return;
     _isLazy = false;
-    _dirty(primaryChanged: false);
+    _dirty();
   }
 
   /// Marks this transform as dirty.
   ///
   /// This causes all of the transform's outputs to be marked as dirty as well.
-  /// [primaryChanged] should be true if and only if [this] was set dirty
-  /// because [primary] changed.
-  void _dirty({bool primaryChanged: false}) {
-    if (!primaryChanged && _state.isNotPrimary) return;
+  void _dirty() {
+    if (_state == _State.NOT_PRIMARY) {
+      _emitPassThrough();
+      return;
+    }
+    if (_state == _State.COMPUTING_IS_PRIMARY || _isLazy) return;
 
     if (_passThroughController != null) _passThroughController.setDirty();
     for (var controller in _outputControllers.values) {
       controller.setDirty();
     }
 
-    if (_state.isDone) {
-      if (primaryChanged) {
-        _process();
-      } else {
-        _apply();
-      }
-    } else if (primaryChanged) {
-      _state = _TransformNodeState.NEEDS_IS_PRIMARY;
-    } else if (!_state.needsIsPrimary) {
-      _state = _TransformNodeState.NEEDS_APPLY;
+    if (_state == _State.APPLIED) {
+      _apply();
+    } else {
+      _state = _State.NEEDS_APPLY;
     }
   }
 
-  /// Determines whether [primary] is primary for [transformer], and if so runs
-  /// [transformer.apply].
-  void _process() {
-    // Clear all the old input subscriptions. If an input is re-used, we'll
-    // re-subscribe.
-    _clearInputSubscriptions();
-    _state = _TransformNodeState.PROCESSING;
-    primary.whenAvailable((_) {
-      _state = _TransformNodeState.PROCESSING;
-      return transformer.isPrimary(primary.asset);
-    }).catchError((error, stackTrace) {
-      // If the transform became dirty while processing, ignore any errors from
-      // it.
-      if (_state.needsIsPrimary || _isRemoved) return false;
+  /// Runs [transformer.isPrimary] and adjusts [this]'s state according to the
+  /// result.
+  ///
+  /// This will also run [_declareOutputs] and/or [_apply] as appropriate.
+  void _isPrimary() {
+    syncFuture(() => transformer.isPrimary(primary.id))
+        .catchError((error, stackTrace) {
+      if (_isRemoved) return false;
 
       // Catch all transformer errors and pipe them to the results stream. This
       // is so a broken transformer doesn't take down the whole graph.
@@ -203,62 +208,96 @@ class TransformNode {
 
       return false;
     }).then((isPrimary) {
+      if (_isRemoved) return null;
+      if (isPrimary) {
+        return _declareOutputs().then((_) {
+          if (_isRemoved) return;
+          if (_isLazy) {
+            _state = _State.APPLIED;
+            _onDoneController.add(null);
+          } else {
+            _apply();
+          }
+        });
+      }
+
+      _emitPassThrough();
+      _state = _State.NOT_PRIMARY;
+      _onDoneController.add(null);
+    });
+  }
+
+  /// Runs [transform.declareOutputs] and emits the resulting assets as dirty
+  /// assets.
+  Future _declareOutputs() {
+    if (transformer is! DeclaringTransformer) return new Future.value();
+
+    var controller = new DeclaringTransformController(this);
+    return syncFuture(() {
+      return (transformer as DeclaringTransformer)
+          .declareOutputs(controller.transform);
+    }).then((_) {
       if (_isRemoved) return;
-      if (_state.needsIsPrimary) {
-        _process();
-      } else if (isPrimary) {
-        _apply();
-      } else {
-        _clearOutputs();
-        _emitPassThrough();
-        _state = _TransformNodeState.NOT_PRIMARY;
-        _onDoneController.add(null);
+      if (controller.loggedError) return;
+
+      _consumePrimary = controller.consumePrimary;
+      _declaredOutputs = controller.outputIds;
+      var invalidIds = _declaredOutputs
+          .where((id) => id.package != phase.cascade.package).toSet();
+      for (var id in invalidIds) {
+        _declaredOutputs.remove(id);
+        // TODO(nweiz): report this as a warning rather than a failing error.
+        phase.cascade.reportError(new InvalidOutputException(info, id));
+      }
+
+      if (!_declaredOutputs.contains(primary.id)) _emitPassThrough();
+
+      for (var id in _declaredOutputs) {
+        var controller = transformer is LazyTransformer
+            ? new AssetNodeController.lazy(id, force, this)
+            : new AssetNodeController(id, this);
+        _outputControllers[id] = controller;
+        _onAssetController.add(controller.node);
       }
+    }).catchError((error, stackTrace) {
+      if (_isRemoved) return;
+      phase.cascade.reportError(_wrapException(error, stackTrace));
     });
   }
 
   /// Applies this transform.
   void _apply() {
-    assert(!_onAssetController.isClosed);
+    assert(!_isRemoved && !_isLazy);
 
     // Clear input subscriptions here as well as in [_process] because [_apply]
     // may be restarted independently if only a secondary input changes.
     _clearInputSubscriptions();
-    _state = _TransformNodeState.PROCESSING;
-    primary.whenAvailable((_) {
-      if (_state.needsIsPrimary) return null;
-      _state = _TransformNodeState.PROCESSING;
-      // TODO(nweiz): If [transformer] is a [DeclaringTransformer] but not a
-      // [LazyTransformer], we can get some mileage out of doing a declarative
-      // first so we know how to hook up the assets.
-      if (_isLazy) return _declareLazy();
-      return _applyImmediate();
-    }).catchError((error, stackTrace) {
-      // If the transform became dirty while processing, ignore any errors from
-      // it.
-      if (!_state.isProcessing || _isRemoved) return false;
-
-      // Catch all transformer errors and pipe them to the results stream. This
-      // is so a broken transformer doesn't take down the whole graph.
-      phase.cascade.reportError(_wrapException(error, stackTrace));
-      return true;
-    }).then((hadError) {
+    _state = _State.APPLYING;
+    _runApply().then((hadError) {
       if (_isRemoved) return;
 
-      if (_state.needsIsPrimary) {
-        _process();
-      } else if (_state.needsApply) {
+      if (_state == _State.NEEDS_APPLY) {
         _apply();
-      } else {
-        assert(_state.isProcessing);
-        if (hadError) {
-          _clearOutputs();
+        return;
+      }
+
+      assert(_state == _State.APPLYING);
+      if (hadError) {
+        _clearOutputs();
+        // If the transformer threw an error, we don't want to emit the
+        // pass-through asset in case it will be overwritten by the transformer.
+        // However, if the transformer declared that it wouldn't overwrite or
+        // consume the pass-through asset, we can safely emit it.
+        if (_declaredOutputs != null && !_consumePrimary &&
+            !_declaredOutputs.contains(primary.id)) {
+          _emitPassThrough();
+        } else {
           _dontEmitPassThrough();
         }
-
-        _state = _TransformNodeState.APPLIED;
-        _onDoneController.add(null);
       }
+
+      _state = _State.APPLIED;
+      _onDoneController.add(null);
     });
   }
 
@@ -276,123 +315,95 @@ class TransformNode {
       }
 
       _inputSubscriptions.putIfAbsent(node.id, () {
-        return node.onStateChange.listen((_) => _dirty(primaryChanged: false));
+        return node.onStateChange.listen((_) => _dirty());
       });
 
       return node.asset;
     });
   }
 
-  /// Applies the transform so that it produces concrete (as opposed to lazy)
-  /// outputs.
+  /// Run [Transformer.apply] as soon as [primary] is available.
   ///
-  /// Returns whether or not the transformer logged an error.
-  Future<bool> _applyImmediate() {
+  /// Returns whether or not an error occurred while running the transformer.
+  Future<bool> _runApply() {
     var transformController = new TransformController(this);
     _onLogPool.add(transformController.onLog);
 
-    return syncFuture(() {
-      return transformer.apply(transformController.transform);
+    return primary.whenAvailable((_) {
+      if (_isRemoved) return null;
+      _state = _State.APPLYING;
+      return syncFuture(() => transformer.apply(transformController.transform));
     }).then((_) {
-      if (!_state.isProcessing || _onAssetController.isClosed) return false;
+      if (_state == _State.NEEDS_APPLY || _isRemoved) return false;
       if (transformController.loggedError) return true;
-
-      _consumePrimary = transformController.consumePrimary;
-
-      var newOutputs = transformController.outputs;
-      // Any ids that are for a different package are invalid.
-      var invalidIds = newOutputs
-          .map((asset) => asset.id)
-          .where((id) => id.package != phase.cascade.package)
-          .toSet();
-      for (var id in invalidIds) {
-        newOutputs.removeId(id);
-        // TODO(nweiz): report this as a warning rather than a failing error.
-        phase.cascade.reportError(new InvalidOutputException(info, id));
-      }
-
-      // Remove outputs that used to exist but don't anymore.
-      for (var id in _outputControllers.keys.toList()) {
-        if (newOutputs.containsId(id)) continue;
-        _outputControllers.remove(id).setRemoved();
-      }
-
-      // Emit or stop emitting the pass-through asset between removing and
-      // adding outputs to ensure there are no collisions.
-      if (!newOutputs.containsId(primary.id)) {
-        _emitPassThrough();
-      } else {
-        _dontEmitPassThrough();
-      }
-
-      // Store any new outputs or new contents for existing outputs.
-      for (var asset in newOutputs) {
-        var controller = _outputControllers[asset.id];
-        if (controller != null) {
-          controller.setAvailable(asset);
-        } else {
-          var controller = new AssetNodeController.available(asset, this);
-          _outputControllers[asset.id] = controller;
-          _onAssetController.add(controller.node);
-        }
-      }
-
+      _handleApplyResults(transformController);
       return false;
+    }).catchError((error, stackTrace) {
+      // If the transform became dirty while processing, ignore any errors from
+      // it.
+      if (_state == _State.NEEDS_APPLY || _isRemoved) return false;
+
+      // Catch all transformer errors and pipe them to the results stream. This
+      // is so a broken transformer doesn't take down the whole graph.
+      phase.cascade.reportError(_wrapException(error, stackTrace));
+      return true;
     });
   }
 
-  /// Applies the transform in declarative mode so that it produces lazy
-  /// outputs.
+  /// Handle the results of running [Transformer.apply].
   ///
-  /// Returns whether or not the transformer logged an error.
-  Future<bool> _declareLazy() {
-    var transformController = new DeclaringTransformController(this);
-
-    return syncFuture(() {
-      return (transformer as LazyTransformer)
-          .declareOutputs(transformController.transform);
-    }).then((_) {
-      if (!_state.isProcessing || _onAssetController.isClosed) return false;
-      if (transformController.loggedError) return true;
-
-      _consumePrimary = transformController.consumePrimary;
+  /// [transformController] should be the controller for the [Transform] passed
+  /// to [Transformer.apply].
+  void _handleApplyResults(TransformController transformController) {
+    _consumePrimary = transformController.consumePrimary;
+
+    var newOutputs = transformController.outputs;
+    // Any ids that are for a different package are invalid.
+    var invalidIds = newOutputs
+        .map((asset) => asset.id)
+        .where((id) => id.package != phase.cascade.package)
+        .toSet();
+    for (var id in invalidIds) {
+      newOutputs.removeId(id);
+      // TODO(nweiz): report this as a warning rather than a failing error.
+      phase.cascade.reportError(new InvalidOutputException(info, id));
+    }
 
-      var newIds = transformController.outputIds;
-      var invalidIds =
-          newIds.where((id) => id.package != phase.cascade.package).toSet();
-      for (var id in invalidIds) {
-        newIds.remove(id);
-        // TODO(nweiz): report this as a warning rather than a failing error.
-        phase.cascade.reportError(new InvalidOutputException(info, id));
+    if (_declaredOutputs != null) {
+      var missingOutputs = _declaredOutputs.difference(
+          newOutputs.map((asset) => asset.id).toSet());
+      if (missingOutputs.isNotEmpty) {
+        _warn("This transformer didn't emit declared "
+            "${pluralize('output asset', missingOutputs.length)} "
+            "${toSentence(missingOutputs)}.");
       }
+    }
 
-      // Remove outputs that used to exist but don't anymore.
-      for (var id in _outputControllers.keys.toList()) {
-        if (newIds.contains(id)) continue;
-        _outputControllers.remove(id).setRemoved();
-      }
+    // Remove outputs that used to exist but don't anymore.
+    for (var id in _outputControllers.keys.toList()) {
+      if (newOutputs.containsId(id)) continue;
+      _outputControllers.remove(id).setRemoved();
+    }
 
-      // Emit or stop emitting the pass-through asset between removing and
-      // adding outputs to ensure there are no collisions.
-      if (!newIds.contains(primary.id)) {
-        _emitPassThrough();
-      } else {
-        _dontEmitPassThrough();
-      }
+    // Emit or stop emitting the pass-through asset between removing and
+    // adding outputs to ensure there are no collisions.
+    if (!_consumePrimary && !newOutputs.containsId(primary.id)) {
+      _emitPassThrough();
+    } else {
+      _dontEmitPassThrough();
+    }
 
-      for (var id in newIds) {
-        var controller = _outputControllers[id];
-        if (controller != null) {
-          controller.setLazy(force);
-        } else {
-          var controller = new AssetNodeController.lazy(id, force, this);
-          _outputControllers[id] = controller;
-          _onAssetController.add(controller.node);
-        }
+    // Store any new outputs or new contents for existing outputs.
+    for (var asset in newOutputs) {
+      var controller = _outputControllers[asset.id];
+      if (controller != null) {
+        controller.setAvailable(asset);
+      } else {
+        var controller = new AssetNodeController.available(asset, this);
+        _outputControllers[asset.id] = controller;
+        _onAssetController.add(controller.node);
       }
-
-      return false;
-    });
+    }
   }
 
   /// Cancels all subscriptions to secondary input nodes.
@@ -421,7 +432,9 @@ class TransformNode {
     if (_passThroughController == null) {
       _passThroughController = new AssetNodeController.from(primary);
       _onAssetController.add(_passThroughController.node);
-    } else {
+    } else if (primary.state.isDirty) {
+      _passThroughController.setDirty();
+    } else if (!_passThroughController.node.state.isAvailable) {
       _passThroughController.setAvailable(primary.asset);
     }
   }
@@ -441,75 +454,58 @@ class TransformNode {
     }
   }
 
+  /// Emit a warning about the transformer on [id].
+  void _warn(String message) {
+    _onLogController.add(
+        new LogEntry(info, primary.id, LogLevel.WARNING, message, null));
+  }
+
   String toString() =>
     "transform node in $_location for $transformer on $primary";
 }
 
 /// The enum of states that [TransformNode] can be in.
-class _TransformNodeState {
-  /// The transform node is running [Transformer.isPrimary] or
-  /// [Transformer.apply] and doesn't need to re-run them.
+class _State {
+  /// The transform is running [Transformer.isPrimary].
   ///
-  /// If there are no external changes by the time the processing finishes, this
-  /// will transition to [APPLIED] or [NOT_PRIMARY] depending on the result of
-  /// [Transformer.isPrimary]. If the primary input changes, this will
-  /// transition to [NEEDS_IS_PRIMARY]. If a secondary input changes, this will
-  /// transition to [NEEDS_APPLY].
-  static final PROCESSING = const _TransformNodeState._("processing");
-
-  /// The transform is running [Transformer.isPrimary] or [Transformer.apply],
-  /// but since it started the primary input changed, so it will need to re-run
-  /// [Transformer.isPrimary].
+  /// This is the initial state of the transformer. Once [Transformer.isPrimary]
+  /// finishes running, this will transition to [APPLYING] if the input is
+  /// primary, or [NOT_PRIMARY] if it's not.
+  static final COMPUTING_IS_PRIMARY = const _State._("computing isPrimary");
+
+  /// The transform is running [Transformer.apply].
   ///
-  /// This will always transition to [Transformer.PROCESSING].
-  static final NEEDS_IS_PRIMARY =
-    const _TransformNodeState._("needs isPrimary");
+  /// If an input changes while in this state, it will transition to
+  /// [NEEDS_APPLY]. If the [TransformNode] is still in this state when
+  /// [Transformer.apply] finishes running, it will transition to [APPLIED].
+  static final APPLYING = const _State._("applying");
 
-  /// The transform is running [Transformer.apply], but since it started a
-  /// secondary input changed, so it will need to re-run [Transformer.apply].
+  /// The transform is running [Transformer.apply], but an input changed after
+  /// it started, so it will need to re-run [Transformer.apply].
   ///
-  /// If there are no external changes by the time [Transformer.apply] finishes,
-  /// this will transition to [PROCESSING]. If the primary input changes, this
-  /// will transition to [NEEDS_IS_PRIMARY].
-  static final NEEDS_APPLY = const _TransformNodeState._("needs apply");
+  /// This will transition to [APPLYING] once [Transformer.apply] finishes
+  /// running.
+  static final NEEDS_APPLY = const _State._("needs apply");
 
   /// The transform has finished running [Transformer.apply], whether or not it
   /// emitted an error.
   ///
-  /// If the primary input or a secondary input changes, this will transition to
-  /// [PROCESSING].
-  static final APPLIED = const _TransformNodeState._("applied");
+  /// If the transformer is lazy, the [TransformNode] can also be in this state
+  /// when [Transformer.declareOutputs] has been run but [Transformer.apply] has
+  /// not.
+  ///
+  /// If an input changes, this will transition to [APPLYING].
+  static final APPLIED = const _State._("applied");
 
   /// The transform has finished running [Transformer.isPrimary], which returned
   /// `false`.
   ///
-  /// If the primary input changes, this will transition to [PROCESSING].
-  static final NOT_PRIMARY = const _TransformNodeState._("not primary");
-
-  /// Whether [this] is [PROCESSING].
-  bool get isProcessing => this == _TransformNodeState.PROCESSING;
-
-  /// Whether [this] is [NEEDS_IS_PRIMARY].
-  bool get needsIsPrimary => this == _TransformNodeState.NEEDS_IS_PRIMARY;
-
-  /// Whether [this] is [NEEDS_APPLY].
-  bool get needsApply => this == _TransformNodeState.NEEDS_APPLY;
-
-  /// Whether [this] is [APPLIED].
-  bool get isApplied => this == _TransformNodeState.APPLIED;
-
-  /// Whether [this] is [NOT_PRIMARY].
-  bool get isNotPrimary => this == _TransformNodeState.NOT_PRIMARY;
-
-  /// Whether the transform has finished running [Transformer.isPrimary] and
-  /// [Transformer.apply].
-  ///
-  /// Specifically, whether [this] is [APPLIED] or [NOT_PRIMARY].
-  bool get isDone => isApplied || isNotPrimary;
+  /// This will never transition to another state.
+  static final NOT_PRIMARY = const _State._("not primary");
 
   final String name;
 
-  const _TransformNodeState._(this.name);
+  const _State._(this.name);
 
   String toString() => name;
 }