Add more metadata to non-programmatic barback exceptions.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index 9ebca8e608ab57273230922ffd5d3fe3334d64d3..a379ae516f41217a23475e2ee8123c7ffb0f7aa6 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -7,7 +7,10 @@ library barback.errors;
 import 'dart:async';
 import 'dart:io';
 
+import 'package:stack_trace/stack_trace.dart';
+
 import 'asset_id.dart';
+import 'transformer.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
@@ -18,32 +21,99 @@ class AssetNotFoundException implements Exception {
   String toString() => "Could not find asset $id.";
 }
 
-/// Error thrown when two transformers both output an asset with [id].
-class AssetCollisionException implements Exception {
+/// The interface for exceptions from the barback graph or its transformers.
+///
+/// These exceptions are never produced by programming errors in barback.
+abstract class BarbackException implements Exception {}
+
+/// Error thrown when two or more transformers both output an asset with [id].
+class AssetCollisionException implements BarbackException {
+  /// All the transforms that output an asset with [id].
+  final Set<TransformInfo> transforms;
   final AssetId id;
 
-  AssetCollisionException(this.id);
+  AssetCollisionException(Iterable<TransformInfo> transforms, this.id)
+      : transforms = new Set.from(transforms);
 
-  String toString() => "Got collision on asset $id.";
+  String toString() => "Transforms $transforms all emitted asset $id.";
 }
 
 /// Error thrown when a transformer requests an input [id] which cannot be
 /// found.
-class MissingInputException implements Exception {
+class MissingInputException implements BarbackException {
+  /// The transform that requested [id].
+  final TransformInfo transform;
+  final AssetId id;
+
+  MissingInputException(this.transform, this.id);
+
+  String toString() => "Transform $transform tried to load missing input $id.";
+}
+
+/// Error thrown when a transformer outputs an asset to a different package than
+/// the primary input's.
+class InvalidOutputException implements BarbackException {
+  /// The transform that output the asset.
+  final TransformInfo transform;
   final AssetId id;
 
-  MissingInputException(this.id);
+  InvalidOutputException(this.transform, this.id);
+
+  String toString() => "Transform $transform emitted $id, which wasn't in the "
+      "same package (${transform.primaryId.package}).";
+}
+
+/// Error wrapping an exception thrown by a transform.
+class TransformerException implements BarbackException {
+  /// The transform that threw the exception.
+  final TransformInfo transform;
 
-  String toString() => "Missing input $id.";
+  /// The wrapped exception.
+  final error;
+
+  TransformerException(this.transform, this.error);
+
+  String toString() => "Transform $transform threw error: $error\n" +
+    new Trace.from(getAttachedStackTrace(error)).terse.toString();
 }
 
-/// Error thrown when a transformer outputs an asset with the wrong package
-/// name.
-class InvalidOutputException implements Exception {
-  final String package;
+/// Error thrown when a source asset [id] fails to load.
+///
+/// This can be thrown either because the source asset was expected to exist and
+/// did not or because reading it failed somehow.
+class AssetLoadException implements BarbackException {
   final AssetId id;
 
-  InvalidOutputException(this.package, this.id);
+  /// The wrapped exception.
+  final error;
+
+  AssetLoadException(this.id, this.error);
+
+  String toString() => "Failed to load source asset $id: $error\n"
+      "${new Trace.from(getAttachedStackTrace(error)).terse}";
+}
+
+/// Information about a single transform in the barback graph.
+///
+/// Identifies a single transformation in the barback graph.
+///
+/// A transformation is uniquely identified by the ID of its primary input, and
+/// the transformer that is applied to it.
+class TransformInfo {
+  /// The transformer that's run for this transform.
+  final Transformer transformer;
+
+  /// The id of this transform's primary asset.
+  final AssetId primaryId;
+
+  TransformInfo(this.transformer, this.primaryId);
+
+  bool operator==(other) =>
+      other is TransformInfo &&
+      other.transformer == transformer &&
+      other.primaryId == primaryId;
+
+  int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
-  String toString() => "Invalid output $id: must be in package $package.";
+  String toString() => "$transformer on $primaryId";
 }