Only run at most 10 transformers at once in barback.

pkg/barback/lib/src/transform_node.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 107 matching lines @@
     var transform = createTransform(this, newOutputs, _log);
 
     // Clear all the old input subscriptions. If an input is re-used, we'll
     // re-subscribe.
     for (var subscription in _inputSubscriptions.values) {
       subscription.cancel();
     }
     _inputSubscriptions.clear();
 
     _isDirty = false;
-    return transformer.apply(transform).catchError((error) {
+
+    return phase.cascade.graph.transformerPool.checkOut().then((item) {

[Alan Knight, 2013/10/23 00:24:06]

That seems like a rather awkward way to get to the pool

[nweiz, 2013/10/23 04:11:25]

I agree. I'm planning a refactor that will make the "vertical" dependencies
(e.g. PackageGraph <-> AssetCascade <-> Phase <-> TransformNode) in barback more
clear.

+      return transformer.apply(transform).whenComplete(item.release);

[Alan Knight, 2013/10/23 00:24:06]

Is it worth defining a pool API that automatically acquires and releases. e.g.
   transformerPool.withResource( (resource) => transformer.apply(transform));
?

Also, is it right that in this expression we have a transformerPool, but we
don't obtain the transformer, we already have it, and we don't use item other
than implicitly. It might make sense if we're just limiting the number running
at once, but just wanted to be sure.

[nweiz, 2013/10/23 04:11:25]

Done.
Yeah; really what the pool is controlling is calling [apply]. I'm going to
rename it to [transformPool] to make that a bit clearer.

+    }).catchError((error) {
       // If the transform became dirty while processing, ignore any errors from
       // it.
       if (_isDirty) return;
 
       if (error is! MissingInputException) {
         error = new TransformerException(info, error);
       }
 
       // Catch all transformer errors and pipe them to the results stream.
       // This is so a broken transformer doesn't take down the whole graph.
@@ skipping 72 matching lines @@
   }
 
   void _log(AssetId asset, LogLevel level, String message, Span span) {
     // If the log isn't already associated with an asset, use the primary.
     if (asset == null) asset = primary.id;
     var info = new TransformInfo(transformer, primary.id);
     var entry = new LogEntry(info, asset, level, message, span);
     _onLogController.add(entry);
   }
 }