Use backtracking when solving dependency constraints.

utils/pub/path_source.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 path_source;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../../pkg/pathos/lib/path.dart' as path;
@@ skipping 15 matching lines @@
 
   Future<Pubspec> describe(PackageId id) {
     return defer(() {
       _validatePath(id.name, id.description);
       return new Pubspec.load(id.name, id.description["path"],
           systemCache.sources);
     });
   }
 
   bool descriptionsEqual(description1, description2) {
-    // Compare real paths after normalizing and resolving symlinks.
-    var path1 = new File(description1["path"]).fullPathSync();
-    var path2 = new File(description2["path"]).fullPathSync();
-    return path1 == path2;
+    try {

[nweiz, 2013/03/29 01:58:25]

I don't understand why this change is in this CL.

[Bob Nystrom, 2013/03/30 00:15:55]

The previous code will fail if you call descriptionsEqual() and either path
doesn't exist. We have tests specifically for working with path deps to
non-existent paths. I believe they passed coincidentally because the old solver
happened to not hit this before it failed to find a solution, but the new solver
does end up comparing descriptions and would crash.

I think this change is a decent one: I don't think descriptionsEqual() should
ever throw given valid arguments.

+      // Compare real paths after normalizing and resolving symlinks.
+      var path1 = new File(description1["path"]).fullPathSync();
+      var path2 = new File(description2["path"]).fullPathSync();
+      return path1 == path2;
+    } on FileIOException catch (ex) {
+      // If either of the files couldn't be found, fallback to just comparing

[nweiz, 2013/03/29 01:58:25]

"fallback" -> "fall back"

[Bob Nystrom, 2013/03/30 00:15:55]

Done.

+      // the normalized paths.
+      var path1 = path.normalize(description1["path"]);
+      var path2 = path.normalize(description2["path"]);

[nweiz, 2013/03/29 01:58:25]

Shouldn't you make these absolute here?

[Bob Nystrom, 2013/03/30 00:15:55]

Done.

+      return path1 == path2;
+    }
   }
 
   Future<bool> install(PackageId id, String destination) {
     return defer(() {
       try {
         _validatePath(id.name, id.description);
       } on FormatException catch(err) {
         return false;
       }
 
@@ skipping 62 matching lines @@
     // confused as files.
     if (fileExists(dir)) {
       throw new FormatException(
           "Path dependency for package '$name' must refer to a "
           "directory, not a file. Was '$dir'.");
     }
 
     throw new FormatException("Could not find package '$name' at '$dir'.");
   }
 }