Download to a temp dir and rename on success.

utils/pub/hosted_source.dart

 // Copyright (c) 2012, 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 hosted_source;
 
 import 'dart:io' as io;
 import 'dart:json';
 import 'dart:uri';
 
@@ skipping 55 matching lines @@
   /**
    * Downloads a package from the site and unpacks it.
    */
   Future<bool> install(PackageId id, String destPath) {
     var parsedDescription = _parseDescription(id.description);
     var name = parsedDescription.first;
     var url = parsedDescription.last;
 
     var fullUrl = "$url/packages/$name/versions/${id.version}.tar.gz";
 
-    return Futures.wait([httpGet(fullUrl), ensureDir(destPath)]).chain((args) {
-      return timeout(extractTarGz(args[0], args[1]), HTTP_TIMEOUT,
+    print('Downloading $id...');
+
+    // Download and extract the archive to a temp directory.
+    var tempDir;
+    return Futures.wait([httpGet(fullUrl), createTempDir()]).chain((args) {
+      tempDir = args[1];
+      return timeout(extractTarGz(args[0], tempDir), HTTP_TIMEOUT,
           'Timed out while fetching URL "$fullUrl".');
-    }).transformException((ex) {
-      // If the install failed, delete the directory. Prevents leaving a ghost
-      // directory in the system cache which would later make pub think the
-      // install succeeded.
-      // TODO(rnystrom): Use deleteDir() here when transformException() supports
-      // returning a future. Also remove dart:io import then.
-      new io.Directory(destPath).deleteRecursivelySync();
-
-      throw ex;
-    });
+    }).chain((_) {
+      // Now that the install has succeeded, move it to the real location in
+      // the cache. This ensures that we don't leave half-busted ghost
+      // directories in the user's pub cache if an install fails.
+      return renameDir(tempDir, destPath);
+    }).transform((_) => true);
   }
 
   /**
    * The system cache directory for the hosted source contains subdirectories
    * for each separate repository URL that's used on the system. Each of these
    * subdirectories then contains a subdirectory for each package installed
    * from that site.
    */
   String systemCacheDirectory(PackageId id) {
     var parsed = _parseDescription(id.description);
@@ skipping 43 matching lines @@
 
     var name = description["name"];
     if (name is! String) {
       throw new FormatException("The 'name' key must have a string value.");
     }
 
     var url = description.containsKey("url") ? description["url"] : defaultUrl;
     return new Pair<String, String>(name, url);
   }
 }