watch for file system changes after context created

pkg/analysis_server/lib/src/context_manager.dart

 // Copyright (c) 2014, 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 context.directory.manager;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 426 matching lines @@
   }
 
   /**
    * Create a new empty context associated with [folder].
    */
   _ContextInfo _createContext(
       Folder folder, File pubspecFile, List<_ContextInfo> children) {
     _ContextInfo info = new _ContextInfo(
         folder, pubspecFile, children, normalizedPackageRoots[folder.path]);
     _contexts[folder] = info;
+    UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
+    info.context = addContext(folder, packageUriResolver);
+    info.context.name = folder.path;
+    // Subscribe to file changes after context has been set
+    // so that no changes will be received before we have a context
+    // (e.g. _computePackageUriResolver throws an exception)

[Paul Berry, 2015/06/18 20:24:42]

I don't understand why this is necessary.  Subscribing to file changes before
the context has been set is safe; what's unsafe is *processing* the file changes
after the context has been set.  But that's ok because we don't process change
notifications until the next time the event loop runs, so even without this CL,
we should be fine--by the time we process the notification, the context will
have been set.  Are you quite sure that this is what's going on with bug 23671? 
It seems like it can't possibly be what's happening.

Delaying the subscription until after creating the context has an additional
disadvantage: it creates a race condition.  If the filesystem changes after
creating the context but before we start listening for changes, we'll miss those
changes.  This isn't just an academic concern--we see exactly these kinds of
issues when someone is doing a git operation like a rebase and it causes the
analyzer to rebuild the context.

[danrubel, 2015/06/22 11:22:56]

Good point. Per discussion I reworked this to add the watcher first, then
unsubscribe if there is an exception before the context is assigned.

     info.changeSubscription = folder.changes.listen((WatchEvent event) {
       _handleWatchEvent(folder, info, event);
     });
-    UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
-    info.context = addContext(folder, packageUriResolver);
-    info.context.name = folder.path;
     return info;
   }
 
   /**
    * Potentially create a new context associated with the given [folder].
    *
    * If there are subfolders with 'pubspec.yaml' files, separate contexts are
    * created for them and excluded from the context associated with the
    * [folder].
    *
@@ skipping 331 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }