watch for file system changes after context created

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index bfa8cb6fbd0c7fd383aefbde12a7d6ab2d3be71c..3abc54de15ab441685f44dff9ac57ce8f835edc8 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -444,12 +444,15 @@ abstract class ContextManager {
     _ContextInfo info = new _ContextInfo(
         folder, pubspecFile, children, normalizedPackageRoots[folder.path]);
     _contexts[folder] = info;
-    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;
+    // 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);
+    });
     return info;
   }