Change AboutResourceLoader::GetAboutResource to wait for inflight update task.

chrome/browser/chromeos/drive/change_list_loader.h

Index: chrome/browser/chromeos/drive/change_list_loader.h
diff --git a/chrome/browser/chromeos/drive/change_list_loader.h b/chrome/browser/chromeos/drive/change_list_loader.h
index 3715c860fecb323235890f5175ac08da4110caa8..c5c193882b5ede63597b5f2048469cb1c5271d57 100644
--- a/chrome/browser/chromeos/drive/change_list_loader.h
+++ b/chrome/browser/chromeos/drive/change_list_loader.h
@@ -85,37 +85,46 @@ class AboutResourceLoader {
     return cached_about_resource_.get();
   }
 
-  // Gets the about resource from the cache or the server. If the cache is
-  // availlavle, just runs |callback| with the cached about resource. If not,
-  // calls |UpdateAboutResource| passing |callback|.
+  // Gets the 'latest' about resource and asynchronously runs |callback|. I.e.,
+  // 1) If the last call to UpdateAboutResource call is in-flight, wait for it.
+  // 2) Otherwise, if the resource is cached, just returns the cached value.
+  // 3) If neither of the above hold, queries the API server by calling
+  //   |UpdateAboutResource|.
   void GetAboutResource(const google_apis::AboutResourceCallback& callback);
 
   // Gets the about resource from the server, and caches it if successful. This
   // function calls JobScheduler::GetAboutResource internally. The cache will be
   // used in |GetAboutResource|.
-  void UpdateAboutResource(
-      const google_apis::AboutResourceCallback& callback);
+  void UpdateAboutResource(const google_apis::AboutResourceCallback& callback);
 
  private:
   // Part of UpdateAboutResource().
   // This function should be called when the latest about resource is being
-  // fetched from the server. The retrieved about resoure is cloned, and one is
-  // cached and the other is passed to |callback|.
+  // fetched from the server. The retrieved about resource is cloned, and one is
+  // cached and the other is passed to callbacks associated with |task_id|.
   void UpdateAboutResourceAfterGetAbout(
-      const google_apis::AboutResourceCallback& callback,
+      int task_id,
       google_apis::GDataErrorCode status,
       scoped_ptr<google_apis::AboutResource> about_resource);
 
   JobScheduler* scheduler_;
   scoped_ptr<google_apis::AboutResource> cached_about_resource_;
+
+  // Identifier to denote the latest UpdateAboutResource call.
+  int current_update_task_id_;
+  // Mapping from each UpdateAboutResource task ID to the corresponding
+  // callbacks. Note that there will be multiple callbacks for a single task
+  // when GetAboutResource is called before the task completes.
+  std::map<int, std::vector<google_apis::AboutResourceCallback> >
+      pending_callbacks_;
+
   base::WeakPtrFactory<AboutResourceLoader> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(AboutResourceLoader);
 };
 
 // ChangeListLoader is used to load the change list, the full resource list,
-// and directory contents, from WAPI (codename for Documents List API)
-// or Google Drive API.  The class also updates the resource metadata with
-// the change list loaded from the server.
+// and directory contents, from Google Drive API.  The class also updates the
+// resource metadata with the change list loaded from the server.
 //
 // Note that the difference between "resource list" and "change list" is
 // subtle hence the two words are often used interchangeably. To be precise,