Make HistoryDBTask not refcounted, and ensure it's destroyed on its origin thread.

chrome/browser/history/history_backend.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_HISTORY_HISTORY_BACKEND_H_
 #define CHROME_BROWSER_HISTORY_HISTORY_BACKEND_H_
 
 #include <set>
 #include <string>
 #include <utility>
@@ skipping 33 matching lines @@
 struct DownloadRow;
 
 // The maximum number of icons URLs per page which can be stored in the
 // thumbnail database.
 static const size_t kMaxFaviconsPerPage = 8;
 
 // The maximum number of bitmaps for a single icon URL which can be stored in
 // the thumbnail database.
 static const size_t kMaxFaviconBitmapsPerIconURL = 8;
 
+// Keeps track of a queued HistoryDBTask. This class lives solely on the
+// DB thread.
 class QueuedHistoryDBTask {
  public:
   QueuedHistoryDBTask(
-      scoped_refptr<HistoryDBTask> task,
+      scoped_ptr<HistoryDBTask> task,
       scoped_refptr<base::SingleThreadTaskRunner> origin_loop,
       const base::CancelableTaskTracker::IsCanceledCallback& is_canceled);
   ~QueuedHistoryDBTask();
 
   bool is_canceled();
-  bool RunOnDBThread(HistoryBackend* backend, HistoryDatabase* db);
-  void DoneRunOnMainThread();
+  bool Run(HistoryBackend* backend, HistoryDatabase* db);
+  void DoneRun();
 
  private:
-  scoped_refptr<HistoryDBTask> task_;
+  scoped_ptr<HistoryDBTask> task_;
   scoped_refptr<base::SingleThreadTaskRunner> origin_loop_;
   base::CancelableTaskTracker::IsCanceledCallback is_canceled_;
+
+  DISALLOW_COPY_AND_ASSIGN(QueuedHistoryDBTask);
 };
 
 // *See the .cc file for more information on the design.*
 //
 // Internal history implementation which does most of the work of the history
 // system. This runs on a background thread (to not block the browser when we
 // do expensive operations) and is NOT threadsafe, so it must only be called
 // from message handlers on the background thread. Invoking on another thread
 // requires threadsafe refcounting.
 //
@@ skipping 286 matching lines @@
       const std::vector<SearchRow::ColumnID>& projections,
       const std::string& selection,
       const std::vector<base::string16>& selection_args,
       const std::string& sort_order);
 
 #endif  // defined(OS_ANDROID)
 
   // Generic operations --------------------------------------------------------
 
   void ProcessDBTask(
-      scoped_refptr<HistoryDBTask> task,
+      scoped_ptr<HistoryDBTask> task,
       scoped_refptr<base::SingleThreadTaskRunner> origin_loop,
       const base::CancelableTaskTracker::IsCanceledCallback& is_canceled);
 
   virtual bool GetAllTypedURLs(URLRows* urls);
 
   virtual bool GetVisitsForURL(URLID id, VisitVector* visits);
 
   // Fetches up to |max_visits| most recent visits for the passed URL.
   virtual bool GetMostRecentVisitsForURL(URLID id,
                                          int max_visits,
@@ skipping 467 matching lines @@
   base::Closure backend_destroy_task_;
 
   // Tracks page transition types.
   VisitTracker tracker_;
 
   // A boolean variable to track whether we have already purged obsolete segment
   // data.
   bool segment_queried_;
 
   // List of QueuedHistoryDBTasks to run;
-  std::list<QueuedHistoryDBTask> queued_history_db_tasks_;
+  std::list<QueuedHistoryDBTask*> queued_history_db_tasks_;
 
   // Used to determine if a URL is bookmarked; may be NULL.
   //
   // Use GetHistoryClient to access this, which makes sure the bookmarks are
   // loaded before returning.
   HistoryClient* history_client_;
 
 #if defined(OS_ANDROID)
   // Used to provide the Android ContentProvider APIs.
   scoped_ptr<AndroidProviderBackend> android_provider_backend_;
@@ skipping 11 matching lines @@
 
   // Listens for the system being under memory pressure.
   scoped_ptr<base::MemoryPressureListener> memory_pressure_listener_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // CHROME_BROWSER_HISTORY_HISTORY_BACKEND_H_