Add the ability for objects which derive from RefCountedThreadSafe to specify...

chrome/browser/chrome_thread.h

 // Copyright (c) 2009 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_CHROME_THREAD_H_
 #define CHROME_BROWSER_CHROME_THREAD_H_
 
 #include "base/lock.h"
 #include "base/task.h"
 #include "base/thread.h"
@@ skipping 107 matching lines @@
   //   When running under unit-tests, this will return true if you're on the
   //   main thread and the thread ID you pass in isn't running.  This is
   //   normally the correct behavior because you want to ignore these asserts
   //   unless you've specifically spun up the threads, but be mindful of it.
   static bool CurrentlyOn(ID identifier);
 
   // If the current message loop is one of the known threads, returns true and
   // sets identifier to its ID.  Otherwise returns false.
   static bool GetCurrentThreadIdentifier(ID* identifier);
 
+  // Use these templates in conjuction with RefCountedThreadSafe when you want
+  // to ensure that an object is deleted on a specific thread.  This is needed
+  // when an object can hop between threads (i.e. IO -> FILE -> IO), and thread
+  // switching delays can mean that the final IO tasks executes before the FILE
+  // task's stack unwinds.  This would lead to the object destructing on the
+  // FILE thread, which often is not what you want (i.e. to unregister from
+  // NotificationService, to notify other objects on the creating thread etc).
+  template<ID thread>
+  struct DeleteOnThread {
+    template<typename T>
+    static void Destruct(T* x) {
+      if (CurrentlyOn(thread)) {
+        delete x;
+      } else {
+        DeleteSoon(thread, FROM_HERE, x);
+      }
+    }
+  };
+
+  // Sample usage:
+  // class Foo
+  //     : public base::RefCountedThreadSafe<
+  //           Foo, ChromeThread::DeleteOnIOThread> {
+  struct DeleteOnUIThread : public DeleteOnThread<UI> { };
+  struct DeleteOnIOThread : public DeleteOnThread<IO> { };
+  struct DeleteOnFileThread : public DeleteOnThread<FILE> { };
+  struct DeleteOnDBThread : public DeleteOnThread<DB> { };
+  struct DeleteOnWebKitThread : public DeleteOnThread<WEBKIT> { };
+
  private:
   // Common initialization code for the constructors.
   void Initialize();
 
   static bool PostTaskHelper(
       ID identifier,
       const tracked_objects::Location& from_here,
       Task* task,
       int64 delay_ms,
       bool nestable);
 
   // The identifier of this thread.  Only one thread can exist with a given
   // identifier at a given time.
   ID identifier_;
 
   // This lock protects |chrome_threads_|.  Do not read or modify that array
   // without holding this lock.  Do not block while holding this lock.
   static Lock lock_;
 
   // An array of the ChromeThread objects.  This array is protected by |lock_|.
   // The threads are not owned by this array.  Typically, the threads are owned
   // on the UI thread by the g_browser_process object.  ChromeThreads remove
   // themselves from this array upon destruction.
   static ChromeThread* chrome_threads_[ID_COUNT];
 };
 
 #endif  // #ifndef CHROME_BROWSER_CHROME_THREAD_H_