Add truncate and cancel for FileWriter; write and tests will come in later CL...

chrome/browser/file_system/file_system_dispatcher_host.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/file_system/file_system_dispatcher_host.h"
 
 #include "base/file_path.h"
 #include "base/thread.h"
 #include "base/time.h"
 #include "chrome/browser/browser_process.h"
@@ skipping 220 matching lines @@
   GetNewOperation(request_id)->TouchFile(
       path, last_access_time, last_modified_time);
 }
 
 void FileSystemDispatcherHost::OnCancel(
     int request_id,
     int request_id_to_cancel) {
   fileapi::FileSystemOperation* write =
       operations_.Lookup(request_id_to_cancel);
   if (write) {
-    write->Cancel();
-    Send(new ViewMsg_FileSystem_DidSucceed(request_id));
+    // The cancel will eventually send both the write failure and the cancel
+    // success.
+    write->Cancel(GetNewOperation(request_id));
   } else {
+    // The write already finished; report that we failed to stop it.
     Send(new ViewMsg_FileSystem_DidFail(
         request_id, base::PLATFORM_FILE_ERROR_INVALID_OPERATION));
   }
 }
 
 void FileSystemDispatcherHost::Send(IPC::Message* message) {
   DCHECK(ChromeThread::CurrentlyOn(ChromeThread::IO));
   if (!shutdown_ && message_sender_)
     message_sender_->Send(message);
   else
@@ skipping 20 matching lines @@
       dispatcher,
       ChromeThread::GetMessageLoopProxyForThread(ChromeThread::FILE));
   operations_.AddWithID(operation, request_id);
   return operation;
 }
 
 void FileSystemDispatcherHost::RemoveCompletedOperation(int request_id) {
   DCHECK(operations_.Lookup(request_id));
   operations_.Remove(request_id);
 }