[fsp] Do not allow write operations on read only file systems.

chrome/browser/chromeos/file_system_provider/operations/truncate.cc

 // Copyright 2014 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/chromeos/file_system_provider/operations/truncate.h"
 
 #include <string>
 
 #include "chrome/common/extensions/api/file_system_provider.h"
 #include "chrome/common/extensions/api/file_system_provider_internal.h"
@@ skipping 10 matching lines @@
     : Operation(event_router, file_system_info),
       file_path_(file_path),
       length_(length),
       callback_(callback) {
 }
 
 Truncate::~Truncate() {
 }
 
 bool Truncate::Execute(int request_id) {
+  if (!file_system_info_.writable())
+    return false;
+
   scoped_ptr<base::DictionaryValue> values(new base::DictionaryValue);
   values->SetString("filePath", file_path_.AsUTF8Unsafe());
   values->SetDouble("length", length_);
 
   return SendEvent(
       request_id,
       extensions::api::file_system_provider::OnTruncateRequested::kEventName,
       values.Pass());
 }
 
 void Truncate::OnSuccess(int /* request_id */,
                          scoped_ptr<RequestValue> /* result */,
                          bool has_more) {
   callback_.Run(base::File::FILE_OK);
 }
 
 void Truncate::OnError(int /* request_id */,
                        scoped_ptr<RequestValue> /* result */,
                        base::File::Error error) {
   callback_.Run(error);
 }
 
 }  // namespace operations
 }  // namespace file_system_provider
 }  // namespace chromeos