AU: Speed up updates by using buffered writes.

file_writer.h

 // Copyright (c) 2009 The Chromium OS 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_WRITER_H__
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_WRITER_H__
 
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
@@ skipping 36 matching lines @@
   virtual ~DirectFileWriter() {}
 
   virtual int Open(const char* path, int flags, mode_t mode);
   virtual ssize_t Write(const void* bytes, size_t count);
   virtual int Close();
 
   int fd() const { return fd_; }
 
  private:
   int fd_;
-  
+
   DISALLOW_COPY_AND_ASSIGN(DirectFileWriter);
 };
 
 class ScopedFileWriterCloser {
  public:
   explicit ScopedFileWriterCloser(FileWriter* writer) : writer_(writer) {}
   ~ScopedFileWriterCloser() {
     int err = writer_->Close();
     if (err)
       LOG(ERROR) << "FileWriter::Close failed: "
                  << utils::ErrnoNumberAsString(-err);
   }
  private:
   FileWriter* writer_;
-  
+
   DISALLOW_COPY_AND_ASSIGN(ScopedFileWriterCloser);
 };
 
 }  // namespace chromeos_update_engine
 
 #endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_WRITER_H__