[fsp] Improve performance for reading small chunks of data.

chrome/browser/chromeos/file_system_provider/fileapi/buffering_file_stream_reader.h

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_FILEAPI_BUFFERING_FILE_STREAM_READER_H_
 #define CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_FILEAPI_BUFFERING_FILE_STREAM_READER_H_
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "webkit/browser/blob/file_stream_reader.h"
 
 namespace net {
 class IOBuffer;
 }  // namespace net
 
 namespace chromeos {
 namespace file_system_provider {
 
 // Wraps the file stream reader implementation with a prefetching buffer.
 // Reads data from the internal file stream reader in chunks of size at least
-// |buffer_size| bytes (or less for the last chunk, because of EOF).
+// |preloading_buffer_length| bytes (or less for the last chunk, because of
+// EOF). If total number of bytes to be downloaded is known, it should be passed
+// as |bytes_to_read|, otherwise it must be -1.
 //
-// The underlying inner file stream reader *must not* return any values
+// The underlying internal file stream reader *must not* return any values
 // synchronously. Instead, results must be returned by a callback, including
 // errors.
 class BufferingFileStreamReader : public webkit_blob::FileStreamReader {
  public:
   BufferingFileStreamReader(
       scoped_ptr<webkit_blob::FileStreamReader> file_stream_reader,
-      int buffer_size);
+      int preloading_buffer_length,
+      int64 bytes_to_read);
 
   virtual ~BufferingFileStreamReader();
 
   // webkit_blob::FileStreamReader overrides.
   virtual int Read(net::IOBuffer* buf,
                    int buf_len,
                    const net::CompletionCallback& callback) OVERRIDE;
   virtual int64 GetLength(
       const net::Int64CompletionCallback& callback) OVERRIDE;
 
  private:
   // Copies data from the preloading buffer and updates the internal iterator.
   // Returns number of bytes successfully copied.
   int CopyFromPreloadingBuffer(scoped_refptr<net::IOBuffer> buffer,
                                int buffer_length);
 
   // Preloads data from the internal stream reader and calls the |callback|.
   void Preload(const net::CompletionCallback& callback);
 
+  void OnReadCompleted(const net::CompletionCallback& callback, int result);
+
   // Called when preloading of a buffer chunk is finished. Updates state of the
   // preloading buffer and copied requested data to the |buffer|.
   void OnPreloadCompleted(scoped_refptr<net::IOBuffer> buffer,
                           int buffer_length,
                           const net::CompletionCallback& callback,
                           int result);
 
   scoped_ptr<webkit_blob::FileStreamReader> file_stream_reader_;
-  int buffer_size_;
+  int preloading_buffer_length_;
+  int64 bytes_to_read_;
+  int64 bytes_read_;
   scoped_refptr<net::IOBuffer> preloading_buffer_;
   int preloading_buffer_offset_;
-  int buffered_bytes_;
+  int preloaded_bytes_;
 
   base::WeakPtrFactory<BufferingFileStreamReader> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(BufferingFileStreamReader);
 };
 
 }  // namespace file_system_provider
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_FILEAPI_BUFFERING_FILE_STREAM_READER_H_