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

webkit/browser/fileapi/file_system_backend.h

 // Copyright (c) 2012 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 WEBKIT_BROWSER_FILEAPI_FILE_SYSTEM_BACKEND_H_
 #define WEBKIT_BROWSER_FILEAPI_FILE_SYSTEM_BACKEND_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 17 matching lines @@
 class AsyncFileUtil;
 class CopyOrMoveFileValidatorFactory;
 class FileSystemURL;
 class FileStreamWriter;
 class FileSystemContext;
 class FileSystemFileUtil;
 class FileSystemOperation;
 class FileSystemQuotaUtil;
 class WatcherManager;
 
+// Maximum value of length arguments used for
+// FileSystemBackend::CreateFileStreamReader().
+const int64 kMaximumLength = std::numeric_limits<int64>::max();

[hashimoto, 2014/09/04 11:21:25]

Why is this constant needed?

[mtomasz, 2014/09/05 01:04:40]

Having a call:

new FileStreamReader(0, kMaximumLength) is saying more than
new FileStreamReader(0, std::numeric_limits<int64>::max()).

It's also shorter, since we don't need to include <limits> everywhere we want to
use a maximum length for a file stream reader.

Finally, if we change the type from int64 to uint64, then we can just do it in
one place.

+
 // An interface for defining a file system backend.
 //
 // NOTE: when you implement a new FileSystemBackend for your own
 // FileSystem module, please contact to kinuko@chromium.org.
 //
 class STORAGE_EXPORT FileSystemBackend {
  public:
   // Callback for InitializeFileSystem.
   typedef base::Callback<void(const GURL& root_url,
                               const std::string& name,
@@ skipping 53 matching lines @@
   // read/write implementation.
   virtual bool HasInplaceCopyImplementation(FileSystemType type) const = 0;
 
   // Creates a new file stream reader for a given filesystem URL |url| with an
   // offset |offset|. |expected_modification_time| specifies the expected last
   // modification if the value is non-null, the reader will check the underlying
   // file's actual modification time to see if the file has been modified, and
   // if it does any succeeding read operations should fail with
   // ERR_UPLOAD_FILE_CHANGED error.
   // This method itself does *not* check if the given path exists and is a
-  // regular file.
-  // The |length| argument says how many bytes are going to be read using the
-  // instance of the file stream reader. If unknown, then equal to -1.
+  // regular file. At most |limit| bytes can be fetched from the file stream
+  // reader.
   virtual scoped_ptr<storage::FileStreamReader> CreateFileStreamReader(
       const FileSystemURL& url,
       int64 offset,
+      int64 limit,

[hashimoto, 2014/09/04 11:21:25]

It's not unclear from the word |limit| if this value is measured from the
beginning of the file, or from the offset.
How about using something like "max_length", "max_bytes_to_read", or anything
which clearly says this value is the number of bytes measured from |offset|?

[mtomasz, 2014/09/05 01:04:40]

This naming OFFSET and LIMIT is used widely in databases, and it means a limit
starting from the offset. max_length is not much better here since it may mean a
length of the file. max_bytes_to_read makes sense. Let's go with it.

       const base::Time& expected_modification_time,
       FileSystemContext* context) const = 0;
 
   // Creates a new file stream writer for a given filesystem URL |url| with an
   // offset |offset|.
   // This method itself does *not* check if the given path exists and is a
   // regular file.
   virtual scoped_ptr<FileStreamWriter> CreateFileStreamWriter(
       const FileSystemURL& url,
       int64 offset,
@@ skipping 28 matching lines @@
         const std::string& extension_id) = 0;
   // Gets virtual path by known filesystem path. Returns false when filesystem
   // path is not exposed by this provider.
   virtual bool GetVirtualPath(const base::FilePath& file_system_path,
                               base::FilePath* virtual_path) = 0;
 };
 
 }  // namespace storage
 
 #endif  // WEBKIT_BROWSER_FILEAPI_FILE_SYSTEM_BACKEND_H_